Articles

Global Positioning System

Introduction

The Global Positioning System (GPS) comprises the NAVSTAR network of 24 satellites orbiting at a height of 11,000 nautical miles, five control stations located on the ground and the user community throughout the world. The orbit of each satellite, which takes 12 hours, is arranged so that six satellites are usable from the ground most of the time.

GPS was developed by the US Department of Defence for military use, but the system was made accessible for civilian use under ther presidency of Ronald Reagan in the 1980s. The first satellite was launched in 1978 as part of a 10 strong experimental system but a second phase of launches between 1989 and 1994 built of the success of the experimental network and has resulted in the present day constellation of 24 satellites. Each satellite is expected to last for up to ten years.

The GPS system is now commonly used through the world and has a number of applications such as navigation from a vehicle while on the move (projecting position onto an electronic road map), aviation & marine (both commercial and recreational), large contruction projects, time sources for computer networks (where the time can be derived from GPS with an accuracy of 1 microsecond or better), expeditions, and individual recreational activities (hiking, fishing, orienteering, etc...). A modern GPS receiver is a handheld battery device (often spashproof or waterproof) which can provide 24 hours of battery operation and has numerous features on it (such as 12 parallel channels, waypoints, speed & direction when moving, proximity warnings and altitude measurements).

Garmin GPS12XL receiver

At midnight on 1 May 2000, the accuracy of the GPS system was dramatically improved when Selective Availability was switched off by order of President Clinton. This means that civilian users now have access to the full accuracy GPS offers. Typically this means that positions can be accurate to about 10m depending on the number of satellites available.

How position is determined

Each satellite transmits a radio frequency signal (the civilian signal is on 1575.42 MHz) with a power of 50W for reception on the ground without a dish or other form of directional antenna. A pseudo-random binary sequence is sent which is actually a code generated in one of 32 unique ways so that each of the 24 satellites can be identified. The "pseudo-random code number" 1-32 is usually displayed by the receiver for each satellite in use. Ephemeris data giiving the satellite status (healthy/unhealthy) and time & date information is also sent together with almanac data giving the orbital position throughout the day for the satellite itself and all the other satellites in the constellation.

The receiver builds up a picture of where it is on the Earth's surface using the information sent from as many satellites as it can find in the sky. It does this by determining how far each satellite is away by comparing the time each satellite signal was transmitted with the time it was received on the ground. Knowing the speed of light, the receiver is able to determine the distance the radio signal will have travelled and triangulate its position.

The receiver is assumed to have its own accurate time source in order to measure time satellite differences, but it actually uses the satelites' clocks to synchronise its internal clock to the current time. The satellites all carry clocks accurate to 3 nanoseconds.

If there is only one NAVSTAR satellite in view the receiver is limited to computing the distance to the satellite and determining it is somewhere on the surface of an imaginary sphere with the satellite at its centre. This is not enough to provide a position, so more satellite signals are needed. If there are two satellites, the receiver can now repeat this "ranging" calculation but because it knows it has to be on both imaginary spheres at once (one for each satellite), it can now narrow its position to being somewhere on the circular line where the two spheres intersect. This is still not enough but if a third satellite signal is added, the receiver is able to determine that it is on one of two possible locations on the circular intersection. Usually, one of the two positions is impossible and can be eliminated leaving just the true position. If the altitude above the Earth's surface is also required, this requires a fourth satellite signal.

Limits to position accuracy

There are several limitations to the accuracy of the position given by GPS. Some are listed below together with measures that can be taken to counteract them (where possible):

• Selective Availability. This was a deliberate degradation of the accuracy of the system to reserve its highest accuracy for military use by the US DoD. It has now been turned off by Presidential order following new security assessments which showed that the benefits to users outweigh the risks to US security. It was the largest degradation in performance and resulted in typical position accuracy of about 100m, now full accuracy is available positions can be obtained with an accuracy of down to 10m. It is still possible to improve the accuracy of GPS position readings using the following methods: your current position can be read continuously and averaged over a long period of time; more than one operating frequency can be exploited; a differential GPS (DGPS) service can be used (which transmits correction information from a known fixed location). Subscription DGPS services are available in the US on a VHF FM terrestrial network, or via satellite. A free service is available on a network of maritime beacon transmitters operating on LF/MF (283.5-325 kHz) which send correction information at 100bps using minimum shift keying (a form of FSK). Here in Europe, the LF network (restricted to 283.5-315 kHz) is also free and covers most of North Western Europe's coastline from a number of countries. Here in the UK, Trinity House operates six DGPS sites (at Flamborough Head, North Foreland, St Catherines, Lizard, Nash Point & Point Lynas). DGPS can improve position accuracy to 5-10m.

• Satellite Geometry. This is the impact on accuracy of the relative position of the NAVSTAR satellites. If the satellites that are in view from the ground are all more or less in the same part of the sky (eg. because the terrain is obstructing satellites from other directions), it can be difficult for the receiver to triangulate accurately. Position error because of satellite geometry can be as large as 100-170m. Little can be done to overcome this problem, but the receiver is able to estimate (and report) an estimate of the degree of loss of precision (DOP=dilution of position).

• Propagation delays and multi-path. These effects cause the radio signal from satellites to have an apparent unexpected delay in arrival time. Propagation delays with line of sight satellite signals are caused by differences in the speed of light because the radio signal is passing through densly ionised plasma (the ionosphere) and also the lower part of the atmosphere (which has a relative permittivity that is higher than one). If the radio signal were travelling through free space or a medium with know constant properties, its speed would be predictable. The effect of propagation through the ionosphere in particular is difficult to predict as the density of ionisation varies considerably with time of day, season and the 11 year cycle of solar activity. Multi-path occurs where the satellite signal is not received line of sight but has been reflected by buildings or natural features in the terrain so that it has travelled further than expected. The impact of these effects on position accuracy are difficult to determine, however, DGPS is effective at overcoming them.

• Internal clock errors. These cause inaccuracy as a result of the clock inside the receiver being wrong (it should be set from the satellite network). Drift of the clock before re-synchronisation is an example of how the internal clock can be wrong.

National Maritime Electronics Association (NMEA)

The National Maritime Electronics Association was formed in 1980 by a group of individuals to try and provide a way of allowing marine electronics systems to inter-operate. The result was the NMEA interfacing standards which support inter-connection between a variety of maritime systems (such as radio communication & satellite equipment, direction finding apparatus, weather instruments, time systems, radar systems, etc...).

A feature of modern GPS equipment is that it can output a serial data stream containing full satellite status and position information in NMEA 0183 protocol.

NMEA 0183 protocol

Global Positioning System | NMEA 0183 Protocol

This document maintained by simon.collings@cableinet.co.uk.
Material Copyright © 2000 Simon Collings