# LO and Calibration Signal Distribution in a Multi-Antenna Satellite Navigation Receiver +
## 1 Introduction  
Global navigation satellite systems (GNSS) are key to safety-critical applications like automated and connected or tele-operated driving or unmanned flight maneuvering. These applications depend on the reliable availability and functionality of GNSS signaling. Due to their positioning on medium earth orbits (MEO) and the available link budgets, the respective received signal power may be as low as \(-130\) dBm (corresponding to \(10^{-16}\) W) at ground level [1]. This low signal strength makes the receivers prone to deliberate or unintended RF interference (RFI). Sources of unintentional distortions are, e.g., harmonic tones of local oscillators or strong signals transmitted by other wireless services in adjacent frequency bands. Among the deliberately caused disturbances, signal jamming and spoofing attacks are of major concern [2]. The term jamming refers to the transmission of a strong RF signal in or close to the GNSS frequency bands. The signal saturates the analog frontend of the GNSS receiver, thus blocking the reception of the navigation signals. As is widely known though legally prohibited in many countries, truck drivers use so-called personal privacy devices (PPD) to hide their location from third parties [3]. Due to the low GNSS signalpower, jamming is easily achieved with an appreciable areal coverage. The detrimental impact of such jammer devices on safety-critical infrastructure or processes is frequently under-appreciated. Spoofing describes the process of deceiving a GNSS receiver, leading to false position, velocity, and time (PVT) data. Spoofing attacks are more challenging to identify, but are also more complex to implement compared to jamming. Several methods have been proposed to achieve the immunity of a GNSS receiver against intentional or unintentional interference [4]. One possibility is to enhance the robustness of the analog receiver frontend. As an example, a receiver chain architecture able to handle lower output signal levels was less susceptible to saturation [5; 6]. Another approach is the use of multi-antenna receivers in array configuration, enabling beamforming and null steering [7; 8; 9]. Dedicated digital array signal processing helps to suppress unwanted signals by directing radiation nulls towards the interfering source, or estimating the directions-of-arrival (DoA) to verify the authenticity and validity of incoming signals [10]. We previously introduced a 2 \(\times\) 2-element uniform rectangular array (URA) connected to a dual-frequency receiver frontend [11]. This receiver system provides both a high positioning accuracy, due to the dual-band design, and high robustness against interference, due to the array performance. Despite the compact dimensions of the array with the radiating elements separated by a quarter wavelength, these features come at the expense of a large module size of 10 cm \(\times\) 10 cm \(\times\) 10 cm. Such dimensions exceed typical installation spaces, e.g., of passenger cars, and thus prevent their penetration into a mass market. Therefore, our goal was to realize a more compact GNSS receiver system. We proposed to split the full number of four antenna elements into two distributed two-element sub-arrays [12]. This approach effectively increases the number of possible mounting spaces by enabling the installation of sub-arrays in distant parts of a car, such as side mirrors or other suitable car body appliances. A drawback of this approach, however, is the creation of grating lobes due to the electrically large element separation that challenges array signal processing like DoA estimation. This effect can be mitigated by optimizing the arrangement of the sub-arrays [13]. On the RF frontend side, the concept of spatially separated sub-arrays necessitates new concepts for the design and calibration of the receiver hardware.