Evolving GNSS Spectrum Situational Awareness / RADAR Technologies

Feb 2023 | No Comment

The RF spectrum is a critical resource, for both practical military and civilian PNT users

Graeme Hooper

Managing Director GPS Satsystems

T he endless stream of global media articles on GPS/GNSS spectrum vulnerabilities discussing both spectrum blocking (jamming) and counterfeit timing signals (spoofing) attempting to disrupt GPS/GNSS continue to roll out daily. One might easily formulate the impression that the GPS/GNSS service providers are not doing enough to address the problem. However, that is not quite accurate. In late 2019 European GNSS Agency initiated their “Development of an Advanced Interference Detection and Robustness Capabilities System” project, while, in early 2021 a number of US Government policy announcements were also issued.

From the US, first was the White House Jan 15th Space Policy Directive-7 (SPD- 7) to cabinet officers, directing them to take specific steps to protect, toughen and augment GPS signals with other PNT services, encouraging the development of alternative PNT technology and improved security. New technologies such as “quantum sensing, relative navigation and/ or private alternative PNT services” were then promoted.

All very nice, but there is still at least one problem. Other than quantum sensing, all other suggested alternatives still use the RF spectrum, so logically the RF vulnerability problems would still remain with any new RF services. Malicious RFI agents can quickly adapt to these new frequencies and signals. How soon will quantum sensing become a “main street” commercial reality? Who really knows, but practical realisation is still best guess many years into the future.

Also, SPD-7 included the directive to authenticate future GPS signals, notionally to be introduced on future GPS NTS-3 satellites, due for launch this year. Currently, GPS signals do not have authentication features, although new technologies such as the Chips Message Robust Authentication (CHIMERA) signals are expected to be experimentally transmitted from the new NTS-3. This will add both data and signal authentication to the L1C signal, similar to, Galileo’s combined Open Service Navigation Message Authentication (OSNMA) and E6 Commercial Authentication Service (CAS) initiatives.

At this point, one might believe that combined with daily announcements of new “Jamming/ Spoofing (J&S) Resistant Receiver technologies combined with new signal authentication strategies, that all PNT spectrum vulnerability problems are solved. Well, NO, the problem is NOT solved. Neither of these solutions do anything for the billions of daily users dependent on existing “in-service” Receiver technologies, while also, glosses over ongoing vulnerabilities associated with RF front-end Antenna and Low Noise Amplifiers. If the RF front-end antenna & LNA chain are compromised, then no technology down at the PNT Receiver cable end will still be operational.

Enter the US government’s second announcement late March 2021 by the Army’s Assured Positioning, Navigation and Timing/Space Cross-Functional Team’s (APNT/Space CFT) announcing the new Navigation Warfare Situational Awareness (NAVWAR-SA) development initiative. The development of new technologies with the ability to “sense/ sweep” wide area PNT environments in real-time at a distance. A civilian equivalent would be real time PNTSpectrum-SA management tools that provide the ability to instantly detect, identify and uniquely geolocate multiple malicious J&S (intentional and unintentional) sources with high precision simultaneously.

With this type of technology, there will not be any significant perceived differences between Jamming and/or Spoofing transmission. They will both be simply resolved by any SA sensor network’s directional antennas, as RF energy/signals arriving from unexpected directions (azimuth/elevations), where there should not be any. Furthermore, with these types of SA technologies, where all legitimate GNSS SVs are individually beam locked by directional antenna arrays, any spoofing attempts (signals from wrong directions) become easily detectable and geolocatable.

To implement this real time/wide area PNTSpectrum-SA vision requires significant departures from traditional GNSS ways-of-thinking, through adoption of other advanced technologies not normally associated with the GNSS. These are more aligned with aerospace passive electronically scanned RADAR (RAdio Direction And Ranging) and astronomy’s weak signal object processing technologies, to name a few.

Some might argue that existing multistation CORS networks, GNSS Interference Detectors, and/ or, mobile phone apps cloud sourcing RFI localisation already provide adequate SA. Sadly, this is not the case. Although they might detect small regional RFI problems, they cannot provide the accurate 3D RFI geolocation dat required for an effective SA solution. This requires precise 3D geolocation and signal characterisation intelligence married with appropriate telecommunications EM/ RF propagation models, (eg. ITU’s ITURP.452 and/or ITS_ITM-1.2.2) to produce the desired end-user regional 3D Area Of Influence Prediction (AOIP) projections.

These regional 3D AOIP projections are then viewable much like weather prediction reports, presenting regional RFI spectral power distribution information as a 3D “fog heat map”. With changing colours to represent the differing levels of RF power intensities predicted across wider regions. These projections would include the combined effects of all RFI signal sources in the region of interest, (many 10s of square kms). For the AOIP projections to be both accurate and meaningfully, the PNTSpectrum-SA sensor network needs long range 3D scanning detection and ranging functions, just like weather radar hence the reference GPS/GNSS Radar.

Extending the PNTSpectrum-SA AOIP vision further, the Geospatial industry came to the rescue with their well evolved 3D display/ visualisation tools and agility to manage large datasets efficiently and quickly. Within geospatial software environments, the AOIP 3D projections are then overlayed 2D/3D digital twin representations of the terrestrial area of interest. This area might for example be an airport or container terminal, etc. Visualising RFI hot spots and their relative terrestrial location then becomes easily discernible to system operators and other site managers.

Like for weather reports, automated PNTSpectrum-SA warnings can then be issued to mission critical/ safety GNSS users, while also, presenting meaningful and accurate RFI geospatial information to the officials responsible for maintaining the regional GNSS spectrum integrity. Quickly and efficiently eradicating the RFI sources, whether or not, either intentional or unintentional origins.

This new March 2021 US APNT/Space CFT initiative, is a very welcome government acknowledgment that the RF spectrum is a critical resource, for both practical military and civilian PNT users. Total dependency lasting for many years to come. Making new PNTSpectrum-SA technologies available to the policing and enforcement authorities will ensure ongoing access to a free and unadulterated GNSS spectrum, for all PNT users and their legacy equipment.

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