What does the future hold for military and commercial systems dependent on current GPS?

With Assured Positioning, Navigation, and Timing (APNT) and Low Earth Orbit PNT (LEO PNT) going from strength to strength, what does the future hold for military and commercial systems dependent on the current GPS configuration? Should military and commercial platforms be modified to include APNTs, for now, with a view to adding LEO PNTs in the future? Should they integrate these two systems or rely on one or the other as standalone systems?

Government and industry agree that jamming of GPS and all GNSS is a growing threat with the development of jamming and spoofing technologies. This prompted the government to support APNT to strengthen GPS. An Executive Order dated February 12, 2020, requires a comprehensive update of national policy on PNT services from the federal government and critical infrastructure owners and operators to strengthen critical infrastructure resilience.

Research, development and manufacturing have improved the performance—positioning, timing and (desired) accuracy—of GNSS PNTs and the ability to operate in RF-challenged environments. APNT gives the US military a reliable way to supplement GPS or act as an alternative to it by using other sensors such as inertial navigation systems, differential GPS, visual sensors, lidar, radar, radios and star trackers that complement GPS.

The near-term expansion of Internet service to include commercial broadband LEO satellites also provides the potential for robust PNTs using their waveforms as signals of opportunity (SOOP). GPS and other GNSS have infrastructure to maintain very accurate time in their constellations, as well as satellites with specially designed transmitters, clocks and waveforms dedicated to the PNT function. Conversely, SOOPs are in space for a different purpose and are not optimized for PNT. Therefore, the challenge is to exploit the characteristics of SOOP waves by designing innovative techniques to determine the range to each satellite and to provide users with a reliable PNT. The LEO PNT approach may have applications for ground forces and for air, munitions, missile, and commercial applications requiring higher levels of PNT security and integrity.

GPS receivers for future military platform designs may use Software Defined Radio (SDR) approaches and may include LEO PNT signals. This technology, although designed to work independently, can be used to complement existing navigation sensors commonly used in navigation systems, including APNT. Expanding to the use of multiple constellations will serve to optimize performance and resilience in RF-challenged environments. However, the closer proximity of LEO satellites to Earth and their signal structures allow for higher signal powers, thus making them more resistant to jamming. With all these separate systems or synthesis via SDR, how does the receiver ensure signal integrity or accuracy? The SDR qualification test will include an unlimited number of scenarios.

One hallmark of the GPS program is that it facilitates a comprehensive systems engineering effort by managing Interface Control Documents (ICDs) in a single location for alternative systems developed by various program offices across the country. This makes both system integration and receiver development extremely difficult and complex.

“The new SPD-7 [Space Policy Directive 7, the United States Space-based Positioning, Navigation and Timing Policy, dated Jan. 15, 2021] focusing on interoperability and APNT is an essential document to address a realized threat and a path forward,” said Bernie Gruber, former head of the GPS Directorate (now the Military Communications and PNT Directorate). “To this end, the combination of SDR and data fusion potentially offers a distinct advantage for leveraging the diversity of signals and sensors, thereby improving the robustness of critical PNT information.”