Do we need GNSS alternatives, OR, should we protect what we already have?

Graeme Hooper Managing Director GPSat Systems Australia

In the real estate market they say it’s all about location, well, it’s the same in satellite navigation but for many different reasons. Precise location of objects using GNSS has been and will continue to be, the primary focus of the industry. However future trends for the 2020s onwards, will see “location” extended into the more peripheral challenged areas of precisely geolocating Radio Frequency Interference (RFI) spectrum threats. By example, the substantial reduction in aircraft hijacking is attributed to the broadscale introduction of X-Ray screening machines at airports, similar philosophies adopting more advanced radio technologies deployed terrestrially, we can also substantially improve protection for our precious GNSS satellite signals.

GNSS an incredibly effective global utility, but it’s vulnerable to either deliberate or accidental Radio Frequency Interference (RFI) jamming of it’s terrestrial signals. Jamming and/or spoofing (J&S) effects have the potential to severely cripple/ interrupt important mission critical GNSS safety of life operations, such as GBAS at airports, marine AIS or autonomous machines used in modern industrial applications.

For these mission critical applications, there are three key strategies available for increasing GNSS senor resilience in these J&S threats environments, these being;

1. Avoidance: Defence, hardening the navigation sensor by utilising CRPA antenna technologies, enhanced signal processing (Mcode, adaptive RF digital front ends), etc.

2. Augmentation, aiding the navigation process through the integrated use of a dissimilar sensors such as IMUs, Radar, Lidar, etc.

And/Or,

3. Engagement: instigating a process of “offence” by utilising tailored technologies to deal with the J&S RFI signals directly. In a GNSS environmental context, this translates to accurately and timely geolocating the RFI emission point/s and then take the required interdictory actions to eliminate the problem entirely.

All above strategies have an important place in modern GNSS enabled mission critical autonomous operations. However, history has taught many times that “avoidance and augmentation” are NOT enduring strategies alone. Only entirely effective solution for permanently dealing with persistent spectrum interference issues is active “engagement”. Many great leaders have quipped, “the BEST defence is a GOOD offence”.

For GNSS a good offence is direct interdictory responses, extending to many options, including law enforcement officers apprehending offending criminals, airport staff rectifying errant hanger GNSS signal re-radiators, mine workers rectifying faulty machinery, and/ or military kinetic explosive combat responses. It’s only through instantaneous precision geolocation determination of threatening RFI source transmission points, coupled with prompt direct interdiction responses can resilient GNSS PNT can ever be meaningfully achieved.

Do we need GNSS alternatives, OR, should we protect what we already have? Australia’s decade long response through several Australian Research Council (ARC), Defence and GPSat System investments adopts the “do more for GNSS protection and engagement philosophies”.

For automation projects requiring “mission critical” safety and integrity level (SIL) certifications, GNSS technology signal vulnerabilities presents several challenges for achieving formal operational regulatory approvals. Systematic failures due signal atmospheric disturbances, multipath and/ or terrestrial RFI, effective control must be extended to ALL aspects, including the GNSS RF environment.

Industrial Safety Standards such as Aviation DO-178C and IEC61508, or specific industry derived standards (eg., rail EN50126, automotive ISO26262, etc) all require “vulnerability to undesired EM signals” to be appropriately address during safety requirements (SRS) establishment. Numerous references within IEC61508 relate to “Increase of interference immunity” (Part 7 – A.11.3) and “Measures against Physical Environment” (Part 7-A.14). Requirements Specification Part 7-B.2 requires the “competent GNSS expert” to address ALL vulnerabilities, and then, implement effective mitigation processes to ensure SIL systems are “complete, free from mistakes, free from contradiction and simple to verify”. Comprehensively devoid of ANY inherent systematic failure modes impacting ongoing safety. In summary, it’s essential that future automation GNSS professionals extend their responsibilities beyond exclusive USER PNT product/ services boundaries to also include meaningful spectrum policing and control. Rapidly and precisely geolocating RFI J&S threats, and then, promptly implementing appropriate interdiction responses. Even the more sophisticated GNSS “avoidance” technologies are neither adequate nor appropriate to meet entire GNSS SIL certification mandates alone.

Is it possible to effectively regionally police the terrestrial RF spectrum in the GNSS bands, quickly detecting, characterising and precisely geolocate RFI threats to GNSS operations.? Yes absolutely, providing the right equipment and technologies are carefully fashioned to meet the unique nature of GPS RF signals arriving from space at very low levels, well below the ambient/ background noise.

Military radar and astronomy sciences for many decades have dealt with very weak RF signals arriving from any direction. Employing phased array antenna receiver reception combinations for directional signal amplification, and then, precisely determining Direction of Arrival (DOA). Digital advanced spectrum cleansing/ whitening signal processing techniques for subtracting “known RF sources” (eg, GNSS signals, 4G harmonics etc.) improving RFI sensitivity are essential ingredients for any future regional spectrum GNSS policing system.

To complete essential ingredient list, GNSS engineers need to invert their Time Difference Of Arrival (TDOA) skills away from “multiple satellite RF sources into single reception antenna” thinking, to embrace, “single RFI source into multiple spatially distributed reception antenna” thinking. Using a network of remote synchronised monitoring stations (multiple RF antennas & receivers) all working cooperatively receiving RFI source transmission signals at different locations. Each remote monitoring station’s DOA and TDOA contributions can then be used centrally for very rapid and accurate RFI signal geolocation and Area of Influence (AOI) determinations.

Does AOA phased array, TDOA and advanced spectrum cleansing processes work for effective regional GNSS spectrum policing? Absolutely! Through a dedicated team of engineers (several PhDs) from GPSat Systems, UniAdelaide, and UniNSW effectively combining each’s expertise in RF electronics design, geodetic spatial sciences, defence radar phased array engineering, weak RF signal numerical processing and numerous others technologies, have all carefully been blended into the very unique GRIFFIN project. Having successfully completing past (2017) Australian Defence testing and demonstrations, GRIFFIN is now undergoing defence initial production, with deliveries anticipated Q2 2020.

For future 2020 and onwards GNSS trends, systems like GRIFFIN and issues associated with spectrum protection are sure to gain both greater industry focus and adoption, particularly in SIL related applications.