GPS World

Author: GPS World Staff

  • GPS Adventure Game Out the Window

    GPS Adventure Game Out the Window

    GPS DATA maintains a 3D model that keeps a car correctly positioned while passengers hunt down werewolves.
    GPS DATA maintains a 3D model that keeps a car correctly positioned while passengers hunt down werewolves.

    Look out the window — is a werewolf hiding behind that tree?

    Looking out the window while on a long car trip can be an adventure with a new GPS-based game being developed by The Interactive Institute in Stockholm, Sweden. The Backseat Playground uses GPS to turn passing sights like forests, buildings, and rivers into locations for in-game characters and events, reports New Scientist magazine. Backseat Playground consists of a GPS receiver, a handheld computer, and headphones — all connected to a laptop in the trunk of the car. A database of geographical information is used to match events in the game to suitable locations. Game characters and events are all generated dynamically during the car trip.

    The game begins with a radio newsflash, relayed by the handheld computer, which places a passenger at the start of a murder mystery or a werewolf thriller. As the car travels along its route, the player receives further phone calls and messages from in-game characters.

    Players interact mostly by listening; minimal graphics are involved, according to the designers. “It’s like a novel,” said John Bichard, who created Backseat Playground with colleagues Liselott Brunnberg and Oskar Juhlin. “We are trying to suggest spaces and places and events and have the user fill in the gaps to build a narrative.”

    For now, a prototype now being evaluated only works over an area of 35 square kilometers in Stockholm. Initial feedback has been positive, and the team hopes to add voice recognition to the set-up. “Being able to talk directly to the characters would be great,” Bichard said.

    We’ve always wondered what those werewolves had to say.

  • Philips Exits PND Market Before Entry

    Philips Electronics, the Netherlands-based electronics giant that is Europe’s largest consumer electronics company, said in June 2006 that it would enter the personal navigation device (PND) market in the fall, to compete with Garmin, TomTom, Magellan, and other PND makers. But it abruptly pulled the plug on that effort in early December, stating that it was no longer interested. A spokesperson confided that the company had watched the market closely and decided it was too crowded.

    This marks the second time Philips has retreated in this sector. Although its Carin system was an early dedicated in-vehicle nav system (circa 1990), and the company was an early investor in NavTeq, it later abandoned that market.

    The booming European PND market, which analysts say could double to about 5 million units, has attracted Japanese consumer-electronics giants as well as many smaller Taiwanese manufacturers. Hardly a day goes by without a trumpeting of a new PND, often from a company heretofore unheard of in GPS and nav circles.

    “It’s a very competitive market and it puts a lot of pressure on profit margins,” stated the Philips spokeperson. “We decided we need some focus, and navigation devices like these don’t fit within this focus.”

    The company remains interested in GPS in general, but not for stand-alone products. “We don’t want to go further with GPS as a single device, but it’s an interesting technology to implement in other products,” she added. Mobile phones or digital music players remain as possible candidates for GPS capabilities.

  • Cingular Launches LBS with TeleNav

    Cingular Wireless has launched its first generally available, location-based service with TeleNav Inc’s GPS Navigator. Cingular business and government customers can get turn-by-turn voice and onscreen GPS directions, while driving or walking, on Cingular business devices including the HP iPAQ hw6920 and hw6500 Mobile Communicators, the Cingular 8125 Pocket PC, and the Palm Treo 650. The latter two devices require a Bluetooth GPS receiver. A GPS receiver is built into the Mobile Communicators. Features include:

    • full-color moving maps
    • a “Biz Finder” for locating nearby businesses such as cash machines, restaurants, hotels, and gas stations
    • a spot marker for locating a parked car
    • a fuel finder for finding low gas prices
    • pedestrian mode.

    Pricing is $5.99 per month for up to 10 trips, or $9.99 per month for unlimited trips.

    “Location-based services on wireless phones have disrupted the navigation market and created a more versatile solution for both businesses and consumers,” said Ken Hyers, principal mobile wireless analyst at ABI Research, of the launch of TeleNav GPS Navigator.

  • Driving for Dollars: Urban Challenge Purse Put at $3.5 Million

    The Defense Advanced Research Projects Agency (DARPA) will award $2 million, $1 million, and $500,000 awards to the top three robotic finishers who complete its new Urban Challenge course in November 2007.

    Under Secretary of Defense for Acquisition, Technology and Logistics Kenneth J. Krieg approved the cash prizes, evidencing the Department’s interest in making one-third of all combat vehicles — principally supply vehicles — driver-less by 2015.

    DARPA has staged two desert Grand Challenges, in 2004 and 2005 in the Mojave, with significant difficulties posed by geography and terrain. The 2007 Urban Challenge will feature fully autonomous ground vehicles conducting simulated military supply missions in a mock urban area. The race will take place on November 3, 2007, at a location to be announced later, in the western United States.

    Robotic vehicles will attempt to complete a 60-mile course through traffic in less than six hours, operating under their own computer-based control. Vehicles must obey traffic laws while merging into moving traffic, navigating traffic circles, negotiating busy intersections, and avoiding obstacles.

    Learning Curve. Participants in past Grand Challenges have truly risen to the occasion, learning and innovating as they go. The number of sensors and software applications integrated into most of the experimental vehicles increased dramatically between the first and second races. Inertial sensors proliferated, with cost, size, and power consumption going down, while performance went up. Inertial systems, along with various camera/vision apparati, function as the workhorses covering the ground in most of the vehicles. GPS generally teams with a central processing unit (CPU) to act as the brain guiding the process.

    William “Red” Whittaker of Carnegie Mellon University’s Robotics Institute and Louis Nastro of Applanix Corporation co-authored a detailed technical article in September 2006 GPS World on their experience with the students of Carnegie Mellon’s Red Team Racing effort, designing and outfitting two vehicles that competed and placed in the 2005 race.

    To qualify for the race or simply watch, see the Grand Challenge website for additional information and rules for the Urban Challenge.

  • Letters to the Editor – December 2006

    Need Clear Vision

    Well done on the editorial this month (“Skies Cloudy All Day,” October). I totally agree with you regarding the lack of clear vision for our GPS future. And I’m not saying there is not vision at the Air Force, but the GPS programs must be backed by the full Congress and its funding. We must put government funding behind that vision to make it a reality. When we as U.S. citizens say GPS is critical to our national security, and even our livelihood more and more each day, we must back that with the bucks to keep it healthy and strong as the utility that it has become.

    – Ellen Hall
    President, Spirent Federal Systems Inc.

    NDGPS Killer App

    This is in response to the October 2006 letter “NDGPS Budget” by Tim Smith and the reply by Erik Gakstatter. There will be a killer application for NDGPS, for ground vehicles to know their position precisely to the meter, especially in the urban canyon setting where satellite reception can be very poor. This effort is spearheaded by the Department of Transportation (DOT) for the Intelligent Transportation System (ITS). In addition, the Vehicle Infrastructure Integration (VII) consortium consists of several state agencies, automobile manufacturers, and vendors to implement vehicle-to-vehicle-to-roadside communications via Dedicated Short Range Communications (DSRC). The primary purpose is to enhance vehicle safety using communications for collision avoidance, signal violation warning, near instantaneous traffic information, and road hazard warnings. In this case accurate GPS positioning is very important and where NDGPS can fill the need. Secondary purposes are to provide a wide range of applications starting with local electronic signage, navigation aids, traveler information, electronic payments (fuel, parking, tollway), ramp metering, and a wide variety of potential consumer applications in the vehicle.

    The article “Safe in Traffic” in the very same issue of GPS World describes the whole project and the use of GPS for ground vehicle transportation, but not the need for accurate GPS positioning which could be provided by NDGPS.

     – Neal W. Probert
    President, ProbeStar Technical Systems

    WAAS in the East

    Could you please tell me when WAAS will be up and running in Eastern Canada?

     – Kenneth Lewis
    Government of New Brunswick

     

    Editor’s Reply: There have been some changes due to geostationary satellite repositioning. See the web page of the University of New Brunswick’s Wide Area Augmentation System Monitoring Station and the Federal Aviation Administration’s “New WAAS GEO Status as of 11/9/06” PDF file.

  • Geo-Enabling the IC with GeoPDF

    This week, TerraGo Technologies announced that it has entered into a strategic agreement with In-Q-Tel – an independent venture capital fund tasked with identifying new technologies for the Central Intelligence Agency (CIA) and the larger intelligence community (IC) – to spark the development of new mapping solutions needed by such IC leaders as the National Geospatial-Intelligence Agency (NGA).

    For several years now, many government agencies (including the US Army Corps of Engineers, USACE), have relied on TerraGo’s innovative GeoPDF proprietary data-sharing format to exchange geospatial data between users of varying skills levels – from engineers on the scene of natural disasters like Hurricane Katrina to soldiers in the field who can use Adobe Reader to manipulate maps. The agreement between TerraGo and In-Q-Tel is expected to advance the development of GeoPDF and related technologies to meet the operational demands of these organizations, as they continue tackling such challenges as the Global War on Terror and emergency response for major natural and manmade disasters.

    Since its first MAP2PDF for Adobe Acrobat product was introduced seven years ago, TerraGo has evolved the technology that allows for the creation and sharing of digital map display files in a user-friendly format. Today’s GeoPDF format allows users to convert raster and proprietary CAD and GIS mapping and database information to text-searchable, georeferenced PDF files. It also allows for the creation of both georegistered and GIS database-embedded PDF files – hence the term GeoPDF.

    With GeoPDF, users can send complex, georegistered maps as PDF files with layers and embedded feature attributes. A GeoPDF can be distributed and used in connected or disconnected modes with the free Adobe Reader and GeoPDF Toolbar software. Users can view finished digital maps, turn layers on and off, query attributes, display coordinates, measure distances, and track locations via GPS . . . all without the need for specialized geospatial knowledge or training.

    For an example of how USACE is using the GeoPDF file format, we can look to the corps’ Topographic Engineering Center (TEC). Currently, TEC is creating unclassified/for official use only digital map displays for regions of the world where the Global War on Terror is being fought. TEC has created DVDs for five countries in support of the war, and it has distributed these DVDs to the military. The project has included taking all of the NGA standard map sheets of Korea, having them scanned and converted to GeoPDFs, and packaging them with an index sheet. By the end of the year, TEC anticipates having created 30-plus country DVDs that consist of all NGA standard products in raster GeoPDF format. The center is also awaiting NGA’s delivery of vector-based GeoPDFs for inclusion in the country DVDs.

    It’s abundantly evident that TEC and NGA have bought into the GeoPDF concept in a big way. Whether TerraGo will be the only GeoPDF name in town remains to be seen, but I think it’s inevitable that the technology itself is here to stay, in both the private and the public sector. In the intelligence community, GeoPDF is clearly a smart way to share geospatial information quickly and easily, especially when the nation’s welfare – and American lives – might be on the line.

    Editor’s Note: Eric Gakstatter, a GPS/GIS consultant with Discovery Management Group LLC, and Ray Caputo, a geographer with the US Army Corps of Engineers Topographic Engineering Center, contributed to this editorial.

  • NDGPS Abandoned?

    The Federal Railroad Administration has relinquished responsibility for national differential GPS (NDGPS), saying that it no longer supports NDGPS for positive train control. DOT’s Research and Innovative Technology Administration (RITA) now has sponsorship of NDGPS, and will hold a meeting on October 4 to assess.

    The outlook for NDGPS appears bleak. “Without funding, the system faces tough choices.” Options reportedly under consideration include moth-balling the system for $2 million or shutting it down for $10 million. NDGPS supporters claim that a large mass of users of NDGPS have not yet been heard.

  • Letters to the Editor – October 2006

    Constellation Viability

    We recently discovered that in a 5-day period three GPS satellites were taken off line indefinitely with no explanation as to possible return to service. This is significant in that the constellation that remains is “non-optimal” in terms of providing more than four satellites at all locations and so on. Specifically we have concerns about RNP Aviation operations that rely on excellent satellite availability to achieve the lower RNP, less than 0.2 capability for final approaches. This seriously impacts airline operations as our calculations indicate that RNP 0.18 operations have been reduced (worldwide) to an availability of about 86 percent. This is a very significant loss of availability. This potentially also impacts LPV and other FAA certified GPS approaches as well.

    Do you have any information as to what has happened to these satellites and whether or not they may be returned to service? May the constellation be “rebalanced” — with satellites being moved to new orbits to make the remaining 26 satellites provide a more optimal configuration that would also substantially improve RNP and other aviation operations availability?

    Any insights or references as to where I may go to learn more about this situation would be greatly appreciated.

    —David Vacanti Aerospace Fellow,
    RF Navigation Surveillance IPSC Lead, Honeywell

    Editor’s reply: As of September 4, three on-orbit GPS satellites had been taken out of service. PRN 15 was taken off line on 21 August; PRN 3 off on 24 August, and PRN 29 off on 26 August. PRNs 15 and 3 were reportedly undergoing clock swaps. PRN 3 had a previous clock swap in June. See “Clipped Birds” in The System news coverage for constellation status as of press time, September 22.

    NDGPS Budget

    Eric Gakstatter’s on line article on NDGPS (Nationwide Differential GPS) is a very poor framing of the issues. I have two main problems with GPS World’s on-line and hardcopy reporting of this issue.

    One, the print article relegates an issue that is one of the most important in more than five years for GPS to a “down-at-the-bottom-of-page-20-issue.”

    Secondly, it allows a pro-WAAS policy wonk such as Mr. Gakstatter to report on it. Very disappointing. This issue should not be framed as an NDGPS vs. WAAS issue as Gakstatter continually tries to make it. This is not the reason that NDGPS funding is an issue. It is much more complicated than that. Additionally, Mr. Gakstatter undermines his own credibility by making light of the user community that requires NDGPS for real-time, 1–5 meter, differential corrections and cannot use WAAS reliably.

    Both of these points illustrate the lack of understanding by him and GPS World of real-world problems that face the GPS user community and policy issues in Washington.

    In an article titled “Managing the GPS Constellation for Today’s Needs” in this month’s GPS World publication (September 2006), Mr. Lavrakas states that “Accuracy is the thing, and real-time accuracy is the most important thing.” I wholeheartedly agree. The NDGPS funding issue concerns a fantastically reliable augmentation system that is both efficient and fiscally responsible. My hope is that some sanity will return to this debate and the right path will be taken for the betterment of the GPS user community and the U.S. taxpayer (“Let’s not forget whose money makes all of this possible,” A. Cameron, Editor, same issue of the magazine).

    Please, GPS World is a great publication, let’s have some accurate and complete reporting rather than opinion being floated as fact.

    —Tim Smith
    GPS Program Coordinator, National Park Service

    Eric Gakstatter replies: Clearly when valuable programs are threatened, emotions run high.

    For the record, I did not claim there were no unique applications for NDGPS. I wrote that the primary threat to NDGPS is the lack of a “killer app” and that WAAS is a key technology that is putting pressure on NDGPS. I think it would be difficult to argue that it is not.

    GPS World’s editor and I invite you to write a more substantive, detailed essay to make “The Case for NDGPS.” I think it would be in the best interest of the GPS user community to hear it.

    Finally, I am neither pro-WAAS nor anti-NDGPS. I am pro-user community. I have used — and own — both WAAS and NDGPS equipment extensively in the field and am very familiar with the capabilities and limitations of both technologies.

     

    Air Traffic

    It was gratifying to read about the result of so many years of hard work to make the vision of GBAS a reality. As Honeywell’s former partner [Pelorus Navigation Systems] in the development of the current systems at SEA-TAC and Moses Lake [Bremen, Malaga, Spain and other locations] I was delighted to read the results of what I knew would be found. My congratulations to Tim and the others at Boeing that had the courage to support LAAS/GBAS/ GRAS from the beginning. It has been a road far too long but the proverbial ILS light at the end of the tunnel will soon be replaced with GBAS.

    —Mike Beamish
    President, Analysis Unlimited
    Calgary, Alberta, Canada

     

    I so much enjoyed the August news story “Next Generation Air Traffic Controlled by GPS.” As I wrote in my October 1991 GPS World article titled “GPS, Aviation, and Airport; The Integrated Solution,” this was the future. The 1991 article includes then-revolutionary concepts of seamless airport operations, Automatic Dependent Surveillance in the terminal area, computer automated safety logic, and precise digital maps used for navigation and air traffic control. ADS-B is an integral part of the 21st century aviation infrastructure modernization.

    The recent Comair, Louisville, Kentucky, disaster could certainly have been prevented with a moving map display showing the right travel path or even a set of navigational waypoints used during the departure taxi and take off. As experienced in the FAA’s Safe Flight 21 and Capstone programs, this technology saves lives, reduces delays, hence lowering airline operating costs, and reduces fuel consumption, thereby reducing pollution, while providing superior cost-effective performance for the air traffic controller and pilot over legacy systems of yesterday.

    There are still hurdles to fully realize the aviation infrastructure of the 21st century and an FAA request for ADS-B funding is certainly a major step. A change in FAA management or a change in the U.S. administration could impact or delay vital enhancements to the country’s aviation infrastructure.

    —Bob Pilley

     

    Race FX

    Just wondered why your article “IndyCar Series Supplements Broadcast with GPS Data” concerns an open wheel racing series, but your photos are from the NASCAR Nextel Cup series. Probably both series use the same technology, but Dale Jarrett, Bobby Labonte and Dale Earnhardt, Jr., drive stockcars, not IndyCars.

    —John Webster
    Selma, Alabama

    Editor’s reply: Good catch. Photos from the IndyCar Series weren’t available, but as you state, both series use the same technology, which was the focus of the article.

  • Blue Marble Offers Applied Geodesy Training in Houston After the GIS for Oil and Gas Show

    September 9, 2006 — Blue Marble Geographics will be offering applied geodesy training in their Geographic Calculator class after the GIS for Oil and Gas Conference on Friday, September 22, 2006. The day-long session is an in-depth training course using the Geographic Calculator, a flexible data-conversion application.

    The course also features education on the basics of geodesy and mapping. Blue Marble will also be exhibiting at the Geographic Information & Technology Association GIS for Oil and Gas Conference at booth number 502.

    The Geographic Calculator converts individual coordinates, point database files, and vector map files from virtually any coordinate system to any other. Users can transform between coordinate systems, calculate the distance and azimuth between two coordinates, and calculate new coordinates at a known distance and azimuth from known coordinates. The Geographic Calculator also automatically computes grid convergence, point scale factor, datum shifts, and grid shifts.

    Call toll free 800-616-2725 to reserve a spot in the class.

  • Leadership Talks: Is Galileo Real, or Not?

    Leadership Talks: Is Galileo Real, or Not?

    Javad Ashjaee, front left, with colleagues in his Moscow office.
    Javad Ashjaee, front left, with colleagues in his Moscow office.

    Javad Ashjaee (JA) is president and CEO, Javad Navigation Systems. Alan Cameron (AC) interviewed him by phone at his Moscow office.

    AC: What details can you give us about the “new beginning” you mentioned in your May profile?

    JA: Things have changed a lot during the past six years. ASICs, microprocessors, and electronic technology in general have progressed significantly. These advances give us much better fundamental tools to implement new signal processing innovations and to design better and smaller products, while at the same time reducing their cost. With new GPS signals, improved GLONASS, and Galileo on the horizon, there are lots of opportunities and challenges to get me going for many years again. I phrased it as a “new beginning” because in the past six years I was not as productive as I wanted to be, due to situations beyond my control.

    AC: How do you see your “complete independence” playing out in the marketplace?

    JA: Now I can start working on GNSS technology again and hope to make a difference in this field again. The last six years were the darkest years of my professional life. The partnership that I had hoped would be a complementary force in pursuing the most innovative technologies turned out mostly frustrating, and it put stumbling blocks in the way of what I wanted to accomplish. By “complete independence” I mean the contractual limitations have expired, and I am free to pursue new technologies on my own and bring innovations to the marketplace that my team and I feel proud of.

    We have just started this new round of technical development and we are trying to define the technical directions. I hope to bring a completely new generation of GNSS products to the market in the second half of 2007. What keeps me motivated and as excited as my first attempts of 25 years ago is the level of support and encouragement that I receive from all my colleagues, friends, and even some of my competitors.

    AC: What technology challenges are you and your team tackling?

    JA: One of my technology challenges is to bring high-end Galileo into the GPS/GLONASS picture. My first decade of GNSS involved GPS, the second decade GPS+GLONASS, and in the third decade I hope for GPS+GLONASS+Galileo. Of course there are also innovations in signal processing in every one of these navigation systems. As I said earlier, enhancements in the electronic industry, as always, keep pushing us to come up with new products, similar to what you see in the computer and mobile communications industries. The life of any product cannot be longer than two years.

    I’m waiting for the Europeans to tell us what is really their plan for Galileo. I want to focus a lot on Galileo but I should admit that with Galileo I’m partially hopeful, but partially disappointed, too.

    AC: Why are you disappointed about Galileo?

    JA: I basically do not know, or I should say do not understand, the current plan of Galileo as presented; and I could not find anyone who could give reasonable answers to my questions. It is not obvious who the forces behind the Galileo project are, who is going to fund it, and who needs it to the extent that they are willing to pay for it. On several occasions, in different forums I tried to raise such questions and subsequently became convinced that unfortunately my worries and questions regarding Galileo were valid. Let me raise these questions once again here with you. I hope all my worries and suspicions regarding Galileo prove to be false and sincerely hope that Galileo to become a reality soon.

    Despite all the questions and uncertainties about Galileo, but due to good publicity, in many communities Galileo has received much more attention than GLONASS, which already has about 20–25 years of on-orbit history and currently about 16 operational satellites on orbit — and recently, with current oil prices, a solid plan to complete the system soon. Until recently many companies were trying to fight or ignore GLONASS, believing that it was not likely that Russia would complete the system. Now Trimble, NovAtel, and Leica have recently announced that they have GLONASS in their receivers, too.

    The attention given to Galileo, even before they have a first real satellite, is because of the credibility that the Europeans have earned. I hope this credibility continues to grow by providing open information to the public. Unfortunately, in my belief, so far they have not done so.

    One of the issues confronting Galileo is the explanation of its funding and management. It has been said that Galileo will be funded as a private business, but no one has ever provided a business model. When I try to guess about the structure of any business plan based on the information I have received, I conclude that it is very unlikely that it can be funded as such. The fate of Iridium — which did have a detailed business plan but overestimated its number of subscriptions — makes us more worried about Galileo, which does not even have a business plan. Iridium had the support of large, experienced industrial names like Motorola.

    Any company whose stock is being traded publicly will suffer a lot as soon as they announce involvement in any project that does not have a solid business plan. The larger the company, the more vulnerable they are to close scrutiny by stock-market analysts.

    Funding in Doubt

    In providing details on Galileo funding, it was said that most of the Galileo funding will come from bank loans. This makes the issue of a business plan much more urgent and critical for Galileo, especially given the fate of Iridium. Alternatively, the money could come from specific governments. This would easily put an end to the discussion of private funding and give much hope to all of us. I hope this will happen soon.

    There are several other flaws in Galileo’s private financial planning. First, GPS tried to make itself a self-funded program but quickly concluded that it was not feasible. Second, if GPS authorities were wrong, and it could have been done as a self-funded project, now with GPS being offered for free, Galileo has a much tougher time to make it as a private enterprise. Note that GLONASS is also there for free. Can you start a pizza business, when you know the guy next door is giving away pizza for free?

    Third, it is extremely unlikely that anyone will buy Galileo-only receivers, which means Galileo wants to piggy-back itself on GPS and receive money from the operational GPS. In our pizza example, imagine that your pizza shop can sell its pizza only if it can add slices of pizza from the free shop next door! Clearly the shop next door will stop providing you free pizza if it sees you are making money — or it will at least want its share of the profit.

    The progress in GLONASS, also offered free of charge, makes it even harder for Galileo’s private plan to succeed. With 30 GPS and 24 GLONASS satellites, at any given instance we have more than 14 visible satellites. The critical number to have reliable and robust RTK is something like 10 satellites. The additional Galileo satellites are a plus, but the European system cannot survive if it bases its financial plan on riding on the shoulders of GPS (or GPS+GLONASS).

    It short, the Galileo private funding scheme assumes free support from GPS — which means Galileo assumes that U.S. tax payers will continue to fund Galileo.

    I am certain that even if GPS authorities do not object, the U.S. taxpayers will. U.S. citizens and organizations like the U.S. GPS Industry Council will take the issue to the proper authorities.

    By collecting the first license fee from users or manufacturers, Galileo authorities open the door for large international disputes that put the fate of Galileo in question and raise the issue of customer liability, if it is going to be modeled financially as I mentioned earlier.

    In the private business model of sharing profit with GPS, Galileo will be at a disadvantage for many years until they have operational satellites comparable to GPS.

    The current situation of Galileo, as I see it, is that some independent and mostly research-oriented organizations have been able to collect enough money to define signals. We have seen some published information, such as Günter Hein’s article in GPS World (“Galileo Signal and Frequency Design,” June 2003) on the signal structure. But now when the ICD is published, the signal structure is drastically different.

    The Military Question

    Similar to GPS and GLONASS, the need for Galileo mostly comes from military establishments and markets. The two recent wars showed that the benefits of satellite navigation systems are no longer a luxury, but a necessity for any country that possesses a certain level of modernized military. The French government needs its own navigation system to independently market its Exocet missiles and Mirage 2000, for example. The Chinese government also has need for such systems, and this could be the reason that they push to fund and participate in the Galileo project.

    Does this mean France will break from NATO and join forces with China? This brings us to the very difficult question of international alliances, much more serious than the financial and funding issues that I raised earlier. I find it unlikely that the United Kingdom may want an independent navigation system separate from the United States. France may be the only major country that is willing and can afford to fund such system. This is an extremely difficult issue. I cannot think of any country in Western Europe that is pushing for the breakup of NATO.

    This is an order of magnitude more difficulty than money. This is an issue of East-West alignment, and the breakup of NATO, which has many more consequences.

    It is very simple-minded if we think Paris taxi drivers are pushing to have their own system because they do not trust the United States!

    Although the European military should be most concerned about the future of Galileo, they have been absent from all the Galileo conferences that I have attended. The only European military uniforms I have seen were those of a German Air Force brass band that played Mozart songs at the Munich ENC-GNSS conference.

    At best, it seems that some large European companies have teamed up to lead the Galileo project and keep space, control, and user segments under their management and control, and then try to attract money from military sources. This scenario also does not seem to have much chance of success. It is unlikely that military organizations will allow leadership and control of their vital navigation systems to remain in private-sector hands. Under any circumstances, the military organizations will be the real force behind the Galileo even though they may hide it publicly, at least for a while.

    Galileo’s best chance of success is for the French government to pay all or at least most of the costs, and not depend on support from other countries who do not have much vital interest and dependence on a new navigation system. I hope this will happen sooner rather than later.

    Galileo is not going to delay because of technical issues. There is no problem for the Europeans to have a good signal structure, or to launch good satellites. There is enough intelligence in Europe, they can solve those quickly. Financial and, more important, political issues will determine the future of Galileo.

    Will Galileo happen or not? I have no answer. I had a chance to share my thoughts with several prominent authorities. They mostly agreed with my concerns but they said it was politically incorrect for them to even discuss these sensitive issues.

    To go forward, I will work on Galileo and will assume that the Galileo authorities will work with the GPS authorities and will make a playing field such that U.S. GPS manufacturers can have the same benefits from Galileo that Europeans have from GPS. Previously we followed the information that was released unofficially by Galileo (by semi-official authorities) and we made chips to track those signals. Next we will follow the new ICD and we will make receivers based on it.

    Meanwhile we will try to tap whatever legal authorities, the GPS authorities in the U.S., to help us make the playing field level, to make the Europeans give us the same prompt access to information that they give to European organizations, without any charge.

    The United States has been so generous and so open in providing complete and honest GPS information to all, that the first GPS satellite was actually tracked in the UK. Dr. Brad Parkinson noted this point in his keynote speech at the European Navigation Conference in Manchester.

    I was so hopeful and supportive of Galileo in the article I wrote for you some months ago. But as I tried to find information regarding the future of Galileo, I developed serious concerns. I pray for Galileo to become a reality soon.

    AC: What percentage of Javad Navigation Systems’ annual budget is devoted to research and development?

    JA: We do not have a fixed percentage. Developing technology is our first priority and has always been a priority for me. We spend whatever it takes to have the best technology, even if we have to borrow money — though we have never had to. Probably this has been the reason that in all my history of involvement with GPS, the past 25 years, we were profitable every single quarter, because our focus was spending on technology, and giving priority to it. I don’t think there is any other GPS company in the world that can claim profitability for every quarter for 25 years. Our cumulative annual growth in each company was 45 percent. Currently we have a staff of over 100. Our Advanced Theoretical Design team alone consists of more than twelve professors and scientists with more than 200 person-years of experience.

    AC: How can we best navigate the next years of new signals, structures, and so on?

    JA: The main thing is that we request all U.S. GPS authorities to help to make this a level playing field.

    If the Galileo authorities really intend to combine GPS with Galileo for commercial markets, they’d better be as gracious and open as GPS authorities were to them.

    Iridium

    Iridium communications service was launched on November 1, 1998, and went into Chapter 11 bankruptcy on August 13, 1999.

    The system was originally to have 77 active satellites (later reduced to 66) to enable worldwide voice and data communications using handheld devices. Its financial failure was largely due to insufficient demand for the service. The system is currently being used extensively by the U.S. Department of Defense for its communication purposes.

  • Expert Advice: Managing the GPS Constellation for Today’s Needs

    Expert Advice: Managing the GPS Constellation for Today’s Needs

    John Lavrakas
    John Lavrakas

    In a recent editorial in GPS World’s Survey & Construction e-newsletter entitled “No Joy in Surveyville,” Eric Gakstatter lamented the performance of the GPS constellation for surveying. He is not alone. In June, the Australian Broadcasting Company reported that farmers in Australia were experiencing major problems with GPS because two satellites had been removed from service.

    For many, GPS is at its best performance ever, with 29 satellites in orbit and user range errors at their lowest levels in years. Yet for others, GPS performance falls short of expectations. What is the real issue here? Is it the number of usable satellites in the constellation — or have the demands of the user community grown?

    Today’s Performance

    Let’s first take a look at the performance relative to the current constellation. The GPS Standard Positioning Service (SPS) Performance Standard identifies 24 nominal orbital slots for a 24-satellite constellation. In this article, I refer to these as the 24 primary slots. My source material for the slot allocations is the U.S. Coast Guard operational advisories.

    Examining GPS performance over the past three years with respect to satellites in the key orbital slots, we see some interesting trends. Figure 1 presents the average number of satellites on orbit as well as the average number of healthy satellites in the 24 primary slots. A healthy satellite is one that has not been removed from service either due a scheduled outage (satellite and clock maintenance) or from an unscheduled anomaly (for example, degraded clock operation or problems with the spacecraft bus).

    Figure 1. Average number of healthy satellites on orbit (blue) and average number of healthy satellites in the 24 primary slots (red).
    Figure 1. Average number of healthy satellites on orbit (blue) and average number of healthy satellites in the 24 primary slots (red).

    The number of usable (healthy) satellites grew from 26 to about 28 on average, but this has not changed substantially in the past three years. It has varied between 27 and 29 satellites, with no significant upward or downward trend over this period. The number of satellites in primary slots, however, does show a noticeable trend, growing steadily through 2003 until late 2004 when it leveled off, after which it began to decrease. This trend recurs in Figure 2, where we view the same metric in half-year increments.

    Figure 2. Average number of healthy satellites in primary slots, shown in half-year increments.
    Figure 2. Average number of healthy satellites in primary slots, shown in half-year increments.

    The reduced number of filled primary slots stems from unscheduled outages. Scheduled outages have no significant impact on number of satellites usable since the operators typically remove a satellite from service for only a few hours, and such maintenance is performed on the order of once a month per satellite. Unscheduled outages, however, can last days and may require significant effort on the part of the satellite operators to resolve.

    The SPS Performance Standard states that 24 operational satellites must be available on orbit with 0.95 probability (averaged over any day). We see this figure has been met at the 100 percent level over the past three and a half years.

    The SPS Performance Standard further states that at least 21 satellites in the 24 nominal plane/slot positions must be set healthy and transmitting a navigation signal with 0.98 probability (yearly average). This figure is met.

    Figure 3 presents the monthly availability of the primary 24-satellite constellation (blue plot), that is, the percent of time over a month that there is a full set of 24 usable satellites in their primary slots. Here we see a marked trend, showing a steady growth from the beginning of 2003 up to the end of 2004, followed by a reduction, but still above the 95 percent level. Figure 3 also shows the monthly availability of 21 or more satellites assigned to the 24 primary slots (red plot), which has been at 100 percent over the past three and a half years.

    Figure 3. Monthly availability of the primary 24-satellite constellation (bue) and of 21 or more satellites assigned to the 24 primary slots (red).
    Figure 3. Monthly availability of the primary 24-satellite constellation (bue) and of 21 or more satellites assigned to the 24 primary slots (red).

    So What’s the Problem?

    If the U.S. government is meeting its commitments, why do users see degraded performance?

    Part of the issue is that the government manages the constellation to a set of metrics that is not up with the times, so to speak. The SPS Performance Standard has a legacy dating from prior to May 2000 when the government imposed Selective Availability, the intentional degradation of the positioning and timing accuracy for civilian users.

    Surveyors back then were considered eccentrics, as it were, living off the crumbs that fell from the table of the basic service. They took advantage of carrier phase tracking, but were content to post-process the data. Work that took days and weeks prior to GPS could now be done in hours. Well, those days are gone, and the push is now to get work done in minutes.

    The familiar adage “Give GPS users a yard and they’ll want an inch” has a corollary: “Give GPS users a process that takes them hours and they’ll want it done in minutes — or seconds!” Users have found they can do their processing much faster, as long as the constellation performance is well above the levels set in the SPS Performance Standard. This has indeed been the case since 2000.

    The GPS program has placed into orbit more satellites than originally anticipated. The general thought was that 27 satellites were sufficient to support the 24 satellite constellation. With 28, 29, and even 30 satellites in orbit, GPS has exceeded expectations, yet now the new expectations are that the government will sustain this level of performance.

    Improvements Happen

    The U.S. Air Force has made significant improvements in GPS operations as well in recent years. The satellite operators have become more user-focused. Prior to taking a satellite offline for maintenance, operators examine the effect of its removal to users worldwide. Also, they have adjusted operational procedures such that anomalies that once took hours to correct are now resolved within minutes. These improvements have directly benefited users, yet despite this, the mindset of the GPS operators is still to provide the service identified in the SPS Performance Standard, which is not the same as day-to-day service that users have come to expect.

    The Presidential Policy on National Space-Based Position, Navigation and Timing (PNT), issued in December 2004, provides high-level guidance on what service users can expect. Among its goals for space-based PNT, the policy states:

    • provide uninterrupted availability of positioning, navigation, and timing services
    • meet growing national, homeland, economic security, and civil requirements, and scientific and commercial demands.

    What “availability” is assumed in the first goal? Is it availability for users employing the 5-degree mask angle (as defined in the SPS Performance Standard), or is it the more stringent demand of mask angles at 10 degrees or higher? Is it availability of four satellites in view to support the generic user or that of six satellites in view to support receiver autonomous integrity monitoring with fault detection and exclusion?

    What “demands” mentioned in the second goal are to be met? Are they the demands of the precision farmer and surveying community? Or perhaps the tighter requirements of the urban user?

    This policy also states that the government will improve the performance of space-based positioning, navigation, and timing services. This implies that the service identified in the SPS Performance Standard will need to be changed to accommodate these improvements.

    To some extent, the U.S. government can only go so far in meeting user needs with the current system. The maximum number of satellites today’s operational control system can support is 30. As of the time of this article, GPS had 29 operational satellites on orbit, although at any given time not all are usable, due to necessary maintenance or unscheduled downtime. Whenever satellites are set unusable, the satellite operators look at the resulting performance, comparing it to the SPS Performance Standard. This is where the issue lies. The SPS Performance Standard assumes a generic user with only a 5-degree mask angle, yet this one assumption no longer represents a significant class of GPS users: the precision users.

    If there are 29 satellites available, why is a reduction to 27 such a big issue? Today’s GNSS users are more demanding than the users were even five years ago. Accuracy is the thing, and real-time accuracy is the most important thing. Today GNSS is used in precision applications such as agriculture, surface mining, and seismic drilling. To get the needed accuracy, users of GNSS exclude low-elevation angle satellites to mitigate the effects of the atmosphere. They set the mask angle in their receivers to 8, 10, even 12 degrees. This higher mask angle reduces the number of available satellites to the users, and correspondingly the dilution of precision and associated positioning error goes up, as illustrated in Figure 4.

    Figure 4. Picture of DOP performance over various mask angles for June 5, 2006.
    Figure 4. Picture of DOP performance over various mask angles for June 5, 2006.

    Is GPS Properly Managed?

    So the issue becomes, is the U.S. Air Force managing the constellation in the best interests of all of its users? Is keeping older satellites in orbit the best policy, and asking the satellite operators to do the best they can with the constellation provided to them? Or is it better to expend taxpayer dollars to replace the older, yet still operational, satellites with newer satellites?

    From a user’s perspective, the newer satellites are better — far better than the older satellites in range accuracy, health, and resistance to integrity failures. The increased reliability produced by their redundant systems also acts as insurance against longterm failures of GPS.

    Today’s users do expect more from GNSS. While today they have but one choice, in the future they will have at least two others, as Galileo and GLONASS come online. So it is important for the U.S. government to continue to adapt GPS operations to support its current user base.

    Recommendations

    There is no easy solution to the problem of ensuring that GPS continues to meet today’s user’s needs, since the field of users and applications is becoming more diverse and demanding. For many, the preferred answer is to launch more satellites, keeping the level at 30 satellites, but there are significant cost implications with this approach.

    On the other hand, relying on an aging constellation to remain operational is also fraught with peril. Many satellites are on their final legs, with key components on a single point of failure. The clocks onboard the satellites are not as stable as they used to be, and require considerable attention from the satellite operators. The cost of losing satellites is significant for certain sectors in our economy, the sectors that employ precision GNSS. In the distant future this problem will be eliminated through the diversity of fully operational Galileo and GLONASS constellations, but for now the issues are immediate and real.

    The U.S. government can and should take several steps to better address the increasing demands on GPS:

    • The government should update the SPS Performance Standard to accommodate other classes of users and bring the metrics up to date with respect to current performance.
    • The satellite operators should refine their assessments of user impact to include a view of how special classes of users are impacted. In particular this should include precision users and aviation applications. This involves using higher mask angles in their assessments and incorporating receiver autonomous integrity monitoring (RAIM) availability.
    • The decision-making authorities in GPS should continue to support an aggressive program to replace aging satellites.

    Such improvements will continue to benefit GPS users worldwide, and help ensure the U.S. government’s goal of providing the best PNT service available.

    John W. Lavrakas is a consultant in satellite navigation. He has spent the past 26 years in GPS, working in satellite command and control, user operations, GPS receiver development, and satellite navigation performance analysis. Contact him at [email protected].

  • Troubleshoot Before Takeoff

    Modeling and simulation, two separate but related activities conducted prior to flight tests of high-performance military navigation systems, can reduce costs, shorten timelines, and remove some uncontrollable variables from the process, to deliver more accurate, verifiable results.

    Flight tests on an outdoor range provide engineers with an abundance of data to improve performance of weapon navigation systems. Whether the guidance package is hosted on a fighter jet, a cruise missile, an artillery shell, or a hypersonic kill vehicle, the navigation performance of a weapon during flight trials closely reflects its suitability for combat. The roar of jet engines, the percussion of a gun barrel, live video feeds, or a radio link to the cockpit can make live flight tests exciting — and sometimes unpredictable.

    While these activities are critical for weapon system qualification and validation, live tests nonetheless have significant limitations. Lengthy flight trials can be quite expensive. The rising price of aircraft fuel combined with test-range costs, travel expenses and data analysis can easily exceed millions of dollars for extended operational test and evaluation (OT&E). Also, test results can vary widely between trials due to a number of uncontrolled variables such as flight dynamics, RF interference, and atmospheric effects. Finally, the tests themselves often involve safety and technical risks and usually require careful planning and coordination, often months in advance.

    For these and other reasons, military navigation system flight tests are generally limited in number and are preceded by extensive modeling and simulation in the lab, where a methodical and disciplined test approach can produce substantial savings and more detailed insight. Successful completion of these activities provides system developers and integrators with the confidence required to proceed to the next stage of testing. This phased approach is useful for a broad range of weapon systems including GPS-guided ground-, sea- and air-launched munitions; high-performance aircraft; and long-range interceptors.

    Model, Then Simulate. Evaluating weapon navigation systems in the lab prior to outdoor testing usually consists of two sequential activities. First, performance of the GPS/inertial navigation system (INS) throughout the flight trajectory is modeled in software using representative data for the satellite constellation, RF signal environment, body masking, antenna pattern, receiver processing algorithms, and other parameters.

    Second, the GPS/INS flight hardware is exercised with controlled inputs to collect data relating to the operation of its code- and carrier-tracking loops, its susceptibility to interference, and the performance of the navigation filter under simulated highly dynamic conditions. Changes in vehicle velocity along its three axes are represented by inertial delta-V (ΔV) terms that are delivered to the INS navigation filter, while changes in attitude are delivered as delta-q (Δθ) terms. For normal flight operations, accelerometer and gyro sensors within the INS measure and report these forces, but in test mode the inertial simulator PC generates these terms based on translational and rotational motion described within the simulated trajectory. Computing these terms involves mathematical translation between the sensor frame and the vehicle body frame, and includes all errors due to sensor bias, drift, and misalignment. The inertial simulator PC delivers these terms to the INS via appropriate electrical interface.

    The fighter aircraft flight discussed here is based on a flight model overlaid on a map of the Fort Worth, Texas, area (FIGURE 1).

    Figure 1 Simulated tactical fighter aircraft flight trajectory over north Texas
    Figure 1 Simulated tactical fighter aircraft flight trajectory over north Texas

    Mod/Sim Process

     

    Before discussing the simulation results, it’s important to understand modeling and data analysis in its proper context. Disciplined performance analysis is like the scientific method — it begins before the test is initiated and concludes by validating preliminary assumptions using results of the experiment.

    Analysis of military navigation system performance begins with software modeling as shown in FIGURE 2, first panel. In this example, software tools predicted, among other things, SV carrier-to-noise values; azimuth and elevation angles; constellation dilution of precision (DOP) values; and GPS receiver acquisition/tracking performance. If required, software may also predict jammer-to-signal (J/S) ratios at the weapon GPS antenna.

    Figure 2 Navigation system performance analysis
    Figure 2 Navigation system performance analysis

    The next step (second panel in Figure 2) enhances software models with hardware-in-the-loop (HWIL) simulations, replacing predictions with actual performance data. Conclusions from RF and computer simulation stage will often aid significantly in field-test set-up. Benefits of simulation include isolating design flaws, validating algorithm performance, and exercising the GPS/INS system with realistic and stressful inputs. These activities may reduce the number of flight trials required, which in turn reduces the potential for program cost and schedule slips.

    Modeling Results

     

    FIGURES 3 AND 4 show predicted results from the commercially available Navigation Tool Kit (NavTK) regarding the navigation performance of the unaided GPS receiver within the fighter aircraft during its highly dynamic flight. STK generated the 6-degree-of-freedom (6-DOF) trajectory used in this test. This trajectory is based on the dynamic limits of the aircraft; representative roll/pitch/yaw rates; typical climbing; cruising and descent speeds; takeoff and landing distances, and so on. This data was subsequently passed to NavTK. Figure 3 shows number of satellites tracked throughout the flight while Figure 4 predicts East-North-Up errors given actual GPS performance (clock data, ephemeris data, and system performance) for July 1, 2005. This detail is possible because NavTK can accept archived GPS constellation data from the GPS Operations Center (GPSOC) to fully represent the effects of geometry, satellite health, ephemeris errors, and clock variations for individual SV signals.

    rder=”0″ align=”left” class=”content_image” src=”/files/gpsworld/nodes/2006/6535/i4_t.jpg” alt=”Figure 3 Number of signals tracked” />
    Figure 3 Number of signals tracked

    Figure 3 highlights flight portions when loss of lock is expected due to platform orientation and accelerations. When the aircraft initiates a steep turn, the accelerations cause cycle slips that result in loss of lock. These results are useful for isolating discrete periods of time during which performance of the INS-only solution can be scrutinized during hardware simulation. At these times the INS navigation filter relies exclusively on ΔV and Δθ terms from the inertial simulator to compute a fix.

    Figure 4 Predicted navigation errors
    Figure 4 Predicted navigation errors

    When PRN 25 disappears below the horizon approximately 25 minutes into the flight, we see a noticeable shift in predicted accuracy. This suggests that DOP values may worsen noticeably at this point, or that clock/ephemeris errors for this satellite had a noticeable effect on the GPS-only solution. These results depend on inputs provided for the receiver models, constellation behavior and tracking loop algorithms, and provide a start point for pre-HWIL evaluation. Again, results from this modeling exercise highlight a unique moment in time in which the performance of the GPS/INS navigation filter can be tested with a hardware simulator.

    Software modeling tools can also provide valuable inputs to the simulation process with trajectory modeling. STK can create flight simulations for military applications including missiles, rockets, spacecraft, and airborne platforms. In this example, software created a 6-DOF fighter trajectory which was stored as a file on a SimGEN PC.

    GPS/INS Simulation

     

    HWIL GPS/INS simulation of many military flight trajectories consists of generating a composite GPS RF signal synchronized with inertial ΔV and Δθ terms. Precise synchronization is achieved via a 1 PPS interface between the RF signal generator and the inertial simulator PC. Latency of the IMU motion data is also specified in microseconds. Together, these RF and inertial inputs are delivered to the navigation system under test.

    FIGURE 5 illustrates the GPS/INS test equipment in more detail. Platform dynamic motion (in this case from STK), GPS constellation data, signal parameters, and GPS/INS error sources are defined within the simulation application (SimGEN). Signal control commands are issued continually from the simulator PC to the RF signal generator via IEEE-488 (GPIB). The RF signal is typically connected to the GPS receiver antenna port via coaxial cable. INS ΔV and Δθ terms are calculated to the vehicle trajectory on a separate PC (SimINERTIAL) and are delivered to the GPS/INS device in test mode via the appropriate interface. The SimINERTIAL PC and signal generator are synchronized via a 1 PPS signal. Barometric altimeter aiding and instrumentation are achieved via MIL-STD-1553B.

    Figure 5 GPS/INS simulation block diagram
    Figure 5 GPS/INS simulation block diagram

    Realistic simulation requires many unique components of the navigation solution to be carefully specified in advance. These include but are not limited to:

    • 1. Trajectory
    • 2. Antenna pattern
    • 3. Constellation geometry
    • 4. Codes and frequencies
    • 5. G-sensitivity
    • 6. Gyro and accelerometer errors
    • 7. Atmospheric effects
    • 8. Multipath
    • 9. Interference

     

    Trajectory. As mentioned earlier, a flight trajectory is usually expressed as a series of time-stamped translational and rotational motion terms. Rotational motion is typically described by Euler angles or quaternions. The GPS/INS simulator uses this trajectory as the basis for calculating SV pseudoranges, Doppler values, and inertial delta-V and delta-q terms. This flight trajectory can be delivered to the simulator remotely via a high-bandwidth interface such as IEEE-488, SCRAMnet or TCP/IP, or, as in this example, may reside as a file on the simulator PC hard drive. Flight times may range from a few minutes to several hours or even days.

    For this simulation, the 45-minute flight over Fort Worth includes take-off, landing, and several steeply banked high-g turns. The airspeed for the majority of the flight is approximately 500 mph.

    Antenna Pattern. If the simulator’s RF signal is injected directly into the antenna port of the GPS/INS system under test, the gain and phase characteristics of the receive antenna must be modeled. For best results, the effects of the body on which the antenna is mounted must be included. Gain and phase pattern measurements are typically collected in an anechoic chamber or outdoor range and are generally available to test engineers evaluating GPS/INS system performance.

    Constellation Geometry. A simulated flight can occur at any time or location to test the effects of constellation geometry on the navigation solution. Historical GPS almanac information is available from the GPSOC and can be loaded into the simulation scenario. This example used historical almanac parameters from July 1, 2005.

    Codes and Frequencies. Most U.S. military GPS/INS systems operate at both L1 and L2 frequencies and track the encrypted P(Y) and unencrypted C/A codes. Future military receivers will also track the M-code on L1 and L2. To fully evaluate unique features of the weapon navigation system including direct-Y acquisition, SAASM functions, and resistance to interference, the simulator must support all modes of classified and unclassified operation.

    G-Sensitivity. Acceleration force can alter the performance of a crystal oscillator. Specifically, changes to the local acceleration vector stemming from high-g maneuvers will change the output frequency of the oscillator. Airframe vibrations may cause frequency modulations of the oscillator output that affect the performance of the tracking loops. To a certain extent, the effects of these forces on the code- and carrier-signal offsets can be modeled. For this scenario, the oscillator G-sensitivity is specified as 2 × 10-11 sec/sec/G per axis, typical for a military-grade oven-controlled crystal oscillator (OCXO).

    Gyro and Accelerometer Errors. Note that GPS/INS simulation does not test the performance of the sensor elements themselves; this exercise is generally conducted with mechanical spin tables or centrifuges. Rather, military INS manufacturers typically include a test interface that allows simulated motion terms to be directly injected into the navigation system. This allows test engineers to evaluate the performance of the Kalman filter during high-dynamic or stressed conditions. For one manufacturer’s system, this in
    terface is proprietary to the Inertial Sensor/Recorder Simulator (ISRS-2); another’s systems accept these test inputs via RS-422. A non-proprietary inertial interface known as NATO Standard Agreement (STANAG) 4572 also accepts simulated ΔV and Δθ terms via RS-422.

    Embedded gyro and accelerometer sensors are imperfect and are subject to a variety of errors. Stochastic errors may be modeled by random walk or by a Gauss-Markov noise estimator, while deterministic errors such as biasing, axis misalignment, and scale- factor errors should also be specified. For this simulation, a second-order Gauss-Markov model is used as described in STANAG 4572 for gyros and accelerometers with unique values for the natural frequency, damping factor, and power spectral density.

    Atmospheric Effects. After Selective Availability was set to zero in May 2000, the primary contributor to GPS ranging error remains ionospheric delay. Dual-frequency receivers can effectively eliminate this effect by processing both carriers, but range measurements at L1 and L2 are still subject to multipath errors. Furthermore, ionospheric scintillation is a complex phenomenon that can affect the tracking performance of dual-frequency receivers. This simulation specifies the historical broadcast Klobuchar coefficients for July 1, 2005, and modifies the SV code and carrier offsets to approximate the effects of the ionosphere for the time, date, and location of the scenario.

    Tropospheric delay varies by latitude and elevation angle and is a function of temperature, humidity, and atmospheric pressure. For more realistic results, the effects of this ranging error can also be simulated. For this simulated flight over Texas the surface refractivity index is set to 324.8.

    Multipath. While the aerial environment is generally free of the same satellite obscuration effects more common on the ground, signals may still be reflected off wings, stabilizers, and flight control surfaces. These path delays affect the performance of the GPS receiver correlators and may introduce position and velocity errors into the navigation solution. GPS simulation allows the tester to specify reflective effects including fixed-delay multipath, Doppler multipath, and vertical plane multipath on a per-satellite basis.

    Interference. Over the last 10 years a variety of techniques have been developed to mitigate the effects of intentional interference on military navigation systems. Adaptive antenna arrays may steer nulls in the direction of jammers or beams in the direction of satellites. Spectral filtering divides the L1 and L2 bandwidths into discrete bins in which interference can be excised using fast Fourier transform (FFT) techniques. Polarization filters attempt to reject interference from sources that are not right-hand circularly polarized (RCHP). Regardless of the mitigation technique, the performance of antijam GPS systems is generally evaluated in the lab prior to conducting flight tests.

    Simulation allows the tester to specify a variety of interference waveforms including broadband Gaussian noise, continuous wave (CW) and swept-CW, AM, FM, and coherent jammers. The relative signal strength of these sources will vary as a function of the distance between the navigation system under test and the jammer.

    HWIL Simulation Results

     

    Military navigation engineers are usually interested in the following performance characteristics:

    • 1. Accuracy
    • 2. Availability
    • 3. Resistance to interference

     

    FIGURE 6 shows example results that can be obtained using this equipment to test INS devices under these simulated highly dynamic test conditions. The results presented here are for illustrative purposes only and show the type of data obtainable to fully characterize the performance of a specific INS under all possible conditions. In this example, latitude (black), longitude (green), and height (blue) errors are plotted separately. Discontinuities in the calculation of latitude and longitude may be observed when high-g turns are initiated. Errors in the height component may correspond to rapid climbs and descents. RMS error for this entire illustrative mission was slightly higher than 16 meters. Again, these results are based on the specific inputs for this simulation. Navigation engineers would need to determine whether similar inputs represent their own flight environment and whether these results will satisfy their operational objectives. In practice, results will vary as a function of simulated gyro/accelerometer errors, oscillator g-sensitivity, pre-flight calibration time, flight dynamics, barometric aiding, and SV visibility.

    Figure 6a INS simulation results: latitude upper left, longitude upper right, height lower left
    Figure 6a INS simulation results: latitude upper left, longitude upper right, height lower left

    As expected, availability of the navigation solution was 100 percent. In contrast to the GPS-only performance modeled in Figure 3, the INS solution used ΔV and Δθ inertial inputs to compute a solution whenever a GPS fix was temporarily unavailable.

    Figure 6B Close-up look at latitude (black), longitude (green) and height errors (blue) from lower right panel
    Figure 6B Close-up look at latitude (black), longitude (green) and height errors (blue) from lower right panel

    Intentional RF interference was not created for this HWIL simulation. However, adding this component to the RF signal is possible with additional equipment, and a similar chart could be created to illustrate the effects of jamming on the INS solution.

    In addition to this simple error plot, truth data from the GPS/INS SimINERTIAL simulator were also delivered in WGS-84 ECEF coordinates to STK for graphical performance analysis. Time-stamped latitude, longitude, and height data extracted from the military GPS/INS MIL-STD-1553B interface were also provided to STK. Several key performance parameters were then analyzed including carrier-to-noise levels (C/N0), tracking states, pseudorange error per channel, and instantaneous position/velocity errors. FIGURE 7 and the opening graphic compare truth location at a single moment in time as prescribed by the simulator (gray aircraft) versus the actual GPS/INS computed location (color-coded per figure of merit, or FOM). STK can be used to visualize these outputs in real time. Instantaneous position error is projected on the red vector between the aircraft centers of gravity. Vectors pointing toward each SV tracked may be color-coded to J/S, while line type may be selected based on tracking state. Instantaneous pseudorange error for each channel is also provided. Many other data items can also be displayed graphically or in text.
    =”articlecaption”>
    Figure 7 INS simulation results
    Figure 7 INS simulation results

     

    Conclusion

     

    Field tests generally provide the most indisputable evidence of military navigation system performance. When a GPS/INS-guided gravity weapon is released above a test range and impacts the ground near its target, we may physically measure the miss distance. We may also assume a similar weapon released at a future time under similar conditions will achieve approximately the same results. But how can we be sure? One approach would be to perform hundreds of trials, if costs and schedules were of no concern.

    However, this is rarely the case. GPS receiver performance models used with the HWIL SimINERTIAL simulator allow the test engineer to minimize flight tests while achieving the same level of confidence. As we have shown, modeling and simulation activities may be tailored for a specific objective — for example, highly dynamic aircraft motion with a fixed satellite constellation geometry while using inertial sensors with known bias and drift characteristics. This simulation may be valuable in conducting analysis of alternatives for different user equipment mixes (receivers, antennas, inertial sensors, platforms, and so on) or to ensure the system meets requirements for all missions.

    Other modeling and simulation objectives may differ from those described in this article. FIGURE 8 enhances our aircraft flight test model to include intentional jamming, an advanced digital antenna, and future modernized GPS space capabilities. Similar enhancements can be added to the RF simulation.

    Figure 8 Real-time 4D data display
    Figure 8 Real-time 4D data display

    Whatever the application, a variety of commercial products allow navigation engineers to prepare for live flight trials on the test range. GPS receiver performance analysis models can be used to create specific GPS/INS simulations that focus on key performance characteristics and environmental challenges. Graphical analysis software can simplify the task of combing through thousands of data records to isolate specific periods during which performance is most important. With careful planning and a disciplined test approach, military and defense industry engineers can use these tools to ensure their weapon systems will deliver at the most critical times on the battlefield.

    Manufacturers

     

    Analytical Graphics, Inc. developed the Navigation Tool Kit and STK commercial software products used for this simulation. GPS/INS test results were collected using the Spirent GSS7700 simulator with SimINERTIAL.

    CURTIS HAY is director of GPS business development for Spirent Federal Systems. He has master’s degrees in electrical engineering and business administration and served eight years as an officer in the United States Air Force in a variety of GPS-related assignments.

    COLIN FORD is a senior software engineer at Spirent Communications and is the lead architect for the SimINERTIAL navigation test suite.

    GREG GERTEN supports satellite navigation and electronic warfare programs for Analytical Graphics, Inc. He received a master’s degree in electrical engineering from the University of Dayton and has completed graduate courses in GPS at the Air Force Institute of Technology.