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

GPS receivers come in a variety of shapes and prices, the more expensive the unit the more satellites can be observed and the higher the precision possible through post-processing of data. Units like the Trimble Ensign cost around $100 and can give positions to around 20-50m without selective availability. These units can give around 100-200m with selective availability but, as civilian users, we do not know whether this phenomenon is present. The accuracy is more than adequate for people who wish to know where they are and where they are going from the point of view of navigation. Other units with similar accuracies are available as an antenna/OEM board for a computer configuration, an antenna and PCMCIA card for lap-tops or installed as one of the options on luxury cars like the BMW Series 7 and Toyota Landcruiser Stationwagons.

This accuracy is not often adequate from the point of view of mapping or survey coordination, and certainly not for high precision cadastral surveying or engineering measurement. In order to achieve the sort of positional accuracy geomatics professionals are accustomed to, we need to purchase other hardware.

More expensive units like the Ashtech XII and the Leica System 300 are designed to store all the data that is received by the unit from all the available satellites, allowing the signals to be computed at a later stage to achieve much higher precisions and accuracies. The Leica unit also allows communication between the various components thus real-time differential positioning can be undertaken. Multi channel geodetic quality receivers cost from $25,000 to $50,000.