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General / GPSTheoryAndBackground

Introduction

The global positioning system (GPS) is a satellite-based navigation system consisting of a network of 24 satellites which are orbiting in space approximately 11000 miles from the surface of the Earth. The satellites make two complete orbits of the Earth every day.

The first GPS satellite was launched by the US Military in February 1978 and civilian use began in the early 1990s. In May 2000 the US Military turned off selective availability, which reduced GPS positional errors from 30m to just a few meters.

Each GPS satellite transmits a message containing three pieces of information, the satellite number, its position in space and the time at which the message was sent. The GPS receiver reads the message and saves the information.

To calculate its position, the GPS receiver compares the time at which a signal was transmitted by a satellite with the time it was received by the GPS receiver. This allows it to determine how far away that particular satellite is. Using information from four satellites the GPS receiver can calculate its 3D position and the time. If the receiver already knows the time to within a few nanoseconds only three satellite signals are required, however this is not normally the case and in practice a minimum of four satellites signals are required for a “useful” solution.

The main unavoidable errors in the GPS positioning system are due to four main sources, all of which have approximately equal impact:

The last of these errors Ionosphere and Troposphere are caused by the atmosphere delaying the radio signals from the GPS satellites. All of these errors are present in the GPS "system" before the signal is received, so they are receiver independent. Typically the resultant error in position due to these sources is in the range 2m-3m. There are another two important sources of positional error in GPS positioning:

In an open environment with no local obstructions, multipath is quite low and not a major problem, however, in built up environments it can quickly become the dominant source of error and is particularly difficult to eliminate.

The raw satellite range comes from the receiver tracking the signal information from the satellite; the carrier phase comes from tracking the actual radio wave oscillation. All modern GPS receivers track both of these signals. Using modern electronics, receiver sourced errors are now very small. For raw satellite ranges the error is in the range of about 20cm, for carrier phase the errors are less than 1mm. Good positional accuracy relies on both accurate ranges and carrier, whilst speed and distance are predominantly based on carrier only information

In conclusion, most of the errors in GPS systems are caused by the signals themselves and not by the GPS receiver itself. Therefore, the raw positional accuracy from a good quality GPS system from Manufacturer "A" compared with Manufacturer "B" is likely to be very similar indeed.