The Benefit Of Low Cost Accelerometers For Gnss Anti-Spoofing

Global navigation satellite systems (GNSS) are integral to aviation navigation and precise landings. It will be fundamental to many other safety of life applications such as railway and autonomous vehicles. To truly protect these systems and maintain safety, we need to not just address nominal hazards but also deliberate threats such as spoofing. Many methods have been proposed to address spoofing. A natural suggestion is to compare GNSS with other independent sensors, particularly inertial sensors. This paper develops a GNSS spoof monitoring methodology that uses comparisons with an inertial sensor. The methodology differs from prior methods developed and uses comparisons of accelerometers and GNSS acceleration (rather than position or velocity). This structure keeps the two instruments independent which makes it a steady state test and allows for tests over time. This paper develops and examines the use of acceleration from low cost sensors to detect GNSS spoofing in safety of life applications. We examine if there are suitably random accelerations and whether low inertial sensors can provide useful comparison to GNSS. Then we examine how we can use these measurements to provide robust spoof detection.First, we examine whether there are adequate signals from the vehicle and whether they are sufficiently measurable by both GNSS and low cost accelerometers. The conjecture is that it is difficult for malicious spoofer to predict and hence replicate the accelerations experienced by these vehicles. For example, an aircraft experiences random accelerations due to wind gusts as well as semi-random accelerations due to configurations changes (flap, landing gear position). Similarly, a railcar may experience potentially random accelerations along-track and in the vertical direction. Measurements from flight tests show that there are often significant and unique accelerations that can be measured by both GNSS and consumer grade accelerometer. In cryptographic terms, the acceleration information can provide a "one time pad" that is difficult for most spoofers to determine. To reasonably compare accelerations, the measurements from GNSS and the accelerometer need to be aligned. Accelerometers measure in the body frame while GNSS provides results in inertial or local frames. GNSS is used to estimate the body frame and align the two measurements. While the alignment is not exact, the results indicate that it may be generally sufficient for approach.Next, having an unpredictable and high frequency acceleration signature allows for the development of robust spoof detection monitoring. Guiding principles in the initial design are simplicity and low false alert rates. High false alert rate will cause high unavailability and, even worse, general distrust of the system. Hence, the paper develops a simple statistically based test for detection. It demonstrates the performance of the test method on the flight test data collected.