# Detecting and Mitigating Attacks on GPS Devices
## 6 Countermeasures and Their Comparison
### Countermeasures for GPS-Denied Environment--Alternate Positioning
#### 6.3.4 Satellite-Based Approaches  
There is a growing amount of research on using other satellite systems to augment GPS-based navigation, of which there are two primary categories of approaches: (1) multi-GNSS approaches and (2) commercial LEO constellation-based approaches, such as mega-constellations. Commercial multi-GNSS chipsets can be configured to receive signals from GPS, GLONASS (Russia), Galileo (European Union), and BeiDou (China). Multi-GNSS chipsets inherently mitigate the threat of attack on any one GNSS, with a limited number of papers in the literature that quantitatively support this claim. The authors of [156; 157] conducted analyses and experimentation with multi-GNSS receivers to characterize their performance, establishing that multi-GNSS approaches can outperform any single GNSS system. The authors of [157] analyzed the performance of multi-GNSS receivers in the presence of jamming and established that position accuracy is largely unaffected due to the availability of other GNSS systems, even if a single GNSS signal is made unavailable. The authors of [158; 159] proposed methods for automatically selecting which GNSS to use for position estimation based on observed conditions. The authors of [159] proposed a method to treat all GNSS constellations as sub-systems of a single system and then selectively choose which satellites from each constellation would be used for position estimation. These papers all demonstrated high position estimation accuracy, as expected, with errors typically of five meters or less. It is intuitively obvious that a multi-GNSS receiver will generally outperform a dedicated GNSS receiver; however, this approach has one key limitation. Other GNSS systems, such as GPS, are susceptible to jamming and spoofing attacks. Consequently, these systems cannot necessarily be trusted any more than GPS. The diversity of employing multiple GNSS systems will improve system robustness; however, an adversary capable and willing to launch an attack on GPS is likely willing and able to do the same against all the other systems. Developing a multi-GNSS jamming or spoofing attack with the SDR-based attack systems found in the literature would be relatively straightforward.  
There is an increasing amount of research in the literature that is focused on the use of non-GNSS satellite systems such as Starlink, OneWeb, OrbComm, and Iridium for navigation [160], with Starlink receiving the most interest in the literature to date. These constellations are extremely large (thousands of satellites) and very fast-moving from the perspective of a ground-based observer due to their Low Earth Orbit (LEO) positions, which largely mitigates a physical attack scenario. Much of the work in the literature has been performed by [PERSON] and his research team, the authors of [160] and numerous other papers on the subject [161; 162; 163; 164; 165]. The authors investigated using numerous features of the downlink signals from these constellations, with a focus on Starlink. Some of the more promising signal characteristics are carrier phase and carrier Doppler shift.  
Other research teams have recently begun studying the use of these commercial mega-constellations for navigation purposes [166; 167; 168; 169]. Many of the proposed methods would require modification to the downlink signal of these commercial systems, which is unlikely to happen; however, many other proposed approaches are completely passive and utilize the as-is downlink signals. The results presented in these papers are promising and demonstrate that this is a feasible concept; however, the downlink signals from these systems will likely be susceptible to jamming or spoofing attacks, especially given the capability and flexibility of SDR-based GPS attack systems.