Although there are many variations in GPS receiver design, all receivers must perform certain basic functions. We will now discuss these functions in detail, each of which appears as a block in the diagram of the generic receiver shown in Fig. 4.1.
The purpose of the receiver front end is to filter and amplify the incoming GPS signal. As was pointed out earlier, the GPS signal power available at the receiver antenna output terminals is extremely small and can easily be masked by interference from more powerful signals adjacent to the GPS passband. To make the signal usable for digital processing at a later stage, RF amplification in the receiver front end provides as much as 35–55 dB of gain. Usually the front end will also contain passband filters to reduce out-of-band interference without degradation of the GPS signal waveform. The nominal bandwidth of both the L1 and L2 GPS signals is 20 MHz (±10 MHz on each side of the carrier), and sharp-cutoff bandpass filters are required for out-of-band signal rejection. However, the small ratio of passband width to carrier frequency makes the design of such filters infeasible. Consequently, filters with wider skirts are commonly used as a first stage of filtering, which also helps prevent front-end overloading by strong interference, and the sharp-cutoff filters are used later after downconversion to intermediate frequencies (IFs).