Interference to GPS from UWB Transmitters

Ultra Wide Band (UWB) technology is based on very short pulses of radio energy. In theory, its wide signal bandwidth yields excellent multipath immunity. Hence, UWB has been used in a variety of applications, including communication and ranging, and is expected to see increased use in the future. Since signals from GPS satellites have very low power levels (-130 dBm or -160 dBW) near the surface of the Earth, potential interference from UWB to GPS receivers (and therefore to GPS-based system such as aeronautical safety-critical flight systems) is a serious concern. Stanford University has designed and conducted a series of accuracy tests on an aviation-grade GPS receiver to study the impact of UWB. These tests quantify the impact of UWB signals relative to white noise of equal power. In other words, they determine whether a given UWB signal has more or less effect on accuracy than an equivalent amount of white noise. Here, white noise refers to continuous noise from a noise diode that has a power spectral density much broader than the RF front end of the GPS receiver. This noise is used to model thermal noise in the receiver, sky noise, and any other wideband interference processes other than UWB. UWB signals also have bandwidths that are greater than the front end of the GPS receiver, but they have an additional structure that may cause their effect to be very different than that of white noise. Accuracy is the metric of choice for aviation receivers because the most demanding precision approach operations require airborne pseudorange accuracies of approximately 15 cm [2]. Accuracy measurements also include the deleterious effects of cycle slips and are the most appropriate metrics for precision approach. The effect of the UWB signal is sensitive to the details of the UWB signal design. This paper presents test results for the following UWB parameters: PRF (0.1, 1.0, 20.0 MHz), modulation (no modulation, random OOK, random PPM), burst duty cycle (10%, 50%, and 100%), burst-on time (10 us, 1 ms, 10 ms). In each case, the UWB power in the GPS L1 band was swept from –115 dBm to about −50dBm.