The purpose of the receiver front end is to ®lter 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 ampli®cation in the receiver front end provides as | Global Positioning Systems Inertial Navigation and Integration Mohinder S. Grewal Lawrence R. Weill Angus P. Andrews Copyright 2001 John Wiley Sons. Inc. Print ISBN 0-471-35032-X Electronic ISBN 0-471-20071-9 __4 Receiver and Antenna Design RECEIVER ARCHITECTURE 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. . Radio-Frequency Stages Front End 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 bandwith of both the LA and 2 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 to prevent front-end overloading by strong interference and the sharp cutoff filters are used later after downconversion to intermediate frequencies IFs . 80 RECEIVER ARCHITECTURE 81 Antenna Fig. Generic GPS receiver. Frequency Downconversion and IF Amplification After amplification in the receiver front end the GPS signal is converted to a lower frequency called an intermediate frequency for further amplification and filtering. .
