# Spoofing Detection Algorithm Based on Pseudorange Differences
## 1 Introduction  
As global satellite navigation systems (GNSSs) play an increasingly important role in society and industry, the security of these systems is a crucial component. Intentional and unintentional spoofing interference affects normal use of navigation and timing terminals. Unlike jamming, the goal of spoofing is to take control of the user receiver. The receiver captures the spoofing signal and uses it for the calculation of an incorrect positioning. In [1], the authors analyzed the vulnerability of the satellite signal and the GNSS to attacks, illustrating how the spoofing signal enters and takes over the receiver. To facilitate a threat analysis, the authors divided the spoofing threat into three categories: simplistic, intermediate and sophisticated spoofing attacks [2]. In addition to these three categories, there is another case: meaconing. Meaconing is the interception and replay of navigation signals on the received frequency, typically with a power higher than the original signal, to confuse the navigation terminal [3]. The main form and the purpose of a spoofer is to generate a similar and false satellite signal and make the receiver capture it. The spoofer is defined as a system containing three elements: the receiving antenna, the spoofing signal generator and the transmitting antenna. The receiving antenna receives the signal and inputs it into the spoofing signal generator. Then, the generator generates the spoofing signal, which has the same form as the satellite signal according to the input and the spoofinginterference intention and is finally transmitted by the transmitting antenna. The signal input to the generator and the signal output from the generator are not the same, but are similar. The main form of the meaconer is to delay the transmission of the real signal by the delay module. A meaconer contains the same three elements as the spoofer; however, the meaconer has a signal transponder with a delay module instead of the spoofing signal generator. The whole process can be described as: the receiving antenna receives the signal and inputs it into the signal transponder, and the signal is then transmitted by the transmitting antenna after a certain delay. The signal input into the transponder and the signal output from the transponder are the same. The structure of the signal in this spoofing scenario is not changed. Compared with the spoofer, meaconing has the advantage that it is easier to implement and has a lower cost. The coarse/acquisition code (C/A code) of the satellite signal is open and transparent, and its structure is well known to the public. Therefore, the spoofing signal, which is very similar to the real satellite navigation signal, can mislead the receiver from the correct position. The precision code (P code) is encrypted and cannot be simulated easily. Even so, meaconing still can use it by the time delay module. For spoofing interference, the spoofer should first destroy the connection between the receiver and the real signal and should then make the receiver capture the spoofing signal, preferably by a higher signal power than the normal one. In [4], the authors point out that the tracking of the real signal can be destroyed as long as the power of the spoofing signal is at least four decibels (dB) higher than the real signal in the condition that the pseudo-code rate difference between the real signal and the spoofing signal is 1/3 Hertz (Hz) and the receiver's coherent accumulated time is 1 ms. To suppress spoofing interference, the existence of the spoofing signal must first be accurately detected. Previous detection technologies mainly focused on signal distortion detection, by considering the signal power [5; 6], the spatial distribution properties [7; 8] and by observing the change rates of ranges and the clock offset/drifts [1]. The signal power is influenced by many factors during transmission, and transient power increase does not mean the spoofing signal exists.