Application of Signal Processing in GPS Signal Detection Using USRP

The Universal Software Radio Peripheral (USRP) is slowly becoming a very popular piece of hardware used in different universities and research labs across the world. It is inexpensive, which attracts a lot of attention, along with its variety of applications and capabilities. The USRP connects to a host-computer through a high-speed USB or Gigabit Ethernet interface. Another reason for the increasing popularity of the USRP is its ability to respond to multiple programming software such as GNU Radio, Matlab/Simulink, and LabView. There is a broad range of capabilities of the USRP one of which includes receiving GPS signals. Each GPS satellite transmits data on two frequencies, L1 (1575.42 MHz) and L2 (1227.60 MHz). We focused on the L1 band that transmits at 1575.42 MHz. The main objective was to engage in signal analysis with the carrier signal of the L1 band. This task proved to be challenging but not impossible. The carrier signal is essentially to carry information modulated on to it; in this case the navigation message from the GPS satellite along with other codes that are irrelevant to use at this point. After receiving the signal via USRP and with a simple extraction of the carrier signal, we were able to record the signal and reconstruct it using its In-phase and Quadrature phase (IQ) data. With just the carrier signal, one is able to do multiple things. One can modulate his/her own information onto the signal and transmit it through the USRP. Further analysis on the characteristics of the signal can be done. For example, one can compare the strength of a direct to the indirect signal. In theory, one can determine the characteristics of the surrounding area when comparing direct and indirect carrier signal. The possibilities are endless.