Read Richard Langley’s introduction column, Innovation Insights: What is a carrier phase?
An approach for ground vehicles using carrier-phase and inertial measurement data
The combination of easily accessible low-cost GNSS spoofers and the emergence of increasingly automated GNSS-reliant ground vehicles prompts a need for fast and reliable GNSS spoofing detection. To underscore this point, Regulus Cyber, an Israeli cybersecurity company, recently spoofed a Tesla Model 3 on autopilot mode, causing the vehicle to suddenly slow and unexpectedly veer off the main road.
Among GNSS signal authentication techniques, signal-quality monitoring (SQM) and multi-antenna could be considered for implementation on ground vehicles. However, SQM tends to perform poorly on dynamic platforms in urban areas where strong multipath and in-band noise are common, and multi-antenna spoofing detection techniques, while effective, are disfavored by automotive manufacturers seeking to reduce vehicle cost and aerodynamic drag. Thus, there is a need for a single-antenna GNSS spoofing detection technique that performs well on ground vehicles, despite the adverse signal-propagation conditions in an urban environment.
In a concurrent trend, increasingly automated ground vehicles demand ever-stricter lateral positioning to ensure safety of operation. An influential study calls for lateral positioning better than 20 centimeters on freeways and better than 10 centimeters on local streets (both at a 95% probability level). Such stringent requirements can be met by referencing lidar and camera measurements to a local high-definition map, but poor weather (heavy rain, dense fog or snowy whiteout) can render this technique unavailable.
On the other hand, progress in precise (decimeter-level) GNSS-based ground vehicle positioning, which is impervious to poor weather, has demonstrated surprisingly high (above 97%) solution availability in urban areas. This technique is based on carrier-phase differential GNSS (CDGNSS) positioning, which exploits GNSS carrier-phase measurements having millimeter-level precision but integer-wavelength ambiguities.
Key to our promising results is the tight coupling of CDGNSS and inertial measurement unit (IMU) data, without which high-accuracy CDGNSS solution availability is significantly reduced due to pervasive signal blockage and multipath in urban areas. Tight coupling brings millimeter-precise GNSS carrier-phase measurements into correspondence with high-sensitivity and high-frequency inertial sensing. Our particular estimation architecture incorporates inertial sensing via model replacement, in which the estimator’s propagation step relies on bias-compensated acceleration and angular rate measurements from the IMU instead of a vehicle dynamics model.
As a consequence, at each measurement update, an a priori antenna position is available whose delta from the previous measurement update accounts for all vehicle motion sensed by the IMU, including small-amplitude high-frequency motion caused by road irregularities. Remarkably, when tracking authentic GNSS signals in a clean (open-sky) environment, the GNSS carrier-phase predicted by the a priori antenna position and the actual measured carrier phase agree to within millimeters.
The research described in this article pursues a novel GNSS spoofing-detection technique based on a simple but consequential observation: it is practically impossible for a spoofer to create a false ensemble of GNSS signals whose carrier-phase variations, when received through the antenna of a target ground vehicle, track the phase values predicted by inertial sensing. In other words, antenna motion caused by factors such as road irregularities or rapid braking or steering is sensed with high fidelity by an onboard IMU but is unpredictable at the sub-centimeter-level by a would-be spoofer.
Therefore, the differences between IMU-predicted and measured carrier-phase values offer the basis for an exquisitely sensitive GNSS spoofing-detection statistic. What is more, such carrier-phase fixed-ambiguity residual cost is generated as a byproduct of tightly coupled inertial-CDGNSS vehicle position estimation.
Two difficulties complicate the use of fixed-ambiguity residual cost for spoofing detection. First is the integer-ambiguous nature of the carrier-phase measurement, which causes the post-integer-fix residual cost to equal not the difference between the measured and predicted carrier phases (as would be the case for a typical residual), but rather modulo an integer number of carrier wavelengths. Such integer folding complicates development of a probability distribution for a detection test statistic based on carrier-phase fixed-ambiguity residual cost.
Second, the severe signal multipath conditions in urban areas create thick tails in any detection statistic based on carrier-phase measurements. Setting a detection threshold high enough to avoid false spoofing alarms caused by mere multipath could render the detection test insensitive to dangerous forms of spoofing. Reducing false alarms by accurately modeling the effect of a particular urban multipath environment on the detection statistic would be a Sisyphean undertaking, requiring exceptionally accurate up-to-date 3D models of the urban landscape, including materials properties.
Our work takes an empirical approach to these difficulties. It does not attempt to develop a theoretical model to delineate the effects of integer folding or multipath on its proposed carrier-phase fixed-ambiguity residual cost-based detection statistic. Rather, it develops null-hypothesis empirical distributions for the statistic in both shallow and deep urban areas, and uses these distributions to demonstrate that high-sensitivity spoofing detection is possible despite integer folding and urban multipath.
