In Portland, Oregon, and in Berlin, Germany, the two largest and most important international conferences on GPS, GNSS, PNT, survey, mapping and geodesy take place this year on exactly the same dates — just 5,177 miles apart. Now that’s bad timing. Our strategy is to divide our forces and send key personnel to interact with industry leaders at each gathering — to bring you the news and developing stories you need to keep on the forefront of change.
If you’re at ION GNSS+ or Intergeo, look for these faces, come up and introduce yourselves. We want to talk with you! If you’re not fortunate enough to attend either conference, look to our website, newsletters and this magazine for product launches, videos and in-depth stories filed from the developing frontiers of PNT. We’ll be reporting !!Live!! and for weeks, even months, to come.
Attending Intergeo in Berlin:
Burch
Barwacz
Joyce
Gerard
Tim Burch is our survey editor; in his day job he’s a professional surveyor and board of directors secretary of that profession’s national society.
Allison Barwacz is digital media content producer for North Coast Media (NCM, that’s us) with a passion for videography and writing.
Mike Joyce and Ryan Gerard, senior account manager and account manager, respectively, work closely with our marketing partners, who make this magazine and multi-media communications channel possible.
Attending ION GNSS+ in Portland:
Stoltman
Whitford
Mitchell
Cozzens
Harms
Sabau
Limpert
Cameron
Langley
Kevin Stoltman is founder and president of NCM, with a distinguished career in business-to-business publishing.
Marty Whitford is editorial director and publisher; earlier, he actually worked at GPS World and attended ION-GNSS 2004.
Michelle Mitchell is account manager for GPS World and senior marketing and event manager for NCM. She knows the GPS industry landscape and players extremely well.
Tracy Cozzens is our managing editor, with her hands on all the controls.
Joelle Harms is an award-winning digital media manager, focused on content planning and creation.
Joe Sabau is an account manager with a keen eye for market trends.
Kelly Limpert is a digital media content producer developing a strong online and social media presence for all of our partners.
Richard Langley is GPS World’s innovation editor and a professor at the University of New Brunswick.
Surrounding sounds may not be a common way of determining location. But on the battlefield, warfighters need to know the direction of gunshots to enable a proper response.
Weighing 12 ounces, the Boomerang Warrior-X by Raytheon BBN Technologies provides immediate hostile fire location awareness to individual soldiers and gives unit leaders shooter grid coordinates, according to the company. These situational awareness enhancements improve coordinated team responses to hostile fire.
Incoming shot announcements are transmitted to a built-in speaker or an earpiece while a lightweight display provides range and azimuth of the shooter position. As the soldier moves, the system compensates for the soldier’s motion and continually updates the threat’s location on a wrist display.
The Boomerang Warrior X system.
This summer, an undisclosed Gulf nation has awarded a direct commercial sales contract to Raytheon BBN Technologies valued at more than $10 million for the delivery of 2,000 Boomerang Warrior-X systems during the next 12 months.
“This technology is a proven life saver on the battlefield,” said Ed Campbell, president of Raytheon BBN Technologies. “Boomerang delivers the best performance of any available shooter detection system today at the lowest cost.”
Raytheon BBN Technologies is a wholly owned subsidiary of Raytheon Company.
GPS expert Scott Pace has been chosen by the White House to serve as executive secretary of the National Space Council. Pace is currently director of the Space Policy Institute and Professor of Practice of International Affairs at George Washington University (GWU).
He also serves as a special counselor to the National Space-Based Positioning, Navigation and Timing (PNT) Advisory Board.
Pace has a long career in space policy and is well known and highly respected in the community. Ever since the Trump Administration indicated that it would reestablish the Space Council, his is virtually the only name rumored to be in the running to serve as the head of its staff, according to the announcement on Space Policy Online.
The council was officially reestablished on June 30, and is chaired by Vice President Mike Pence. Pace was spotted at Kennedy Space Center last week where Pence addressed the KSC workforce, further fueling speculation that he would be appointed as head of the Space Council.
In its announcement, the White House said Pace has “honed his expertise in the areas of science, space, and technology” citing his career at GWU, NASA, the White House Office of Science and Technology Policy (OSTP), and the RAND Corporation’s Science and Technology Policy Institute.
Pace received a bachelor’s degree in physics from Harvey Mudd College, a master’s in Aeronautics and Astronautics and Technology and Policy from MIT, and a Ph.D. in policy analysis from the RAND Graduate School.
During the George W. Bush Administration’s second term, Pace was NASA’s Associate Administrator for Program Analysis and Evaluation under then-NASA Administrator Mike Griffin. He was closely involved in formulating the Constellation program to return humans to the surface of the Moon and then going on to Mars.
His expertise is much broader, however. He was deputy director and acting director of the Office of Space Commerce at the Department of Commerce from 1990 to 1993, when that office reported to the Deputy Secretary of Commerce (instead of being part of NOAA as it is today).
He has been very active on GPS issues for many years, including protecting GPS spectrum at World Radiocommunications Conferences (WRCs) organized by the International Telecommunication Union (ITU). He was a member of the U.S. delegation to the WRCs in 1997, 2000, 2003 and 2007.
He also has served as a member of the U.S. delegation to the United Nations Committee on Peaceful Uses of Outer Space (2009 and 2011-2015). Today he is vice-chair of NOAA’s Advisory Committee on Commercial Remote Sensing, of which he has been a member for several years.
John Logsdon, who founded GWU’s Space Policy Institute and is Professor Emeritus there, said via email that he could think of “no one more qualified” to take on the “essential task of crafting a strategic approach to using U.S. space capabilities to advance this country’s geopolitical interests and to forge productive collaboration among all government space actors and the private sector.”
Mary Lynne Dittmar, president and CEO of the Coalition for Deep Space Exploration (CDSE), also praised the announcement.
“Dr. Pace’s unique combination of experience in government, the private sector, and academia, and his internationally-recognized expertise in space policy, make him an exemplary selection” for the position. She added that CDSE looks forward to working with “the Council, its staff, and the vice president’s office to support U.S. leadership and strategic interests in space.”
CDSE is an alliance of space industry businesses and advocacy groups that support deep space human exploration and science.
GPS signals are so weak, they cannot be used reliably where they are obstructed such as indoors or in concrete canyons. But if the satellites were much closer, their signals would be much stronger. The low Earth orbit Iridium constellation is already orbiting and providing a PNT service. This month we learn about its current capability and future promise.
By David Lawrence, H. Stewart Cobb, Greg Gutt, Michael O’Connor, Tyler G.R. Reid, Todd Walter and David Whelan
(A shortened version of “Innovation Insights” appeared in the magazine.)
INNOVATION INSIGHTS with Richard Langley
WHOA CANADA! July 1st marks Canada’s sesquicentennial. In 1867, four Canadian provinces, Ontario and Quebec (up to then known as the single Province of Canada), Nova Scotia and New Brunswick, joined together to form The Dominion of Canada — the name suggested by New Brunswick’s Sir Leonard Tilley. Other provinces came on board later with the last, Newfoundland and Labrador, joining in 1949.
Apart from my interest in educating all and sundry about the origins of the “true north, strong and free,” what has this got to do with GNSS or allied technologies? Well, it turns out that Canada has played and continues to play an important role in the development of communications and navigation technologies.
It started on Christmas Eve, 1906, when Canadian inventor Reginald Fessenden carried out the first amplitude modulation radio broadcast of voice and music. And in 1925, Edward “Ted” Rogers, a Canadian pioneer in the radio industry, invented a radio tube using alternating current that became a worldwide standard in radio circuits.
Many other developments in terrestrial communications took place in Canada over the years including microwave repeater technology and shortwave radio broadcasting from the famed transmitter plant (now defunct, unfortunately) established near Sackville, New Brunswick, during World War II.
There have also been significant Canadian advances in satellite technology. The first Canadian satellite, Alouette (French for “skylark”), was launched in September 1962 to study the ionosphere. Launched by the United States, it was the first satellite to be constructed by a country other than the U.S. or the Soviet Union. Several other Canadian ionospheric research satellites have been orbited since including CAScade, Smallsat and IOnospheric Polar Explorer or CASSIOPE, launched in September 2013. CASSIOPE carries eight instruments for studying the ionosphere including the University of New Brunswick’s GPS Attitude, Positioning, and Profiling instrument.
Canada has also been a leader in satellite communications technology. The first Anik geostationary satellite was launched in November 1972. (Anik means “little brother” in Inuktitut.) Eight more Anik satellites were launched subsequently including Anik F1R, which is also used to broadcast Wide Area Augmentation System information to GPS receivers. And the first satellite to explore the 14/12-GHz band for direct broadcasting to homes and businesses was Canada’s Communications Technology Satellite, dubbed Hermes, launched in January 1976.
And, of course, we don’t need to mention the Remote Manipulator System on the International Space Station, commonly known as Canadarm, nor the work of celebrity Canadian astronaut Col. Chris Hadfield.
In the area of satellite navigation, Canada is known for its development of techniques to use the U.S. Navy Navigation Satellite System or Transit for one-meter positioning accuracy permitting establishment of geodetic control points such as in Canada’s far north. Canada was also an early adopter of GPS and with software and hardware developments by industry, government and academia has made its mark in the world of precision positioning, navigation and timing.
Another Canadian initiative is the Aerion satellite-based air traffic surveillance system that will use the enhanced low Earth orbit Iridium constellation.
And we shouldn’t forget that Canada is slated to provide the search and rescue package for the GPS III satellites.
Speaking of GPS, we all know what a great technology it is, providing the “gold standard” in global satellite navigation. But it does have one dominant problem: the weakness of the signals. The signals are so weak that they cannot be used reliably where they are obstructed such as indoors or in concrete canyons. The problem stems from the fact that these medium Earth orbit satellites are far away and their energy is significantly spread out during their passage to Earth. If the satellites were much closer to the Earth, their signals would be much stronger. Mind you, you would need more satellites to provide global coverage. Fantasy? No. There is already a constellation of satellites in orbit providing such a PNT service. It is Iridium–the same constellation that will provide the Canadian-initiated aircraft tracking system–and in this month’s column we will learn about is current capability and future promise. Pretty neat, eh?
With the advent of smartphones, there are now more than four billion devices that make use of GNSS. These satellite navigation systems provide not just the blue dot representing location on our phones, but also support the critical infrastructure we rely upon.
The U.S. Department of Homeland Security recognizes that all 16 sectors of U.S. critical infrastructure depend on GPS — 13 of which have critical dependence. A recent report by London Economics estimates the cost of a GNSS outage to the U.K. alone would be over £1B per day.With autonomous systems on the rise, our reliance on GNSS will only be increasing.
As we become more dependent on this technology, we become vulnerable to its limitations. One major shortcoming is signal strength. Designed to work in an open-sky environment, GNSS is severely limited in deep attenuation environments, with little or no service in dense cities or indoors. Furthermore, we are susceptible to jamming where a 20-watt GNSS jammer can deny service over a city block.
The proximity of low Earth orbit (LEO) has the potential to provide much stronger signals than the distant GNSS core-constellations like GPS in medium Earth orbit (MEO). Today, the only LEO system with global coverage is the Iridium constellation used primarily for communications.
FIGURE 1 shows the 31-satellite GPS constellation in contrast with the 66-satellite Iridium network. The scale of the difference in distance (several Earth radii) is extraordinary. The result is that Iridium signals are 300 to 2,400 times stronger than GNSS signals on the ground, making them attractive for use in position, navigation and timing (PNT) applications where GNSS signals are obstructed.
FIGURE 1. The 66-satellite Iridium constellation in low Earth orbit and 31-satellite GPS constellation in medium Earth orbit.
LEO-based PNT is now mainstream, in the form of real-time signals that have been delivered over the Iridium satellite network since May 2016. This service is made possible by Satelles in partnership with Iridium Communications Inc. in a service called Satellite Time and Location (STL), a non-GNSS solution for assured time and location that is highly resilient and physically secure. Consumers, businesses and governments are already using these LEO-based signals in environments with high GNSS interference or occlusion.
The security features of these signals are also used to reliably validate GNSS PNT solutions in real time to help mitigate potential spoofing. Furthermore, the fast LEO orbits of Iridium generate Doppler-frequency-shift signatures significantly stronger than GPS, increasing the utility of the STL signal for positioning applications.
STL field tests demonstrate a positioning accuracy of 20 meters and timekeeping to within 1 microsecond, all in deep attenuation environments indoors. This adds substantial robustness in augmenting the GNSS core constellations like GPS and also allows for a standalone backup in many applications.
LEO Constellations: Past, Present, Future
In 1964, Transit (or the U.S. Navy Navigation Satellite System) became the first operational satellite navigation system. This constellation typically consisted of five to 10 satellites placed in polar orbits with an altitude of about 1,100 kilometers. Unlike many terrestrial radio navigation systems, a position fix was not instantaneous. It required 10 to 16 minutes of observation as a satellite passed overhead to achieve the needed geometric diversity. There was also latency; users had to wait for a satellite to come into view, which could take from 30 to 100 minutes.
The trade-off was accuracy; early performance was a few hundred meters and was later improved to 20 meters (and even down to about 1 meter for multiple-pass fixed-site surveys), the best performance of its day. In 1967, Transit became open for civilian use and remained operational until 1996 when GPS was at full operational capability.
The Soviet Union developed a system similar to Transit known as Parus/Tsikada, with first satellites on orbit in 1967. Parus/Tsikada operated on the same passive Doppler observation principle as Transit, on similar frequencies and in similar polar orbits.
Today, the largest satellite constellation with constant global coverage is Iridium. With 66 LEO satellites delivering worldwide satellite connectivity, including the poles, this system has tenfold more satellites than Transit had. Along with its strong signals compared to the GNSS core-constellations in MEO, Iridium’s global coverage makes it ideal for use in PNT applications where GNSS is obstructed.
Figure 1 shows the scale of the difference in altitude with Iridium at 780 kilometers and GPS at 20,200 kilometers. This has substantial implications not only for signal strength, but also for coverage.
Though Iridium has twice as many satellites as GPS, at the Equator users can often only see one satellite at a time, whereas they can see 10 from GPS. This was one of the fundamental trades considered in the design of the GPS constellation. The higher the altitude, the more each launch cost; the lower, the more satellites had to be built to provide coverage. To put this in perspective, global coverage for one satellite in view at all times requires fewer than 10 satellites in MEO, but requires closer to 100 in LEO.
Future LEO Constellations
The hundreds of LEO satellites needed to match the coverage of GPS may be coming. In late 2014 and early 2015, the International Telecommunication Union reported a half-dozen filings for spectrum allocation for large constellations of LEO satellites.
In January 2015, OneWeb announced a partnership with Virgin and Qualcomm to produce a constellation of 648 LEO satellites to deliver broadband Internet globally. This represents the next order of magnitude, with tenfold more satellites than Iridium.
Within days of this announcement, SpaceX, with support from Google, announced a similar ambition for a constellation of more than 4,000 LEO satellites.
In August 2015, Samsung expressed interest with a proposal for a LEO constellation of 4,600. Boeing joined the race in June 2016, announcing plans for a LEO constellation of nearly 3,000 satellites.
These LEO constellations are being proposed to keep up with the rising demand for broadband, not to replace ground infrastructure, and will provide Internet access to the 54% of the global population that lack that access.
TABLE 1 compares the GNSS core constellations in MEO to the big (Iridium), broadband (OneWeb, SpaceX, Boeing) and early navigation (Transit, Parus/Tsikada) LEO constellations.
TABLE 1. Constellation comparison.
LEO versus MEO
Low and medium Earth orbit each have their individual strengths and weaknesses in the context of navigation as summarized by TABLE 2.
TABLE 2. Comparison of LEO and MEO systems for navigation.
Closer to Earth, LEO offers less spreading loss and improved signal strength on the ground. FIGURE 2 shows that the signal spreading (or space) loss for Iridium is between –140 and –130 dB compared to GPS at –160 dB.
This stems from Iridium being 25 times closer to Earth than GPS, resulting in a gain in the neighborhood of 252, which is approximately 30 dB (1,000 fold). This is confirmed by field tests where the carrier-to-noise-density ratio (C/N0) is typically 45 dB-Hz for GPS but closer to 80 dB-Hz for Iridium.
FIGURE 2. Slant range and spreading loss as a function of orbital altitude and user elevation angle (GSO = geostationary orbit).
Now, we face the drawback of LEO proximity: coverage. Being closer to Earth means that satellites have much smaller footprints as shown in FIGURE 3.
FIGURE 3. Comparison of medium and low Earth orbit satellite distance and footprints (drawn to scale).
FIGURE 4 shows the satellite-footprint radius as a function of orbital altitude and user elevation mask angle. This plot shows the GPS footprint to be threefold larger than Iridium’s, corresponding to nine times more area covered. Hence, to achieve the same coverage as GPS with Iridium’s altitude, a LEO constellation requires an order of magnitude more satellites.
FIGURE 4. Satellite footprint radius as a function of orbital altitude and elevation angle (GSO = geostationary orbit).
Another major difference between LEO and MEO is speed. A GPS satellite completes one Earth revolution every 12 hours, while Iridium does so in only 100 minutes. The shorter the orbital period, the faster the angular rate (also called mean motion) and the more quickly satellites pass overhead. The Earth-centered angular rate of Iridium is seven times faster than GPS.
As a result, users on Earth’s surface see LEO Iridium satellites traverse the local sky in just over 10 minutes compared to hours with satellites in MEO. This characteristic gives rapid changes in geometry and several benefits for navigation.
The swift motion whitens multipath (making it more random, like white noise) as reflections are no longer effectively static over short averaging times. Geometric diversity also leads to effective Doppler positioning as was once leveraged by Transit and now by STL using Iridium. Geometric diversity is also desirable for carrier-phase differential GNSS, allowing for much more rapid resolution of integer cycle ambiguities.
Iridium-Satelles STL Service
As previously mentioned, the STL service has been in operation since May 2016. Many from industry and government are already using this service to achieve a more robust PNT solution. This service will only continue to improve with the Iridium NEXT satellites under deployment — the first 10 were successfully launched in January.
STL is a non-GNSS solution for assured time and location that is highly resilient and physically secure. STL utilizes the Iridium constellation to transmit specially structured time and location broadcasts. Due to their high RF power and signal-coding gain, the STL broadcasts are able to penetrate into difficult attenuation environments, including deep indoors. Like GNSS signals, these broadcasts are specifically designed to allow an STL receiver to obtain precise time and frequency measurements to derive its PNT solutions.
STL is able to augment or serve as a back-up to existing GNSS PNT solutions by providing secure measurements in the presence of high attenuation (deep indoors), active jamming and malicious spoofing. Unlike the MEO GNSS satellites, Iridium uses 48 spot beams to focus its transmissions on a relatively small geographic area. The complex overlapping spot beams of Iridium combined with randomized broadcasts give a unique mechanism to provide location-based authentication that is extremely difficult to spoof.
Two main technical innovations are applied to the existing Iridium quadrature phase-shift keying (QPSK) transmission scheme to facilitate precision measurements. First, the QPSK data at the beginning of an STL burst is manipulated to form a continuous wave (cw) marker, which can be used for burst detection and coarse measurement. Second, the remaining QPSK data in the burst is organized into pseudorandom sequences, reducing the effective information data rate while providing a mechanism for precise measurement via correlation with locally generated sequences.
The processing gain of the sequence correlation operation also enhances the capability of the STL signal to penetrate buildings and other occlusions. STL is designed such that a receiver can reliably decode the bursts and perform precise Doppler and range measurements at attenuations of up to 39 dB relative to unobstructed reception. This is sufficient to penetrate buildings and other occlusions, providing coverage in most deep indoor and urban canyon environments.
In environments where both GNSS and STL time and location fixes are available, the GNSS fixes will generally be more accurate. The key advantage of STL is its ability to provide time and position fixes where GNSS is not available because of occlusions, spoofing or other reasons. In this respect, GNSS and STL can be seen as complementary technologies, and it is apparent that receivers supporting both are highly desirable when practical. An example of a combined GNSS + STL receiver board is shown in FIGURE 5 and is available from Satelles.
To test the signal penetration of STL, trials of the system were undertaken at multiple locations inside an urban high-rise building. For these tests, locations with little or no GPS reception were chosen to measure the impact of such an environment on STL signal reception.
Two GPS receivers were used, a smartphone with assisted GPS and a standalone consumer receiver using Bluetooth communications without assistance data. Similarly, STL was used with and without assistance. For these tests, STL assistance included real-time, out-of-band delivery of satellite clock and orbit data and message payload contents. These test locations ranged from the top (13th) to the bottom (2nd) floor as shown in FIGURE 6.
FIGURE 6. Iridium-based STL test locations. These are indoor and deep attenuation environments where GPS is unavailable.
The results show that only upper floors near windows were able to track at most one to two GPS satellites while lower floors could see none. STL, on the other hand, always experienced strong signals. Even on the lowest floor, with many layers of steel and concrete between the antenna and the sky, the C/N0 from Iridium was between 35 and 55 dB-Hz. GPS, by comparison, is typically between 35 and 50 dB-Hz in an open sky environment.
Indoor Time-Transfer Capability
To evaluate the timing performance of STL in a static, indoor environment, a custom STL receiver board was configured to generate a pulse-per-second (PPS) output. The difference in timing between the STL PPS was then compared to the timing output of a GNSS “truth” reference — in this case, a timing receiver that has nominal timing performance at least an order of magnitude better than the STL-based timing we were attempting to measure.
FIGURE 7 shows the timing difference between the PPS signals generated by the STL receiver and the GNSS receiver, showing the STL ability to provide sub-microsecond timekeeping even in a deep attenuation environment.
FIGURE 7. Iridium-based STL timekeeping results based on data from a 30-day indoor trial. This compares indoor STL timing with a GPS feed from outdoors. This shows STL’s timekeeping to be within 1 microsecond in a deep attenuation environment.
While sub-microsecond timing is sufficient for many applications, higher timing accuracy is desired by some. It has been further demonstrated that STL is capable of achieving sub-100-nanosecond timekeeping in a stand-alone configuration with a rubidium-based STL receiver with an unknown static location indoors.
Indoor Positioning Performance
Unlike the time-transfer capability of STL, positioning requires satellite motion over time to achieve a reasonable 4D time-and-location fix. Therefore, understanding the convergence properties of STL positioning accuracy over time is important to understanding the applicability of STL for various potential uses.
To study these convergence properties, STL data was collected over a 24-hour period in a one-story office environment. The data was then post-processed in a series of trials that each represented a different starting time in the data set — each trial offset to begin 5 seconds ahead of the previous trial’s start time. In this way, the 24-hour data set could be used to generate a statistically significant set of trial runs in which positioning convergence characteristics could be evaluated.
We found out from the results of the post-processed trials that after 10 minutes of convergence, the STL solution had converged to an accuracy of better than 35 meters for 67% of the trials. After sufficient time, typically an accuracy of 20 meters can be achieved in deep attenuation environments such as indoors. The vertical accuracy of STL, in the absence of other measurements or vertical constraints, is comparable to the horizontal accuracy.
Looking Forward
We see the benefit of LEO in navigation with the operational STL using Iridium, where stronger signals allow for operation deep indoors and in other GNSS-challenged environments. Though extremely valuable as a complement to GPS, Iridium lacks the numbers to fully replace GPS as a standalone navigation system in all capacities as only one satellite at a time is typically in view.
However, these numbers may be coming in LEO with the unprecedented scale of the recently announced Broadband constellations of OneWeb, SpaceX, Boeing and others summarized in Table 1. OneWeb’s constellation is nearly as large as the total number of operational satellites in LEO today and is an order of magnitude larger than Iridium. SpaceX’s and Boeing’s proposed constellations each have more than twice the total number of operational satellites in orbit in 2017.
The unparalleled number of satellites in these proposed broadband LEO constellations gives rise to better geometry than any of the GNSS core-constellations in MEO by at least threefold, as shown by FIGURE 8.
FIGURE 8. Comparison of geometric dilution of precision (98th percentile) as a function of constellation size and altitude (MEO = medium Earth orbit; GSO = geostationary orbit).
This plot represents the 98th percentile geometric dilution of precision a user would experience on Earth as a function of constellation size and altitude, assuming a 5-degree elevation mask angle. This stronger geometry allows for relaxation of the signal-in-space user range error, while still matching the user position accuracy of GPS. This enables the use of lower than traditional cost satellite clocks and more amenable orbit determination levels.
When combined with the more benign LEO radiation environment compared to MEO, satellite navigation payloads could be built using commercial off-the-shelf components in place of specialized space-hardened ones, greatly reducing cost. By partnering with these LEO constellation providers, much like Satelles has done with Iridium, a PNT service comparable to GPS could be achieved though with the added benefits of LEO including stronger signals and rapid changes in geometry.
Conclusion
Robust PNT services from LEO are here today, providing augmentation to GPS where GPS isn’t available. The addition of navigation signals from LEO provides a number of benefits. The faster LEO motion provides geometric diversity, giving rise to multipath whitening, faster initialization times for carrier-phase differential GNSS, and Doppler-based positioning.
Perhaps most importantly, LEO constellations have the advantage of being closer to the Earth than the GNSS core constellations in MEO, experiencing less path loss and delivering signals 1,000 times (30-dB) stronger. This makes them more resilient to jamming and more capable in deep attenuation environments such as in urban canyons and indoors.
This extra power allows the LEO-based Satelles STL using Iridium to achieve timekeeping within 1 microsecond and a positioning accuracy of 20 meters, all while deep indoors where GNSS is unavailable. This adds indispensable resilience and security to GNSS that we are increasingly reliant upon, creating a comprehensive satellite navigation system that truly works everywhere.
This PNT service using Iridium is perhaps a sign of things to come. We’ve seen a progression in LEO use since the dawn of the Space Age, namely, an order of magnitude increase in constellation size every 30 years. Transit first offered an occasional position update based on a constellation of six satellites in the 1960s.
Built in the 1990s, Iridium, with an order of magnitude more satellites at 66, now offers global coverage. On the horizon are constellations like OneWeb, which promise the next order of magnitude with 648+ satellites, slated for the 2020s. This most recent scale gives rise to better satellite geometry than GPS today with the added benefits of LEO.
The STL signal using Iridium sets a precedent that could lead to unparalleled navigation services that are robust due to the improved signal strength and precise due to the huge number of LEO satellites coming, each moving quickly and giving the geometric diversity needed to enable fast carrier-phase differential GNSS.
The need for such a service is already clear. It would enable a diversity of future technologies and applications, such as safety-critical autonomous vehicles under development that must operate in challenging urban environments.
Acknowledgments
This article is based on a book chapter to be released in a new generation of GPS “Blue Books” entitled 21st Century Navigation Technologies: Integrated GNSS, Sensor Systems, and Applications to be published by Wiley-IEEE.
The article was also based on the following Institute of Navigation conference publications by the authors:
“Differential and Rubidium Disciplined Test Results from an Iridium-Based Secure Timing Solution” by S. Cobb, D. Lawrence, G. Gutt and M. O’Connor in Proceedings of the 2017 International Technical Meeting of The Institute of Navigation, Monterey, California, 2017.
“Test Results from a LEO-Satellite-Based Assured Time and Location Solution” by D. Lawrence, H.S. Cobb, G. Gutt, F. Tremblay, P. Laplante and M. O’Connor in Proceedings of the 2016 International Technical Meeting of The Institute of Navigation, Monterey, California, 2016.
“Orbital Diversity for Satellite Navigation” by D. Lawrence, H.S. Cobb, G. Gutt, F. Tremblay, P. Laplante and M. O’Connor in Proceedings of ION GNSS 2012, the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation, Nashville, Tennessee, 2012.
“Leveraging Broadband LEO Constellations for Navigation” by T.G.R. Reid, A.M. Neish, T.F. Walter and P.K. Enge in Proceedings of ION GNSS+ 2016, the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, 2016.
Manufacturers
The unassisted Bluetooth receiver used was a Dual Electronics XGPS150A Universal Bluetooth GPS Receiver; the assisted-GPS smartphone used was a Samsung Galaxy S4. Timing output was evaluated with a Trimble Thunderbolt GNSS timing receiver.
DAVID LAWRENCE is the principal navigation architect for Satelles. In addition to authoring over 20 papers and over 30 patents, Lawrence has developed high-performance navigation software that has been deployed in aircraft landing, precision agriculture, mining, transportation, and machine automation.
H. STEWART COBB is the principal hardware architect for Satelles. Dr. Cobb has made a diverse range of contributions to the PNT community, including inventing and delivering the first commercial implementation of pseudolites as a principal hardware engineer at Novariant.
GREG GUTT is the president and chief technology officer of Satelles. As a graduate student, Gutt Developed ultra-low-noise superconducting sensors for NASA’s Gravity Probe B program. He later went on to become a Boeing technical fellow and is the original principal inventor of the Satelles time and location technology.
MICHAEL O’CONNOR is the chief executive officer of Satelles. As a graduate student, O’Connor developed the world’s first GPS-based precision steering system for farm vehicles. He went on to bring this technology to market with Novariant and helped launch the precision agriculture industry.
TYLER G.R. REID just completed his Ph.D. in the GPS Research Laboratory in the Department of Aeronautics and Astronautics at Stanford University. He is an alumnus of the International Space University and will soon be starting as a research scientist at Ford Motor Company on their autonomous driving team.
TODD WALTER is a senior research engineer in the Department of Aeronautics and Astronautics at Stanford University where he received his Ph.D. in applied physics. His research focuses on implementing high-integrity air navigation systems.
DAVID WHELAN was the vice president and chief technologist for Boeing Defense, Space & Security. Whelan earned his Ph.D. and MS in physics from the University of California Los Angeles and his B.A. from the University of California San Diego.
FURTHER READING
Authors’ Conference Publications
“Differential and Rubidium Disciplined Test Results from an Iridium-Based Secure Timing Solution” by S. Cobb, D. Lawrence, G. Gutt and M. O’Connor in Proceedings of the 2017 International Technical Meeting of The Institute of Navigation, Monterey, California, Jan. 30 – Feb. 1, 2017, pp. 1111–1116.
“Leveraging Commercial Broadband LEO Constellations for Navigation” by T.G.R. Reid, A.M. Neish, T.F. Walter and P.K. Enge in Proceedings of ION GNSS+ 2016, the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, Sept. 12–16, 2016, pp. 2300–2314 (best presentation award).
“Test Results from a LEO-Satellite-Based Assured Time and Location Solution” by D. Lawrence, H.S. Cobb, G. Gutt, F. Tremblay, P. Laplante and M. O’Connor in Proceedings of the 2016 International Technical Meeting of The Institute of Navigation, Monterey, California, Jan. 25–28, 2016, pp. 125–129.
“Orbital Diversity for Satellite Navigation” by P. Enge, B. Ferrell, J. Bennet, D. Whelan, G. Gutt and D. Lawrence in Proceedings of ION GNSS 2012, the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation, Nashville, Tennessee, 17–21 Sept., 2012, pp. 3834–3846 (best presentation award).
Global Navigation from Low Earth Orbiting Satellites
“Analysis of Iridium-Augmented GPS for Floating Carrier Phase Positioning” by M. Joerger, L. Gratton, B. Pervan and C. E. Cohen in Navigation, Vol. 57, No. 2, Summer 2010, pp. 137–160, doi: 10.1002/j.2161-4296.2010.tb01773.x.
“Overview of IRIDIUM Satellite Network” by K. Maine, C. Devieux and P. Swan in Proceedings of IEEE WESCON’95, the Microelectronics Communications Technology Producing Quality Products Mobile and Portable Power Emerging Technologies Conference (formerly Western Electronics Show and Convention), San Francisco, California, Nov. 7–9, 1995, pp. 483–490, doi: 10.1109/WESCON.1995.485428.
Transit, the U.S. Navy Navigation Satellite System
The Legacy of Transit, a special edition of the Johns Hopkins APL Technical Digest edited by V.L. Pisacane, Vol. 19, No. 1, Jan.–March 1998.
“A History of Satellite Navigation” by B.W. Parkinson, T. Stansell, R. Beard and K. Gromov in Navigation, Vol. 42, No. 1, Spring 1995, pp. 109–164, 10.1002/j.2161-4296.1995.tb02333.x.
“The Navy Navigation Satellite System: Description and Status” by T.A. Stansell, Jr. in Navigation, Vol. 15, No. 3, Fall 1968, pp. 229–243, 10.1002/j.2161-4296.1968.tb01612.x.
GPS and other Global Navigation Satellite Systems
Springer Handbook of Global Navigation Satellite Systems, edited by P.J.G. Teunissen and O. Montenbruck, published by Springer International Publishing AG, Cham, Switzerland, 2017.
Editor’s Note: This online preview presents brief highlights from the upcoming July cover story in GPS World, “Navigation from LEO: Current Capability and Future Promise.” The article is by David Lawrence, H. Stewart Cobb, Greg Gutt, and Michael O’Connor of Satelles, Tyler G. R. Reid and Todd F. Walter of Stanford University, and David Whelan.
Robust position, navigation, and timing services from low Earth orbit (LEO) are here today, providing augmentation to GPS where GPS isn’t available. The addition of navigation signals from LEO provides a number of benefits. The proximity of LEO satellites has the potential to provide much stronger signals than the distant GNSS core-constellations like GPS in medium-Earth orbit (MEO).
Today, the only LEO system with global coverage is the Iridium constellation used primarily for communications.
Figure 1 shows the 31-satellite GPS constellation in contrast with the 66-satellite Iridium network. The scale of the difference in distance (several Earth radii) is extraordinary. The result is that Iridium signals are 300 to 2400 times stronger than GNSS signals on the ground, making them attractive for use in position, navigation, and timing (PNT) applications where GNSS signals are obstructed.
Figure 1. The 66 satellite Iridium constellation in low Earth orbit and 31 satellite GPS constellation in medium Earth orbit.
LEO-based PNT is now mainstream, in the form of real-time signals that have been delivered over the Iridium satellite network since May 2016. This service is made possible by Satelles in partnership with Iridium Communications Inc. in a service called the Satellite Time and Location (STL), a non-GNSS solution for assured time and location that is highly resilient and physically secure. Consumers, businesses, and governments are already using these LEO-based signals in environments with high GNSS interference or occlusion.
The security features of these signals are also used to reliably validate GNSS position, navigation and time (PNT) solutions in real time to help mitigate potential spoofing. Furthermore, the fast LEO orbits of Iridium generate Doppler-frequency signatures significantly stronger than GPS, increasing the utility of the STL signal for positioning applications.
STL field tests demonstrate a positioning accuracy of 20 meters and timekeeping to within 1 microsecond, all in deep attenuation environments indoors. This adds substantial robustness in augmenting the GNSS core-constellations like GPS and also allows for a standalone backup in many applications.
Along with its strong signals compared to the GNSS core-constellations in MEO, Iridium’s global coverage makes it ideal for use in PNT applications where GNSS is obstructed. Figure 1 shows the scale of the difference in altitude with Iridium at 780 kilometers and GPS at 20,200 kilometers. This has substantial implications not only for signal strength but also for coverage.
Though Iridium has twice as many satellites as GPS, at the equator users can often only see one satellite whereas they can see ten from GPS. This was one of the fundamental trades considered in the design of the GPS constellation. The higher the altitude, the more each launch cost; the lower, the more satellites had to be built to provide coverage. To put this in perspective, global coverage for one satellite in view at all times requires fewer than ten satellites in MEO but requires closer to one hundred in LEO.
Future LEO Constellations
The hundreds of LEO satellites needed to match the coverage of GPS may be coming. In late 2014 and early 2015, the International Telecommunication Union (ITU) reported a half dozen filings for spectrum allocation for large constellations of LEO satellites.
In January 2015, OneWeb announced a partnership with Virgin and Qualcomm to produce a constellation of 648 LEO satellites to deliver broadband Internet globally. This represents the next order of magnitude, with tenfold more satellites than Iridium. Within days of this announcement, SpaceX, with support from Google, announced a similar ambition for a constellation of more than 4,000 LEO satellites.
In August 2015, Samsung expressed interest in a proposal for a LEO constellation of 4,600. Boeing joined the race in June 2016 announcing plans for a LEO constellation of nearly 3,000 satellites. These LEO constellations are being proposed to keep up with the rising demand for broadband, not to replace ground infrastructure.
LEO versus MEO
Low- and medium-Earth orbit each have their individual strengths and weaknesses in the context of navigation. Closer to Earth, LEO offers less spreading loss and improved signal strength on the ground. On the other hand, being closer to Earth means that satellites have much smaller footprints. The GPS footprint is threefold larger than Iridium, corresponding to nine times more area covered. Hence, to achieve the same coverage as GPS with Iridium’s altitude, the LEO constellation requires an order of magnitude more satellites.
Another major difference between LEO and MEO is speed. A GPS satellite completes one Earth revolution every 12 hours while an Iridium one does so in only 100 minutes. The shorter the orbital period, the faster the angular rate (also called mean motion) and the more quickly satellites pass overhead.
The swift motion whitens multipath (making it more random–like white noise) as reflections are no longer effectively static over short averaging times. Geometric diversity also leads to effective Doppler positioning and is also desirable for carrier-phase differential GNSS, allowing for much more rapid resolution of integer cycle ambiguities.
Iridium-Satelles Satellite Time and Location (STL)
The STL service has been in operation since May 2016. Many from industry and government are already using this service to achieve a more robust PNT solution. This service will only continue to improve with the Iridium NEXT satellites under deployment; the first ten satellites of this generation were successfully launched in January 2017.
STL is a non-GNSS solution for assured time and location that is highly resilient and physically secure. STL utilizes the Iridium constellation to transmit specially structured time and location broadcasts. Due to their high RF power and signal-coding gain, the STL broadcasts are able to penetrate into difficult attenuation environments, including deep indoors.
Like GNSS signals, these broadcasts are specifically designed to allow an STL receiver to obtain precise time and frequency measurements to derive its PNT solutions. STL is able to augment or serve as a backup to existing GNSS PNT solutions by providing secure measurements in the presence of high attenuation (deep indoors), active jamming, and/or malicious spoofing.
Unlike the MEO GNSS satellites, Iridium uses 48 spot beams to focus its transmissions on a relatively small geographic area. The complex overlapping spot beams of Iridium combined with randomized broadcasts give a unique mechanism to provide location-based authentication that is extremely difficult to spoof.
The July cover story in GPS World magazine will explore all the above topics in more technical detail, and go further into the areas of signal strength in challenging environments, indoor time-transfer capability, and a section on looking forward.
The PNT service using Iridium is perhaps a sign of things to come. On the horizon are constellations like OneWeb which promise the next order of magnitude with 648+ satellites, slated for the 2020s. This most recent scale gives rise to better satellite geometry than GPS today with the added benefits of LEO.
The STL signal using Iridium sets a precedent that could lead to unparalleled navigation services that are robust due to the improved signal strength and precise due to the huge number of LEO satellites coming, each moving quickly and giving the geometric diversity needed to enable fast carrier-phase differential GNSS.
The need for such a service is already present. This would be enabling for the safety-critical autonomous vehicles under development that must operate in challenging urban environments and to a diversity of other future technologies and applications as well.
GNSS and PNT markets continue changing, sometimes very rapidly. No news there. Technology advances relentlessly, opening up new application areas and new price points as it goes.
The market for inertial navigation systems (INS), a subset of that PNT universe, is no exception. The number of available options in inertial has grown substantially. Micro-electrical-mechanical systems (MEMS) sensors lead the charge. Smaller, lighter, lower power and less expensive than previous inertial measurement units, they are truly changing the game and exploding past their previously limited deployment.
So much so that I now need to find a MEMS expert to join our editorial board, advise me on article selection, and attend an ever-widening spectrum of PNT-relevant conferences on behalf of this magazine.
That’s not the only smaller, faster, lighter, cheaper advance warping the speed of change in PNT. Our reporter Robin Wrinn got a look at the 33rd Annual Space Symposium at the ways 3D printing is changing how GPS satellites are put together. I had to rub my eyes when I read her account. Yes, GPS satellites. The column in The System of Systems (page 10) touches only lightly upon this phenomenon. I had to edit all the rest out as we are so short of space in this issue. But go online, where space no longer constrains us, for a fuller account and startling photos.
If this trend goes on much longer, I’ll need a 3D printing expert on the editorial board as well. Indeed, we gave some consideration a few years back to bringing 3D printing “under the umbrella,” so to speak, inside the magically expanding tent — like something out of Harry Potter — that encloses all the technologies we must cover, just to keep up with you folks.
I sense something else lurking about, awaiting an entrance. And for this I’ll really need an expert adviser. I don’t even know what to call it. Somehow it combines virtual reality and gamification. Yes, really. Games are about to begin playing a role in PNT. First in mapping, through the visualization of data; this is explored in our May Defense PNT & Geointelligence Insidernewsletter column.
Gamification is “the application of game-design elements and game principles in non-game contexts.” To what purpose? To improve productivity, of course. Though we may call it accuracy, or availability, or robustness in our realm. It begins with crowdsourcing, probably. Though I feel the ice getting thinner, the limb weaker beneath me as I climb out upon it. Think I’ll stop now.
Most activity so far in the PNT community has centered around the questions of “Where am I?” and “Where am I going?” and “How fast am I going?” Positioning, navigation and timing. Seemingly that should about cover it. But no.
Mapping comes into the picture: “What fixed objects are in my environment?” This is actually a corollary of “Where am I?” though let’s not put too fine a point on it.
All this “I” business. To get to driverless cars and other autonomous vehicles, we will have to look beyond the first person singular, what some researchers call “the ego vehicle.” We must know, with a high degree of precision and certainty, “Where are other moving vehicles?” and “Where are they going?” and “How fast are they moving?” Another order of magnitude, if not several. PNT squared, as it were.
In the fast oncoming intelligent transportation systems (ITS), future driving (very much present and evolving now) will rely on accurate, reliable and continuous knowledge of the position of other so-called road participants. That’s not just cars, trucks, motorcycles and buses, but includes pedestrians and bicycles and who knows what else — skateboards?
The first approaches to this requirement use on-board ranging sensors such as camera/vision systems, radar, laser scanners and more. (Some of this “more” is explored in this May’s print cover story, “Look Around.”) This already calls for a significant level of integration with GNSS and inertial systems of the ego.
But it’s still not enough. A cooperative approach must develop, in which the other road participants actively support the continuous estimation of all relative positions. Not only must they have all the sensors the ego possesses, they must continually communicate all that data with the ego, and conversely. This is what’s called “connectivity.”
It’s almost as if vehicles are becoming sentient, expressive beings. A bit like us. Bringing new meaning to the expression “the automobile as an extension of the self.”
Do anything interesting today? Specifically, did you do something interesting involving positioning, navigation or timing (PNT)?
GPS World is always on the look-out for case studies — stories of how you, our readers, used PNT or GNSS equipment, or applied related technologies, to solve a problem. Each month in our Market Watch and Updates sections, I try to include a few case studies. We always provide news about new products or company and industry announcements, but it’s the case studies that often “bring it home” to our readers.
We’ve taken a look at thermal mapping at the South Pole and a one-man survey project on a remote tropical island, using both a UAV (unmanned aerial vehicle) and a receiver on a pole. We also share how lifeguards can use UAVs to save people who are drowning. Previously, we discussed how avalanches were being mapped and how a state transportation department was making the move to tablets for 3D mapping. We showed how UAVs could speed cell-tower recovery after floods.
So, tell us what you’re up to. We want to hear about it. With pictures. Email me at [email protected].
Micro-Technology for Positioning, Navigation, and Timing towards PNT everywhere and always; slide from a 2014 DARPA presentation to the Space-Based Positioning, Navigation and Timing National Advisory Board (Image: Robert Lutwak, DARPA Micro-Technology Office). Click to enlarge.
The U.S. Defense Advanced Research Projects Agency (DARPA) has initiatives underway with a dizzying number of technologies, all seeking to reduce reliance on GNSS in challenged environments. Using cold atom interferometry and other techniques to reduce the size, weight and power consumption (SWAP) as well as cost of inertial sensors, employing other signals of opportunity (SOI), chip-scale atomic clocks (CSAC), micro-electro-mechanical systems (MEMS) and more, the Micro-Technology Office (MTO) and the Adaptable Navigation Systems (ANS) projects press relentlessly forward to provide U.S. forces with PNT “everywhere and always.”
DARPA’s ANS initiative explores tools to enable use of the many sensors available to warfighters and first reponders. “Over the past two decades, the field of robotics has done a lot for extracting features out of imagery and tracking those features as the robot moves through a given environment,” said Lin Haas says, program manger at the Strategic Technology Office. “We’ve been building upon those capabilities and using the capabilities for the purposes of navigation.”
ANS seeks to provide GPS-quality PNT to military users regardless of the operational environment. It addresses three basic challenges through its Precision Inertial Navigation Systems (PINS) and All Source Positioning and Navigation (ASPN) efforts:
better inertial measurement units (IMUs) that require fewer external position fixes;
alternate sources to GPS for those external position fixes;
new algorithms and architectures for rapidly reconfiguring a navigation system with new and non-traditional sensors for a particular mission.
PINS is developing an inertial measurement unit (IMU) that uses cold atom interferometry for high-precision navigation without dependence on external fixes for long periods of time. Atom interferometry involves measuring the relative acceleration and rotation of a cloud of atoms within a sensor case, with potentially far greater accuracy than today’s state-of-the-art IMUs.
A company called AOSense has applied cold-atom interferometry to IMUs and demonstrated sensors that support system drifts of 5 meters per hour, by using quantum physical properties to measure the relative acceleration and rotation of a cloud of laser-cooled atoms. The next challenge is shrinking the lasers to microsystem size, because the concept requires three lasers generating five beams to cool and move the atoms through interferometers to determine movement and rotation of the device.
Because even long-duration IMUs require an eventual position fix, the ASPN effort is developing sensors that use signals of opportunity — non-navigation signals from sources like television, radio and cell towers, and satellites, as well as natural phenomena, such as lightning.
“Our navigation systems tend to be finely tuned, and as a result they are fairly brittle in terms of accepting new sensors without a lot of hands-on time to make it work,” said Haas.
Flexible Combinations. Integrating and tuning these diverse sensors, maps and other components into a navigation system is expensive and slow, producing platform and mission-specific solutions. The ASPN effort is also developing new fusion algorithms and plug-and-play processing architectures for rapid integration and near-real-time reconfiguration or upgrading of sensors, IMU devices, maps and databases on a navigation system. With flexible combinations of existing and new navigation sensors, ASPN can produce improvements in accuracy, robustness and cost of navigation systems across a range of platforms, environments and missions.
PINS is working towards a final subsystem demonstration in fiscal year 2017. ASPN has completed multiple field demonstrations on air, land and sea platforms, with final demonstrations scheduled in fiscal 2017.
Chip-Scale Atomic Clocks. Meanwhile, last year DARPA launched the Atomic Clocks with Enhanced Stability project under the direction of Robert Lutwak (recipient of GPS World’s Leadership Award for Products in 2012). “If ACES is successful, virtually every Defense Department system will benefit,” Lutwak said.
ACES seeks to create palm-sized, battery-powered atomic clocks that perform up to 1,000 times better than the current generation, employing experts and techniques from atomic physics, optics, photonics, microfabrication and vacuum technology. “All of our modern communications, navigation and electronic warfare systems as well as our intelligence, surveillance and reconnaissance systems depend on accurate time-keeping,” Lutwak added.
Pseudolites. In other, non-DARPA initiatives around the Department of Defense, the Command and Control Directorate of the Army’ Communications-Electronics Research, Development and Engineering Center (CERDEC) is “very concerned about what happens when we lose GPS,” according to Paul Olson. CERDEC is developing vehicle-based, dismounted and anti-jam antenna pseudolite systems.
The pseudolites have completed feasibility testing and entered acquisition for transmitters, receivers and command-and-control. Rockwell International and L-3 are developing the transmitters. The effort seeks to use current military GPS receivers with software modified to accept pseudolite signals.
This article draws on interview quotes that appeared in Signal magazine of the Armed Forces Communications and Electronics Association.
According to ION, JNC is the largest U.S. military positioning, navigation and timing (PNT) conference of the year with joint service and government participation. The event will focus on technical advances in PNT, emphasizing joint development, test and support of affordable PNT systems, logistics and integration.
From an operational perspective, the conference will focus on advances in battlefield applications of GPS; critical strengths and weaknesses of field navigation devices; warfighter PNT requirements and solutions; and navigation warfare.
The event, which will feature a technical exhibit and showcase of guidance, navigation and control technology products, will include more than 200 operational presentations, ION reports.
The ION Joint Navigation Conference will take place at the Dayton Convention Center, as well as a classified environment on June 8 at the Air Force Institute of Technology.
I have mixed emotions as I write this column. Delighted, absolutely, to be given the opportunity to write for GPS World on topics that I am so passionate about; but also sad that we will not see any more articles from Don Jewell, whose excellent columns I followed so religiously over the years. I never had the opportunity to meet Don personally but, to me, he is irreplaceable. But let’s talk about the changing face of defense positioning, navigation and timing (PNT) — not in the editorial sense, but in the technology sense.
As we all know, PNT and GPS are no longer synonymous. With a host of innovative technologies on the horizon, PNT is about so much more than GPS these days, and the military knows it. Sure, GPS has been the workhorse of PNT for many years, and it’s not going anywhere anytime soon. I’ll be clear on that: GPS is not going anywhere. But it’s not a complete solution either.
Let me paraphrase what a friend in the infantry tells me, by saying GPS is a 60 percent solution to their navigation needs. What does that mean? Well, it goes something like this:
60 percent of the time: GPS is great, it does what we need.
20 percent of the time: We are indoors or underground, and GPS is simply not available.
15 percent of the time: We’re in an urban canyon. GPS availability is intermittent, and the accuracy is poor.
4 percent of the time: We’re in forests or dense vegetation, and GPS is sporadic.
1 percent of the time: GPS is jammed.
You can argue the numbers depending on the mission, but you get the idea. What, then, is the answer for the soldier? Well, first things first: We don’t want to reinvent the good 60 percent so, once again, GPS is here to stay. The question is how do we push past that 60 percent figure and get ourselves closer to 100 percent? Let’s go from the bottom up, and address GPS jamming.
Overcoming interference
The classic solution to jamming is an adaptive antenna, also known as a controlled radiation pattern antenna (CRPA). More on this another time but, for now, suffice it to say that CRPAs are a well-understood and mature technology, and can offer very high levels of jamming resistance.
The often-cited disadvantage of a CRPA antenna is its size, weight and power: As CRPAs employ multiple antenna elements, they are inherently larger and heavier. The electronics can pretty much be covered by a single chip these days, leaving the antennas themselves as the problematic aspect, but advances in antenna technology have also made big hurdles.
For airborne platforms, conformal antennas designed as part of the structure or fuselage can be used; whilst for the dismounted soldier, the trend is towards wearables, where the antennas may be an inherent part of the clothing or helmet design.
Aside from adaptive antennas there are a whole host of other techniques in your anti-jam kit bag, including receiver-based techniques.
It’s a numbers game
For forests and urban canyons, this is where multi-frequency multi-GNSS comes into its own. It really is a numbers game: The more constellations you use, the more satellites you can choose from, and the greater your chances of seeing enough satellites to derive a reasonable navigation solution. You also have more options for mitigating the effects of multipath and other errors.
Of course, this gives rise to a potentially difficult question for some governments: In defense applications, do you want to rely on foreign GNSS constellations as part of your PNT solution? The attitude here depends on your own country’s policy and a trade-off of perceived gains against perceived threats. The UK, for example, has chosen to embrace all available constellations and frequencies in future military navigation systems.
That’s probably about as far as GNSS gets you, because now we’re looking at the 20 percent of the time where the user is indoors or underground. In other words, environments where GNSS simply isn’t available. This 20 percent is perhaps more tricky to address, and is the realm of alternative and complementary PNT technologies.
Beyond GNSS
Fusing different sensor modalities to create a combined navigation solution is anything but a new idea. The benefits of combining GPS with an inertial sensor were recognized a long time ago, and this classic pairing continues to be the subject of research today.
The two technologies are highly complementary in various ways: GNSS offers absolute position, low short-term accuracy, and high long-term accuracy. On the other hand, an inertial sensor offers the opposite: relative position, high short-term accuracy, and low long-term accuracy. It’s a match made in heaven.
But whilst GNSS plus inertial may be a good choice for, say, airborne platforms, it doesn’t solve the in-building and underground problem. Without GNSS, you need something else.
Indoor navigation has been one of the hottest research topics of recent times, but there are really two types of indoor scenario: the first is when you’re in a shopping mall or airport. You can use an inertial sensor, Wi-Fi, mobile base stations, and various other bits of infrastructure to help you navigate.
The second scenario is the military one: You’re in an unfamiliar enemy compound or underground tunnel complex. In this case, there is no GNSS, no Wi-Fi, no mobile communications; and, for navigation, you can only really rely on the sensors you bring with you.
So what other sensor works underground, and complements inertial?
Visual/inertial integration
Visual odometry is an established, yet often overlooked, navigation technology that is undergoing a resurgence of interest, in both military and civilian applications. In simple terms, visual odometry uses sequential camera images to determine motion in a six degrees of freedom reference frame. Using either single or multiple cameras a platform can estimate both its 3D position and orientation, providing much the same information as an inertial sensor — but with a few added benefits.
Visual/inertial sensing allows 3D reconstruction of a road incident. (Screenshot: Roke)
Because cameras and associated vision-processing algorithms are capable of detecting corners and features, a 3D model of the environment in which the soldier is operating can also be built up. In other words, we can perform simultaneous localization and mapping (SLAM).
But like any navigation technology, visual odometry has its limitations. It likes well-defined features in the environment, such as corners, but can get confused by moving objects like trees and clouds. Its performance also depends on factors such as the quality of the camera and lens, and how well the system is calibrated. Like an inertial sensor, it provides a relative positioning solution and is subject to accumulation of errors over time. It’s a great technique, but it really comes into its own when combined with another navigation sensor, such as an inertial unit.
And it’s not just the military guys who are taking advantage of visual/inertial integration. Just take a look at Google’s Tango project, or what Qualcomm is doing, or Roke’s black box for driverless cars, to name but a few examples.
Bringing it all together
Over the course of the last decade or two, the operational landscape for soldiers has changed significantly, with far greater focus on urban warfare. The military realized some years ago that the answer to robust navigation for dismounted soldiers was going to require a range of sensor modalities: no single navigation technology is ideal in all environments. That’s why this has been the focus of so many defense programs of recent years.
By way of example, the UK Ministry of Defence (MoD) initiated a research program in 2013 called Dismounted Close Combat Sensors (DCCS). The contract addressed a range of soldier capabilities, one of which was the ability to provide reliable soldier position and orientation in all environments.
The DCCS programme evaluated a whole bunch of technologies, but eventually converged to an integration of three primary sensors: multi-constellation GNSS, a low-cost inertial measurement unit (IMU) and a video camera. The single monocular video camera was used to strap down the IMU, in a very tightly-coupled system. It makes sense: when GNSS is available, use it. When GNSS isn’t available, the integrated visual/inertial navigation sensor continues to provide both location and orientation for the duration of the mission. As it should be for a tightly integrated navigation system, the performance of the combined system outperforms any individual sensor in isolation.
Whilst integrated sensor systems enable our soldiers to position, orientate and navigate themselves, the performance of individual sensors continues to be pushed to new limits. Inertial technology is advancing all the time, and defense is again pushing the boundaries. Take a look at what DARPA is up to, as an example.
The missing ‘T’
Haven’t we missed something? Ah yes, there’s a “T” in PNT. So whilst there would seem to be various options for achieving a robust positioning and navigation solution, we mustn’t forget precise timing for those applications that need it. Quantum technology is flavor of the month here and, once more, the defense agencies are furthering developments: DARPA with its ACES program, and MOD/DSTL via the Quantum Technology Program, to illustrate just a couple of examples.
So whilst GPS will continue to remain the workhorse, defense PNT is migrating from GPS-only to being a many-faced beast. And I haven’t even gotten started on pseudolites, signals of opportunity, eLoran, and cooperative navigation.
The future of defense PNT looks pretty good to me.
Surrounding sounds may not be a common way of determining location. But on the battlefield, warfighters need to know the direction of gunshots to enable a proper response.
