The annual ACSM (American Congress on Surveying and Mapping) isn’t what it used to be. Attendance was way down and the number of exhibitors is way down. The technical content, however, was still pretty good. In fact, I’ve included links to several videos I recorded at the ACSM/GITA conference.
This year, the ACSM conference was co-located with the GITA (Geospatial Infrastructure & Technology Association) annual conference. This made the trip worthwhile. By themselves, both conferences are becoming too small for most attendees (and therefore, exhibitors) to attend.
GITA is a GIS conference targeted at the global geospatial community, but in reality it attracts infrastructure geospatial users such as electric/gas/water utilities and local government.
This mix of ACSM and GITA is interesting and was a great opportunity for surveyors. While the economy is starving surveyors who are in the typical boundary and land development markets, the GITA crowd, in my estimation, are in dire need of a GIS-versed land surveyor.
There are many topics that were interesting and I thoroughly enjoyed most of the ones I attended, but there are two points I want to address about this conference:
Surveying/GIS collaboration discussion
Surveying Body of Knowledge discussion
If I can write fast enough, there is a third I’d like to tackle regarding the Driven By Data discussion. If not in this column, I’m sure I will touch on it in a future column or maybe in my Geospatial Solutions Weekly column.
Surveying/GIS Collaboration
One of the major benefits of co-locating the ACSM and GITA conferences is that it gives attendees a chance to mix it up with the “other side.” History has consistently demonstrated that it’s always easier to view the “other side” with a certain level of antipathy from afar. However, when one learns more intimately about the adversary’s intentions and struggles, that antipathy eventually turns towards empathy and appreciation. I recall listening to a US Veteran of World War II talking about fighting the enemy. I’m paraphrasing, but it went something like this:
“I believed in what we were doing and fighting the enemy was just doing my job. In those circumstances, we were enemies. Under peaceful circumstances, however, we may have been neighbors and we may have even been good friends.”
Land surveyors and GIS folks should be good friends. They both have a lot to gain from a positive relationship and a lot to lose with an adversarial relationship, with the former standing to lose the most.
Rudy Stricklin presented a very good session entitled “Professional Land Surveyors and Geospatial Professionals Building Bridges in Arizona.” In the presentation, he describes the process surveyors and GIS folks went through in Arizona to collaborate and find a common ground to work from. I’m not saying I necessarily agree with everything that was presented or enacted in Arizona, but Rudy’s consistent and often used terms like “collaboration” and “inclusive” certainly conveyed the team-building spirit and positive attitude needed to build a long-term relationship. The bridge-building process presented by Rudy is a model that would be difficult to go wrong with in a similar endeavor by another state, province or local/regional government.
I recorded the presentation in its entirety. It’s in five parts with each being about 10 minutes in length. I suggest listening to the first segment as he paints the broad picture. However, the entire presentation is well worth your time.
There was also a discussion panel entitled “GIS/Surveying Geospatial Collaboration.” On the panel was Gene Trobia, Arizona State Cartographer, Jack Avis, PLS, and Bill Coleman, PLS. Jack and Bill are both land surveyors who offer GIS services.
Gene has some great stories about the early ESRI years and GIS challenges. He recalled there were 37 people at the first ESRI User Conference he attended.
Watch this 85 second description by Gene of the challenge faced by GIS managers explaining why some are myopic.
I posed a couple of questions to the panel.
The first was the subject of a National Parcel Database with references to the First American parcel database.
The second question I posed was how can a small surveying firm that is focused on boundary and mortgage surveys (and is starving) can transition to offering GIS services.
The focus of this presentation/discussion was to define the role (Body of Knowledge) of professional surveyors in the 21st century.
Why develop a Surveying Body of Knowledge (BoK)?
According to the committee (the folks above plus Bob Burton, PLS, PE and Bob Dahn, PLS), the Surveying BoK was developed to:
Formulate a scope of the surveying profession.
Promote recognition for the need for college education.
To help surveyors in business development.
To develop a surveying scholarship
To help promote the surveying profession.
To define the distinctiveness of the surveying profession.
The Surveying BoK Committee has defined the surveying profession to encompass the following disciplines:
Positioning
Imaging
GIS
Law
Land development
The discussion was led by Josh Greenfeld with Earl and Joe presenting on Positioning, Josh presenting for Robert Burtch on Imaging, Wendy presenting on Law, Josh presenting on GIS, and Wendy presenting on Land Development.
Often referred to as the world’s second-oldest profession, it’s ironic that land surveyors are trying to redefine themselves after thousands of years. But, technology has forced them to face reality. I can’t say I wouldn’t do the same thing. I would say, however, that it’s late in the game for this. Of course, hindsight is 20-20, but this effort really should have begun 10 years ago. Someone dropped the ball.
Regardless, I think they’ve got the right idea. The BoK committee consists of pe
ople who are highly respected in the surveying profession. The BoK document is not perfect (and they recognize that and are looking for input), but it’s a step in defining the future of the surveying professional.
I think expanding the horizons of the land surveyor to include the five disciplines (positioning, imaging, GIS, law and land development) is a great idea. This would expand the profession significantly as it would paint a much more current and accurate picture of the knowledge and skillset a student could strive to achieve if they chose surveying as a profession to pursue. A Surveying Body of Knowledge (BoK) doesn’t exist today so it’s difficult to paint a picture and describe the knowledge and skillset much beyond that of boundary surveyor.
However, I want to make what I feel is a very important point
I mentioned this during the discussion and I’ll write it here. If one of the purposes of this document is to take it and run to the state legislature to have it legally define the land surveyor’s domain (and therefore eliminate others from operating in that space), I would vehemently oppose it. Honestly, I got that weird feeling when Dr. Greenfeld made a comment early in his presentation that one of the Surveying BoK purposes was to be used “to define the distinctiveness of the profession against those who are trying to encroach on our profession [because] there are a lot of cases like this.” In other words, he’d like positioning, imaging, GIS, law (as related to surveying) and land development to be the exclusive domain of the land surveyor. That would be a mistake, a HUGE mistake. After the discussion group, I asked Dr. Greenfeld about this remark. He dismissed the premise with the thought that laws can be changed and that a larger group with more resources could overturn such a law if there was enough dissent.
The reason I think it would be a huge mistake is because it limits competition. It’s common knowledge that competition breeds innovation. Henry Ford said “you can have any color (automobile) you want, as long as it’s black.” Without competition, you may still be driving a black automobile without air conditioning. Of course, all-out competition is not the answer either. Just like in politics, the right answer is not at either extreme, but somewhere in the middle.
As a side note, here is a short clip from the audience regarding the importance of communication skills in the education of land surveyors.
General C. Robert Kehler, Commander of the U.S. Air Force Space Command
Editor Don Jewell Talks with the Air Force General Heading Space Command: His Views, Use, and Plans for GPS
Defense editor Don Jewell is a retired Air Force officer who served for 30 years; many of his former peers and contemporaries are currently senior officers in today’s U.S. Air Force. Don sat down recently with General C. Robert Kehler, Commander of the U.S. Air Force Space Command, whom he has known and worked with for more than 20 years, to discuss GPS from the four-star point of view.
Don Jewell (DJ): General Kehler, thanks for taking the time to have this discussion today. I would like to keep this very informal, more of a conversation, like the days when you and I and Willie Shelton [now Lt. Gen. Shelton, USAF] sat around on your lanai, sharing a brew, telling war stories, and solving the world’s problems.
General Kehler (GK): Believe me, Don, there are days when I wish we were still doing that. I appreciate the opportunity to have a conversation with you.
DJ:Great. Sir, to get to the crux of the matter, as the senior warfighter for space, how do you see GPS in the future, and how does it contribute to the joint fight?
GK: Don, you know this, as may many of your readers at GPS World, but I don’t believe we can say it often enough: GPS is the primary source of position, navigation, and timing (PNT) information for the Department of Defense, and it will remain that way at least until the year 2030. This has been a remarkably successful program, supporting the joint warfighter in nearly every aspect of joint operations. How GPS supports joint operations, whether it’s the individual soldier, sailor, airman, Marine, or Coast Guardsman, who is on the ground or inflight or who happens to be in the dark in a mountainous region somewhere or in the flat expanse of the desert — it doesn’t much matter. GPS has been their constant companion now for many years. They have come to rely on GPS in ways that help them do their job better, and it allows them to perform missions that in the past they would not have been able to perform in this kind of a manner, with this kind of perfectness.
GPS is going to remain the foundation of the PNT strategy. And with the modernization effort that we have underway in GPS, we are going to make sure that it remains the world’s premier source of position, navigation, and timing information, and in particular that it remains woven through the fabric of the joint warfighting network.
DJ:This portends an excellent future for GPS, despite comments by the Air Force Chief of Staff and Gen. “Hoss” Cartwright, vice chairman of the Joint Chiefs of Staff, that we should move away from GPS. Do the Chief’s comments cause you any concern?
GK:They do not cause me any concern. We are committed to keeping GPS the gold standard. We have a commitment in that regard. I understand exactly what the Chief of Staff said and why. I will be happy to discuss that more.
DJ: We’ll table that for now, and get to it later if we have the time. I have often heard you say in your GPS update and status briefings that GPS is one of your good systems. Indeed, you have described it as one of the systems you don’t have to worry about too much, because it works. It would be interesting to ascertain how you know when you are doing a good job with GPS. How do you know it works? For example, do you receive comments, e-mails, or letters from warfighters?
GK: I think there are really two big ways that we know we are doing a good job with GPS. First of all, we measure our performance against the standard. What the users see, of course, is accuracy and satellite availability. Those have become our two primary standards. We make sure we are performing up to those standards. And in fact, as you know, we continually outperform those documented standards and the requirements that we have.
We also look, not only at the satellites, but at the ground command and control (C2) system and the ground support network. We make sure those elements are always up and running as well. From a numbers standpoint, from a “how well are we meeting the standards we have set for ourselves” standpoint, we exceed those standards. We exceed in terms of accuracy and availability, both the satellite system and the ground-supporting infrastructure as well.
But these days, I will tell you, I think the numbers are interesting, but what I think we look at just as hard is how the public talks about GPS.
And if you look today, GPS, at least in my opinion, is everywhere in the public conscience. I was saying earlier today, you really don’t have to go much farther than your television set. Almost any evening you turn the TV on you’ll hear something about GPS. You’ll either hear people who are equating their product to GPS, or you’ll hear in a television show someone mention GPS or their GPS device. And that is without it being a program about the satellites themselves, or the U.S. Air Force, or the things we do at Schriever Air Force Base to make it all work.
My view is that the fact that we get this informal public feedback constantly, and that it’s positive, says a lot about how good a job we are doing as well. When your program becomes a new word in the English language, I think that says something about success. Any more, if you say GPS to people they might not point to a satellite, they might point to the little device they are holding in their hand, but they understand somebody is providing that for them and that it is working well.
The final piece to that is also our civil partners. You know we have a GPS Executive Committee (PNT ExCom) inside the government that meets periodically to have conversations about the way ahead on GPS for the entire government, and by extension for the United States. The feedback that we get at those meetings, and unfortunately I can’t get to every one of them, but in those that I have attended, the feedback has been universally positive.
We just had a Civil Focus Day recently, and the feedback we got was universally positive. Are there things we can do better? Yes, of course there are, there are always things you can do better, but I can tell that we are doing a good job with GPS, not only because of the numbers that we look at but because of the feedback that we get, and the way GPS has been accepted and adopted, if you will, as part of the lexicon.
DJ: You’re absolutely right about the positive feedback. I attended Civil Focus Day, wearing a different hat, as you know, and I agree, everybody was onboard and positive about GPS.
The next topic revolves around how your scorecard is graded by the joint community, and do you have a way of actually getting feedback from the warfighter?
GK: Yes, as I said, we are graded or we grade ourselves primarily on accuracy and availability as they are documented for us in the performance standards. In watching those numbers, we know that we are exceeding the performance standards that we set for ourselves. But we also receive feedback directly from the warfighters. We receive feedback from the military users through the GPS Operations Center (GPSOC). You know, and I think most of your readers know, that there is a way that you can directly contact what we call the GPSOC 24 hours a day, seven days a week, and we find that both our military and civilian users do that.
Another way that we receive feedback is through the Coa
st Guard Navigation Center (NAVCEN), where they are specifically watching and helping us watch the performance of GPS. We get feedback directly from them as well. But much like the prior topic, there are also other ways that we get feedback.
For example, in each of our theaters of operations, for each of our combatant commanders, the joint or combined force air component commander is also designated as the space coordinating authority. And working for that space coordinating authority in the AOC (Air Operations Center) is someone called the director of space forces, an Air Force officer who is responsible for making sure that the space support is there when it needs to be and in the fashion that it needs to be. Those directors of space forces also have a small staff working with the combined force air component commanders.
They are getting direct feedback from the warfighters as well. They are either getting it as a normal course of business, on a day-in day-out basis, or they are asking for it specifically as well. We are also getting direct feedback from the units themselves. We have made contact through a number of our forward space people. We work with Army Space and Missile Defense Command and as a matter of fact we have talked with the Marines and others directly. We don’t wait for their feedback, we go out and solicit it also, and we actually help them solve some very difficult problems that we had early on in the conflict with some of our weapons systems that we have now fixed.
We are mindful, we know when certain operations are underway, we deconflict that with activities in the [GPS] constellation, making sure that we are providing the very best service all the time. We are embedded through the planning process in the theaters with military operations and with space professionals who are in the planning cells and Air Operations Centers. We are very comfortable. We are getting constant feedback from the warfighters in addition to the scoring we do ourselves and against the performance standards.
DJ: As you know, in many of my articles I frequently comment that where GPS is concerned, geometry and numbers matter. In that regard you recently approved a 24+3 GPS constellation change. Now we get a good many letters from warfighters at GPS World, and some letters are all about GPS accuracy as you spoke of earlier, but actually more letters mention GPS availability as being critical. Where do you stand on the debate of what is more critical, accuracy or availability, as far as the warfighters are concerned?
GK: We don’t separate the two children here, availability and accuracy. Obviously, it doesn’t mean a lot to us if you have high availability and not high accuracy, or if you have high accuracy and not high availability. They go together, and we work both of those issues. We try to make sure that we have the highest availability and accuracy. The accuracy numbers have been very good, as you know. We have been trying to improve availability, particularly for users in impeded environments. We are doing that by taking advantage of the largest constellation of operational GPS satellites we have ever had on orbit. We have begun to adjust the way we have configured the on-orbit constellation.
You called it 24+3, and we were all calling it 24+3 for a while. Now we are calling it Expandable 24, because those are the words that are actually in the Standard Positioning Service Performance Standard. We are expanding the available operational useful slots from 24 in the constellation to 27, and that movement is underway. This should result in improved availability for users in challenged areas like mountainous terrain, deep canyons, and in some cases urban terrain. It improves those kinds of availability numbers worldwide for everyone, for all users. This is not just for warfighters, it’s for all users.
We have begun the movement of the satellites (SVs), and because we are trying to balance on-orbit longevity with movement, it will take us a period of months to move the satellites to the new locations. That movement is underway, and the availability numbers should begin to improve as the movement begins; you don’t have to wait until they are all in their final locations.
By the way, as an aside, just last night, I was driving in Washington [D.C.] and I was using the navigation feature in my cell phone. One of the things it tells you is how many satellites are in view as you are driving along. Now, just to be clear, I was not driving, I was a passenger in the car, so I was not distracted by trying to drive. But I sat there with the thing in my lap, watching it while we were driving through the streets of Washington, D.C., and there were never less than nine satellites in view. At best I noticed that there were 12.
So I thought about that for a minute. Half of the constellation was occasionally in view as we were driving around the streets of Washington. This is pretty powerful, and we are talking about availability. I sat there thinking to myself, yo, if we can help somebody out there — turn that availability when they need it into the right number of satellites — this is a pretty powerful movement that we’ve got going.
DJ: It is, and what you just said about being in the back of the car reminds me about what General Chuck Horner (USAF, ret.) said after he retired as commander in chief, Space Command. He said you know you are truly retired as a four-star general when you go out and get in the back of the car in the morning, and nothing happens.
GK: You’re exactly right. I have a new officer aide who had never been stationed in Washington, and can’t survive in Washington without some kind of a GPS navigation device. He had one going in the front seat, and I had mine going in the backseat, and we were comparing notes as we drove along. It really is pretty remarkable.
DJ: Our readers will he happy to hear that you also have dueling GPSs. I have readers write and say they have up to three or four going at one time on long trips, comparing different GPS device accuracies and interfaces.
GPS has truly been a life-changing event for many of our users, especially the warfighters. I receive hundreds of letters and e-mails from warfighters and this move to Expandable 24 is meeting with unanimous approval.
GK: That’s good to know, and I must say that originated here. Actually, that originated with the IRT [GPS Independent Review Team], as you well know. We then took that to Strategic Command, and Strategic Command embraced it. General Chilton embraced it immediately, and I think that we have done the right thing here. The downside risk here did not outweigh the positive impact that we think we can have on people who need expanded availability.
DJ: Sir, as I said before, wearing a different hat, I attended your Civil Focus Day and I thought it was outstanding. Do you have any comments you would like to make concerning that event, and do you think you achieved your goals?
GK: We did achieve our goals, because our primary goal is improving communication and cooperation, as well as making sure we’ve got a stronger working relationship between the civil and military GPS communities. In that regard I think our goal was achieved. We addressed a lot of crucial concerns that impact both communities. We emphasized that the ongoing GPS modernization and enhancement efforts are going to be transparent to the civil users, and in fact will result in pretty dramatic improvements for civil users:more signals and other enhancements that I think are going to be useful as time goes by. In that regard I was very pleased.
We had a number of very senior people throughout the government who expressed their interest in GPS with their attendance. We had seen, as you know, additional commitment from some of the other [U.S.] government agencies to be supportive in helping to invest in GPS, which I think is very positive. I just think that in general terms we want to make ourselves more transparent in terms of how we are dealing with the constellation and the future of the constellation.
We recognize in Air Force Space Command the unique role that we have for this global utility that the United States of America provides free of charge for everyone else on planet Earth. We recognize that with the use of this and the increasing impact it has on all our lives, comes a unique responsibility for stewardship. We have embraced that responsibility, and that means we have to be transparent and we have to have a collaborative team that we work with, and that was a large part of the Civil Focus Day.
DJ: Many of the proposed systems that may or will one day compete with or complement the GPS are on hold, delayed, or still not at full operational capability. What is your viewpoint on where we stand in relationship to these systems, such as GLONASS, Galileo, and Beidou, for example?
GK: Our objective from an Air Force standpoint has been to support the U.S. government’s goal of wanting to engage in cooperative activities related to space-based PNT, and I think the focus of that cooperation has been to try and ensure that we have compatibility between GPS and other space-based PNT systems. There is a goal on our part to make sure we can be compatible and interoperable. There is a goal on our part to make sure we are protecting our national security interests and that we are maintaining a level playing field in the global market for space-based PNT goods and services.
Those are our objectives, those are the national objectives of the U.S., and the Air Force is supporting those objectives through our management and operation of the GPS constellation. That will continue to be our posture: to make sure, as best we can, to have fostered successful relationships on space-based PNT.
DJ: You certainly can’t ask for more than that. The objectives are laudable, but on the surface they don’t necessarily fit well with the recent comments by the chief of staff of the USAF, and I guess that brings us to the topic we briefly discussed earlier. Do you fully understand where the chief was going with his comments concerning GPS at Tufts University last month, and do you have any comments that might help our readers put the chief’s remarks in the proper perspective?
GK: I do. I was present when General Schwartz made his comments, and honestly I understood what he was saying and why. I think that he was misunderstood in implication. I think what he said was misapplied by some. In my view, General Schwartz fully supports GPS. What he was doing, though, is he was talking about GPS and its value for military operations.
What we know is that, like any other military capability that we rely on for important pieces of our warfighting force, GPS will be challenged by a determined enemy that is interested in trying to defeat U.S. forces on the field of battle somewhere. He was reminding us that we need to be mindful of that:adversaries could potentially exploit GPS as a vulnerability because of the way we have come to rely on our GPS for our own American way of warfare. And because it is such a critical system to the warfighter, it will be an attractive target to any would-be enemy.
Having said that, his point was, with which I fully agree, we have to be diligent in finding ways to operate with the same accuracy and precision in the event that GPS is degraded. That’s exactly what the GPS Modernization Program is designed to do. But this goes beyond GPS as well, it goes into other things, for example, missiles are guided to targets or munitions are guided to targets in some cases by GPS, in some cases by inertial systems, and in some cases by a combination of both. It would be foolish for us to not have provided for the eventuality where GPS will be jammed. But again he was talking about a military environment here; he was not talking about the global environment, he was talking about the military environment.
I recommend to people sometimes that they should go look at, well, pick your search engine of choice on your home computer, and type in “GPS jammers” and see what you get. There is a proliferation of GPS jammers around the world, everything from the sizes that will plug into the cigarette lighter in your car to large devices that are sold internationally for military purposes. We know that GPS will be contested when or if we are involved in any military conflict. The chief was warning us that we need to take that into account, and I believe he was exactly right to do it.
DJ: Thank you, sir, that helps clarify the Chief’s remarks considerably. I just wish he had said what you said versus what he said. Sometimes senior leaders are just too close to the problem and they erroneously assume their audience has information, knowledge, or insights that they in fact just do not possess, and it skews their perception of the senior leader’s remarks.
The last topic I would like to discuss concerns the infamous AEP 5.5C update that did not go quite as well as planned. Again in this instance, the public perception may be skewed by a lack of information and a lack of communication. I know you are fully up to speed on this issue; what are your thoughts?
GK: I would make a couple of points about upgrading the ground software. First, with this latest version of the ground software, AEP 5.5 and all of its iterations, we learned a lot about the complexity of the GPS system, how complex it has become. We learned a lot about standards, and what happens if you make receivers and you don’t follow the standards, because there was nothing wrong with the [AEP] 5.5 software in this case. The issue was in the receivers — a very small percentage of our military receivers — where the manufacturers did not comply with the standards. We hold ourselves to a set of standards, we publish those standards, as you well know, and it is important for people who are making GPS devices to follow those standards.
Now here’s what we learned, though. We learned that not only is it important to follow the standards, but we learned that we can do better in how extensively we test prior to installing software. By that I mean — not that we didn’t test extensively before — increase the population of receivers that we test against and the rigor with which we test them, would be a better way to say this.
The other thing we learned is that collaboration and cooperation needs to be more robust, such that we are doing these upgrades on an active basis, not a passive basis. What we had been doing before is we would publish a NANU and say that we were about to do an upgrade to the ground software. We would then do the upgrade. We would wait to find out what was happening. What we learned this time was, that is probably too passive as we go to the future. Not only will we test more extensively across a broader range of GPS devices, but we will also put [receivers] in place, in a series of predetermined locations, if you will, where we will contact them actively to find out as we are progressing whether they are encountering any difficulties. We did learn a lot here.
We also learned that these upgrades need to be done in a fashion that is repeatable, so that every time we do this we will have a process in place that allows us to treat them roughly the same, depending on the magnitude and risk associated with the change, if you will, in terms of how we intend to go forward. I think we learned a lot about vetting and we learned a lot about execution. We
reminded ourselves again why standards are so important, and we reminded ourselves why partnerships are so important and why rapid feedback is important: so that we can deal with problems as they emerge.
We also learned something for the longer term, Don. We learned that we probably need better simulation tools as we look to the future, because you know there is only one active system, and it is the active system. It has become so complicated that there are hundreds of millions of receivers out there, as you well know, and the likelihood that we can characterize all of them in advance of a software drop is pretty low. We are going to have to get better at following a simulation as we go forward.
The most significant piece of data, though, from all this was there was nothing wrong with AEP 5.5. It performed exactly the way it was designed. The issues that were encountered were anomalies in user equipment, and that user equipment was identified because it did not follow the standards.
DJ: General Kehler, do you have any closing remarks for our readers, a message you want to make sure gets heard?
GK: Don, we understand the unique position that we are in as stewards of GPS. This is unusual, I believe, throughout the U.S. military, that a military service would have this type of responsibility for a system that has this kind of global impact. And it has that global impact 24 hours a day, seven days a week, 365 days a year. We recognize that unique responsibility that we have.
We know that means we have to be transparent about the way we conduct our business. We think that we are doing much better at that, and we will get better at that even more as we look to the future.
Our bottom line is that we believe that GPS is the gold standard today for the world. We intend to keep it that way as we look to the future, and we will allow the performance of the GPS system to speak for itself. We are very, very proud of the job that we do regarding GPS.
The young — many very young — men and women who operate and fly that constellation everyday, the outstanding technical people we have who design and build the satellites, the phenomenal launch team that we have that gets them to the Cape and gets them successfully on orbit — all of these pieces that are taken together along with, by the way, a civil group of participants from across the government who work very hard at all of this, along with independent folks who are on our review teams and elsewhere as well as the industry, the broader industry —this is a remarkable success story that has now influenced virtually everything we do, everywhere on the face of the planet. I think we ought to be very proud of that, and I can tell you that this Command is extraordinarily proud of it and recognizes that this puts a unique burden on us to deliver. We are going to continue to do just that.
DJ: That’s a great message and a very important one. In closing, might I ask you about your future? Rumor has it that there are plans afoot for you to move onward and upward.
GK: Don, my wife keeps saying that we go to Myrna — she is the dry cleaner and tailor down the street here — to find out where we are going.
I don’t know. I have been here two and a half years, Don, and typically this assignment will last about three years. That will take us into late summer, early fall, and I honestly, honestly do not know what happens with us next. We are going to have to wait and see what the pleasure is of my superiors and how all the pieces sort of fit together.
I think you know, when you get to be a four-star, there are a lot of factors that come to bear. At this point we will just have to wait and see. The only thing that I am worried about right now is the job that I’ve got, and I will be very, very pleased to stay here. We could stay here for 10 more years, and I would be delighted to stay here because this is a magnificent command.
We are doing phenomenally important work, and I am very proud of the people in Air Force Space Command. This is a wonderful, wonderful group of people.
DJ: You should be proud of them, sir. We get a lot of mail about what a great job the Air Force is doing as the steward of GPS. Our mail is always very positive concerning Air Force Space Command. I want you to know, sir, in closing, that working with Colonel Ford and Colonel Buckman has been a real pleasure. Your folks have been just super.
GK: I think so, too, and I don’t tell them that enough, really. We’ve got a great team here at headquarters, and we’ve got a great team across this command. We are delighted to have cyber responsibilities now, and there is clearly a relationship between space and cyberspace, and we see it. Every time I get a chance to commend the people in the Command, I like to take the opportunity to do so.
DJ: Thank you for your time today, sir. I know how busy you are, and I think we should find the time soon to sit down and have another discussion, possibly on cyberspace.
It’s been a tough couple of weeks for SBAS (Satellite-Based Augmentation System), namely the USA’s WAAS program and India’s GAGAN program. WAAS and GAGAN have taken big hits recently that threaten the integrity of the programs. Both events were totally unexpected and are causing disruptions of GPS correction services.
Let’s Start with WAAS
First of all, consider the following infrastructure graphic describing WAAS.
WAAS Infrastructure (note: GEO satellites positioning not geographically correct in graphic)
At the moment, WAAS uses two geostationary satellites (referred to as GEOs) to broadcast GPS corrections throughout the WAAS service area, which covers the U.S., Mexico, and most of Canada. The user’s GPS receiver must be able to “see” at least one of the WAAS GEOs in order to receive the GPS corrections. Currently, one WAAS GEO (PRN 135) is located at 133°W longitude and one (PRN 138) is located at 107°W longitude. They are positioned, for the most part, to provide “dual coverage” in case one fails as the following graphic illustrates. The solid line represents the visibility above the horizon of PRN 138 (107°W). The dashed line represents the visibility above the horizon of PRN 135 (133°W). In New York, for example, PRN 138 is visible at 30°+ above the horizon while PRN 135 is visible at ~15° above the horizon.
The Federal Aviation Administration (FAA) is the WAAS steward. WAAS (and SBAS) was designed for aviation use and paid for by the FAA. The fact that surveying and mapping users benefit from WAAS is a by-product. The FAA owns and controls most of the WAAS infrastructure, such as the 38 WAAS reference stations located throughout the U.S., Canada, and Mexico. About the only thing they don’t own are the WAAS GEO satellites, and this has been the source of most of the problems with WAAS in the past few years.
Lease vs. Buy
It would be prohibitively expensive for the FAA to own GEO satellites that were exclusively used by WAAS. Instead, the agency leases bandwidth from owners of commercial satellites. These are the same commercial satellite owners who lease bandwidth to media (e.g., television) customers. It’s not unlike a utility pole you see along the road with many different wires and devices attached to the pole from different companies who pay to lease space on the pole, except it’s a very expensive pole orbiting in space.
If you’ve been using WAAS for a number of years, you’ll remember back in 2006 there was a hiccup with the WAAS GEOs at that time. The FAA was leasing space on two Inmarsat satellites (AOR-W and POR). They began transitioning to the current WAAS GEOs but before the transition was complete, Inmarsat began moving AOR-W. This was a headache for some WAAS users and really showed the vulnerability of WAAS.
Losing Control
The vulnerability reared its ugly head again last week when one of the commercial satellite operators (Intelsat) that the FAA leases space from announced it had lost contact with its Galaxy 15 (G-15) satellite, which is the GEO that WAAS PRN 135 is broadcast from. Intelsat reported it had lost the ability to send commands to G-15. Without the ability to control the satellite, it will slowly drift out of orbit until it becomes unusable. The FAA estimates this will occur in one to three weeks.
Solutions?
Intelsat’s answer was to bring in an older generation backup satellite (G-12), which was in a backup orbit at 122°W. It arrived at 133°W around April 14. Intelsat said that G-12 has virtually an identical C-band package as the G-15 and they could transfer C-band customers to the G-12. The problem is that there is no L-band package (which WAAS needs) on the G-12, so the FAA was out of luck.
Since Intelsat’s G-12 backup won’t help WAAS, the FAA is looking at other alternatives:
Contract with Inmarsat to bring back POR (178°E). The FAA says that will take 12-18 months. Personally, I don’t think it’s a good solution. It’s too far to the east to help much at all. Its coverage footprint barely covers the western U.S.
Speed up the testing on the new PRN 133 (98°W) and bring it into service more quickly than the original December 2010 schedule. The FAA says it can accelerate testing by one to two months. This is good and I see the benefit, but it still doesn’t help Alaskan users.
The replacement backup satellite being moved to 122°W to backup G-12 may be a solution. It will be a few weeks before it is known what is possible. That would be the best scenario from a coverage footprint standpoint. The question is how long it would take to bring it into service.
On another note, the FAA stated that with the money they are saving with G-15 going out of service, they will be able to accelerate the acquisition of another WAAS GEO. I have no doubt that this has put a new level of fear into the FAA folks, and they have to realize that they can’t be running thin on WAAS GEOs. If you weren’t aware, the future of aviation navigation is based on GPS, WAAS, LAAS, etc. These sorts of hiccups would be an absolute nightmare if the National Airspace System (NAS) was already dependent on GPS.
GAGAN
GAGAN (GPS-Aided Geo Augmentation Navigation) is India’s SBAS. It has been under development for many years and is quite far along in development. It is funded through implementation by the Airport Authority of India with the Indian Space Research Organization. In 2008, GAGAN was broadcasting a test signal from an Inmarsat GEO with reasonable results.
India’s intent was to launch its new GSAT-4 communication satellite with part of its purpose being a GAGAN GEO satellite. GSAT-4 was to be India’s first rocket with an Indian-designed and built cryogenic-fueled third stage. Apparently it is a very difficult technology to master as it reportedly took India 16 years to develop.
Last week, after much anticipation, the rocket with GSAT-4 onboard was brought to the launch pad. Liftoff was reportedly flawless. At 8:25 minutes into flight, the rocket failed and the entire rocket, GSAT-4 and all, ended up splashing into the Bay of Bengal. It’s a crushing blow to India’s GAGAN SBAS program, which has suffered a number of delays.
P.S. Veeraraghavan, director of the Vikram Sarabhai Space Centre in Thiruvananthapuram, said “Our target is to fly a GSLV with our indigenous cryogenic engine within one year. But it will be tough.”
Following is a video report from an India news organization describing the event:
Webinar Tomorrow
If you don’t receive this too late (or you can access the archive if you do miss it), you might want to catch my 60-minute webinar “GPS, GLONASS and SBAS Constellation Updates.” It’s free and full of the latest information. I’ll also be answering a number of questions from people who registered. I hope to see you there!
Trimble has introduced an innovative Global Navigation Satellite System (GNSS) reference receiver for infrastructure, precise scientific, and network applications. The Trimble NetR9 GNSS reference receiver is a Continuously Operating Reference Station (CORS) receiver that can support the demanding applications for the earth science community and for the surveying, construction, mapping, and agricultural industries, Trimble said, adding that the NetR9 was designed to provide the user with maximum features and functionality from a single receiver.
The Trimble NetR9 reference receiver offers 440 channels for robust GNSS constellation tracking. The receiver supports a wide range of satellite signals, including GPS and GLONASS signals. In addition, Trimble is committed to providing Galileo-compatible products in advance of Galileo system availability, the company said. In support of this plan, the Trimble receiver is capable of tracking the experimental Galileo GIOVE-A and GIOVE-B test satellites for signal evaluation and test purposes.
The Trimble NetR9 reference receiver can be used as a standalone receiver or as part of a network solution. Specific applications include high-accuracy positioning as part of a Trimble VRS network, as a mobile field base station or CORS for real-time kinematic (RTK) corrections, as a scientific reference station collecting information for specialized studies, as a field campaign receiver for post-processing applications, and as support for Differential Global Positioning System (DGPS) coastal beacons. In addition, the Trimble NetR9 reference receiver can be used for monitoring the integrity of VRS networks as well as the deformation of physical infrastructure such as bridges, dams, mines, oil platforms, and other natural and manmade structures.
The Trimble NetR9 reference receiver’s large internal memory (8 GB) allows post-processed results for base stations to be computed after survey completion, improving the accuracy of the survey. The highly compressed secure internal memory allows for more than 20 years of 15-second dual-frequency GPS data storage. In addition, the NetR9 also has USB logging capability for additional storage capacity, Trimble said.
The receiver supports the new CMRx communications protocol, which provides correction compression for optimized bandwidth and full utilization of all satellites in view. This gives the customer more robust positioning data and reliable positioning performance, Trimble said.
Optimized for field use with built-in rechargeable batteries, the NetR9 reference receiver consumes very little power and can be used for projects with remote connectivity and in extreme weather conditions. It has an IP67 rating, which means it is sealed against dust and can survive immersion in up to a meter of water for approximately 30 minutes. It also meets MIL-STD 810F standard for drops, vibration, and temperature extremes.
The Trimble NetR9 has its physical memory built into the circuit board, providing greater protection of data, particularly under extreme conditions. Multiple built-in serial ports supply communications and power to support field use, whether connecting to a radio for RTK surveys, direct communication with a satellite phone for remote operations, or for ancillary input devices such as inclinometers and meteorological sensors, and it offers Bluetooth communication with a cell phone for real-time data streaming. In addition, both power and Ethernet can be supplied over a single cable using Power over Ethernet (PoE) technology.
Updated: Friday, April 9 11:00am US Pacific. I added more specific information regarding signing up for Space Weather Prediction Center email alerts. See below.
It’s time to touch on the solar activity subject again, as there was an event earlier this week and rumors began to fly. The mainstream press jumped on a story back in January when the first solar flare of Solar Cycle 24 occurred. Of course, journalists were writing about worst-case scenarios in the event of extreme solar events that could cause power grids to fail, GPS to stop working, etc.
While that is true, it’s a real stretch and the typical “sky is falling” reporting. In reality, the solar flare back in January had no effect on GPS operations. In fact, it would take an event 10-20 times stronger than last January’s to begin to notice any effect on GPS operations. Earlier this week (Monday 0800 GMT), the first geomagnetic storm of Solar Cycle 24 occurred.
Geomagnetic storms are the ones that will give GPS users problems, although this one didn’t because it was relatively minor. The last geomagnetic storm strong enough to noticeably affect GPS users occurred in December 2006. During such an event, it might interrupt your GPS receiver for 10-15 minutes. Most users would not notice or they might attribute it to a local system malfunction. By the time they investigate and reset the system, the event would have passed and the user is back in operation. It would be barely noticeable, if at all.
According to Joe Kunches of the NOAA Space Weather Prediction Center, a geomagnetic storm is a global event (as opposed to a regional event) that is caused by a highly energized solar wind that is fast and embedded with a strong magnetic field. In the following chart, you can see how this week’s event illustrates this.
Source: NOAA Space Weather Prediction Center
In the above chart, the top panel illustrates how the magnetic field becomes much more turbulent starting at 0700 GMT. The fourth panel on the chart denotes the solar wind speed, which ramped up to approximately 2,000,000 mph (3,218,688 kph) at its peak.
There needs to be very turbulent solar wind that disturbs the Earth’s geomagnetic field in order for GPS operations to be affected. For those of you who are familiar with the Total Electron Count (TEC), a dynamic TEC density in the ionosphere is what really messes up GPS operations. If the TEC is stable, the ionospheric models work fine and we get really good GPS performance like we’ve seen in the past few years in between solar cycles.
GPS L1 users are affected most by a dynamic TEC density in the ionosphere. These are users of WAAS, DGPS, and commercial L1 correction services like OmniSTAR VBS (not their XP or HP service). During the extreme geomagnetic event in October 2003, published simulations (Yousuf, Skone, Coster, University of Calgary, ION NTM 2005) that illustrated the WAAS maximum horizontal error (95th percentile) blew out to 25 meters while single baseline DGPS maximum horizontal error (95th percentile) blew out to 18 meters. This extreme event lasted for several days.
This doesn’t mean you’re going to have major problems in the future if you are using WAAS (or another SBAS) or DGPS, but just that high-performance GPS L1 receivers are the most susceptible to extreme solar events. In the case of the December 2006 event, SBAS and DGPS users might have experienced 10-15 minutes of unusual behavior depending on their locations. According to Kunches, high latitude geographic regions (60+ degrees latitude) and the region within 10 degrees of the geomagnetic equator (as opposed to the geographic equator) are affected the most by geomagnetic storms.
GPS L1/L2 receivers are less susceptible to extreme solar events because they can actively model the affects of the ionosphere, but they are not immune. Extreme events such as in October 2003 can cause a loss of phase lock, especially on L2 with GPS receivers that utilize codeless/semicodeless techniques, which are virtually all of the dual-frequency GPS receivers on the market today. The L2 signal-to-noise (SNR) ratio on L2 is quite a bit lower due to the codeless/semicodeless technique so it is more susceptible.
GPS L1/L2 receivers using L2C will be less affected (assuming a sufficient number of GPS satellites are broadcasting L2C) due to a stronger SNR.
Not the time to panic
The reason I wrote this article is to share what I’ve learned about the effects of solar storms on GPS operations from speaking with a number of different scientists. This isn’t meant to be a warning of impending doom for GPS users or anything or that sort. Extreme events typically occur near the solar peak and then again during the decline of the cycle. The peak is estimated to occur around May 2013, so the typical extreme events affecting GPS would likely occur in 2013, 2014, and 2015. It’s too early to start worrying much about it now.
However, as Solar Cycle 24 ramps up, we’ll see more and more geomagnetic storm activity. If you’re a high-performance GPS user (meter or sub-meter level GPS L1 and GPS L1/L2), I think it’s a good idea to monitor space weather now. Fortunately, the NOAA Space Weather Prediction Center (where Kunches works) provides a service that will notify you of unusual space weather by e-mail. You can sign up to receive e-mail alerts at http://www.swpc.noaa.gov
Following are detailed instructions for signing up for alerts:
RF ID (Radio frequency Identification) in Survey Monuments
If you haven’t been followi
ng my Geospatial Solutions Weekly newsletter (sign up here for free), you might want to sign up and read the article I wrote on how RF ID is going to be a technology very much used by surveyors in the future. You can read the article by clicking here.
Webinar later this month (April 22, 10 a.m. Pacific time, 6 p.m. GMT): GPS, GLONASS, and SBAS Constellation Updates
There’s been a lot of infrastructure changes with GPS, GLONASS, and SBAS in the past six months. We’ve already got several hundred people registered for this webinar. It’s going to be a good one. Here are some of the questions I’ve received already and will be addressing:
When and where will the new FAA WAAS GPS Satellite cover?
Will the accuracy of hand-held units be increased with these latest changes?
What developments will make GPS & GLONASS work better together? In terms of RTK accuracy.
There have been some questions as to whether you can receive continuing education credit (PDH, CEUs, etc.) by attending the webinar. Please e-mail me directly with these requests and I will do my best to accomodate.
APP PLANET featured 100 exhibitors and a lounge for old-fashioned social networking.
By Moni Malek
It’s that time of year, around Valentine’s Day, when most of the who’s who in the mobile phone industry meet at the Mobile World Congress. I have been attending this event for nearly 15 years, and have seen the location change from Cannes to Barcelona, and the name change from GSM World Congress to 3GSM World Congress to Mobile World Congress.
At the same time, the number of mobile phone users shot up from the millions to the billions. A new feature this year was the App Planet hall. The attendance of 47,000 was only marginally down from the 49,000 visitors in 2009, making it still a very busy a event, with no sign of the recession compared to other shows I’ve seen. It’s still the best place to meet companies in the mobile space — I met 25 in three days, as well as running into ex-colleagues and contacts who, like me, have been attending for years.
Smartphone Entry. The trend of the last year or so has been the burst entry of smartphones. First started by Apple iPhone for consumers and to some extent Blackberry for professionals (the so-called fruit phones), operating systems (OS) have evolved to include Android from Google, Palm Pre’s webOS, Nokia and Intel merging their top-end smartphone operating systems, and Symbian going open source. Microsoft has people excited with Windows Phone 7, with the first handsets running on it scheduled to hit the markets around the holiday season.
Most of the smartphones are GPS-enabled, and as these phones increase the market penetration of GPS, GPS use will increase, leading to more use of location-based applications.
Deep Pockets. For those of you who think GPS personal navigation device market pricing is tough, the mobile phone market is cut throat. Volumes are out of this world, and in lots of countries around the globe, the volumes are more than the population! These volumes require deep pockets to keep up the investment to make money on decreasing margins.
There has been a trend toward consolidation in the GPS chip industry. Less than a year or two ago in Barcelona booths represented eRide (acquired by Furuno), Global Locate (acquired by Broadcom), GloNav (acquired by NXP, then wound up in ST Ericsson), Nemerix (which seems to have disappeared, though it’s rumored some assets went to another chip company), and finally SiRF (now part of CSR-SiRF). CSR-SiRF’s booth was more like a fortress, but at least I got to talk to the SiRF founder.
It will be interesting to see what a Bluetooth-GPS company with a lot of cash in the bank plans as a next move. As for survivors, u-blox still had a booth (they weren’t acquired; they did an Initial Public Offering), and CellGuide had a small section of the Israel booth.
App Planet. Since I first attended this show, global mobile-phone technology has gone from GSM voice to GPRS data to 3G voice/data to HSPA. Now comes LTE (Long Term Evolution), which is really a packet data network that can use VoIP. Together, 3G and smartphones give us an environment which lets apps become a new business model worth billions. The Apps Planet hall showcased a lot of these models. The hall didn’t exist last year, but this year had 100 exhibitors. It easy to predict this number will grow.
There are so many applications, they will need to differentiate to stand out from the crowd and gain mass. I think location-based apps need to get better, and I see that happening at the show. deCarta allows searches for places based on real walking distances or near the route you are traveling. Aloqa has clients for every smartphone with channels that you can choose for your interest. Mireo impressed me with not only natural text guidance (“turn left after the Apple store”) but its super-fast routing in less that 2 seconds, as opposed to 30-seconds-plus on other devices. It features algorithms with pre-stored routes to major junctions, so only the rest is routed. In any case, the net effect is you are routed before you have to think which way to drive or walk. I always say mobile phone users have short attention spans and expect instant gratification, and fast routing certainly helps.
Finally, an Audi A5 Cabriolet displayed a solution for the European Commission’s eCall emergency call initiative, a car which automatically sends your position after an accident to a Public Safety Answering Point. eCall should be implemented in Europe by 2014, but Qualcomm is looking to put the system into the Audi A8 this year.
Moni Malek is CEO of ML-C MobileLocation-Company GmbH, a new company integrating location and communication in a system platform.
Motorola’s Christian Kurzke discusses Android with developers.
By Ana P. C. Larocca, Ricardo E. Schaal, and Augusto C. B. Barbosa, University of São Paulo
Multipath makes it difficult to detect very low-frequency structural vibrations, ranging from 0.05 to 1 Hz, important in characterizing dynamic loads and determining safe structural lifetimes. The authors have developed a phase-residual method for use with very high-frequency data to distinguish receiver noise, multipath, and the periodic displacements that are most structurally significant. The methodology can apply to bridges, tall buildings, and towers.
Civil engineers continuously seek reliable methods and tools to improve the quality and lifetime of large structures. Most studies in this field have been based on static loading. Nowadays, dynamic loading has become a particular concern, and GPS offers direct measures of dynamic displacements of large structures induced by traffic, wind, and earthquakes.
Precisely characterizing the vibrations that are a common behavior of large structures such as bridges, tall buildings, and towers undergoing dynamic loads facilitates structural analysis studies. It is feasible to detect structural vibrations using a computational model and GPS sensors. The critical vibration frequencies of bridges detectable with different GPS positioning techniques (real-time kinematic, static, quasi-static) range from 0 to 0.3 Hz.
However, the unavoidable presence of multipath signals in the same frequency range makes it difficult to detect very low-frequency vibrations, mostly ranging from 0.05 up to 1 Hz, for short- to medium-span bridges.
Our preliminary results show that the structural vibration measurements, mixed with random amplitude and frequency signals generated by electronics and the ionosphere, together with slowly varying signals generated by multipath, can be better detected with an oversampled GPS data set. This hypothesis relies on fact that the structure oscillation is reasonably stable during the data-collecting period.
The analyses of GPS time series used were done by mathematical addition of well-known sine waves in the raw phase of a 100-Hz data set collected from a short baseline. This strategy simulates the antenna vibrating vertically on a structure, for example at the deck’s midpoint of a bridge.
Methodology
The methodology used to collect and analyze GPS data was developed for providing low-cost high-accuracy monitoring with single-frequency GPS receivers. The technique is the interferometry method based on the analysis of the L1 double-difference phase residuals of regular static observations. In this data-processing, one satellite is considered as a reference, and its selection is according to the direction of the vibration to be measured. The satellite not taken as a reference — located in the same direction as the vibration movement — has the residual values that contain information about bridge deckvibrations (phase changes). In 2001, we named this the phase-residual method (PRM); see “Millimeters in Motion” in GPS World, January 2005.
The residuals incorporate all phase deviations from the adjusted double-difference position during the observation. These phase deviations are due to electronic receiver noise, multipath, small dynamic antenna movements, and other error sources. Converting the residuals to the frequency domain by the fast Fourier transform (FFT) associated with a continuous wavelet transform (CWT), it is possible to see the different behaviors of the receiver phase noise,
multipath, and periodic vibration, enabling the distinction between them. The periodic displacement presents a peak due to the fundamental vibration mode, while the receiver noise presents a white-noise spectrum, and the multipath presents a broad spectrum close to zero frequency. The last feature is very dependent on how the antennas “see” their vicinity. As PRM does not need well-known coordinates epoch-by-epoch to determine the amplitude and the frequency values of the oscillations, it is possible to get reliability.
The spectrum analyses were done by FFT, which provides a design of the vibration’s peak amplitude values; the CWT was used to detect the variation of the frequency value during the timespan of observations, and for validating the results.
Simulation and Filtering
The preliminary investigation was done by the mathematical addition of sine waves on satellite signals close to zenith, which are the most affected by a vertical amplitude vibration in a real situation. The double-difference phase was calculated, taking as reference the lowest satellite.
The mathematically generated sine wave had peak-to-peak amplitude of 1 millimeter and frequency values ranging from 0.06 Hz up to 1 Hz. The analyses for sine-wave detection were done by applying the FFT and the CWT with the Morlet Wavelet, which deserves a short description.
The CWT was used because structural vibration signals with small peak-to-peak amplitudes in the low frequency region are not well represented in time and frequency by the FFT methods. A particular wavelet, Morlet, was used and is defined as
(1)
where wo is dimensionless frequency and η is dimensionless time. When using wavelets for feature extraction purposes, the Morlet wavelet is a good choice, because it provides a good balance between time and frequency localization.
The idea behind the CWT is to apply the wavelet as a band-pass filter to the time series. The CWT of a time series (f (t),t = 1,…,N) with uniform time steps dt, is defined as the convolution of f (t) with the complex combination of the mother wavelet scaled and normalized, as:
(2)
where Wj,k(t) represents the similarity between wavelet function and the analyzed time series f (t); that is, the higher the value of Wj,k(t), the greater the similarity between the analyzed function and the mother wavelet function that modulates the analyzed signal. The CWT was implemented in MATLAB software.
100-Hz Phase Data
Regarding the detection of low frequencies due to a small peak-to-peak amplitude vibration, it is important to show the L1 double-difference residuals of a 100-Hz data rate (Figure 1) and its spectrum before mathematically adding the sine-wave signal due to periodic vibrations. The figure shows the raw phase residuals of 20 seconds of data between two satellites, SV05 (lowest) and SV20 (highest).
FIGURE 1. Raw L1 double-difference phase residuals from a time series at a 100-Hz data rate.
Figure 2 presents a 1-second data span for better visualization of peak-to-peak amplitude of the raw double-difference phase residuals, which is lower than 3 millimeters.
FIGURE 2. Residuals from L1 double-difference phase residual.
Figure 3 was produced to verify the variability of 100-Hz residuals and the probability of errors in the signal that can contribute to degrading the identification of the sine-wave vibration peaks. The resulting histogram is close to a bell curve of a Gaussian distribution, demonstrating the good quality of the 100-Hz data. Figure 4 shows the Morlet CWT computed to identify the low-frequency bias term and a high-frequency noise term. The 5-percent significance (95-percent confidence) level of significant signal-wave information is delimited by a thick contour. The signal information of double-difference phase residuals was used as a reference for supporting a better distinction between noise and sine-wave signals.
FIGURE 3. The Gaussian distribution of 100-Hz data rate residuals.FIGURE 4. Continuous Wavelet Transform of the residual time series. The 5-percent significance level of sine wave detection is shown as a thick contour.
Zero-Baseline Test
A zero-baseline test was performed to determine the correct operation of a GPS receiver, associated antennas, and cabling. The objective was to verify the precision of the receiver. A 1-minute data sample was collected. Figure 5 shows the residuals of L1 double-difference phase.
FIGURE 5. Zero baseline 100-Hz data rate residuals of L1 double-difference phase.
Figure 6 shows 5 seconds of the zero-baseline data; the peak-to-peak amplitude of residuals is very small, close to 2.0 millimeters. This information leads us to expect detection of very low-frequency vibrations, ranging up to 0.3 Hz with a 1-millimeter amplitude displacement peak-to-peak.
FIGURE 6. Residuals from a zero baseline with 100-Hz data.
Figure 7 shows the spectrum of the zero-baseline residuals; it is possible to observe the region close to zero strongly affected by multipath. This makes the detection of very low frequencies difficult.
FIGURE 7. Power spectrum of a zero-baseline residual.
The CWT was applied to decomposing the zero-baseline double-differenced residuals into a low-frequency bias term and a low-frequency noise term. Figure 8 shows the behavior of the residuals of the 100-Hz phase data, where red regions represent the most suggestive energy level of the measurement noise term.
FIGURE 8. Morlet CWT of zero-baseline residual time series. The 5-percent significance level of sine-wave detection is shown as a thick contour.
Preliminary Simulation Results
Figure 9 illustrates the raw L1 double-difference phase residuals with a periodic sine wave of 1 millimeter peak-to-peak amplitude mathematically added to the time series. It is possible to observe the presence of the periodic signal.
FIGURE 9. Raw L1 residual time series with a sine wave of 1-Hz frequency and 1-millimeter amplitude.
Figure 10 shows that the stronger energy is close to 1 Hz due to the 1-Hz sine wave, as expected. The resulting well-defined peak is due to the high sampling rate provided by 100-Hz receivers. Figure 11 shows details of the peak due to the sine wave of 1 Hz added to the residuals.
FIGURE 10. Spectrum of L1 double-difference phase residuals with a sine wave of 1 Hz and 1 millimeter.FIGURE 11. Close-up of region with the most power at 1 Hz.
We analyzed these data with the Morlet CWT to find events to compared when other low frequencies had been simulated, helping separate noise from signal. Figure 12 presents the standardized time-series residuals, showing a region with highest power level. The continuous red region corresponds to a 1-Hz sine wave, and the spread-out red-orange regions may be due to electronic noise and multipath. The region outside the cone, delimited by the thick contour, indicates the detection of significant signal information but without the 95-percent confidence.
FIGURE 12. Morlet CWT of time series of residuals with 1-Hz sine wave with 1 millimeter amplitude. The 5-percent significance level of sine-wave detection is shown as a thick contour.
0.5-Hz Sine Wave. The second sine wave generated had the same peak-to-peak amplitude, 1 millimeter, and the frequency value of 0.5 Hz. Figure 13 illustrates the raw L1 double-difference phase residuals with a periodic 0.5-Hz sine wave mathematically added to the time series.
FIGURE 13. Raw L1 double-difference phase residuals with a sine wave of 0.5 Hz.
Figure 14 shows an energy peak at a frequency of approximately 0.5 Hz, also with a well defined peak.
FIGURE 14. Spectrum of L1 double-difference phase residuals with a sine wave of 0.5 Hz.
Figure 15 shows details of the peak.
FIGURE 15. Close-up of region with the most power at 0.5 Hz.
The CWT in Figure 16 shows that the intensity energy level represented by the red continuous region and the spread-out red-orange regions are quite similar to those of the CWT of the 1-Hz sine wave (Figure 12). Note a decrease in energy intensity (orange-yellow) that occurs due to decreased signal sampling of the 0.5-Hz signal (10 cycles) in 20 seconds of data, compared to 1 Hz (12 cycles) in the same 20 seconds.
FIGURE 16. Morlet CWT of time series of residuals with 0.5 Hz sine wave with 1 mm amplitude. The 5-percent significance level of sine wave detection is shown as a thick contour.
0.1-Hz Sine Wave. The third sine wave mathematically generated had the same peak-to-peak amplitude, 1 millimeter, and a frequency of 0.1 Hz. Figure 17 illustrates the raw L1 double-difference phase residuals with the periodic 0.1-Hz sine wave mathematically added to the time series. Figure 18 shows the power at one frequency, approximately 0.10 Hz, still with a well-defined peak.
FIGURE 17. Raw L1 double-difference phase residuals with a sine wave of 0.10 Hz.FIGURE 18. Close-up of region with the most power at 0.10 Hz.
Figure 19 presents identification of the 0.1-Hz sine wave by CWT with the 5-percent significance level shown as a thick contour. A decrease of energy intensity (orange-yellow) occurs due to decreased signal sampling of 0.1 Hz (2.5 cycles) in 20 seconds of data compared to 0.5 Hz (10 cycles) in the same 20 seconds.
FIGURE 19. Morlet CWT of time series of residuals with 0.1-Hz sine wave with 1-millimeter amplitude; 5-percent significance level of sine wave detection shown as a thick contour.
0.08-Hz Sine Wave. We simulated a sine wave of this frequency (Figure 20). Figure 21 presents identification of the 0.08-Hz sine wave by CWT through the 5-percent significance level shown as a thick contour. A decrease in energy intensity (orange-yellow) occurs due to decreased signal sampling of 0.08 Hz (almost two cycles) in 20 seconds of data compared to 0.5 Hz (ten cycles) in the same 20 seconds.
FIGURE 20. Close-up of region with most power at 0.08 Hz.FIGURE 21. Morlet CWT of time series of residuals with 0.08 Hz sine wave with 1-millimeter amplitude; 5-percent level of sine-wave detection shown as a thick contour.
0.06-Hz Sine Wave. Finally, a 0.06-Hz sine wave was simulated and added to the residuals, but the FFT spectral analysis did not present the power peak. This can be attributed due to the sine-wave period providing only 1.5 cycles during 20 seconds and did not generate enough power to be detected by FFT.
Figure 22 presents a close-up view of 0.06-Hz sine-wave power spectrum of the residuals not indicating a significant peak close to the expected frequency region.
FIGURE 22. Power spectrum of double-difference phase residuals with 0.06-Hz sine-wave signal.
The investigation continued with a Morlet CWT. In Figure 23 it is possible to verify the presence of a faded red region close to the period corresponding to 0.06 Hz — at the bottom of figure and under the cone’s thick contour — signalling that the wavelet was able to detect a very low frequency even with a small sampling. However, due to small signal sampling, the detection is not within a 95-percent confidence. Otherwise, if the time series had lasted more than 20 seconds, certainly the sine wave would have been detected.
FIGURE 23. Morlet CWT of time series of residuals with 0.06 Hz sine wave with 1-millimeter amplitude.
These analyses suggest that longer time-series data would enable detection of very low frequencies with 95-percent confidence.
Conclusions
The lack of amplitude accuracy does not constitute a significant restriction in large structure monitoring, as the exactness of its natural oscillating frequency, harmonics, and response to external dynamic forces are more important for identification of a structural problem.
Using 100-Hz receivers to detect very low-frequency vibrations, the combination of 100-Hz data with filtering techiniques enables detection of signal vibrations of very low frequencies. The tests were conducted using a mathematical simulation of sine waves added to raw residuals of L1 double-difference phase.
The results of simulations and filtering techniques indicate that very low frequency vibrations can be detected when the sampling rate of GPS data and the sampling frequency of an embedded sine wave is large.
Additionally, zero baseline and static short baseline trials have been conducted to assess the noise of the receivers that is close to 2.5 millimeters — extremely low and contributing to detection of vibrations with low peak-to-peak amplitude.
Spectral analysis is a fundamental tool for engineering development. Despite such new analysis concepts as FFT and CWT used here, as well as higher-order spectra, basic frequency domain analysis will remain the practical analysis tool in the foreseeable future.
Future tests will be carried out collecting 100-Hz data, sufficient for having oversampling of sine-wave frequencies due to structural vibrations, and using a new methodology with just one GPS receiver.
Acknowledgments
Thanks to the JAVAD GNSS Moscow Research and Development team for providing a Triumph receiver and 100-Hz data through Michael Glutting, whom we also thank. The researchers received a sponsorship from the National Counsel of Technological and Scientific Development Government (CNPq) of the Brazil Federal Government to purchase a pair of 100-Hz data-rate GPS receivers.
Manufacturers
The 20 seconds of data were kindly provided by JAVAD GNSS Moscow Research and Development team and were collected using Javad GNSS Triumph receivers with JNS choke-ring antennas.
Ana P.C. LaRocca is a lecturer in the Department of Transportation Engineering of the Polytechnic School at the University of São Paulo (USP) and holds a Ph.D from that same institution.
Ricardo E. Schaal is an associate professor with a Ph.D. from USP.
Augusto C. B. Barbosa is a Ph.D candidate at the Institute of Astronomy, Geophysics and Atmospheric Sciences, at USP.
By Steven M. Di Naso, Vincent P. Gutowski, Harvey Henson, and Ryan Leonard
During the winter of 1838–39, the great Native American Cherokee Nation trekked across southern Illinois, in a forced removal by the U.S. government from their ancestral homeland in Tennessee. Harried, unequipped, and unsupported by their captors, thousands died on the Trail of Tears. Burial records were not kept, and burial locations remain lost to this day. Local history suggests that some Illinois settlers allowed the Cherokee to bury their dead on small plots of land adjacent to their own family cemeteries. One such plot, the Campground Presbyterian Church cemetery near Anna, Illinois, may contain unmarked Cherokee graves.
Researchers from Southern Illinois University and Eastern Illinois University used GPS to navigate and precisely map probes of a ground-penetrating radar (GPR) instrument in the cemetery. We monumented the geophysical survey grids using real-time kinematic (RTK) DGPS. Site topography was also mapped using GPS, as were the individual cemetery headstones. Adding geographic information systems (GIS) software to our mix to map cemetery headstone distribution and record headstone attributes (dates of death, names), we could determine chronological gaps within the cemetery that coincide with the probable emigration of the Cherokee.
GPR and electromagnetic conductivity produced contour plots of high-resolution magnetic gradient data. Small dipolar anomalies detected are typically related to disruptions within near-surface soil horizons and may correspond to locations of shallow graves: the lost final resting places of many Cherokee.
By close examination of the geophysical survey data and the anomalies produced from them, we were able to present plausible if not possible locations of several gravesites. However, at this time, and for obvious reasons, the actual location must remain secure and cannot be published.
The figure below shows a mosaic of amplitude depth slices at .30–.70 meter intervals from processed interpolated 250-MHz GPR profile data. White rectangles denote known graves. Most marked graves were imaged, although some were represented as more subtle anomalies on this display. Some possible unmarked graves were interpreted at UTM coordinates xxxx, yyyy.
The cemetery is within working distance of CORS station ILCB at Southern Illinois University. Two RTK GPS units communicating with the station via CDMA cellular radio used real-time differential corrections along a variable baseline length of approximately 28.5 kilometers, enabling mapping of the site at centimeter-accuracy resolution.
Survey data were edited, mapped, and analyzed with a GIS. Family genealogy polygons were generated using last names, to produce family distribution plots throughout the cemetery.
Manufacturers
The study, supported by a National Park Service grant with Southern Illinois University at Carbondale, used two Leica 1250 RTK GPS units, a Leica TC802 robotic total station, and Esri ArcGIS ArcInfo. Equipment was provided by Kara Company of Countryside, Illinois.
A couple of weeks ago, I participated in a roundtable discussion at the Land Surveyors Association of Washington (LSAW) annual conference on the subject of RTK Networks (RTN). Gavin Schrock, administrator of the Washington State Reference Network (WSRN), did a good job of selecting a number of industry folks who’ve got personal experience with RTN to be on the panel.
I always enjoy listening to heavy RTK users about their thoughts, their procedures and how they arrived at them. We danced around a number of subjects with one being the “RTN’s biggest flaw.” My first thought was the communications link. That always seems to me to be the biggest problem with RTK in general. When it’s not working, the first thing I check is the communications link.
“Wrong,” said the panel members.
According to them, the biggest weakness of RTK/RTN is the vertical accuracy. They want vertical accuracy to be equal to horizontal. Duh, why didn’t I think of that? My only excuse is that I’m so used to expecting vertical to be 2x-2.5x worst than horizontal that I already have my expectation set and don’t see it improving until we have a lot more satellites in orbit that will bring very low VDOP values. But I guess if I really think about it, vertical accuracy is the Achilles heel (well, maybe behind the line-of-sight limitation).
It was great to hear thoughts from real-life RTK users. Two panel members in particular espoused the value of RTK/RTN in their operations.
Douglas Casement, PLS, a solo land surveyor using a Leica receiver on the Leica Spider Network, talked about the efficiency of RTK/RTN and doing projects in a half-day that would have taken a couple of days using conventional surveying equipment with a two-man crew.
Mike McEvilly, PLS, works for a surveying/engineering firm in Washington State. He uses the WSRN for RTK corrections. He talked about using RTK on most of their projects in one way or another with the limitation being the vertical accuracy on some projects. I asked him if he had any problems with “brownouts” (lack of satellites), he said he didn’t, but then I found out he is using GPS+GLONASS receivers.
Larry Signani, PLS, is responsible for the geodetic framework behind three RTNs in Washington State. He talked about how he constrains the networks and ties them into the National Spatial Reference System (NSRS). This is the behind-the-scenes grunt work that really makes an RTN perform. It really makes me wonder how other RTNs handle this.
Gavin spoke a bit about procedures and the testing they’ve done, with RTN rovers, on NGS Calibrated Baselines (CBL) during the life of the WSRN. They’ve got a myriad of data that they’ve collected and used to develop their RTK operating procedures. It’s fascinating to look at the data they’ve collected…that’s another article altogether, but I will share with you a slide that summarizes their RTK field practice.
There’s always been a lot of discussion about RTK procedures and occupation times. Last year, I wrote an article called “What’s Your Occupation Time?” that garnered quite a few e-mail responses. I want to address that subject again in the next couple of months.
In the meantime, for those who haven’t read it, an extensive report was published by the UK Survey Association regarding RTK performance and procedures. I highly suggest downloading and reading the report. You can download it by clicking here. I would also suggest downloading and reading the National Geodetic Survey’s User Guidelines for Single Base Real Time GNSS Positioning. Although it doesn’t agree with the UK Survey Association on the time splits (the NGS suggests four-hour time splits) for setting project control, it is the most complete “RTK User’s Guide” I’ve run across. I think it’s a must-read for any RTK beginner as well as a refresher for veteran users.
I could write a lot more about this, and will over the coming months. I’d love to hear about your RTK field procedures and how you arrived at them. E-mail me at [email protected] and let me know your procedures for setting control and topo surveying.
Thanks, and see you next time.
Follow me on Twitter at http://twitter.com/GPSGIS_Eric
Edit: Link updated to User Guidelines for Single Base Real Time GNSS Positioning. Previous link was to a draft version of the document.
Thank you for making “GPS for GIS Data Collection – 101” one of the most well-attended webinars we’ve done. It’s the first that was co-hosted by GPS World magazine and Geospatial Solutions online. If you don’t subscribe to my Geospatial Solutions Weekly newsletter, you might want to consider it as I venture into GIS and broader issues that I don’t have the space to cover in this newsletter. Also, the webinar had a record number of sponsors. Thanks to Hemisphere GPS, Laser Technology, and First American. Those folks make it possible for us to bring these webinars to you free of charge.
As customary, the newsletter after the webinar is dedicated to addressing some of the questions and posting the results from the polls I took during the webinar.
Poll Results
I conducted three polls during the webinar. I received some feedback that we aren’t giving folks enough time to respond to the polls. We’ll pay more attention to that in future webinars and allow more time. Following are the results:
Poll #1: Do you currently use GPS for collecting GIS data?
Yes: 68.5%
No: 31.5%
Total votes: 165
Poll #2: What accuracy do you require in a GPS mapping system?
Question #1: How many satellites are transmitting and how many are just for replacement purposes?
Gakstatter: There are 30 operational GPS satellites. Currently, they are configured in a 24-satellite configuration so six of them are orbiting as “back-ups.” There are also three satellites, I believe, that are in inactive reserve that could be brought back into service if required.
However, as covered in my last three newsletters, the DoD is transitioning the GPS constellation to a 27-satellite configuration to improve satellite visibility to users. The process of transitioning started in January will take up to two years to complete. Please see the following articles for details on the 24+3 configuration:
Question #2: I do have a question, but it will take too long right now. How do I contact you later?
Gakstatter: Please feel free to e-mail me with questions any time…[email protected]. I learn a lot from your questions.
Question #3: What about use of iPhones or Blackberries with GPS embedded in the device?
Gakstatter: As smartphones become more powerful and prevalent, I think the use of them for GIS data collection will increase. I have two comments on this:
To this point, the ability to run GIS data collection software is hit or miss. Some smartphones just don’t have the resources (memory, processing speed) to handle running the more powerful data-collection software on the market. Of course, with technology advancing that may not be as much of an issue in the future, and it’s possible that GIS software manufacturers will write streamlined software specifically for smartphones.
The accuracy of GPS receivers built into smartphones will always be pretty rough. I’d put it in the 5+ meter category and I don’t think it will get much better, so adjust your expectation accordingly. However, using Bluetooth you might be able to “tether” the smartphone to a higher performance external GPS receiver.
Question #4: Is there a place for consumer-grade receivers in GIS data collection?
Gakstatter: Yes, I wrote an article on this last year. You can read it here…
Please don’t hesitate to e-mail me more questions about this that may not be answered in the referenced article. I’ve been thinking about a follow-up article on this subject. Question #5: What accuracy would you expect to record from a GPS handheld unit?
Gakstatter: There are high-performance handheld GPS receivers that can deliver centimeter-level positions and there are consumer-type handheld GPS receivers that delivery 5+ meter accuracy. This is typically a direct relationship between accuracy and cost (you’re not going to get sub-meter accuracy from a $200 receiver).
The best way to approach this is to decide what accuracy you require (cm-level, one foot, sub-meter, 1-3 meters, 3-5 meters, 5+ meters) and look at the budget you have available. You might want to take a look at the webinar I conducted last year titled “A Buyer’s Guide to GPS/GIS Mapping Equipment” and a newsletter article I wrote around the same time titled GPS Receivers for GIS Data Collection.
Question #6: We have a Topcon GMS-2 unit using an exteral antenna on a range pole similiar to one of the pictures you had in the presentation. How does the height of the range pole with the external antenna affect the X-Y position? Or does it? Thanks.
Gakstatter: The value of the range pole is that it gives the GPS antenna a clear view of the sky (above your head and other local obstructions). It can only improve your X-Y position. I don’t know how many times I’ve seen users hold a handheld GPS receiver up against their chest, effectively eliminating the use (and degrading accuracy) of GPS satellites behind them.
Question #7: For area determination which is preferred: static or dynamic?
Gakstatter: Personally, I would use dynamic unless you’re talking about a very small parcel of land (less than an acre). I’ve seen a number of reports on this and I believe all of them used dynamic data collec
tion with pretty reasonable results. In other words, I don’t think static buys you much in terms of acreage precision. However, I’ve been in circumstances where I used a combination of both such as when I know there’s a reasonably straight line between two vertices, but it would be very difficult to walk a direct line between them. In that case, I might use static for that leg of the traverse.
Question #8: I thought that PDOP was Positional Dilution of Precision.
Gakstatter: Several of you busted me on this. I mis-typed the presentation slide. I wrote Precision Dilution of Precision, which doesn’t make any sense. It should have been Position Dilution of Precision (PDOP). The horizontal component of PDOP is HDOP (Horizontal Dilution of Precision). The vertical component of PDOP is VDOP (Vertical Dilution of Precision).
Click here for a Wikipedia link that provides a little more information on GPS DOPs.
Question #9: Explain limitations of what type of project you cannot do if not a licensed surveyor.
Gakstatter: Because local laws vary widely, it really depends on where you are working. Even within a country like the U.S., each state has its own statutes that define the roles of the land surveyor.
In some areas, activities as simple as GIS data collection must be supervised by a licensed surveyor. In other areas, high-liability activities such as construction staking can be done by virtually anyone.
Question #10: Could the steel plate in my head cause multipath or obstruct signals when I use the integrated antenna?
Gakstatter: I can safely say (tongue in cheek) that in 20 years of GPS product development, conducting workshops/seminars, attending conferences, and performing GPS fieldwork, I’ve never heard this question. I’m speechless. :-)
Question #11: A presumption that we should avoid is that by default “GIS data collection” implies low accuracy. This is simply not true. Position accuracy is independent of GIS. GIS can handle any level of accuracy the user desires. There is no such thing as a “GIS-grade” or “GIS-accuracy” survey. What relationship does GIS have with accuracy?
Gakstatter: I think Guest Commentator Craig Greenwald and I covered this well in the webinar, but it’s good to reinforce the point. I cringe when I hear someone say GIS stands for Get It Surveyed because it implies that the quality of a GIS is dependent on accuracy. It’s not. In some cases, +/- 500 feet. accuracy is perfectly fine for analysis in a GIS. The accuracy required by a GIS totally depends on the type of analysis you are conducting. Many surveyors typically think of GIS in terms of a land record (parcel) mapping system, but GIS is used for so much more than that. You don’t need cm-level accuracy to find the optimal location for the next McDonald’s restaurant within a city.
Question #12: Do you plan on conducting a webinar that will discuss strictly GPS, i.e., RTK vs. static, data reduction, post processing, etc.
Gakstatter: Yes, if you’re not subscribed to the Survey Scene newsletter, please sign up for that here as well as the Geospatial Solutions Weekly newsletter on the same sign-up page. The price is right…free. You can also look at the webinar archives where I have covered some of these subjects before. I’m also scheduled to conduct at least three more webinars this year (next one in May/June – topic not yet determined).
There were many other questions and I’ll continue including answers to them in the mid-March Survey Scene newsletter. Also, I suggest you sign up for my Geospatial Solutions Weekly newsletter (GSS Weekly) as mentioned above as I tackle GPS/GIS-related issues there, too. Next week, in the GSS Weekly, I’ll continue my discussion on the roles of the surveyor and GIS professional.
Adding GLONASS to GPS gives a total of about 50 satellites, for a significant improvement in navigation availability, reliability, robustness, and convergence time through a new multi-GNSS precise point positioning (PPP) service. System performance and field results demonstrate that there is no need to await future constellations — better performance is available now.
By Tor Melgard, Erik Vigen, Ole Ørpen, Fugro Seastar AS, and Jon Helge Ulstein, Bourbon Offshore Norway AS
Precise point positioning (PPP) stands out as an optimal approach for providing global augmentation services using current and coming GNSS constellations. PPP requires fewer reference stations globally than classic differential approaches, one set of precise orbit and clock data is valid for all users everywhere, and the solution is largely unaffected by individual reference-station failures. There are always many reference stations observing the same satellite because the precise orbits and clocks are calculated from a global network of reference stations. As a result, PPP gives a highly redundant and robust position solution.
The results presented here represent a significant step forward in PPP GNSS research and development. Using GLONASS improves the availability and reliability of the solution. The G2 system’s horizontal positioning accuracy is at the decimeter level. These results derive from increasing the number of satellites in the constellation by 60 percent, from about 30 to 50 satellites. The outcome of the development of the G2 real-time combined GPS and GLONASS PPP service represents a next-generation GNSS augmentation. Further, the later GLONASS-M satellites have improved performance and lifetime over previous GLONASS satellites, so that results will continue to improve as that constellation is replenished.
G2 development has benefited from the close cooperation between Fugro and the European Space Operation Centre (ESOC), an establishment of the European Space Agency (ESA). ESOC has contributed its long experience and expertise on precise orbit and clock processing techniques, while the strength of Fugro is real-time positioning and navigation services.
Based on this work, Fugro has introduced the first real-time GPS and GLONASS precise orbit and clock service. The service utilizes Fugro’s own network of dual-system GNSS reference stations to calculate precise orbits and clocks on a satellite-by-satellite basis for all 50 satellites of the two global navigation satellite systems. The system comprises about 40 dual-frequency GPS and GLONASS reference stations distributed around the world as shown in Figure 1.
Raw GNSS measurement data for all satellites are transmitted to processing centers for calculation of the precise orbit and clock of each GPS and GLONASS satellite (Figure 3). The precise data generated is then broadcast to users via geostationary communications satellites with nearly global coverage, as shown in Figure 2.
FIGURE 1. The G2 reference station network consists currently of 40 GNSS receivers owned and operated by Fugro.FIGURE 2. The G2 precise orbits and clocks are broadcast over redundant geostationary satellite beams together with the other Fugro services.FIGURE 3. Dataflow from the reference stations to the redundant calculation servers producing precise orbits and clocks, then to the satellite uplink stations for broadcast over geostationary satellites to combined G2/GNSS user equipment.
Inside the end-user equipment a dual-frequency carrier-phase-based PPP solution gives horizontal positioning accuracy at the decimeter level. The PPP calculation module is provided by Fugro and is embedded in multiple GNSS receiver manufacturers’ products as well as Fugro’s own product line.
Like any GNSS technique, PPP is affected by satellite line-of-sight obstructions. Even the most precise orbit and clock data is useless if the user cannot track particular satellites. When satellite visibility is partially obstructed, a best possible service can be ensured by using the full range of satellites from both the GPS and GLONASS systems. This can occur during a survey of a dense urban environment, and for urban positioning in general. It can occur under heavy tree cover, when a cruise ship is in a high-sided fjord, when an offshore vessel is close to an oil rig or platform, or during ionospheric disturbances.
The trend clearly lies towards increasing availability of GNSS satellites on orbit; many studies predict the future benefits of combining the constellations of GPS and Galileo. There is no need, however, to wait for future constellations to reap the immediate benefits of access to additional GNSS satellites. The current GLONASS constellation may not have all the features of future GNSS systems, but it is available here and now. Recently, the Russian government has proven its commitment to enhancing the GLONASS constellation. Many receiver manufacturers have also acknowledged this fact and now provide combined GPS and GLONASS receivers.
G2 Accuracy and Statistics
In Figure 4, time-series plots show the 3D accuracy of GPS and GLONASS G2 real-time orbits on August 14, 2009. In the comparison, final orbit data from the International GNSS Service (IGS) is used as reference. PPP positioning is mainly affected by the radial orbit error, which is significantly less than the total 3D error shown here. The 95 percent 3D accuracy for GLONASS (22 centimeters) is more than double that for GPS (10 centimeters). The graph demonstrates how this difference in this case is mainly caused by a few GLONASS satellites being less accurate. Actually, several GLONASS satellites have orbit accuracy very close to the level of GPS for real-time G2 data.
FIGURE 4. GPS and GLONASS orbits compared to IGS final orbits.
Figure 5 shows the clock accuracy of the G2 real-time clocks compared to final IGS clocks. A constant bias has been removed to account for the differences in system reference time. Smaller individual clock biases for each satellite can still be observed. Small biases do not affect the final accuracy of the PPP solution, and achievable position accuracy with these clocks are significantly better than the 21-centimeter 95 percent number for GPS may indicate.
FIGURE 5. GPS clocks compared to IGS final clocks. GLONASS clocks compared to a combined solution based on IGS plus Fugro network to calculate a best possible reference solution.
The lower time series in Figure 5 shows the estimated GLONASS clock accuracy. Currently there is no comparable IGS product with precise GLONASS clocks. A post-processing of all available IGS plus Fugro GNSS stations has been made to establish a reference for the comparison. As shown, the GLONASS clocks are more variable, but still they are stable enough to allow for precise navigation.
Real-Time Positioning Results
Real-time position performance is continuously observed at the G2 operation and monitoring center in Oslo, Norway. The graph in Figure 6 shows typical G2 positioning results with the calculation engine running in dynamic mode at a fixed location for a 24-hour period. The blue lines in the north and east time series are at 20 centimeters and the scale is 61 meter. In the height graph the blue lines indicate the 30-centimeter level. The antenna is in a location with clear view of the sky, and in
dependently calculated reference coordinates are used as reference. 1-sigma accuracy statistics on August 14 are 3, 4, and 8 centimeters in easting, northing and height respectively.
FIGURE 6. G2 GPS-plus-GLONASS position monitoring results in Oslo on August 14, 2009.
Figure 7 shows GLONASS-only real-time positioning with clear view of the sky for the same day as in Figure 6 and the same antenna location. The blue line indicates the 50-centimeter level and the scale is 62 meters. For long periods, the GLONASS-only solution works quite nicely. There are, however, shorter periods with fewer than four satellites being tracked, causing the position output to stop, followed by a period of re-convergence.
FIGURE 7. GLONASS-only real-time PPP solution on August 14, 2009 for a 24-hour period.
Figure 8 displays results from May 11, 2009, when there were slightly more satellites available and just enough to have the GLONASS-only solution running for 24 hours without resets. 1-sigma accuracy statistics for this day are 11, 9, and 16 centimeters in easting, northing, and height respectively. Considering the average number of satellites of 6.14 and periods with high DOP values, this is very promising. In early 2010, 20 GLONASS satellites should be available, and by 2011, 24 are expected. In 2010, a performance similar to or better than that of May 11 should generally be expected with the new satellites. By 2011, even better performance is believed to become the norm of GLONASS-only real-time PPP navigation.
FIGURE 8. GLONASS-only real-time PPP solution on May 11 for a 24-hour period.
Even in some clear-view-of-sky situations, the addition of GLONASS may improve the navigation compared to GPS-only solutions. Figure 9 presents an example of such situations. Here the GPS-only solution suffers some multipath-like effects showing up, especially in the east component. Figure 10 shows the combined GPS+GLONASS solution for the same dataset. The distortion in position is practically eliminated. This is an example where adding GLONASS also improves redundancy and accuracy for navigation with clear view of the sky.
FIGURE 9. GPS-only results for a 3-hour period where some multipath-like effects distort the postition, especially the east component.FIGURE 10. Adding GLONASS improves redundancy and accuracy for the same time period as presented in Figure 9.
The next test further analyzes the same dataset as in Figures 9 and 10 by simulating a virtual wall to the south, blocking all satellites below 40 degrees elevation. Figure 11 illustrates this virtual wall blocking both GPS and GLONASS satellites.
FIGURE 11. GPS and GLONASS satellites blocked between the azimuths 90 and 270 degrees and elevation lower than 40 degrees, effectively establishing virtual wall to the south.
With such data blockage, the GPS-only solution fails for more than 20 minutes, as seen in Figure 12, simply because the number of satellites goes below four. Then a period with slow convergence follows because of few satellites and high DOP.
FIGURE 12. GPS-only solution fails when simulating blockage to the south.
Again, adding GLONASS greatly improves the performance, as shown in Figure 13. Now a sufficient number of satellites are tracked all the time, and there is a continuous solution with the combined GPS+GLONASS throughout the time window when the GPS-only solution failed.
FIGURE 13. GPS+GLONASS solution continues working with simulated blockage to the south.
Even with more than 30 satellites in the GPS constellation, there are situations when the satellite geometry gets poor. This occurred in northwest Europe on February 2, 2010. One of the GPS satellites (PRN17) was not available due to maintenance, and even with five to six usable GPS satellites left, the horizontal dilution of precision (HDOP) was in the range of 7–11 for about 12 minutes (10-degree elevation mask), as shown in figure 14. Such high HDOP values lie above what most user installations are configured to accept, and Fugro received feedback from clients at sea losing positioning. The G2 solution was not affected by the poor GPS geometry and kept the HDOP below 2 during this period, as shown in Figure 15.
FIGURE 14. GPS-only performance during a period with poor GPS satellite geometry in Oslo, February 2, 2010.FIGURE 15. GPS+GLONASS performance during the same period as in Figure 14 in Oslo, February 2, 2010.
Convergence-Time Analysis
As will be shown in the following analysis, adding GLONASS not only improves availability and robustness of the solution, it greatly improves convergence time. Real-time high-accuracy PPP solutions use carrier-phase measurements to achieve high-accuracy positioning. To do so, the carrier-phase ambiguities must be determined. This process takes a certain time depending on the observed satellite geometry and is commonly referred to as cold-start convergence time.
Figure 16 presents a theoretical study of the expected convergence time for a GPS-only compared to a combined GPS+GLONASS solution. The lower graph shows how the expected convergence time varies significantly for a GPS-only solution throughout the day, with a peak of 75 minutes. The combined solution shows much more consistent performance, with expected 50–60 percent average improvement over GPS-only.
FIGURE 16. Theoretical study of expected convergence time with actual GPS-and-GLONASS constellation in view of Oslo on June 26, 2009. Adding GLONASS gives a 50–60 percent theoretical convergence time improvement over GPS-only.
We compare this theoretical study to results using G2 data produced in real time in Figure 17. A cold start is performed every 5 minutes throughout the day, for six consecutive days, giving a total of 1,728 convergence tests. The convergence criterion is the time when the 3D position arrives within 40 centimeters of the reference position and remains there for a minimum of 10 minutes. The average convergence time improvement achieved in Figure 17 is 39 percent, with some variations from day to day. On the better days, the average improvement is almost 50 percent, and close to the expected performance based on the theoretical study. On other days, there is room for further improvement. Mainly two factors are expected to contribute: more and newer GLONASS satellites, and further improvements of the G2 precise GPS and GLONASS orbit and clock product.
FIGURE 17. Convergence results for six consecutive days starting June 24, 2009. Average convergence time of GPS-only is 27 minutes, and GPS+GLONASS is 16.5 minutes, a 39 percent improvement.
Dynamic Environment Results
Since late 2008, the G2 system has been installed on the vessel Bourbon Topaz, making frequent trips into the North Sea and back into port in Norway (see BOX).
All positioning data from both the G2 system and the GPS-only reference systems are logged in real time on the vessel. Figure 18 gives an example plot of the relative height estimated by the G2 GPS-GLONASS solution. In the beginning of the plot, the vessel is out at sea, clearly seen as a noise in the graph that actually is the vessel’s movement in the waves. Then the vessel comes into port and the slower tidal variations are observed for the next 12 hours until the vessel again goes back out to sea.
FIGURE 18. Relative G2 height measurements for a 24 hour period. The vessel is in harbor from 04:00 – 16:00 UTC.
On June 22, 2009, an incident was recorded where the combined GPS-GLONASS G2 solution improves performance. As seen in Figure 19, there is a period starting at 10:00 UTC where the GPS-only reference systems suffer from poorer DOP values, and this is reflected both in horizontal and vertical components of the calculated position. This particular plot shows how the height drifts off by roughly 1 meter while the G2 combined solution remains unaffected for the entire period. Generally, the G2 solution also shows a smoother height than the reference system even when such problems as shown here are not present.
FIGURE 19. Height graph from the Bourbon Topaz while in harbor on June 22, 2009. The GPS-only reference system has a period with poor DOP values while the GPS-plus-GLONASS solution is not affected.
The Bourbon Topaz carries the G2 system on operations in the North Sea, and continuously compares it with the GPS-only reference systems onboard.
Test of G2 onboard Bourbon Topaz
The Bourbon Topaz is a modern supply vessel equipped with the latest dynamic positioning (DP) systems, operating in the North Sea. The North Sea can be a harsh environment in which to operate, and we rely on good tools for maneuvering our vessels.
Early on, we recognized the need for stable, reliable reference systems, and our fleet is equipped with Kongsberg Seatex DPS700 system as standard. When we were asked to test the G2 onboard the Bourbon Topaz, we saw this as an opportunity to follow the development in the industry of such services. The DPS232 receiver was set up in connection with the vessel’s DPS700 system, and all information was logged and sent to Fugro Seastar.
We often experience that the vessel has to operate close to offshore installations, which could block good reception of signals. In these cases, the G2 offers a much better and more reliable signal reception. Our experience of the quality of the G2 system is overall positive.
User Equipment
G2 and the other Fugro services can be received from a variety of different user equipment; both Fugro-branded or manufactured equipment and third-party equipment. In most cases the L-band receiver decoding the data from the geostationary satellites, including Fugro subscription software and position calculation module, is integrated into the same box as the GNSS receiver. Both the GNSS and geostationary satellite signals can be tracked with a single antenna.
FIGURE 20. Receivers supporting the Fugro services. These are only examples, and not all third-party equipment manufacturers are shown. Fugro L-band data reception receiver and positioning/subscription software reside inside the receiver.
Conclusions
Test results confirm decimeter-level position accuracy in real-time navigation with G2, the first real-time combined GPS and GLONASS PPP service. Several examples show how G2 improves availability, robustness, and convergence time compared to GPS-only positioning.
More is better. There is no need to wait for future constellations like Galileo to reap the benefits of access to additional GNSS satellites now.
Tor Melgard is R&D manager at Fugro Seastar in Oslo, Norway. He holds an M.Sc. in electrical engineering from the Norwegian Institute of Technology and wrote his thesis at the Department of Geomatics Engineering, University of Calgary.
Erik Vigen is a senior developer at Fugro Seastar. He received his M.Sc. in Geodesy from the Norwegian Institute of Technology.
Ole Ørpen is senior scientist at Fugro Seastar. He received his M.Sc. from the Norwegian Institute of Technology in electrical engineering.
Jon Helge Ulstein is IT superintendent at Bourbon Offshore Norway AS, a subsidiary of the Bourbon Group, Marseilles, France.
The next-generation GPS ground-control system, known as OCX.
Officials from the Space and Missile Systems Center’s Global Positioning Systems Wing announced today the award of the Next-Generation GPS Control Segment (OCX) contract to Raytheon Company, Intelligence and Information Systems, Aurora, Colorado.
The OCX development contract will be 73 months in duration and with option years for sustainment worth $1,535,147,916. The contract will include development and installation of hardware and software at GPS control stations at Schriever Air Force Base in Colorado and Vandenberg AFB in California, deployment of advanced monitor stations at remote sites, and initial contractor support with sustainment options for five years.
OCX will replace the current GPS Operational Control System, maintaining backwards compatibility with the Block IIR and IIR-M constellation, providing command and control of the new GPS IIF and GPS III families of satellites, and enabling new modernized signal capabilities.
“OCX is urgently needed not only to enable new warfighter capabilities but also to put the new GPS III space vehicles into mission operations,” said Col. Dave Madden, commander, GPSW. “OCX will have a flexible architecture that can rapidly adapt to the changing needs of today’s warfighter and will connect to the Global Information Grid so that warfighters around the globe have immediate access to GPS data and constellation status.”
“OCX will allow AFSPC to effectively and efficiently plan and control full-spectrum precision position, navigation and timing information for all GPS user communities,” Madden said. “OCX will achieve this vision by implementing an incremental development approach that supports the evolving military operational environment, while enabling civil and international users who are employing GPS in innovative applications like transportation.”
The Air Force Space Command’s Space and Missile Systems Center, located at Los Angeles Air Force Base, California, is the U.S. Air Force’s center of acquisition excellence for acquiring and developing military space systems including six wings and three groups responsible for GPS, military satellite communications, defense meteorological satellites, space launch and range systems, satellite control network, space-based infrared systems, intercontinental ballistic missile systems, and space situational awareness capabilities.
