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. INTRODUCTION Spoofing of Global Navigation Satellite Systems (GNSS) signals can have deleterious effects on society given the widespread use and dependence of critical infrastructure on GNSS. However, few commercial receivers have significant anti-spoofing (A/S) mechanisms. Even simple interference events such as jamming and meaconing have resulted in erroneous position outputs from shipboard and airborne receivers [1][2] [3][4]. Spoofing tests have shown that deliberate GNSS spoofing could have significant impact on the GNSS receiver and hence GNSS dependent systems [5][6]. While the extent of the impact is still debated, it is clear that a spoofing event would significantly harm some users. So, the debate over the utility of A/S comes down to the likelihood of spoofing events. We can no longer categorically state that GNSS spoofing, outside a laboratory or military setting, has not occurred. Indeed, GNSS spoofing was observed outside the Kremlin in 2016 [7]. Furthermore, the popularity of location-based games such as Pokémon Go has also induced hackers to build and utilize GNSS spoofers [8]. While the spoofer in [8] uses an expensive GNSS signal generators, other professional security groups have put together GNSS spoofer using low cost software defined radios (SDRs), open source software and some basic GNSS know-how [9]. GNSS spoofing capabilities are no longer solely the realm of navigation experts. And as time goes on, spoofing capabilities will get better and its costs will only decrease. There are many motivations to spoof. Ordinary citizens may spoof to aid their gaming, to protect their privacy or to subvert location based charges (e.g. road tolling) or restrictions. A quick search on the Google Play store shows multiple pages of “Fake GPS” apps. The first app, “Fake GPS Location Spoofer Free”, alone has over 60,000 reviews as of May 2017. This indicates that many people took the time to not only download and use the app but also to comment on its benefits! Given the interests in these apps, it is not a stretch to say that there is public interest in spoofing GNSS. Coupling these two factors the availability of GNSS spoofing equipment or know-how and individuals who have interest in spoofing – means we should expect that there will be more spoofing incidents in the future. And while critical infrastructure may not be the target for most spoofers, it may fall victim as collateral damage. In this paper, we develop and examine GNSS spoofing detection via direct comparison of motion (acceleration) using commercial inertial sensors. The concept developed allows for comparison of the two sensors without coupling GNSS with inertial measurement unit (IMU). The design allows for a robust, steady state spoof detection capability that can be developed as an applique to existing receivers. This paper develops this concept, particularly for use in aviation and is organized as follows. The first part covers prior art and describes the developed technique. The second part describes the test equipment used and experiments conducted to collect data for analysis. The third part examines whether inertial measurements from a low-cost IMU can provide enough information for aviation spoof detection. The fourth part develops and tests a methodology for spoof detection based on the data collected. The final part addresses the use of the technique for other modes of transportation such as railway and automobiles. BACKGROUND: PRIOR ART & DEVLEOPED TECHNIQUE