# Low-Cost COTS GNSS Interference Monitoring, Detection, and Classification System +
## 3 Hardware Design
### Overview  
Figure 3 shows a photo of the demonstration version of the monitoring hardware. The central part of the system is a Raspberry Pi model 4B, 4 GB RAM version (Raspberry Pi, Cambridge, UK) single-board computer (SBC), which does the primary processing, interfacing, and networking.  
GNSS reception is provided by a GNSS Raspberry Pi hat (pHat) containing a uBlox MAX-M8Q (uBlox, Thalwil, Switzerland). This uBlox receiver can process GPS, Galileo, Beidou, GLONASS, QZSS, and SBAS signals in the L1 band, but only a maximum of three concurrent GNSSs. Therefore, the current system setup configures the receiver to use GPS L1 C/ A, Galileo E1 OS-B/C, and GLONASS G1 OS to maximize the spectrum diversity. The SBC connects to the pHat via a universal asynchronous receiver-transmitter (UART) bus over the general purpose input/output (GPIO) interface and decodes the National Marine Electronics Association (NMEA) messages at a 1 Hz rate. The GPS service daemon (GPSD) primarily handles the GNSS receiver. The GNSS pHat can easily be substituted with other GNSS pHats as the pHat uses the standard GPIO interface of the Raspberry Pi. For example, several COTS pHats have dual-band GNSS receivers, such as the uBlox ZED-F9R [79], for a significantly increased cost. In the extreme case, it can even accommodate a geodetic-grade GNSS receiver such as the Septentrio MOSAIC (Septentrio, Leuven, Belgium) [80]. Nevertheless, the current GNSS pHat is ideal for a low-cost design approach.  
The second sensor is a NeSDR SMArt v4 (Nooelec, Wheatfield, NY, USA) that connects over USB 2.0 and contains an RTL2832U digital video broadcasting (DVB) radio-frequency front-end (RFFE). This sensor is reconfigured as a SDR RFFE and delivers 8-bit complex I/Q samples at a maximum sample rate of 3.2 MHz. It is sufficient to receive narrow-band GNSS signals, such as GPS L1 C/ A (1.023 MHz chipping rate), but insufficient to accurately process and localize most GNSS signals. Nevertheless, the bandwidth suffices for detection, monitoring, and classification purposes. The received samples are further processed on the SBC. The GNSS pHat and the NeSDR receivers are supplied with antennas. Hence, no additional antennas are required. The GNSS antenna is a standard low-cost right-hand circular polarized (RHCP) patch antenna with an integrated low-noise amplifier (LNA) and a main lobe towards the sky (zenith). The NeSDR uses a vertically polarized quarter-wave mono-pole antenna (whip antenna). The antenna has a null towards the sky and a main lobe towards the horizon, optimizing for terrestrial interference signals. Figure 4 shows the system outside in the Tupperware enclosure and with the antennas.  
The demonstrator requires 6.1 W on average and 7.5 W peak power, including full processing operation. The current concept demonstrator model includes a power bank.  
Figure 3: Top-view on the system hardware: the SBC, GNSS pHat, NeSDR, and power bank.  
It allows for autonomous operation for up to 8 h and is integrated into an ultra-low-cost Tupperware lunch box. Lastly, the Ethernet or the onboard WiFi connects the monitoring system for interfacing. Other connection approaches are discussed in Section 3.3.