# Impacts of GPS Spoofing on Path Planning of Unmanned Surface Ships
## 3 GPS Attack Designs and Spoofing Analysis
### GPS Attacks
#### 3.2.2 GPS Spoofing  
Compared with GPS jamming, GPS spoofing is more dangerous to ship navigation because it is surreptitious [32]. An attacker makes the receiver believe that it is at the wrong location. It is a very dangerous situation for the path planning of USV based on APF. GPSspoofing is classified into production spoofing and repeater. GPS production spoofing refers to the receiver-spoofer generating spoofing signal, Pseudo Random Number (PRN), and navigation message, which the victim GPS receiver can receive. GPS repeater spoofing refers to the spoofer modifying, modulating, and amplifying the received GPS signal or sending it out after a delay. The production spoofing for C/A is possible because C/A signals are public and not authenticated, but it is difficult for P-code. Based on [39], we can know the composite signal received at the antenna of a GPS receiver is \(S_{ant}=S_{a}+S_{s}+S_{n}\), where \(S_{a}\), \(S_{s}\), and \(S_{n}\) are the authentic signal, spoofing signal, and noise signal, respectively. When \(S_{s}\gg S_{a}\), the signal received at the antenna of GPS receiver can be approximated by \(S_{ant}\approx S_{s}+S_{n}\). The spoofing signals generated by the GPS spoofer successfully override the authentic GPS signals. The attacker can transmit its GPS signals using the publicly known GPS parameters by a simple GPS generator. Although this approach can spoof the receiver, the spoof signals look like noise to the receiver [40].  
Next, we focus on GPS repeater spoofing. The attacker can delay and modify the content of received GPS signals to spoof the victim GPS receiver into calculating a false location, such as a receiver-spoofer designed by [PERSON] [41], as shown in Figures 6 and 7. Based on Equation (28) in Section 3.1, if the attacker changes the arrival time of GPS signals (\(T_{u}\)) by delaying the transmission of received signals, it will eventually lead to an inaccurate receiver position. On the other hand, if the attacker changes the content of GPS (i.e., transmission time or location of the satellite) according to its intention, the ship which depends on GPS for path planning navigates to the position set by the attacker, causing a greater danger. The paper [40] introduces that the Cornell GRID receiver can simultaneously track 12 C/A channels and generate 8 C/A spoofing channels. Such equipment coupled with the simple RF hardware can implement GPS spoofing. In case of GPS spoofing or signal loss, the speed information obtained by the Inertial Navigation System (INS) is used to assist the GPS receiver tracking loop in implementing navigation and positioning functions. The INS is a technology used to obtain instantaneous velocity and instantaneous position data by measuring acceleration and automatic integration operation, one of the key equipment for ships. When the GPS is combined with the INS, it can still complete the output of position and speed in the face of GPS jamming. However, in the face of GPS spoofing, it will lead to the divergence of the GPS/INS navigation system and destroy the overall performance of the integrated navigation system. The integrated navigation system supervises the GPS during unmanned ships' navigation. When there is a difference between the position information calculated from the GPS and the position information estimated from INS according to the previous time, the integrated system will suspect that the GPS signal has been spoofed. Therefore, in GPS spoofing, it is necessary to gradually induce the unmanned ships to deviate from the course step by step. Lidar and navigation radar are the main means of ship obstacle avoidance detection. Similarly, GPS spoofing will affect the path planning after multi-source data fusion from lidar, radar, and sensors.  
Figure 6: A Spoofing attack via a portable receiver-spoofer [41].