A novel method using signals of opportunity from low-Earth orbit (LEO) satellites is redefining what’s possible in satellite-based navigation. Researchers have developed a joint pseudo-range and Doppler positioning technique that taps into signals from constellations like Starlink and Iridium NEXT — without relying on traditional navigation signal structures.
By employing low-cost, wide-beam antennas and a specially designed time–frequency inversion algorithm, the team achieved remarkable accuracy: 3.6 meters in 2D and 6.2 meters in 3D, surpassing Starlink positioning approaches based on parabolic antennas by 35%.
Technical barriers in using signals of opportunity include signal transmission times, low signal power, and imprecise orbital data, all of which hinder accurate positioning. Addressing these challenges demands a new approach to extracting usable navigation data from LEO constellations.
In response, researchers from the Aerospace Information Research Institute introduced a joint pseudo-range and Doppler positioning method using wide-beam antennas to receive LEO satellite SOPs. The approach centers on a signal time–frequency inversion algorithm that reconstructs key signal parameters, alongside a novel accuracy metric called Equivalent Position Dilution of Precision (EPDOP).
Real-world experiments combining Starlink Doppler data and Iridium NEXT pseudo-range signals confirmed strong performance, especially in long-baseline conditions — reinforcing the method’s global applicability.
To overcome the cost and complexity of existing satellite tracking equipment, the team employed low-noise bock (LNB) wide-beam antennas capable of simultaneously receiving signals from multiple Starlink satellites. The core innovation lies in a signal processing algorithm that estimates transmission time and frequency from the received code phase and Doppler shifts — enabling both pseudo-range and Doppler observations without needing exact satellite clock data or real-time ephemeris.
To quantify system performance under real-world errors, the researchers developed the EPDOP metric, adapted to mixed measurement inputs. Tests demonstrated the method’s robustness: 3.6 m 2D and 6.2 m 3D positioning using Starlink Doppler signals, and up to 24 m (2D) and 41 m (3D) accuracy using Iridium NEXT SOPs over a 40 km baseline. Compared to Doppler positioning techniques, the algorithm reduced positioning errors by over one-third and successfully suppressed the impact of orbital inaccuracies inherent in public two-line element set (TLE) datasets.
“This work marks a key step toward accessible, accurate navigation using commercial satellite constellations,” said lead author Ying Xu. “By integrating Doppler and pseudo-range measurements and introducing a flexible precision metric, we can now harness Starlink and Iridium NEXT signals for high-precision positioning, even without access to proprietary signal structures. The proposed low-cost architecture opens new possibilities for resilient navigation in GPS-denied environments.”
Because of its ability to operate with low-cost antennas and weak, unstructured signals, the technique is poised to support a wide range of applications: from autonomous driving and unmanned aerial vehicle (UAV) navigation in remote regions to emergency response and IoT asset tracking. Its resilience to satellite orbital prediction errors and adaptability across different LEO constellations make it a strong contender for next-generation positioning systems. As LEO deployments continue to expand globally, this approach offers a scalable and practical solution for real-time, high-accuracy navigation—promising enhanced capabilities for both civilian infrastructure and defense operations.
The White House is drafting an executive order that would simplify the approval for private launches by clarifying space rules and regulations for private American space companies. This order, which President Biden could sign into law early next year, aims to simplify licensing procedures in accordance with existing federal space laws to routinize space activities, including launching rockets and satellites deployed by private companies. Private space companies such as Elon Musk’s SpaceX and Jeff Bezos’ Blue Origin, are preparing to invest in several projects over the next decade, such as Blue Origin’s Orbital Reef project, which would build a commercial space station in low-Earth orbit (LEO).
Additionally, an online tool is being designed for the Department of Commerce to guide private companies through the licensing requirements from the relevant federal agencies. The order may also include granting certain federal agencies oversight of space activities that are not currently monitored, such as asteroid mining and space junk removal.
The private spaceflight company, SpaceX, has undertaken a partner project in addition to its existing space efforts ranging from sending satellites and people to space, to providing a brand of commercial internet connection to remote areas.
The new partner project, Starshield, will join Starlink in providing secure, broadband internet connection to customers. However, there is a stark difference between the partner projects.
Starlink technology has end-to-end user data encryption to secure its network as it was designed for commercial customers. However, the Starshield project is intended for government use in national security efforts. Therefore, it is equipped with an additional high-assurance cryptographic feature to host payloads and process data in a secure way aligning with government national security requirements.
As reported by SpaceX, Starshield has three focus areas including: Earth observation, global communications, and hosted payloads. Starshield’s satellites can integrate with a variety of different payloads and the constellation has a low-Earth orbit (LEO) design making it robust to on-orbit assets.
SpaceX continues to work closely with the United States Department of Defense by providing innovative space technology.
The Ministerial Council of the member states of the European Space Agency (ESA) has pledged 351 million euros to ESA’s board of directors for navigation to aid in multiple satellite navigation endeavors.
This funding helps support ESA’s FutureNAV program, the Navigation Innovation and Support Program (NAVISP), and the Moonlight Initiative, developed for lunar telecommunications and navigation coverage and innovation.
The FutureNAV program is aimed at addressing the rapidly growing need for more ubiquitous, resilient and reliable positioning, navigation and timing (PNT). Its first mission consists of an initial in-orbit demonstration, small constellation of low-Earth-orbit (LEO) navigation satellites. The LEO-PNT satellites will test a multi-layer approach to deliver more accurate and robust PNT services, supplementing Galileo.
The second FutureNAV mission, GENESIS, will map the moving contours of Earth, while enhancing the accuracy of Galileo.
The expanded funding will also be invested in NAVISP. The program has already began working on over 200 projects relating to satellite navigation, PNT research, and backing Member States in research priorities.
Lastly, funding is being delegated to the Moonlight Initiative, a project designed by ESA to extend satellite navigation and telecommunications coverage to the Moon. The advancement and launch of Moonlight infrastructure on the Moon will undertake several missions bolstering future Moon exploration.
SpirentCommunications has revealed its latest low-Earth-orbit (LEO) satellite solution software named SimORBIT, developed in partnership with space-borne receiver developer SpacePNT. The software is designed to aid developers in determining LEO orbits accurately for GNSS/PNT lab testing.
SimORBIT calculates LEO orbits as well as their environments and intricate characteristics to provide an accurate result to developers for testing. The software replicates LEO orbits so that simulations can provide the realistic environment of a LEO satellite, including gravitational and atmospheric impacts the satellite could encounter in space.
SimORBIT was created in partnership with spaceborne receiver developer SpacePNT. “Until now, PNT testing on LEO applications has been limited due to the lack of an integrated solution that could offer realistic LEO orbital data together with GNSS simulation capabilities,” explained Adam Price, Spirent’s vice president of PNT Simulation. “By working in close collaboration with SpacePNT, we have been able to develop the SimORBIT tool to bring a new level of accuracy and realism to LEO application testing by combining the simulation of precise LEO orbits and highly accurate GNSS signals.”
With Spirent’s release of SimORBIT, developers can create non-ICD signals via I/Q injection, or by the Spirent “Flex” feature, generating space-centered PNT signals to be developed in the lab as realistically as possible.
New PNT satellites will operate in low Earth orbit (LEO). (Image: ESA)
News from the European Space Agency (ESA)
Satellite navigation is headed closer to users. ESA’s Navigation Directorate is planning an in-orbit demonstration with new navigation satellites that will orbit just a few hundred kilometers in space, supplementing Europe’s 23,222-km-distant Galileo satellites.
Operating added-value signals, these novel low-Earth-orbit (LEO) positioning, navigation and timing (PNT) satellites will investigate a new multi-layer satnav system-of-systems approach to deliver seamless PNT services that are much more accurate, robust and available everywhere.
Global in coverage, free for everyone to use, GNSS such as Europe’s Galileo have already transformed our society, and due to their sheer omnipresence their influence continues to grow. In 2021, the population of satnav receivers reached 6.5 billion around the world, and the sector is projected to maintain a 10% annual growth rate in the years ahead. But in various respects the standard GNSS approach is nearing the limits of optimum performance — to get even better, added ingredients are becoming essential.
“Satellite navigation has enabled a vast range of applications in recent years, but this very success is inspiring still more demanding user needs for the coming decade,” said Lionel Ries, head of ESA’s GNSS Evolutions R&D team, overseeing the agency’s LEO-PNT studies.
“For use cases such as autonomous vehicles, ships or drones, robotics, smart cities or the industrial internet of things for control of factory systems, the positioning requirements are growing from the current meter-scale to centimeter scale or even more precise, based on continuously reliable signals that are available anywhere, anytime — even indoors —while able to overcome interference or jamming.
“Up until now we have relied for positioning on the classical solution of GNSS such as Galileo, located in medium Earth orbit and based on L-band signals. Standard GNSS alone is not going to be able to fulfil all these future user demands. Instead Europe needs to seize the opportunity to investigate the potential of the kind of LEO constellations that are already on the way in the global market to enable new kinds of PNT services.”
Simply by virtue of physics, with less of a distance to cover down to Earth, the signals from these LEO-PNT satellites can be more powerful, able to overcome interference and reach places where today’s satnav signals cannot reach.
Additionally, by adopting novel navigation techniques and a wider range of signal bands the satellites can address particular user needs: for instance at lower orbits the satellites themselves move more rapidly relative to Earth’s surface — think of the International Space Station at 400 km that orbits the Earth every 90 minutes — which offers possible advantage in the time needed to reach very accurate positions. Also some bands could offer greater penetration in difficult environments while other bands could offer higher robustness and precision.
Mega-constellations of hundreds or even thousands of low-orbiting satellites offer a means of acquiring continuous coverage for telecommunications services or Earth observation. (Image: ESA)
The purpose of ESA’s plan to perform an in-orbit demonstration of low Earth orbiting satnav satellites is precisely to consolidate the types of signals, enabling technologies and their potential for future services.
The plan is to build and fly an initial mini-constellation of at least half a dozen satellites to test capabilities and key technologies, as well as demonstrating signals and frequency bands for use by a follow-on operational constellation, in the same way that Europe’s GIOVE test satellites paved the way for Galileo. Success will place European industry in pole positions for follow-on commercial undertakings, as well as planned institutional programs.
“Each individual satellite would be comparatively small, below 70 kg in mass, compared to a 700 kg current Galileo operational satellite,” added Roberto Prieto-Cerdeira, Galileo Second Generation satellite payload manager and LEO-PNT project preparation manager as part of ESA’s FutureNAV program.
“They can be comparatively more streamlined because they can benefit from other means to calculate the accurate time without extremely precise atomic clocks on board — including relayed signals from the Galileo satellites above them. These satellites would also be built on a rapid batch production basis to save time and cost — we are targeting three years at the most from signing the contracts to the first satellites in orbit, the same kind of timescale achieved by GIOVE-A in the early 2000s.”
A vision of the future shows layered satellite navigation stretching from Earth to the Moon. (Image: ESA)
“It is ESA’s ambition to ensure Europe maintains a world-class space industry, and navigation today forms the single largest downstream space sector, worth about €150 billion annually and growing at the rate of 10% per year,” said ESA Director of Navigation Javier Benedicto-Ruiz. “Standing still is not an option; instead we need to explore new technical avenues to spur European competitiveness and commercialization.”
An operational version of the LEO-PNT constellation would represent a whole new layer for PNT delivery, combined with traditional GNSS as well as 5G/6G-based positioning on the ground, and fused with data from sensors in the user terminals.
Interest from industry
ESA has been researching core elements of the LEO-PNT concept since 2016. Now, with numerous low Earth orbit constellations already taking shape around the globe, the time is right to move from basic research to in-orbit demonstration.
Interest from European industry in the LEO-PNT project has been very high, shown by a recent Request for Information where ESA presented details of how companies and institutions might participate and a large number of companies registered and presented possible concepts and contributions.
Forward to FutureNAV
LEO-PNT is supported through the ESA Directorate of Navigation’s FutureNAV programme, which also includes the GENESIS satellite to measure the shape of Earth more accurately than ever before while also boosting the positioning performance of satnav satellites. The FutureNAV programme, which includes both GENESIS and the LEO-PNT initiative, is up for decision at ESA’s next Ministerial Conference, taking place in Paris on Nov. 22-23. Read the fact sheet here.
Until now, all navigation satellites have flown in medium-Earth orbit – up at 23,222 km in the case of Galileo, which delivers meter-level accuracy. At such altitudes the satellites move slowly across the sky, helping ensure global availability of satellite navigation signals, albeit at relatively low power.
ESA’s LEO PNT constellation would move to a “multilayer system of systems” approach, with medium-Earth orbit signals supplemented by those from LEO satellites at altitudes of less than 2,000 km — along with additional inputs from terrestrial PNT systems and user-based sensors, made up of approximately a dozen satellites, helping European companies move forward at a time when worldwide commercial interest is high in LEO constellations of all kinds, especially for telecommunications and PNT.
The satellites themselves can be stripped down compared to current navigation satellites, because they would essentially be relaying satnav signals from MEO. This is a key point because there will need to be many more satellites to ensure global coverage — because the lower the orbit the faster each individual satellite will pass across the sky. This fact also opens the way to a more agile “New Space” approach to satellite construction for European firms, with smaller payloads and simplified operations from the ground.
Their signals will be much stronger (potentially able to penetrate indoors), and transmitted on novel frequencies, which – along with the new geometries made possible by LEO satellites – should enhance overall service resilience. LEO PNT will also deliver faster position fixes and enable rapid two-way authentication checks. And overall signal availability will be boosted enormously, especially in high-latitude and polar regions.
As a new member of the DIFI Consortium, WORK Microwave will help advance the digitization of satellite communication ground technologies
WORK Microwave, a leading European manufacturer of advanced satellite communications equipment, today announced that it has joined the Digital Intermediate Frequency Interoperability (DIFI) Consortium, an independent space-industry group that formed to advance interoperability in satellite and ground-system networks.
As a new member of DIFI Consortium, WORK Microwave joins a growing roster of leading organizations in the space industry committed to bringing innovation to the digital transformation of space, satellite and related technologies.
“With the new space boom and LEO constellations emerging, digitization of the ground segment plays a key role in scalability and sustainability,” said Jörg Rockstroh, director of business development and digital products at WORK Microwave. “Being a prime supplier of satellite communications equipment, WORK Microwave actively supports standardization and other industry-wide efforts to simplify the ecosystem. Joining the DIFI Consortium is an excellent opportunity to help shape the future digitization of the satellite communication ground segment.”
WORK Microwave is an early adopter of new technologies, including digital signal processing, modem infrastructures, optical communication and Q-/V-band equipment. As a long-term contributor to industry standardization, the company has a history of helping advance satellite communication ground technology.
“The DIFI Consortium’s goal is to provide a simple, open, interoperable digital IF/RF standard that replaces the natural interoperability of analog IF signals and helps prevent vendor lock-in,” said Stuart Daughtridge, chair of DIFI Consortium. “We welcome WORK Microwave to the group and look forward to seeing how they will contribute to moving interoperability forward across space networks.”
19% of tracked space objects threaten GPS and other GNSS satellites. While there are many fewer objects in MEO than in LEO, the risk in the former is arguably greater because GPS is so critical to almost all of our technology.
The Risk
GNSS satellites, especially GPS satellites, are critical to the well-being and smooth functioning of economies and national security. This is especially true in Europe and the United States, which do not have complementary terrestrial systems able to provide vital positioning, navigation and timing (PNT) services when signals from space are not available.
While the probability of debris damage to GNSS in medium Earth orbit (MEO) is much less than for satellites in low Earth orbit (LEO), the consequences of such an event would be much, much higher. The loss of one satellite would be a concern; that of multiple satellites, a major problem. The unthinkable chaos, national security damage, and severe economic impacts to the $21 trillion U.S. GDP make the risk unacceptable.
For those who think we need not worry about the low probability of collisions at MEO, the Galileo collision avoidance maneuver in March 2021 should be a wakeup call. The problem is here. We need to act now.
Background
Much like a nuclear fission reaction, the problem of space debris starts small then grows exponentially, as each collision creates more pieces that, in turn, can collide with other objects.
The MEO debris environment is 100 times less dense than the LEO. The spatial density of orbital debris in LEO (up to 2,000 km), shown in Figure 1, suggests that LEO is the likely location where a runaway chain reaction will initiate. This could easily result in a region of space so dangerous that it would effectively deny access to MEO, where the GPS constellation resides.
While the debris situation at MEO is much better, there are still 4,021 tracked debris objects that could impact GPS and other GNSS satellites. Because future orbital debris collisions in LEO will be responsible for more debris in MEO, the situation is guaranteed to get worse. The dead and debris objects in highly elliptical, or Molniya, orbits, shown in Figure 2, could be responsible for such collisions pushing LEO debris into MEO.
Contributions to the general MEO debris population come from launch systems and other factors. Early GPS satellites (Block II/IIA/IIR) used internal orbital-insertion motors to avoid leaving uncontrolled stages in the operational orbit range when moving from transfer orbit to MEO. For survivability reasons, they were also deployed with sufficient fuel to make several major orbital moves. Unfortunately, later versions used separate orbital-insertion stages, which were left drifting in the orbital neighborhood and carried less fuel, resulting in fewer possible maneuvers to avoid collisions.
Using the CelesTrak visualization interface to extract space situational awareness data captured by the Combined Force Space Component Command’s 18th Space Control Squadron (18 SPCS) reveals a much more dire image of MEO. Of the 21,266 total tracked objects in Earth’s orbit, 157 are active GNSS satellites, as shown in Figure 3.
Figure 3. Active GNSS satellites. (Image: Celestrak)
However, a total of 4,021 objects reside or pass through MEO, which are either active (331), dead (668), debris (1,761), rocket bodies (1,100) or unknown (161) objects, as shown in this video.
These 4,021 objects represent 19% of the total number of tracked objects from the 18 SPCS space catalog. While the total 21,266 tracked objects is a far cry from the 100 million objects NASA’s Orbital Debris Program Office represents, one can imagine that a significant portion of untracked debris objects, under 10 cm in size, reside or pass through MEO as well. This is significant, according to NASA, which says that objects with a diameter of 1 cm to 10 cm are the most dangerous due to the lack of tracking data, which essentially makes them invisible.
False Sense of Security
The growing orbital-debris concern is a threat too big to ignore. Unfortunately, to date attempts to manage space traffic have amounted to passive measures, such as establishing policy, characterizing the environment, and creating orbital protection guidelines. Even the highly touted, $6 billion U.S. “Space Fence” is a passive measure that contributes nothing active to solve the problem. Not at all a “fence,” it is merely a way to detect the larger and more dangerous debris.
These efforts may, in all actuality, be counterproductive if they instill a false sense of security in the public and government leaders that the problem is being adequately addressed.
A Proactive Solution
Since 1978, the orbital debris population has been touted as our biggest space problem. It is important to do as much as we can with policies and procedures to keep the problem from getting worse faster. However, even if we humans were to completely resist our seemingly natural impulse to pollute everywhere we go, collisions with existing debris would continue to increase the number of dangerous objects in orbit.
Active debris removal (ADR) is the only solution. The sooner it begins, the safer we will all be. Like the oceans and cyberspace, orbital space suffers from the tragedy of the commons. Everyone wants to use it, but no one owns it. No one is responsible for ensuring it is cared for and maintained. As a result, user behavior is difficult to control, and the environment often suffers. Government action, presumably supporting the best interests of all users, is the default answer.
The proposed Space Debris Act of 2021 is a great start. It paves the way for persistent funding and creates an industry responsible for safeguarding humanity’s orbital infrastructure. It would introduce tax credits to incentivize non-government funding contributions and reduce the price of debris removal, so that satellite operators and the emerging space tourism industry can afford to clean up space where they plan to operate.
The bill is currently being presented by OrbitGuardians to members of Congress for sponsorship. Organizations wishing to support these efforts should contact Ken Eppens at OrbitGuardians at [email protected].
GPS/GNSS and other critical space assets are at an unacceptable level of risk from debris. It is time to safeguard orbital infrastructure to protect the interests of the United States and humanity’s future in space.
A free GPS World webinar on Nov. 21 tackles a new frontier, if not the final one, for GNSS. “Developments in Space GNSS Navigation,” sponsored by NovAtel, brings together experts from NASA, ESA, NovAtel and Spire (the CubeSat company) to discuss how they’re taking GNSS capabilities beyond Earth’s boundaries.
Navigating through space has long proven to be challenge for aerospace engineers and professionals because of the complex combination of technology and cost required for success.
However, with advancements in GNSS and receiver technology, organizations and nations around the world are increasingly interested in space exploration activities.
Today, the space industry is seeing GNSS technology used in low-Earth orbit (LEO) and highly elliptical orbit scenarios.
In this webinar, speakers from NASA, ESA (the European Space Agency), NovAtel and Spire will examine emerging trends regarding the usage of GNSS technology in the space industry, including an increasing need for situational awareness while navigating through space and the ability to service satellites while in orbit.
These experts will also provide a look into their own experiences with a variety of ambitious space projects and applications.
Speakers include:
Werner Enderle, Head of Navigation Support Office, ESA European Space Operations Center
Benjamin Ashman, Aerospace Engineer, NASA
Erin Kahr, Critical Safety Systems, NovAtel
Dallas Masters, GNSS Program Manager, Spire
Date: Thursday, November 21, 2019 Time: 1 p.m. EST / 10 a.m. PST / 7 p.m. (1900h) Central European Time
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.
