Navigation from LEO: Current capability and future promise

June 8, 2017  - By

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.

Webinar on Thursday. The Satellite Time and Location service described here will be covered in further detail in an upcoming free webinar on Thursday, June 15: Alternative PNT Services: LEO Satellite Time, Location and More.

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.