# Detecting and Mitigating Attacks on GPS Devices
## 4 Factors Contributing to GPS-Denied or GPS-Disrupted Environments
### Propagation-Induced GPS Degradation  
The very low GPS power levels received at the Earth's surface make the system sensitive to blockages and shadowing. This issue occurs when a manmade structure or natural terrain feature blocks the line-of-sight (LOS) from one or more GPS satellites to the receiver. The need for the precise propagation time estimation required to estimate receiver-to-satellite range for GPS trilateration also makes the performance sensitive to multipath fading propagation conditions. This problem occurs when the GPS signals reflect off manmade structures toward the GPS receiver. The reflected signal components will be delayed in time compared with the LOS signal component due to the longer propagation path taken, resulting in the GPS receiver receiving the LOS component of the GPS signal and these time-delayed reflected signals. These different time-delayed versions of the same signal can either constructively or destructively add at the receiver, depending on the geometry and environment, resulting in large-scale and small-scale signal fading. These degradation factors are problematic in both suburban and (particularly) urban settings. The \"urban canyon\" scenario, which describes the scenario within a large urban area with many tall buildings, has historically proved problematic for GPS signal reception. These propagation-induced degradation factors are depicted in Figure 4.  
Several studies have investigated GPS performance in suburban and urban propagation environments; however, these studies do not typically isolate degradation due to shadowing or multipath fading. Instead, both shadowing and multipath fading are loosely categorized as urban propagation. GPS performance in complex urban environments has been the focus of study in several existing papers [35; 37; 42; 43] with greatly varying reported results. The authors of [43] presented experimental results revealing position accuracies of less than five meters in a built-up urban environment, similar to those observed in the literature for open environments. The authors of [42] also observed good performance in an urban measurement campaign, with reported accuracies of 7-13 m; however, the authors of [37] observed much greater variability in position accuracy in urban environments. The  
Figure 4: Propagation-induced GPS degradation factors, including (**a**) signal shadowing and (**b**) multipath fading. authors of [37] concluded that location-based applications must assume a position error of at least 28 m within their application due to the effects of signal shadowing caused by nearby buildings. The varying results reported in these research papers suggest that performance in urban environments is sensitive to environmental details and subject to high variability. These results could suggest that the precise effects of multipath fading and shadowing on GPS reception may not be easily generalized and might require study for specific regions of interest; however, other frequency bands and signals of interest, such as commercial cellular communications, have generalized channel models that are reasonable across a wide range of complex urban environments. These frequency bands have historically received significantly more interest, with thousands of papers published on cellular channel models and measurement campaigns; therefore, the lack of a high-fidelity generalized GPS urban propagation model may be due to insufficient research and publications. Researchers were in general agreement that urban canyon scenarios, such as very tall buildings in dense city downtowns, yield poor accuracy performance or total GPS signal loss. One problem with all studies on this subject is the lack of experimental details and subsequent lack of reproducibility. The papers on GPS performance in urban environments do not give a sufficient description of the details of their experiments or their test environments. It is challenging to draw generalized conclusions or validate the results of these papers without these details.