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Signal Processing in GPS
GPS modules have to evaluate weak antenna signals from at least four satellites, in order to determine a correct three-dimensional position. A time signal is also often emitted in addition to longitude, latitude and height. This time signal is synchronized with UTC (Universal Time Coordinated). From the position determined and the exact time, additional physical variables, such as speed and acceleration can also be calculated. The GPS module issues information on the constellation, satellite health, and the number of visible satellites etc.
The signals received (1575.42 MHz) are pre-amplified and transformed to a lower intermediate frequency. The reference oscillator provides the necessary carrier wave for frequency conversion, along with the necessary clock frequency for the processor and correlator. The analogue intermediate frequency is converted into a digital signal by means of a 2-bit ADC. 
Signal transit time from the satellites to the GPS receiver is ascertained by correlating PRN pulse sequences. The satellite PRN sequence must be used to determine this time, otherwise there is no correlation maximum. Data is recovered by mixing it with the correct PRN sequence. At the same time, the useful signal is amplified above the interference level [xxv]. Up to 16 satellite signals are processed simultaneously. The control and generation of PRN sequences and the recovery of data is carried out by a signal processor. Calculating and saving the position, including the variables derived from this, is carried out by a processor with a memory facility.


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