## 4 Anti-Jam Capability of GPS SDR for CRPA  
This section demonstrates the anti-jam capability of our GPS SDR for CRPA under a synthetic wideband jammer. Live GPS L1 signals through a four-element antenna array are IF-sampled by four USRP2s and recorded in four SSDs. As mentioned in Section 3.1, we record GPS L1 data with 40 Msps rate to demonstrate the SDR's computational capability for future GPS L5 signal processing. Synthesized code division multiple access (CDMA) jamming signals are added to the recorded L1 signals by post-processing. Wideband CDMA jammers are more challenging to GPS receivers than narrowband continuous-wave, or CW, jammers. The jamming direction is selected as the direction of GPS signals from PRN 10 (Figure 9). This CDMA jammer is simulated using PRN 168, which is currently unused by any satellite, and the interference signal is amplified to have 40 dB jamming-to-signal-power ratio (J/S).  
Figure 9 shows parts of the captured output window of the SDR. Without adding the synthesized CDMA jammer, all tracking channels of the SDR track valid satellites in view as the C/N\({}_{0}\) plot of Figure 9 (middle) demonstrates. There are five tracking channels for each satellite. The first channel of each satellite in Figure 9 is the tracking channel for composite signal beamsteered to that satellite. The remaining four channels are the individual tracking channels for each of the four antennas for that satellite. The beamsteering channels of all satellites show about 6 dB C/N\({}_{0}\) enhancements due to the increased gain resulting from utilizing signals from four antennas rather than one antenna. These directional beams toward satellites provide benefits, such as mitigation of multipath errors, even under nominal conditions without GPS jamming.  
Once the simulated CDMA jammer is injected, most single antenna channels lose tracking as in Figure 9 (right). (The SDR does not show C/N\({}_{0}\) of lost channels.) We modulate alternating 1 s and -1 s in the navigation data of the jamming signals in order to check if the SDR tracks valid satellites instead  
Figure 9: Anti-jam capability of GPS SDR for CRPA under synthesized CDMA jamming. The SDR is capable of tracking 12 satellites but eight satellites are shown here because eight satellites are visible at this epoch.  
of the jammer. In fact, the single antenna channels of PRN 25 track the jammer instead of the correct satellite partly due to high cross correlation. Nevertheless, the beamsteering channels of all satellites including PRN 25 still track the correct satellites, and the receiver outputs correct position solutions. (PRN 10 is an exception because the jammer is injected in this direction.)  
Although our GPS SDR for CRPA rejects moderate to high levels of interference in the tests with live signals and synthetic interference, the saturation effects of the analog front-end due to real jammers cannot be studied in this way. Analytical and numerical studies neglecting these effects can be somewhat optimistic for the real-world performance of the GPS SDR for CRPA. To evaluate the saturation effects, live over-the-air GPS jamming tests, which are difficult to perform due to regulations, are necessary in the future.