Spirent Communications has announced a Robust PNT Test Framework that evaluates GPS and GNSS security vulnerabilities for positioning, navigation and timing (PNT) systems.
Threats to GNSS and related PNT applications are more orchestrated and coordinated, with the motivation to disrupt or cause financial loss. The technology to disrupt GPS has also become much more accessible, resulting in GPS vulnerability gaining attention at hacker conventions.
Spirent’s GSS100D Detector, developed in collaboration with Nottingham Scientific Ltd, enables detection, characterization and analysis of real GNSS threats.
The Robust PNT Test Framework will be used by technology, system and application developers where PNT is critical. Spirent’s framework enables threats to be detected in the field, taken into the lab and re-synthesized along with GPS and other GNSS signals. In addition, Spirent’s threat intelligence library of actual and typical threats provides a wide range of GNSS segment errors and spoofing attacks, as well as space weather and other vulnerabilities for preventive troubleshooting.
“Spirent wants to move beyond talking about the increase of GNSS vulnerabilities and offer a pragmatic approach to enable informed decision making when it comes to evaluating the impact of vulnerabilities,” said John Pottle, marketing director of Spirent’s Positioning Technology Division. “Through our Robust PNT Test Framework Spirent is pulling everything together to enable users to readily audit systems and take practical steps to improve resilience.”
The PolaRxS by Septentrio is a multi-frequency, multi-constellation receiver dedicated to ionospheric monitoring and space weather applications. It features simultaneous high-quality tracking of all visible signals (L1, L2, L5, E5ab/AltBOC GPS/GLONASS/Galileo/Beidou/SBAS) at low noise levels. The receiver outputs an extensive set of GNSS measurements, including signal phase and intensity at up to 100 Hz, with a phase noise standard deviation (phi60) as low as 0.03 rad.
The A Posteriori Multipath Estimator (APME+) tackles short-delay multipath to enhance the measurement quality, while LOCK+ tracking guarantees robust tracking of rapid signal dynamics during scintillation events. Included tools provide continuous total electron content (TEC) and scintillation indices logging for space weather and ionosphere monitoring.
Learn about more Galileo-ready products in our Galileo Product Showcase from the April 2014 issue.
Too Much Sensitivity, Not Enough Robustness, Says Parkinson
Brad Parkinson, the founding architect of GPS, told a UK conference that the system needs to be made more robust to ensure worldwide availability of services to users. His concerns over GPS availability relate to threats such as the loss of authorized frequency spectrum (implicitly creating licensed jammers), space weather due to hyperactive ionospheric conditions, and deliberate or inadvertent jamming of GPS signals.
He warned that GPS is more vulnerable to sabotage or disruption than ever before, and charged that politicians and security chiefs are ignoring the risk. Western governments are “in their infancy in recognizing the problem,” he remarked further in an interview with London’s Financial Times. “[In the United States] I don’t know anyone that is really in charge of it. The Department of Homeland Security should be [but] … they don’t have any people that understand it very well. They’ve got one person without any budget to speak of.”
He also warned that Europe’s €5 billion Galileo system is equally at risk.
Parkinson proposed a three-stage program to:
Protect (legally) the signal and physically eliminate jamming sources;
Toughen the GPS/Galileo receiver’s resistance to interference;
Augment the GPS signals with other satellites or with ground-based transmitters such as eLoran.
To support his proposal, Parkinson stated, “The number one need for all GPS or Galileo users is availability. Over the years, manufacturers of signal receiver technologies have focused too much on sensitivity and not enough on resilience or robustness. The maritime industry is a particular concern where users have taken GPS for granted. They must increase preparedness and backups as they do in aviation or other GNSS using industries.
“Even today, most ships have only GPS and the vision of their crew to guide them when approaching harbours. As you can see from today’s conference there are a wealth of solutions to toughen and backup GPS, many of which are not technologically difficult nor expensive, but still their adoption in sectors such as global shipping is certainly not adequate.”
As part of his protection program, Parkinson urged that penalties for jamming GPS networks be coordinated worldwide. “In Australia, if you cause interference likely to cause prejudice to the safe conduct of a vessel, it’s five years in the jug [jail] and $850,000.” Contrasting this with a U.S. case that may simply impose a forfeiture of the culprit’s jamming device, Parkinson added, “I’m calling for the community of nations to move to the Aussie-type penalties.”
In the toughening regard, Parkinson alluded to integration of GPS data with information derived from an inertial positioning system. “If you combine all of these things, a good set should be able to fly within 1 kilometer of a jammer with a 10-kilometer range,” said Parkinson. “That’s what I call toughening.”
Parkinson made his remarks as the keynote speech at GNSS Vulnerabilities and Resilient PNT 2014, hosted by the Royal Institute of Navigation. He will also deliver the keynote address, “Assured PNT: Assured World Economic Benefits,” for the European Navigation Conference on April 15 in the Netherlands.
Image of the sun on Tuesday, Jan. 7, 2014, from the Solar X-Ray Imager on NOAA’s GOES satellite, taken just after the maximum emission of a solar flare. The eruption came from the middle of the sun and is directed toward Earth. This is the largest solar flare so far this year.
Forecasters at NOAA’s Space Weather Prediction Center said the sun’s coronal mass ejection (CME) that reached Earth on Jan. 9, unsettled the geomagnetic field but did not cause storm conditions to be reached due to the weak magnetic field. While there is still a chance we could see some geomagnetic storming, that threat is greatly diminished. The Space Weather Prediction Center is a division of the U.S. National Oceanic and Atmospheric Administration.
The sunspot in Region 1944 that produced the eruption at 1:32 p.m. EST Tuesday, January 7, has had no significant additional flaring and shows signs of decay.
How space weather affects real-time technology
Economies around the world have become increasingly vulnerable to the ever-changing nature of the sun. Solar flares can disrupt power grids, interfere with high-frequency airline and military communications, disrupt GPS signals, interrupt civilian communications, and blanket the Earth’s upper atmosphere with hazardous radiation.
Monitoring and forecasting solar outbursts in time to reduce their effect on space-based technologies have become new national priorities. And NOAA’s Space Weather Prediction Center (SWPC), part of NOAA’s National Weather Service, is the nation’s official source of space weather forecasts, alerts, and warnings.
Space weather explained (source: NOAA).
Monitoring the Sun
To monitor events on the sun, SWPC staff utilize a variety of ground- and space-based sensors and imaging systems to view activity at various depths in the solar atmosphere. A worldwide network of USAF-sponsored optical observatories also provides space weather forecasters with detailed, plain-language information about activity in and around sunspot groups, as well as other areas of interest on the sun.
Space weather forecasters also analyze the 27-day recurrent pattern of solar activity. Based on a thorough analysis of current conditions, comparing these conditions to past situations, and using numerical models similar to weather models, forecasters are able to predict space weather on times scales of hours to weeks.
With effective alerts and warnings, NOAA is helping to minimize the hazards of space weather on technology. For example, satellite operations can be adjusted, power grids can be modified, and polar flights can be rerouted.
Ten years ago, scientists watching the skies experienced a Halloween fright of cosmic proportions, when space weather degraded GPS signals, affecting land and ocean surveys, and commercial and military aircraft navigation.
The most extreme of what became known as the Halloween Storms hit on October 30, 2003 — ten years ago today. According to the National Oceanic and Atmospheric Agency, the Earth could experience a repeat performance this Halloween, with a 35 percent chance of a major storm at high latitudes.
In mid-October 2003, a bundle of concentrated magnetic energy emerged from the Sun’s interior, forming a large sunspot, a site of seething activity. Enormous solar flares soon followed.
Then, on October 28, the sunspot abruptly ejected a concentrated mass of electrically conducting solar wind, flinging it out into interplanetary space toward the Earth. Less than a day later, on October 29, a geomagnetic storm was initiated as the solar wind disrupted the Earth’s protective magnetosphere.
Over the next three days, the “Halloween magnetic storm” would evolve and grow to become one of the largest such storms in half a century. Magnetic storms are global phenomena, and their effects can be easily seen around the world. During the Halloween storm, for example, magnetic direction in Alaska quickly changed by more than 20 degrees. In other words, the storm was so large that it could be measured with a simple compass. The Halloween magnetic storm also produced spectacular aurora, with green phantom “northern lights” seen as far south as Texas and Florida.
“The aurora was exciting,” said Richard Langley, GPS World’s Innovation editor. “I’ve never seen a better one since.”
This full-sky aurora was observed near Fredericton, New Brunswick, Canada (46 degrees north latitude) on October 30, 2003. (Photo courtesy of Richard Langley.)
Langley explained the effect of the phenomenon in his introduction to the October 2004 Innovation article, “Combating the Perfect Storm: Improving Marine Differential GPS Accuracy with a Wide-Area Network.”
It was previously thought that the mid-latitude North American ionosphere was reasonably benign, with minimal storm effects of relevance for marine DGPS users. However, during ionospheric storms in May and October, 2003, [single-frequency] marine DGPS horizontal position accuracies were degraded by factors of 10–30.These degraded accuracies persisted for hours and were well beyond system tolerances specified for marine DGPS users. Such ionospheric activity is not unusual during the years following solar maximum, and is expected to persist for several years.
Langley provides background on what scientists learned from the Halloween Storms in his February 2011 Innovation column, “GNSS and the Ionosphere: What’s in Store for the Next Solar Maximum?”:
The current solar cycle is referred to as cycle 24. During the last solar cycle, cycle 23, the GNSS community was alert and aware of what could happen, and therefore many events were observed and analyzed. Among the most well-known events is a sequence of storms during October and November 2003, commonly referred to as the Halloween Storms.
The most extreme was the storm on October 30, 2003, which resulted from a CME on October 29 at 20:49 UTC, which subsequently impacted Earth’s magnetic field at 16:20 UTC on October 30 and produced a great geomagnetic storm, which lasted for many hours.
Effects on GPS positioning of this storm have been documented by the GNSS research group of the Royal Observatory of Belgium, where kinematic analyses of data from 36 GNSS stations in Europe showed position errors of more than 10 centimeters in the horizontal and up to 26 centimeters in the vertical between 21:00 and 22:00 UTC on October 30. The position errors were largest for locations in northern Europe including Sweden and Norway. The data analysis was carried out using high-quality carrier-phase data, and the processing was based on using an ionosphere-free linear combination of observations from the L1 and L2 frequencies, whereby the first-order effect of the ionosphere is removed from the results. The position errors are thus caused by mainly higher order ionospheric effects.
For navigation-grade GPS positioning, a U.S. National Atmospheric and Oceanic Administration technical memorandum reported that the Wide Area Augmentation System (WAAS) vertical error limit of 50 meters was exceeded for a period of about 11 hours on October 30, 2003. This means that, in practice, WAAS was not available for precision aircraft approaches during that time. The European Geostationary Navigation Overlay Service (EGNOS) was not transmitting during the storm, but simulations carried out later by ESA showed that the boundary regions of the EGNOS coverage area would have been especially affected by a reduction in service availability of about 20–60 percent during that day.
The simulations also showed, however, that in the center of the EGNOS coverage area (in the vicinity of northern Italy), the effect would have been much smaller with a reduction in service availability of only 5–6 percent over the day.
Such large storms are also often accompanied by displays of aurora (aurora borealis and aurora australis) at lower latitudes than normal.
Another shot of the Halloween 2003 aurora, as seen near Fredericton, New Brunswick. (Photo courtesy of Richard Langley)
Other Innovation columns assessing the ionosphere’s effect on GPS include:
After a two-week delay, a rocket carrying a GPS instrument designed by University of New Brunswick scientists was launched into space aboard the SpaceX Falcon 9 rocket on September 29. The rocket left Vandenberg Air Force base in California as part of the CASSIOPE (Cascade Smallsat and Ionospheric Polar Explorer) mission.
Dr. Richard Langley, GPS World Innovation editor and professor in geodesy and geomatics engineering at the University of New Brunswick, is a principal investigator behind the scientific portion of the CASSIOPE mission. Langley and his colleagues will monitor data from the GPS instrument, which is part of the Enhanced Polar Outflow Probe (e-POP) payload aboard the spacecraft.
E-POP will continue the sequence of Canada’s orbiting space environment sensors, which began with Canada’s first satellite, Alouette 1, launched in 1962 to study the ionosphere. e-POP is, perhaps, the most extensive suite of sensors for studying the ionosphere/magnetosphere/thermosphere yet to be launched, and will provide Canadian and other scientists with the opportunity to better understand the impact and variability the sun has on the space environment — what we call “space weather.”
A static fire retested the Falcon 9 rocket after several problems cropped up during a hotfire of the launcher’s engines during preparation for the original launch date September 15. The launch was then delayed because the U.S. Air Force Western Range, which controls a network of tracking and communications assets based at Vandenberg, was busy with Minuteman ballistic missile testing.
The small hybrid satellite blasted off on board a Falcon 9 rocket developed by SpaceX, a commercial space company. The Canadian Space Agency became one of SpaceX’s first customers when the agency decided years ago to use the private U.S. rocket to deliver the satellite at a reduced cost of $10 million. It cost the space agency $63 million to develop the satellite.
The Falcon 9 rocket, with CASSIOPE inside its fairing, on the way to the launch pad at Vandenberg Air Force Base. (Photo credit: SpaceX).The research satellite CASSIOPE on a test platform at the Canadian Space Agency’s David Florida Laboratory. CASSIOPE hosts the GPS Attitude, Positioning, and Profiling instrument designed by GGE researchers. The four white antennas on the left-facing side of the spacecraft will be used to determine the position, velocity, and attitude of the spacecraft while the antenna on the upper side will be used to profile the ionosphere’s electron density. (Photograph courtesy of MacDonald, Dettwiler and Associates Ltd.)
Being a person who enjoys spending time in the field using RTK and DGPS, I followed up on my column last month, “Sources of Public, Real-time, High-Precision Corrections,” with a trip to the field to test the NGS CORS Streaming service. About a month ago, I made a trip to Colorado to attend the Space Weather Workshop in Boulder, stop by the SPAR conference in Colorado Springs, and visit with some of my colleagues in the Denver area.
When I arrived in Denver, my plan was to meet Tim Smith (GPS Coordinator for the U.S. National Park Service) and travel to the Bakerville GPS test site in the Rocky Mountains, which was at about ~11,000 feet in elevation. My intent was to test the CORS Streaming and PBO real-time streaming that I discussed last month to better understand the accuracy and reliability of those services.
I arrived at the Denver airport early on a Monday ready to rock and roll into the Rockies with some high-precision GNSS equipment. As it turned out, I was denied. In Colorado, the weather is dynamic. It was quickly degrading when I arrived in Denver. Snow was definitely in my future for the next few days. Tim made the decision that we shouldn’t travel to Bakerville. The reason for Tim’s trepidation wasn’t necessarily due to the weather in Bakerville, but rather that the I-70 Interstate might turn into a parking lot and we’d be stuck in traffic for a few hours. Fair enough. The backup plan was to do some local testing in the parking lot adjacent to Tim’s office in Denver.
Tim invited Mel Philbrook to join us. Mel is a long-time GNSS technologist who works for the local Trimble dealer. He brought an SUV full of Trimble GNSS equipment, including one of the new R10 GNSS units as well as a GeoXH handheld with an external antenna.
Mel also had an Intuicom RTK Bridge in the trunk of his SUV that facilitated the different sources of RTK reference data we could use. He could switch from CORS Streaming to the local VRS via NTRIP to UHF at the flip of a switch, sending corrections to both the R10 and the GeoXH. I was particularly interested in seeing how the units performed using CORS Streaming, which is/was a free RTK service (single baseline) that was in beta test phase. In Oregon, I don’t have access to CORS Streaming because the only CORS Streaming station west of the Mississippi River is in Boulder, Colorado. The station is TMGO (Table Mountain CORS).
The baseline distance from TMGO to our location was about 55 km. The R10 was reporting a horizontal precision of about 4 cm. Not bad for a 55-km baseline. I didn’t compare the results to a survey mark (shame on me, but keep reading because I get to that) so I’m trusting the R10’s precision estimate. Tim said he’s run the test before using a GeoXH and a longer baseline and saw sub 10-cm horizontal precision. It’s not what the typical person using short baseline or RTK network is accustomed to, but for the high-precision GIS user who’s mapping utility, transportation, and infrastructure, that’s pretty darn good.
Tim, Mel and I spent an hour or so messing around with the equipment before packing it up. Not a very scientific study, but it confirmed that CORS Streaming was accessible via NTRIP and reasonably accurate.
In the meantime, the snow wasn’t letting up. This is the view as I was leaving Tim’s office to head to Boulder for the Space Weather Workshop:
I wasn’t finished with my CORS Streaming testing yet. My experience at Tim’s office gave me enough confidence to allocate time later in the week to conduct a more detailed test after the Space Weather Workshop. Hopefully, the weather would cooperate (call me a fair-weather field guy).
Space Weather Workshop
Every April, NOAA’s Space Weather Prediction Center in Boulder hosts the Space Weather Workshop (SWW), a gathering that has evolved into the leading conference in the U.S. for space weather-related topics. It attracts attendees, experts and speakers from all over the world. The discussion isn’t centered on GNSS, but GNSS certainly is a topic that is discussed. This year’s central topic was the electric power grid. You can view the SWW program here.
Believe it or not, this month (May 2013) was the predicted “solar maximum” for the current solar cycle (Solar Cycle 24, an 11-year cycle). However, Solar Cycle 24 has been unexpectedly weak. See the following slide presented by Doug Bisecker of the Space Weather Prediction Center. Doug is the Chairman of the Solar Cycle 24 Prediction Panel. His question, “Is there any chance we can still salvage some respectability?” speaks volumes about the difficulty in predicting space weather.
Source: Doug Bisecker presentation at the 2013 Space Weather Workshop
From the above, you can see the actual number of sun spot occurrence has been significantly less than predicted. Although sun spots aren’t what cause GNSS receivers to have problems, sun spots can indicate the amount of solar activity, which can be related to geomagnetic storms. Geomagnetic storms disturb the ionosphere and are the events that cause the most problems for GNSS receivers. Looking at the top chart above, you can see the difference in activity between the last solar maximum (peaked in early 2002) and today. The difference is clearly significant.
Does this mean we, the high-precision GNSS users, get a free pass on Solar Cycle 24?
Not at all.
Historically speaking, the most extreme geomagnetic storms (e.g., Oct/Nov 2002) have occurred after the solar maximum so our sensitivity to this issue should be keen for the next two years. Furthermore, there are orders of magnitude more high-precision GNSS receivers being used than ever before, and in mission-critical applications such as auto-steer in machine control (agriculture, construction, etc.). Most GNSS high-precision users today haven’t experienced the effects of an extreme geomagnetic storm. For a short primer on the effects of solar activity on GNSS/GPS, you might want to take a look at this article I wrote in 2008 as well Richard Langley’s 2011 Innovation column “GNSS and the Ionosphere.” In addition to the content, they both contain some valuable links to relevant articles.
In line with a goal of the workshop, a panel of GNSS professionals looked at issues that users face as they go about their business at solar max. The panel was “Global Navigation Satellite System (GNSS) Services: Research Needed to Fill Operational Gaps.” Joe Kunches (SWPC) moderated the panel that included Dr. Geoff Crowley (Astra), Dr. Anthea Coster (MIT), Capt. Steven Miller (USAF) and myself. We highlighted precision GNSS, satellite navigation for commercial aviation (ADS-B), and current work to better understand the errors the ionosphere imposes on user activities.
Something else I learned at the conference was how tough ionospheric scintillation is on GNSS receivers in Brazil. I feel for those users. When I mentioned I was traveling to Chile for an RTK project, the scientists said it is worse in Chile than the U.S., but still not as bad as Brazil. I’ll be very interested to experience how different it is than the U.S. (or other parts of the world where I’ve traveled).
I keep a pretty close eye on space weather and in contact with NOAA’s Space Weather Prediction Center. When I hear of a space weather event that may affect high-precision GNSS/GPS receivers, I send out a Tweet with the hashtag #SolarActivity. You can follow me on Twitter at https://twitter.com/GPSGIS_Eric.
From Space Weather Back to Local Weather
As the week progressed during the Space Weather Workshop, the snow continued. Boulder looked like Christmas in April.
I really wanted to spend some more time in the field to test the accuracy of the NGS’s CORS Streaming service and I was running out of time. In order to perform the test the way I wanted, I needed to find a local NGS survey mark that was observed using GPS. I checked out the NGS survey mark database and got lucky. There was one (PID = KK2060) located on a vista point parking area off of Highway 36 on the way from my hotel to the Space Weather Workshop. I couldn’t have asked for a better or more convenient survey mark location. I was planning to use a Bluetooth GNSS receiver so I could actually collect data while sitting in my car.
On Thursday morning, Mother Nature cleared her skies for me so I drove to the vista point. Remember, there’s a couple of feet of snow on the ground, so I was really hoping to see some kind of wood lathe that would get me close to the survey mark (no, I didn’t preload the KK2060 coords in my GPS L). Fortunately, a wood stake was near the survey mark. However, I didn’t have a shovel or a metal detector so it was either using my hands to shovel and search under two feet of snow for the mark, or…thanks to the rental car company, the car came with a healthy-sized windshield scraper. After 15 minutes of digging in the snow with a windshield scraper, I found KK2060. I’m sure to the people parked on the vista enjoying the view; I looked very suspicious using a windshield scraper to dig a hole in the snow. I wouldn’t have been surprised if a state trooper had shown up.
KK2060 recovered from under two feet of snow with a windshield scraper.
My final challenge was…no tripod or tribrach. I travel light and didn’t want to pack a set and, of course, I forgot to ask Tim if I could borrow a set. It’s never a good idea to set a GNSS antenna directly on the ground, but the antenna was small (<3” in diameter) and I did have a 5” diameter ground plane with about a 1” post. I was able to place it over the survey mark with reasonable confidence.
As I mentioned before, I was using a Bluetooth GNSS receiver (GPS L1/L2, GLONASS), the SXBlue III GNSS.
To collect the data, I was using an SXPad handheld with an AT&T SIM card for the Internet connection. For data-collection software, I used VisualGPSce, a free GPS data-collection program that collects and displays raw NMEA data. Although it doesn’t display enough digits of precision for the horizontal position, it accomplishes the simple task of collecting NMEA-formatted data without applying any transformation so I get the raw NMEA-formatted data from the receiver. It also displays some useful information such as PDOP, RTK indicator and elevation.
The last piece of data-collection software I used was a free NTRIP client software written by the SXBlue people called SXBlue RTN. I needed an NTRIP client software to access the CORS Streaming mount point. The software manages the IP address, port and login/pwd of the CORS Streaming system.
Logging into the NGS CORS Streaming site was painless, and within a few seconds I had an RTK FIXed position from the GNSS receiver, all from the comfort of my rental car, thanks to long-range Bluetooth. I collected ~45 minutes of NMEA data (1-Hz data rate) without interruption.
When I returned to the office, I began the process of comparing the results from CORS Streaming to the NGS survey mark coordinate. I checked with NGS and they reported that CORS Streaming is referenced to the ITRF00 (epoch 1997.0) datum. The KK2060 coordinate is published in NAD83/2011 (epoch 2010.0). I needed to reconcile the datum difference before performing any analysis so I used the NGS HTDP (Horizontal Time Dependent Positioning) online tool to accomplish this.
Finally, I used NMEA Analyzer (custom-built software for performing statistical analysis on GNSS NMEA data to NSSDA horizontal accuracy standards) to calculate accuracy (not precision) values of the data. I set up the NMEA Analyzer software to randomly select 200 epochs out of the ~2,700 collected to mitigate any bias due to filtering or other receiver “tricks”. Following are the horizontal results:
Not bad for an antenna sitting on the ground and an 18-km baseline using a $6,000 GNSS receiver and a free RTK base station. Folks, this is the direction that GNSS technology is heading. The continued proliferation of high-precision GNSS infrastructure (RTK networks, real-time PPP, etc.) and the falling prices of RTK GNSS receivers will dramatically increase the availability of high-precision technology to those who previously could not afford to make the investment.
I didn’t get a chance to test the PBO real-time streaming while I was in Colorado, but fortunately there are many PBO real-time stations that I can test from the comfort of my home office here in Oregon. In fact, there are so many in Oregon and Washington that I can test many different baseline distances to understand what accuracy users can expect. Look for my test results on that sometime this summer.
National Geodetic Survey (NGS) Suffering
Only a week after I did my field test of NGS’ CORS Streaming system in Colorado, NGS announced it was shutting down the CORS Streaming service effective April 26. On April 23, NGS issued the following notice by email:
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The National Geodetic Survey’s prototype Real Time GNSS Data Service (Streaming CORS) will be discontinued effective April 26, 2013. The prototype was introduced a few years ago as a small research project to gauge interest and usage as well as test a proof of concept with the RTCM communities. However, due to low usage of this prototype service and staff limitations within the National Geodetic Survey, we have decided to discontinue the prototype. There were many contributing factors that lead to this decision but the following recent series of events has had a significant impact on project support and operations:
— Funds were cut due to sequestration and rescission
— Upcoming furloughs will impact all National Geodetic Survey Personnel
— A NOAA-wide hiring freeze is in effect
— Our only real-time expert will retire on April 30, 2013
If you have any questions or comments to share, please contact Neil Weston at 301-713-3191 or by email – [email protected].
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I think the action was premature. Hardly anyone knew about the CORS Streaming service and it was only deployed in a small number of locations, which was not enough to cover a significant geographic area or major metro areas.
Nonetheless, I think this action points to bigger problems at the NGS. To all of us in the U.S. (and those in other countries), the NGS has been a tremendous source of GNSS technical expertise, products and services. The problem is that they are losing expertise at a faster rate than they are gaining. Just in the past few months, Dave Doyle and Bill Henning have both retired. Those two were a big part of the NGS user community outreach “boots on the ground” effort.
Furthermore, as the notice indicates, NGS’s only “real-time expert” (Bill Henning) is now retired. That’s a problem. As real-time, high-precision GNSS is gaining traction quickly in industries beyond surveying and engineering, the resources for NGS to support this trend should also expand, not contract. On the other hand, the use of GNSS post-processing is not increasing, yet NGS has loads of resources allocated to support post-processing. As technology trends shift, resources need to be redistributed in alignment with those trends.
The Future of NDGPS Open for Public Comment
The U.S. NDGPS program is on the chopping block again. However, this time it’s much more serious. The last time this issue surface was in 2007 when funding for some of the NDGPS sites was being threatened. At that time, only some of the inland sites were facing decommissioning. The U.S. Coast Guard DGPS part of NDGPS was safe and funded.
However, that’s not the case this time. Even the U.S. Coast Guard is starting to question the value of the DGPS system it created and has been using for more than 15 years. The FAA’s WAAS (Wide Area Augmentation System) has proven to be a viable alternative to NDGPS and is used by thousands of sport mariners and commercial marine pilot associations across the U.S., as well as high-precision users in GIS and surveying/engineering. To further complicate the issue, the use of GLONASS is not supported by NDGPS. Like what we’ve seen in high-precision surveying/engineering receivers, GLONASS is becoming an important feature in receivers used by commercial mariners who have to deal with terrain and structures that impede satellite visibility. Even though WAAS doesn’t support GLONASS, some newer GNSS receivers are able to integrate GLONASS data into the WAAS solution, further increasing the value of WAAS over NDGPS.
It’s likely that you aren’t an NDGPS user, but you might still be affected if the NDGPS is decommissioned. There are a total of 86 NDGPS stations across the Continental U.S., Alaska and Hawaii. As well as being NDGPS signal broadcasters, they are also part of the NGS CORS program that is used by the NGS’s OPUS online post-processing service. If you are using OPUS or NGS CORS for post-processing, you might be using NDGPS CORS data and not realize it. Following is a map of all NDGPS stations in the U.S.:
U.S. NDGPS coverage map.
If you’re interested in reading an explanation from the U.S. Coast Guard and Department of Transportation about the request for public comment and submitting a comment, click here. To be considered, comments must be submitted by July 15.
Monitoring the Ionosphere with Integer-Leveled GPS Measurements
By Simon Banville, Wei Zhang, and Richard B. Langley
INNOVATION INSIGHTS by Richard Langley
IT’S NOT JUST FOR POSITIONING, NAVIGATION, AND TIMING. Many people do not realize that GPS is being used in a variety of ways in addition to those of its primary mandate, which is to provide accurate position, velocity, and time information.
The radio signals from the GPS satellites must traverse the Earth’s atmosphere on their way to receivers on or near the Earth’s surface. The signals interact with the atoms, molecules, and charged particles that make up the atmosphere, and the process slightly modifies the signals. It is these modified or perturbed signals that a receiver actually processes. And should a signal be reflected or diffracted by some object in the vicinity of the receiver’s antenna, the signal is further perturbed — a phenomenon we call multipath.
Now, these perturbations are a bit of a nuisance for conventional users of GPS. The atmospheric effects, if uncorrected, reduce the accuracy of the positions, velocities, and time information derived from the signals. However, GPS receivers have correction algorithms in their microprocessor firmware that attempt to correct for the effects. Multipath, on the other hand, is difficult to model although the use of sophisticated antennas and advanced receiver technologies can minimize its effect.
But there are some GPS users who welcome the multipath or atmospheric effects in the signals. By analyzing the fluctuations in signal-to-noise-ratio due to multipath, the characteristics of the reflector can be deduced. If the reflector is the ground, then the amount of moisture in the soil can be measured. And, in wintery climes, changes in snow depth can be tracked from the multipath in GPS signals.
The atmospheric effects perturbing GPS signals can be separated into those that are generated in the lower part of the atmosphere, mostly in the troposphere, and those generated in the upper, ionized part of the atmosphere — the ionosphere. Meteorologists are able to extract information on water vapor content in the troposphere and stratosphere from the measurements made by GPS receivers and regularly use the data from networks of ground-based continuously operating receivers and those operating on some Earth-orbiting satellites to improve weather forecasts.
And, thanks to its dispersive nature, the ionosphere can be studied by suitably combining the measurements made on the two legacy frequencies transmitted by all GPS satellites. Ground-based receiver networks can be used to map the electron content of the ionosphere, while Earth-orbiting receivers can profile electron density. Even small variations in the distribution of ionospheric electrons caused by earthquakes; tsunamis; and volcanic, meteorite, and nuclear explosions can be detected using GPS.
In this month’s column, I am joined by two of my graduate students, who report on an advance in the signal processing procedure for better monitoring of the ionosphere, potentially allowing scientists to get an even better handle on what’s going on above our heads.
Representation and forecast of the electron content within the ionosphere is now routinely accomplished using GPS measurements. The global distribution of permanent ground-based GPS tracking stations can effectively monitor the evolution of electron structures within the ionosphere, serving a multitude of purposes including satellite-based communication and navigation.
It has been recognized early on that GPS measurements could provide an accurate estimate of the total electron content (TEC) along a satellite-receiver path. However, because of their inherent nature, phase observations are biased by an unknown integer number of cycles and do not provide an absolute value of TEC. Code measurements (pseudoranges), although they are not ambiguous, also contain frequency-dependent biases, which again prevent a direct determination of TEC. The main advantage of code over phase is that the biases are satellite- and receiver-dependent, rather than arc-dependent. For this reason, the GPS community initially adopted, as a common practice, fitting the accurate TEC variation provided by phase measurements to the noisy code measurements, therefore removing the arc-dependent biases. Several variations of this process were developed over the years, such as phase leveling, code smoothing, and weighted carrier-phase leveling (see Further Reading for background literature).
The main challenge at this point is to separate the code inter-frequency biases (IFBs) from the line-of-sight TEC. Since both terms are linearly dependent, a mathematical representation of the TEC is usually required to obtain an estimate of each quantity. Misspecifications in the model and mapping functions were found to contribute significantly to errors in the IFB estimation, suggesting that this process would be better performed during nighttime when few ionospheric gradients are present. IFB estimation has been an ongoing research topic for the past two decades are still remains an issue for accurate TEC determination.
A particular concern with IFBs is the common assumption regarding their stability. It is often assumed that receiver IFBs are constant during the course of a day and that satellite IFBs are constant for a duration of a month or more. Studies have clearly demonstrated that intra-day variations of receiver instrumental biases exist, which could possibly be related to temperature effects. This assumption was shown to possibly introduce errors exceeding 5 TEC units (TECU) in the leveling process, where 1 TECU corresponds to 0.162 meters of code delay or carrier advance at the GPS L1 frequency (1575.42 MHz).
To overcome this limitation, one could look into using solely phase measurements in the TEC estimation process, and explicitly deal with the arc-dependent ambiguities. The main advantage of such a strategy is to avoid code-induced errors, but a larger number of parameters needs to be estimated, thereby weakening the strength of the adjustment. A comparison of the phase-only (arc-dependent) and phase-leveled (satellite-dependent) models showed that no model performs consistently better. It was found that the satellite-dependent model performs better at low-latitudes since the additional ambiguity parameters in the arc-dependent model can absorb some ionospheric features (such as gradients). On the other hand, when the mathematical representation of the ionosphere is realistic, the leveling errors may more significantly impact the accuracy of the approach.
The advent of precise point positioning (PPP) opened the door to new possibilities for slant TEC (STEC) determination. Indeed, PPP can be used to estimate undifferenced carrier-phase ambiguity parameters on L1 and L2, which can then be used to remove the ambiguous characteristics of the carrier-phase observations. To obtain undifferenced ambiguities free from ionospheric effects, researchers have either used the widelane/ionosphere-free (IF) combinations, or the Group and Phase Ionospheric Calibration (GRAPHIC) combinations. One critical problem with such approaches is that code biases propagate into the estimated ambiguity parameters. Therefore, the resulting TEC estimates are still biased by unknown quantities, and might suffer from the unstable datum provided by the IFBs.
The recent emergence of ambiguity resolution in PPP presented sophisticated means of handling instrumental biases to estimate integer ambiguity parameters. One such technique is the decoupled-clock method, which considers different clock parameters for the carrier-phase and code measurements. In this article, we present an “integer-leveling” method, based on the decoupled-clock model, which uses integer carrier-phase ambiguities obtained through PPP to level the carrier-phase observations.
Standard Leveling Procedure
This section briefly reviews the basic GPS functional model, as well as the observables usually used in ionospheric studies. A common leveling procedure is also presented, since it will serve as a basis for assessing the performance of our new method.
Ionospheric Observables. The standard GPS functional model of dual-frequency carrier-phase and code observations can be expressed as:
(1)
(2)
(3)
(4)
where Φijis the carrier-phase measurement to satellite j on the Li link and, similarly, Pijis the code measurement on Li. The term is the biased ionosphere-free range between the satellite and receiver, which can be decomposed as:
(5)
The instantaneous geometric range between the satellite and receiver antenna phase centers is ρ j. The receiver and satellite clock errors, respectively expressed as dT and dtj, are expressed here in units of meters. The term Tj stands for the tropospheric delay, while the ionospheric delay on L1 is represented by I j and is scaled by the frequency-dependent constant μ for L2, where . The biased carrier-phase ambiguities are symbolized by and are scaled by their respective wavelengths (λi). The ambiguities can be explicitly written as:
(6)
where Ni jis the integer ambiguity, bi is a receiver-dependent bias, and bi j is a satellite-dependent bias. Similarly, Bi and Bi j are instrumental biases associated with code measurements. Finally, ε contains unmodeled quantities such as noise and multipath, specific to the observable. The overbar symbol indicates biased quantities.
In ionospheric studies, the geometry-free (GF) signal combinations are formed to virtually eliminate non-dispersive terms and thus provide a better handle on the quantity of interest:
(7)
(8)
where IFBr and IFB j represent the code inter-frequency biases for the receiver and satellite, respectively. They are also commonly referred to as differential code biases (DCBs). Note that the noise terms (ε) are neglected in these equations for the sake of simplicity.
Weighted-Leveling Procedure. As pointed out in the introduction, the ionospheric observables of Equations (7) and (8) do not provide an absolute level of ionospheric delay due to instrumental biases contained in the measurements. Assuming that these biases do not vary significantly in time, the difference between the phase and code observations for a particular satellite pass should be a constant value (provided that no cycle slip occurred in the phase measurements). The leveling process consists of removing this constant from each geometry-free phase observation in a satellite-receiver arc:
(9)
where the summation is performed for all observations forming the arc. An elevation-angle-dependent weight (w) can also be applied to minimize the noise and multipath contribution for measurements made at low elevation angles. The double-bar symbol indicates leveled observations.
Integer-Leveling Procedure
The procedure of fitting a carrier-phase arc to code observations might introduce errors caused by code noise, multipath, or intra-day code-bias variations. Hence, developing a leveling approach that relies solely on carrier-phase observations is highly desirable. Such an approach is now possible with the recent developments in PPP, allowing for ambiguity resolution on undifferenced observations. This procedure has gained significant momentum in the past few years, with several organizations generating “integer clocks” or fractional offset corrections for recovering the integer nature of the undifferenced ambiguities. Among those organizations are, in alphabetical order, the Centre National d’Études Spatiale; GeoForschungsZentrum; GPS Solutions, Inc.; Jet Propulsion Laboratory; Natural Resources Canada (NRCan); and Trimble Navigation. With ongoing research to improve convergence time, it would be no surprise if PPP with ambiguity resolution would become the de facto methodology for processing data on a station-by-station basis. The results presented in this article are based on the products generated at NRCan, referred to as “decoupled clocks.”
The idea behind integer leveling is to introduce integer ambiguity parameters on L1 and L2, obtained through PPP processing, into the geometry-free linear combination of Equation (7). The resulting integer-leveled observations, in units of meters, can then be expressed as: (10)
where and are the ambiguities obtained from the PPP solution, which should be, preferably, integer values. Since those ambiguities are obtained with respect to a somewhat arbitrary ambiguity datum, they do not allow instant recovery of an unbiased slant ionospheric delay. This fact was highlighted in Equation (10), which indicates that, even though the arc-dependency was removed from the geometry-free combination, there are still receiver- and satellite-dependent biases (br and b j, respectively) remaining in the integer-leveled observations. The latter are thus very similar in nature to the standard-leveled observations, in the sense that the biases br and b jreplace the well-known IFBs. As a consequence, integer-leveled observations can be used with any existing software used for the generation of TEC maps. The motivation behind using integer-leveled observations is the mitigation of leveling errors, as explained in the next sections.
Slant TEC Evaluation
As a first step towards assessing the performance of integer-leveled observations, STEC values are derived on a station-by-station basis. The slant ionospheric delays are then compared for a pair of co-located receivers, as well as with global ionospheric maps (GIMs) produced by the International GNSS Service (IGS).
Leveling Error Analysis. Relative leveling errors between two co-located stations can be obtained by computing between-station differences of leveled observations:
(11)
where subscripts A and B identify the stations involved, and εl is the leveling error. Since the distance between stations is short (within 100 meters, say), the ionospheric delays will cancel, and so will the satellite biases (b j) which are observed at both stations. The remaining quantities will be the (presumably constant) receiver biases and any leveling errors. Since there are no satellite-dependent quantities in Equation (11), the differenced observations obtained should be identical for all satellites observed, provided that there are no leveling errors. The same principles apply to observations leveled using other techniques discussed in the introduction. Hence, Equation (11) allows comparison of the performance of various leveling approaches.
This methodology has been applied to a baseline of approximately a couple of meters in length between stations WTZJ and WTZZ, in Wettzell, Germany. The observations of both stations from March 2, 2008, were leveled using a standard leveling approach, as well as the method described in this article. Relative leveling errors computed using Equation (11) are displayed in Figure 1, where each color represents a different satellite. It is clear that code noise and multipath do not necessarily average out over the course of an arc, leading to leveling errors sometimes exceeding a couple of TECU for the standard leveling approach (see panel (a)). On the other hand, integer-leveled observations agree fairly well between stations, where leveling errors were mostly eliminated. In one instance, at the beginning of the session, ambiguity resolution failed at both stations for satellite PRN 18, leading to a relative error of 1.5 TECU, more or less. Still, the advantages associated with integer leveling should be obvious since the relative error of the standard approach is in the vicinity of -6 TECU for this satellite.
FIGURE 1. Relative leveling errors between stations WTZJ and WTZZ on March 2, 2008: (a) standard-leveled observations and (b) integer-leveled observations.
The magnitude of the leveling errors obtained for the standard approach agrees fairly well with previous studies (see Further Reading). In the event that intra-day variations of the receiver IFBs are observed, even more significant biases were found to contaminate standard-leveled observations. Since the decoupled-clock model used for ambiguity resolution explicitly accounts for possible variations of any equipment delays, the estimated ambiguities are not affected by such effects, leading to improved leveled observations.
STEC Comparisons. Once leveled observations are available, the next step consists of separating STEC from instrumental delays. This task can be accomplished on a station-by-station basis using, for example, the single-layer ionospheric model. Replacing the slant ionospheric delays (I j) in Equation (10) by a bilinear polynomial expansion of VTEC leads to:
(12)
where M(e) is the single-layer mapping function (or obliquity factor) depending on the elevation angle (e) of the satellite. The time-dependent coefficients a0, a1, and a2 determine the mathematical representation of the VTEC above the station. Gradients are modeled using Δλ, the difference between the longitude of the ionospheric pierce point and the longitude of the mean sun, and Δϕ, the difference between the geomagnetic latitude of the ionospheric pierce point and the geomagnetic latitude of the station. The estimation procedure described by Attila Komjathy (see Further Reading) is followed in all subsequent tests. An elevation angle cutoff of 10 degrees was applied and the shell height used was 450 kilometers. Since it is not possible to obtain absolute values for the satellite and receiver biases, the sum of all satellite biases was constrained to a value of zero. As a consequence, all estimated biases will contain a common (unknown) offset. STEC values, in TECU, can then be computed as:
(13)
where the hat symbol denotes estimated quantities, and is equal to zero (that is, it is not estimated) when biases are obtained on a station-by-station basis. The frequency, f1, is expressed in Hz. The numerical constant 40.3, determined from values of fundamental physical constants, is sufficiently precise for our purposes, but is a rounding of the more precise value of 40.308.
While integer-leveled observations from co-located stations show good agreement, an external TEC source is required to make sure that both stations are not affected by common errors. For this purpose, Figure 2 compares STEC values computed from GIMs produced by the IGS and STEC values derived from station WTZJ using both standard- and integer-leveled observations. The IGS claims root-mean-square errors on the order of 2-8 TECU for vertical TEC, although the ionosphere was quiet on the day selected, meaning that errors at the low-end of that range are expected. Errors associated with the mapping function will further contribute to differences in STEC values. As apparent from Figure 2, no significant bias can be identified in integer-leveled observations. On the other hand, negative STEC values (not displayed in Figure 2) were obtained during nighttimes when using standard-leveled observations, a clear indication that leveling errors contaminated the observations.
FIGURE 2. Comparison between STEC values obtained from a global ionospheric map and those from station WTZJ using standard- and integer-leveled observations.
STEC Evaluation in the Positioning Domain. Validation of slant ionospheric delays can also be performed in the positioning domain. For this purpose, a station’s coordinates from processing the observations in static mode (that is, one set of coordinates estimated per session) are estimated using (unsmoothed) single-frequency code observations with precise orbit and clock corrections from the IGS and various ionosphere-correction sources. Figure 3 illustrates the convergence of the 3D position error for station WTZZ, using STEC corrections from the three sources introduced previously, namely: 1) GIMs from the IGS, 2) STEC values from station WTZJ derived from standard leveling, and 3) STEC values from station WTZJ derived from integer leveling. The reference coordinates were obtained from static processing based on dual-frequency carrier-phase and code observations. The benefits of the integer-leveled corrections are obvious, with the solution converging to better than 10 centimeters. Even though the distance between the stations is short, using standard-leveled observations from WTZJ leads to a biased solution as a result of arc-dependent leveling errors. Using a TEC map from the IGS provides a decent solution considering that it is a global model, although the solution is again biased.
FIGURE 3. Single-frequency code-based positioning results for station WTZZ (in static mode) using different ionosphere-correction sources: GIM and STEC values from station WTZJ using standard- and integer-leveled observations.
This station-level analysis allowed us to confirm that integer-leveled observations can seemingly eliminate leveling errors, provided that carrier-phase ambiguities are fixed to proper integer values. Furthermore, it is possible to retrieve unbiased STEC values from those observations by using common techniques for isolating instrumental delays. The next step consisted of examining the impacts of reducing leveling errors on VTEC.
VTEC Evaluation
When using the single-layer ionospheric model, vertical TEC values can be derived from the STEC values of Equation (13) using:
(14)
Dividing STEC by the mapping function will also reduce any bias caused by the leveling procedure. Hence, measures of VTEC made from a satellite at a low elevation angle will be less impacted by leveling errors. When the satellite reaches the zenith, then any bias in the observation will fully propagate into the computed VTEC values. On the other hand, the uncertainty of the mapping function is larger at low-elevation angles, which should be kept in mind when analyzing the results.
Using data from a small regional network allows us to assess the compatibility of the VTEC quantities between stations. For this purpose, GPS data collected as a part of the Western Canada Deformation Array (WCDA) network, still from March 2, 2008, was used. The stations of this network, located on and near Vancouver Island in Canada, are indicated in Figure 4. Following the model of Equation (12), all stations were integrated into a single adjustment to estimate receiver and satellite biases as well as a triplet of time-varying coefficients for each station. STEC values were then computed using Equation (13), and VTEC values were finally derived from Equation (14). This procedure was again implemented for both standard- and integer-leveled observations.
FIGURE 4. Network of stations used in the VTEC evaluation procedures.
To facilitate the comparison of VTEC values spanning a whole day and to account for ionospheric gradients, differences with respect to the IGS GIM were computed. The results, plotted by elevation angle, are displayed in Figure 5 for all seven stations processed (all satellite arcs from the same station are plotted using the same color). The overall agreement between the global model and the station-derived VTECs is fairly good, with a bias of about 1 TECU. Still, the top panel demonstrates that, at high elevation angles, discrepancies between VTEC values derived from standard-leveled observations and the ones obtained from the model have a spread of nearly 6 TECU. With integer-leveled observations (see bottom panel), this spread is reduced to approximately 2 TECU. It is important to realize that the dispersion can be explained by several factors, such as remaining leveling errors, the inexact receiver and satellite bias estimates, and inaccuracies of the global model. It is nonetheless expected that leveling errors account for the most significant part of this error for standard-leveled observations.
For satellites observed at a lower elevation angle, the spread between arcs is similar for both methods (except for station UCLU in panel (a) for which the estimated station IFB parameter looks significantly biased). As stated previously, the reason is that leveling errors are reduced when divided by the mapping function. The latter also introduces further errors in the comparisons, which explains why a wider spread should typically be associated with low-elevation-angle satellites. Nevertheless, it should be clear from Figure 5 that integer-leveled observations offer a better consistency than standard-leveled observations.
FIGURE 5. VTEC differences, with respect to the IGS GIM, for all satellite arcs as a function of the elevation angle of the satellite, using (a) standard-leveled observations and (b) integer-leveled observations.
Conclusion
The technique of integer leveling consists of introducing (preferably) integer ambiguity parameters obtained from PPP into the geometry-free combination of observations. This process removes the arc dependency of the signals, and allows integer-leveled observations to be used with any existing TEC estimation software. While leveling errors of a few TECU exist with current procedures, this type of error can be eliminated through use of our procedure, provided that carrier-phase ambiguities are fixed to the proper integer values. As a consequence, STEC values derived from nearby stations are typically more consistent with each other. Unfortunately, subsequent steps involved in generating VTEC maps, such as transforming STEC to VTEC and interpolating VTEC values between stations, attenuate the benefits of using integer-leveled observations.
There are still ongoing challenges associated with the GIM-generation process, particularly in terms of latency and three-dimensional modeling. Since ambiguity resolution in PPP can be achieved in real time, we believe that integer-leveled observations could benefit near-real-time ionosphere monitoring. Since ambiguity parameters are constant for a satellite pass (provided that there are no cycle slips), integer ambiguity values (that is, the leveling information) can be carried over from one map generation process to the next. Therefore, this methodology could reduce leveling errors associated with short arcs, for instance.
Another prospective benefit of integer-leveled observations is the reduction of leveling errors contaminating data from low-Earth-orbit (LEO) satellites, which is of particular importance for three-dimensional TEC modeling. Due to their low orbits, LEO satellites typically track a GPS satellite for a short period of time. As a consequence, those short arcs do not allow code noise and multipath to average out, potentially leading to important leveling errors. On the other hand, undifferenced ambiguity fixing for LEO satellites already has been demonstrated, and could be a viable solution to this problem.
Evidently, more research needs to be conducted to fully assess the benefits of integer-leveled observations. Still, we think that the results shown herein are encouraging and offer potential solutions to current challenges associated with ionosphere monitoring.
Acknowledgments
We would like to acknowledge the help of Paul Collins from NRCan in producing Figure 4 and the financial contribution of the Natural Sciences and Engineering Research Council of Canada in supporting the second and third authors. This article is based on two conference papers: “Defining the Basis of an ‘Integer-Levelling’ Procedure for Estimating Slant Total Electron Content” presented at ION GNSS 2011 and “Ionospheric Monitoring Using ‘Integer-Levelled’ Observations” presented at ION GNSS 2012. ION GNSS 2011 and 2012 were the 24th and 25th International Technical Meetings of the Satellite Division of The Institute of Navigation, respectively. ION GNSS 2011 was held in Portland, Oregon, September 19–23, 2011, while ION GNSS 2012 was held in Nashville, Tennessee, September 17–21, 2012.
SIMON BANVILLE is a Ph.D. candidate in the Department of Geodesy and Geomatics Engineering at the University of New Brunswick (UNB) under the supervision of Dr. Richard B. Langley. His research topic is the detection and correction of cycle slips in GNSS observations. He also works for Natural Resources Canada on real-time precise point positioning and ambiguity resolution.
WEI ZHANG received his M.Sc. degree (2009) in space science from the School of Earth and Space Science of Peking University, China. He is currently an M.Sc.E. student in the Department of Geodesy and Geomatics Engineering at UNB under the supervision of Dr. Langley. His research topic is the assessment of three-dimensional regional ionosphere tomographic models using GNSS measurements.
FURTHER READING
• Authors’ Conference Papers
“Defining the Basis of an ‘Integer-Levelling’ Procedure for Estimating Slant Total Electron Content” by S. Banville and R.B. Langley in Proceedings of ION GNSS 2011, the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, September 19–23, 2011, pp. 2542–2551.
“Ionospheric Monitoring Using ‘Integer-Levelled’ Observations” by S. Banville, W. Zhang, R. Ghoddousi-Fard, and R.B. Langley in Proceedings of ION GNSS 2012, the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation, Nashville, Tennessee, September 17–21, 2012, pp. 3753–3761.
• Errors in GPS-Derived Slant Total Electron Content
“GPS Slant Total Electron Content Accuracy Using the Single Layer Model Under Different Geomagnetic Regions and Ionospheric Conditions” by C. Brunini, and F.J. Azpilicueta in Journal of Geodesy, Vol. 84, No. 5, pp. 293–304, 2010, doi: 10.1007/s00190-010-0367-5.
“Calibration Errors on Experimental Slant Total Electron Content (TEC) Determined with GPS” by L. Ciraolo, F. Azpilicueta, C. Brunini, A. Meza, and S.M. Radicella in Journal of Geodesy, Vol. 81, No. 2, pp. 111–120, 2007, doi: 10.1007/s00190-006-0093-1.
• Global Ionospheric Maps
“The IGS VTEC Maps: A Reliable Source of Ionospheric Information Since 1998” by M. Hernández-Pajares, J.M. Juan, J. Sanz, R. Orus, A. Garcia-Rigo, J. Feltens, A. Komjathy, S.C. Schaer, and A. Krankowski in Journal of Geodesy, Vol. 83, No. 3–4, 2009, pp. 263–275, doi: 10.1007/s00190-008-0266-1.
“Undifferenced GPS Ambiguity Resolution Using the Decoupled Clock Model and Ambiguity Datum Fixing” by P. Collins, S. Bisnath, F. Lahaye, and P. Héroux in Navigation: Journal of The Institute of Navigation, Vol. 57, No. 2, Summer 2010, pp. 123–135.
The National Oceanic and Atmospheric Administration (NOAA) announced that effective Wednesday, July 25, 2012, at 1600 UTC (10:00 AM MDT), the Space Weather Prediction Center (SWPC) will modernize its geomagnetic storm watch products. These products will now be issued relative to the highest expected geomagnetic storm category (NOAA Scale) and will be based on the 3-hour geomagnetic K-index rather than the 24-hour A-index.
According to the announcement, SWPC watch products will still be valid for the entire UTC day, just as they are under the A-based watches today. This change will better align SWPC's geomagnetic watch products with its geomagnetic warning and alert products and NOAA Scale designations. Product Subscription Service customers are not required to take any action regarding this change. The current A-based watches contain expected geomagnetic storm scale (G-level) information so all subscriptions will be automatically transferred to the new G-based watch products.
You may not have noticed it, but last Friday we experienced the first serious geomagnetic storm in this solar cycle (Solar Cycle 24), which began in 2009. Not all types of solar activity (sun spots, solar flares, solar burst, and solar radiation) affect GPS receiver operations. Geomagnetic storms are the ones that can cause problems for GPS receivers if those storms are powerful enough.
Last Thursday, I received e-mail from Joe Kunches at NOAA (National Oceanic and Atmospheric Administration).
“The Sun has been erupting and looks like a storm — say G3 level — could be on for Friday, August 5.”
Joe was right, it hit about a day later, on Friday, August 5, as he predicted.
The good news is that Joe says we generally have at least a 24-hour warning before a geomagnetic storm starts disturbing the ionosphere.
That’s what it comes down to, the ionosphere.
GPS signals being delayed as they pass through the ionosphere end up being the largest source of error in GPS positioning. The signals must pass through the ionosphere, which is full of free electrons. The density of these electrons in the ionosphere affects the speed at which GPS signals travel. If the density of the electrons in the ionosphere was consistent, then it would be straight-forward to create a model and largely mitigate its effects. However, that’s not always the case. The ionosphere has been relatively benign since the last solar cycle, and that’s one of the reasons that GPS accuracy has been so good, especially GPS L1 SBAS systems like WAAS/EGNOS/MSAS, which rely on modeling the ionosphere.
The problem is geomagnetic storms. They wreak havoc on the free electrons in the ionosphere, making it difficult to accurately determine how much the GPS signals have been delayed.
The NOAA Space Weather Prediction Center (SWPC) is one of the foremost agencies that monitors the Total Electron Count (TEC) in the ionosphere. With Joe’s help, I was able to obtain dynamic plots of the TEC from last Friday so I could illustrate to you what happened. I was also able to obtain plots from Gavin Schrock at the Washington State Reference Network (WSRN) showing how it impacted the WSRN. I compiled the plots, added some text, and produced the following Youtube video.
As I wrote in the Youtube video, to get the most updated solar activity information that’s related to high-precision GPS users, you should follow me on Twitter at GPSGIS_Eric.
If you’re looking for a good backgrounder on how the ionosphere affects GPS, you might want to read this April 1991 GPS World column. Although it’s dated in some respects, the fundamental concepts are solid.
Last Push on LightSquared
There’s been some confusion on the FCC comment period regarding the LightSquared/GPS interference issue. The comment period was not extended. The public comment period was July 1 to July 30, 2011. The reply comment period is from August 1 to August 15, 2011. However, it appears the FCC is still logging new comments even after the July 30 cut-off date. Either way, do not hesitate to submit your comments before August 15; just mark it (dropdown menu) as a reply to comments.
This is your last chance to speak out and let your government know how important GPS is to your orgnization.
To date, there have been more than 2,900 individual comments electronically filed as well as more than 15,000 submitted in writing to the FCC (15,000 alone from the Boat Owners Association of the United States). The vast majority of the comments support GPS.
This week I’m following up on my article from a couple of weeks ago about the potential effects of LightSquared’s plans. As a user of high-precision GPS receivers (particularly GPS L1 sub-meter, but also dual-frequency), you should be particularly concerned about this issue. I’ll tell you why. Also, I have a note on recent the solar activity.
The reasons you should be concerned about LightSquared’s plans are two-fold:
1. Consumer GPS receivers and professional-grade GPS receivers designed for higher performance (mapping, surveying, etc.) aren’t necessarily designed the same way. High-performance GPS receivers use a wider bandwidth radio design.
For example, the GPS L1 frequency is 1575.42 MHz. Many high-performance GPS receivers use a wide bandwidth radio that scans +/- 20 MHz from 1575.42 MHz. That equates to a range of 1555 MHz to 1595 MHz. LightSquared’s frequency spectrum is 1525 MHz to 1559 MHz. Clearly, there’s overlap, which is another word for interference. On top of that, LightSquared plans on a broadcast strength of 1,500 watts from a tower located down the street. The GPS broadcast signal strength is about 30 watts from a satellite located some 19,000 kilometers away in outer space. Who’s going to win that battle?
I’m not an aerospace engineer or an RF (radiofrequency) engineer, but I don’t think it takes one to see the potential impact of LightSquared’s service on high-performance GPS receivers. At the very least, it warrants an in-depth technical study.
2. Neither the policymakers nor LightSquared know about or understand the user community of high-performance GPS receivers comprised of hundreds of thousands of high-end GPS receivers. They think the GPS user community is comprised of auto navigation and mobile-phone users. They don’t understand that we are the infrastructure people. We use GPS in a way that they don’t understand, but is so critical to our infrastructure. It’s not their fault, but you can’t assume they know, so it’s up to us to inform them. You have to speak up.
Here’s a perfect example. Click on the following link to view a report presented by LightSquared last week in Taipei, Taiwan, at a 3GPP conference.
The best part about this report is the following statement from the Executive Summary:
“Although results have been provided to date of a limited number of devices (6), LightSquared proposes to close the study at this stage as a more comprehensive study, covering a wider variety of GPS receivers than those involved in cellular applications, has now been initiated under the auspices of the FCC [2]. This study will be conducted by a cross-industry group led by LightSquared and USGPSIC, the reports of the study having complete public visibility.”
Granted, I understand the Taipei conference was focused on the impact of LightSquared’s plan on mobile phones using GPS, but if this is the extent of their testing, it’s alarming. Furthermore, it’s relatively easy to acquire and operate an inexpensive consumer GPS receiver. Can you picture LightSquared attempting to test a sub-meter GPS L1 receiver or a RTK setup? GPS, GLONASS, SBAS, DGPS, real-time, post-processing, and the myriad of receivers on the market need to be tested. Although it’s likely not possible to test all equipment on the market, it’s not prudent to leave anything to chance. If, one year from now, you wake up and find out your $10,000 RTK receiver doesn’t work like it used to, it will be too late to do much about it. It takes very little time to voice your concern now to your elected officials so the appropriate attention is given to high-precision users.
The good news is that Trimble Navigation is involved, along with the Federal Aviation Administration, with the U.S. GPS Industry Council and will be working closely with LightSquared in a Technical Working Group to better understand the impact that LightSquared’s system would have on GPS. Trimble and the FAA aren’t the only parties involved in the working group, but they are the parties that understand the needs of the high-precision user.
The Technical Working Group’s first report is due March 15, 2011. Time is short, so don’t delay.
Use these guidelines to take action. It is a call to action from Dr. Joe Paiva, veteran of surveying since the 1980s with whom many of you are familiar.
Solar Activity
As you’ve probably heard, we’re entering the next solar cycle, which is due to peak in May 2013.
I want to periodically touch on this subject as the solar activity is going to increase over the next few years, and if the solar activity (geomagnetic storms, not sunspots) is severe enough, it will have an effect on GPS accuracy and tracking. Regardless of what you’ve heard in the mainstream media in recent months, the last event serious enough to affect GPS operations was in December 2006. That’s not to say that things aren’t heating up.
But the recent activity does highlight the fact that “the Sun has become, somewhat suddenly, more eruptive,” according to Joe Kunches, of NOAA’s Space Weather Prediction Center. “We’ve been fortunate so far, in that the terrestrial effects — and impacts to GPS — have been very minimal. The most obvious sign of this has been the brilliant auroras up north.”
“The video shows a large prominence eruption — billions of tons of plasma being strewn off the Sun. Some of it is drawn by gravity and rains back to the surface — the rest of it escapes. It’s the blown-away plasma that forms the coronal mass ejections that, when properly pointed, go by the Earth and cause problems for GPS,” said Kunches.
Click on the following image to view a 15-second video of a solar flare that occured on February 24, 2011.
Credit: NASA/GSFC/SDO
From NASA:
When a rather large-sized (M 3.6 class) flare occurred near the edge of the Sun, it blew out a gorgeous, waving mass of erupting plasma that swirled and twisted over a 90-minute period (Feb. 24, 2011). This event was captured in extreme ultraviolet light by NASA’s Solar Dynamics Observatory spacecraft . Some of the material blew out into space and other portions fell back to the surface. Because SDO images are super-HD, we can zoom in on the action and still see exquisite details. And using a cadence of a frame taken every 24 seconds, the sense of motion is, by all appearances, seamless. Sit back and enjoy the jaw-droppi
ng solar show.
Last year, the first GPS IIF satellite was launched. It became the first GPS satellite to broadcast the new L5 civilian signal/frequency. At 1176 MHz, it is further separated from L1 and L2 and located in the protected Aeronautical Radionavigation Services band, so there is no possibility of commercial interference like we see today with theLightSquaredcontroversy. The availability of GPS L5 will usher in a new era of inexpensive, accurate GNSS receivers and will be the future of high-precision GNSS receivers, and quite possibly single-frequency receivers. I will also discuss the international support of L5 from other GNSS in development such as Galileo, Compass, QZSS, as well as SBAS (WAAS/EGNOS/MSAS).
I’ll be presenting some interesting new material in the webinar such as graphics illustrating how many satellites (GPS and others) are projected to be broadcasting L1 and L5 just four years from now. It will be well worth 60 minutes of your time.
You might have heard reports this week about a solar storm this week. This is part of the new solar cycle (Solar Cycle 24) that I’ve written about several times. I want to periodically touch on this subject as the solar activity is going to increase over the next few years, and if the solar activity (geomagnetic storms, not sunspots) is severe enough, it will have an effect on GPS accuracy and tracking. Here’s the scoop on this week’s solar activity.
First of all, I’ll let you in on a secret. I’m working on a new solar activity notification system specifically designed for GPS users. The problem is that people see reports in the mainstream media about solar activity and they automatically assume that it’s going to affect their GPS. Not all solar storms affect GPS; in fact only very specific ones (geomagnetic storms) of sufficient strength will affect GPS operations. I’m working on a notification system that will be tailored to both GPS L1 and GPS L1/L2 users (they are affected differently) so GPS users can have a reliable and specific source of information on solar activity without having to wade through the mainstream media noise.
Stay tuned for details this fall in this newsletter to learn more about my notification system and how to and access it. If you’ve ever used some of the GPS hardware/software products I helped design, you know my top priority is to make it easy to use and understandable.
This week’s event was probably the strongest geomagnetic storm of this solar cycle and of recent years (edit: actually, the storm in early April 2010 was a little stronger), maybe since late 2006. It will create some beautiful “northern lights,” but as strong as that may seem, it still wasn’t strong enough to elicit even a “cautionary” warning to GPS users (neither GPS L1 nor GPS L1/L2).
NASA video of sun’s activity on August 2, 2010
The last geomagnetic storm that adversely affected GPS users was in December 2006. It affected some GPS users for 10-15 minutes. For such a short time, 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 has passed and the user is back in operation. It was barely noticeable, if at all.
On the other hand, a severe geomagnetic storm such as the one that occurred in October 2003 can last for days and wreak havoc on precision GPS receivers. During extreme geomagnetic storms like that one, GPS accuracy suffers a lot, especially with GPS L1 users. During that event, simulations from the University of Calgary showed that WAAS maximum horizontal error (95th percentile) reached 25 meters while single baseline DGPS maximum horizontal error (95th percentile) reached 18 meters.
Dual-frequency users aren’t affected as much by extreme events but aren’t immune. Extreme events such as October 2003 can cause a loss of phase lock, especially with L2 on receivers that utilizing codeless and semicodeless techniques, which are virtually all of the dual-frequency GPS receivers on the market as of today.
For GPS users, nothing can be done to mitigate the effects of a strong geomagnetic storm. The next best step is to try to predict when they will occur so GPS users know what to expect. Fortunately, these storms are not common and scientists can reasonably predict when an event will occur.
There are some good websites to reference when checking up on solar activity. A great place for Europeans to do this is at the Royal Meteorological Institute of Belgium’s website. The U.S. National Weather Service also operates the Space Weather Prediction Center. The Australian Space Weather Agency operates a Space Weather Prediction Center, too. Also, note that for those users along the equator and at higher latitudes, your area is more susceptible to stronger geomagnetic storm activity.
The websites listed above are chock full of information and predictive systems on space weather. In fact, I believe it’s too much information for most GPS users to efficiently interpret. The goal with my new initiative is to provide GPS users with a quick summary so they are able to make informed decisions in a few seconds. Again, stay tuned this fall for the rollout.
Conference/Webinar Presentations
Between webinars and conferences, I’ve put together a fair number of Powerpoint presentations. I’m in the process of uploading many of them, some dating back years, to our website. Currently, I’ve uploaded ones that date back to April 2010. I hope you enjoy them.
The following presentations have all been converted to PDF format and are copyrighted. Feel free to incorporate them (or parts of them) into your documents if you like, just please remember to attribute each page you use to my name, Eric Gakstatter, and GPS World/Geospatial Solutions.
2010 (July San Diego, California) ESRI Surveying and Engineering GIS Summit luncheon keynote presentation: Get It Surveyed (GIS).
2010 (June, Seattle, Washington) Asia-Pacific Economic Cooperation meeting: Mapping and Surveying with SBAS+GPS.
2010 (June, Portland, Oregon) Webinar: GIS Mapping for Forestry, Agriculture, and Other Natural Resource Professionals.
Note that for the following webinar, you can also download an audio portion of the webinar free of charge by clicking here.
2010 (April, Phoenix, Arizona) GITA Annual Conference: How the Evolution of GPS is Transforming Surveying and Mapping (along with Pamela Fromhertz of NGS).
Part 1 – GNSS Mapping/Surveying Technology Update
2010 (April, Phoenix, Arizona) GITA Annual Conference: How the Evolution of GPS is Transforming Surveying and Mapping (along with Pamela Fromhertz of NGS).
Part 2 – Machine Control Using GNSS
2010 (April, Phoenix, Arizona) GITA Annual Conference: How the Evolution of GPS is Transforming Surveying and Mapping (along with Pamela Fromhertz of NGS).
Part 3 – Sub-Meter Mapping Using GPS
2010 (April, Phoenix, Arizona) GITA Annual Conference: How the Evolution of GPS is Transforming Surveying and Mapping (along with Pamela Fromhertz of NGS).
