Global Positioning
    System
 The System
 The Position Solution
 Hardware
 Ionospheric And
    Atmospheric Delays
 The Signals
 Dynamic Positioning
 Static Positioning
 Coordinate Systems
 Conclusions

Satellites transmit the main positioning codes at two different frequencies. As mentioned previously, these frequencies were chosen partly so the effect of the ionosphere and troposhere on the transmitted signal are minimised. The frequencies used are, 1575.42MHz and 1227.6MHz (multiples of the fundamental frequency of 10.23MHz), modulated with the navigation message and the carrier wave.

The signals sent by the satellites to earth are fairly low powered and are barely discernible from the background radiation or noise of the heavens. If, say, we were receiving satellite television we would need a large parabolic dish to receive the signal, just look at the roof of any hotel that receives Sky Channel. An antenna of this design would severly limit the portability of GPS receivers. Instead the satellites generate pseudo-random code which can be deciphered only by receivers that generate a similar signal. Our GPS units shift their version of the pseudo-random code around the incoming signal until a pattern is seen that makes the signal stand out from the noise. As a result of this, only small antennae are required. Another benefit is that the US military can change this code to exclude access to the system.