# Detecting and Mitigating Attacks on GPS Devices
## 1 Introduction  
Modern systems and devices commonly rely on the Global Positioning System (GPS) for positioning, navigation, and timing (PNT). Manned and unmanned airplanes, ships, and ground vehicles typically rely on GPS for positioning and navigation. Unmanned autonomous systems, such as unmanned aerial systems (UASs), rely on GPS to support navigation. Cellular mobile devices utilize these navigation signals for self-positioning and supporting location-based services and applications. Reliable and accurate positioning and navigation are paramount for safe and reliable UAS operations. A UAS must typically maintain an accurate record of where it has been, where it currently is, and where it is going to successfully complete its task. An unmanned sensor often needs to know its location for its measurements to be useful. A UAS must be capable of conducting accurate navigation to maintain flight safety. Autonomous unmanned operations are not possible without reliable positioning and navigation capabilities. A common approach to providing reliable UAS positioning and navigation is through the use of a Global Navigation Satellite System (GNSS), such as a GPS, which provides a series of signals from space-based satellites that ground or air-based systems use to determine their location and typically serves as a primary input into the system's guidance and navigation system. These receivers are now integrated into billions of manned and unmanned commercial and military devices; however, GPS signals are susceptible to a wide variety of factors and may not always be available or trustworthy. Positioning accuracy can be significantly degraded incomplex propagation environments that induce multipath fading or signal shadowing effects. These navigation signals can also be degraded or completely disrupted by intentional or unintentional interference. For example, GPS signals are vulnerable to intentional jamming and spoofing attacks, which can lead to a loss of positioning and navigation integrity. These issues can lead to significant vulnerabilities in commercial and military infrastructure and the degradation or malfunction of vehicles and systems, such as UASs. The loss of GPS signal integrity is a serious issue for manned and unmanned systems; however, it is particularly problematic for unmanned systems. A human operator can use other means for navigation if GPS-based positioning is unavailable in a manned system, including the use of on-hand information such as maps, landmark recognition such as street signs, the use of other navigation devices such as a magnetic compass, or collaboration with others, such as asking others for their current location. Human operators also have the cognitive means to fuse these alternate data sources to provide a reliable position estimation. Unmanned systems may have access to additional data sources, other sensors, and perhaps even collaboration opportunities with other manned or unmanned systems; however, they must possess the intelligence to process and fuse this information to estimate position autonomously. Furthermore, unmanned systems come in various form factors and cost points, which may limit the types of onboard sensing or processing available for positioning and navigation. Many studies reported in the literature have attempted to characterize the performance of GPS positioning in complex propagation environments and proposed methods to augment GPS-based positioning with other sensor types that may be onboard a system, such as visual methods using an onboard camera. Numerous approaches have been proposed to provide autonomous methods to detect or mitigate the effects of hostile jamming and spoofing. Furthermore, many non-GNSS methods of positioning and navigation in GPS-denied environments exist. Many surveys and systematic reviews on GPS security and alternate positioning and navigation methods have been published; however, many only briefly discuss GPS security within a larger cybersecurity discussion.