Design and implementation of a GNSS signal collection system using direct RF sampling

State-of-the-art commercial wideband ADCs (analog to digital converter) already have the qualification for direct RF (radio frequency) sampling, especially for bandpass sampling for L band GNSS (global navigation satellite system) receivers. Compared with IF (intermediate frequency) sampling, this approach has outstanding features of simple structure, high flexibility, as well as low distortion. This paper presents the design and implementation of a GNSS signal collection system using direct RF sampling, where only the amplifiers and filters are employed between the antenna and the ADC, and the collected data are transmitted to a computer via gigabit Ethernet for storage and processing. As for the most crucial issues of direct RF sampling, the impacts of aliasing noise and clock jitter are addressed, and an expression is given to calculate the requirements for aliasing noise. Then the design of FPGA (field programmable gate array) code and computer software is described. By means of a GNSS software receiver, the acquisition results and position solutions based on the signals collected from the live satellites as well as a GNSS RF signal simulator demonstrate that the system works effectively and provides the flexibility for multi-constellation multi-signal structure applications.