# A Survey on Satellite Communication System Security
## 5 Defense in Satellite Communication Systems
### Solutions for Integrity
#### 5.3.2 Hardware-Dependent Parameter Analysis Methods  
(i) Antenna Array Analysis: The role of antenna arrays in anti-spoofing and jamming resistance for GNSS receivers is critical, as highlighted in various studies. These arrays, comprising multiple closely spaced antennas, can either form a fixed receive beam width or adaptively modify it, enhancing the resistance to jamming. In [104], NavAtel's GAJT uses a seven-element controlled reception pattern, and QinetiQ employs a null-forming algorithm, both recognized for their jamming resistance. However, implementing adaptive modification, usually in safety-critical applications, can be complex and costly. One challenge is that measurements in receivers with an antenna array and beamforming equipment can be distorted due to time processing, affecting high-accuracy applications like carrier phase measurements. In another approach, in ref. [105], the focus shifts to double-antenna systems, which are particularly useful in scenarios where antennas are not synchronized. While synchronizing measurements typically require two receivers connected to a common oscillator or precise timestamped measurements, this can be impractical or unreliable in low-cost receivers or spoofing attacks. To overcome synchronization challenges, this study proposes a detection method based on double-antenna power measurements using the correlation outputs of the code-tracking loop for spoofing detection, suitable for typical anti-spoofing applications [106; 107]. Both approaches assume that spoofing signals originate from a single point source and employ spatial domain processing techniques to isolate and discard these signals. This is implemented before the despreading and acquisition stages in a GNSS receiver, significantly reducing the computational load by eliminating the need for extensive searching in the code and Doppler domains to differentiate between authentic and spoofing signals. Also, the system models assume an arbitrary configuration of an N-element antenna array, where the distance between adjacent elements is less than half of a GNSS signal carrier wavelength. Also, they use the same in their experimental setups. This describes a complex baseband representation of received signals at the array, considering both authentic and spoofing signals involving steering vector matrices and noise vectors. (ii) Direction of Arrival: The principle of direction-of-arrival (DOA) sensing is a pivotal element mitigation method against GNSS spoofing. In [108], a system developed in [PERSON]'s lab is discussed, which utilizes interferometry principles for spoofing detection. This system employs software and two antennas to measure the carrier phase, a property critical in discerning signal characteristic variations between antennas. This variation is instrumental in determining the signal's angle of arrival. By analyzing these angles, the system can identify inconsistencies indicative of spoofing based on the fact that while a spoofer transmits false signals from a single antenna, authentic GPS signals originate from multiple satellites and thus arrive from different directions. Similarly, ref. [109] focuses on a novel spoofing detection technique that relies on DOA estimation. This approach involves categorizing emitters and selecting those with high elevation angles. DOA estimation is crucial for distinguishing between authentic and spoofing signals, especially when they emanate from different spatial directions. The technique's reliance on the DOA allows for more precise identification of spoofed signals by contrasting the spatial characteristics of authentic and counterfeit signals. (iii) Subspace Projection: Subspace projection introduces versatility to techniques for GNSS security. Whether it is adapting to dynamic jamming characteristics or efficiently mitigating spoofing attacks in single-antenna systems, subspace projection provides a method for enhancing the reliability and security of GNSS receivers against sophisticated interference threats. Ref.