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GPS Signals and usage of SPS ranging signal  
GPS Signal
GPS Signal
An electromagnetic signal originating from an operational satellite. The SPS ranging signal consists of a Pseudo Random Noise (PRN) Coarse/Acquisition (C/A) code, a timing reference and sufficient data to support the position solution generation process.

There two primary transmission signals that each GPS operational satellite emits which are broadcasted on two primary frequencies. The GPS signal comprised of
a. Pseudo Random Noise (PRN) Coarse/Acquisition (C/A) code
b. Timing reference and sufficient data to support the position solution generation

First signal is at 1575.42 MHz; is also known as a Right Hand Circularly Polarized (RHCP) - L1. This signal transmission power is just right to ensure minimal signal power of -160 dBw at the surface of earth. L1 is Bipolar-Phase Shift Key or BPSK modulated with a Pseudo Random Noise or PRN 1.023 MHz code known as the Coarse/Acquisition or C/A code. This Coarse/Acquisition code sequence repeats each millisecond. Primary reason for this code introduction into signal is to corrupt signal so that it’s not as precise as pure signal. This is done by DoD which tries to prevent full system accuracy to unauthorized users. They call it Selective Availability which is intentional corruption of the signal and GPS satellite clock. DoD also uses encryption of the signal in order to safe-guard it from being intentionally corrupted by jamming. Please read GPS Jamming Article about this in details.

The SPS receiver demodulates the code from the L1 satellite, and detects the differences between the transmitted and the receiver-generated code. The SPS receiver uses an exclusive-or truth table to reconstruct the navigation data, based upon the detected differences in the two codes.

Second ranging signal is transmitted at 1227.60 MHz and is known as L2. This signal transmission power is just right to ensure minimal signal power of -166 dBw at the surface of earth. DOD does not consider this signal to be a part of SPS, even though civil GPS receivers use it in their design. It allows them to use L2 to support two frequency corrections.

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