With the increasing transition in modern warfare to fully autonomous vehicles, drones and marine vessels, navigation systems that provide reliable and accurate position, velocity, timing and attitude data in contested areas have become essential.
Helix Geospace are a leader in providing resilient PNT (positioning, navigation & timing) solutions and is the natural choice for military uncrewed surface vessels (USVs) needing a navigation capability that is able to operate reliably over prolonged durations within increasingly complex, contested maritime environments.
A layered approach using a GNSS-assisted inertial navigation system (INS) is a common way of meeting the navigation needs, but in contested environments GNSS/GPS services are constantly under attack from adversaries seeking to disrupt, deny or spoof them.
Loss of GNSS service is catastrophic for USV missions, with position starting to drift and potentially degrade to ~100m or more within 1-2hrs making it unreliable over prolonged periods and especially given the high speeds often used by military USVs.
By far the best approach is to improve the resiliency of GNSS itself; doing so delivers a 5-10x improvement in positional accuracy compared to sole reliance on INS given its inherent drift, and reinstates GNSS availability during those periods of heavy jamming in which the USV would otherwise experience service denial - in essence, shrinking large zones of GNSS-denial into much smaller pockets of GNSS outage during which the INS can 'plug the gap'.
Some solutions attempt to improve GNSS resiliency through filtering out the jammers within the GNSS receiver, but such an approach leaves the system wide open to high-power attacks that saturate the receiver (Automatic Gain Control hijacking), or attacks that employ complex chirp and swept waveforms that defy efficient filtering. GNSS receivers are similarly vulnerable to PRN denial of service (DoS) attacks in which the receiver is flooded with fake PRN codes that consume resources and overwhelm the receiver’s ability to track legitimate GNSS satellites.
GNSS receivers intrinsically struggle to tackle such attack vectors; a much better approach is to suppress interference & jamming at the RF level as a first line of defence, the GNSS receiver then only needing to handle any residual interference.
A common way of suppressing jamming at the RF level is through sophisticated controlled reception pattern antenna (CRPA) arrays that dynamically alter their reception pattern to neutralise jammers and maximise signal strength from the GNSS satellites.
In the case of navigation systems though, simply substituting the GNSS antennas for commercial off-the-shelf CRPAs doesn’t work.
Navigation systems employ two GNSS antennas to provide position, velocity and heading information to the USV and these antennas need to be very closely matched, with consistent phase centre offset and variation across a range of GNSS signal azimuths and elevations.
Achieving this with CRPAs though is difficult. Each CRPA comprises an array of antenna elements; any small differences between the elements will be accentuated by the array resulting in phase characteristics that differ across the two CRPAs hence distorting the measurements.
Moreover, CRPA operation relies on adaptation of the signals from each antenna element to dynamically shape the CRPA reception pattern and this will also disturb phase stability and accuracy. This is especially problematic in those CRPAs that have adopted STAP-based digital filtering which moves the spatial processing and filtering into the digital domain and in doing so disturbs the phase coherence within the GNSS signal.
By focusing their efforts on jamming suppression, many CRPAs effectively rule themselves out of being used for navigation, or for any high-precision GNSS service where phase coherence is paramount.
Helix Geospace have taken a different approach, designing CRPA technology that delivers military-grade jamming suppression for USVs, but also maintaining phase coherence and GNSS signal linearity to support their navigation needs.
Through optimising antenna design and RF engineering, Helix Geospace have been able to simplify the amount of spatial processing needed in their CRPA thereby maintaining phase coherence.
Core to achieving this is Helix Geospace's patented DielectriX technology and innovative helical design that creates small antenna elements for the CRPA array that are near-identical and highly accurate thereby ensuring a highly consistent phase behaviour across the CRPA units used within the navigation system.
Designed with input from defence partners, Helix's technology provides military-grade jamming suppression (>50dB) to enable the USV to get up to 95% closer to a jamming source without losing GNSS, and is non-blocking by design to suppress interference and jamming irrespective of the number of sources.
The DielectriX antenna technology also delivers a strong reception pattern across all azimuths and elevations thereby enabling optimal satellite tracking and reducing Time to First Fix (TTFF) when moving in and out of GNSS-denied areas. Dual-band support (L1/E1; L2/L5/E5a+b) as well as multi-constellation (GPS & Galileo) further boost satellite availability, and M-code support enables higher signal robustness in military applications.
Using this advanced CRPx technology in combination with a high-performance INS and anti-spoofing algorithms, Helix have been able to produce a Resilient Navigation System (RNS) that achieves military-grade robustness in a sufficiently compact and low SWaP (size, weight & power) package for small USV deployment – something that none of the competition have yet been able to achieve.
Read on for more information on Helix's Resilient Navigation System.