How GPS works
Transmitted GPS Signals
The principle of position determination by GPS and the accuracy of the positions strongly depend on the nature of the signals. A variety of criteria was considered in the development of a suitable signal structure. In consequence the GPS signal is quite complex and offers the possibility of determining the following parameters: one-way (passive) position determination, exact distance and direction determination (Doppler effect), transmission of navigation information, simultaneous receiving of several satellite signals, provision of corrections for ionospheric delay of signals and insusceptibility against interferences and multi path effects. In order to fulfil all these requirements, the signal structure described below was developed.
Choice of the carrier frequency
To transport data signals, a suitable carrier frequency is required. The choice of the carrier frequency is submitted to the following requirements:
Frequencies should be chosen below 2 GHz, as frequencies above 2 GHz would require beam antennae for the signal reception
Ionospheric delays are enormous for frequency rages below 100 MHz and above 10 GHz
The speed of propagation of electromagnetic waves in media like air deviates from the speed of light (in vacuum) the more, the lower the frequency is. For low frequencies the runtime is falsified.
he PRN-codes (explained below) require a high bandwidth for the code modulation on the carrier frequency. Therefore a range of high frequencies with the possibility of a high bandwidth has to be chosen.
The chosen frequency should be in a range where the signal propagation is not influenced by weather phenomena like, rain, snow or clouds.
Based on these considerations, the choice of two frequencies proved to be advantageous.Each GPS satellite transmits two carrier signals in the microwave range, designated as L1 and L2 (frequencies located in the L-Band between 100 and 200 MHz).Civil GPS receivers use the L1 frequency with 1575.42 MHz (wavelength 19.05 cm). The L1 frequency carries the navigation data as well as the SPS code (standard positioning code). The L2 frequency (1227.60 MHz, wavelength 24.45 cm) only carries the P code and is only used by receivers which are designed for PPS (precision positioning code). Mostly this can be found in military receivers.