In the second of a series of articles on GPS, Andrew Coleman looks at the factors affecting accuracy

On their journey from space GPS transmissions are disrupted by the Earth’s atmosphere, tall buildings, trees and radio interference. Add in electronic errors and astronomical ephemeris and the accuracy of an uncorrected GPS signal is only fifteen metres. GPS units can fail to get any fix.

Manufacturers have developed an arsenal of techniques to further reduce uncertainty. That extra smart software and processing power comes at a price. Good general purpose handheld GPS devices are now correct to five to ten metres and there are several options which offer greater accuracy. Differential GPS (DGPS) uses multiple ground stations located at accurately-surveyed points to measure the errors in GPS satellite signals and produce a  constantly updated ‘corrections map’. This map can be used to adjust the local readings made by roaming GPS units. Sub-metre real-time accuracy is possible with DGPS.

Terrestrial DGPS uses ground based transmitters to broadcast corrections for your area to your GPS unit. Commercial subscriptions to this are available in many countries including the UK. Satellite Based Augmentation Systems (SBAS) work much the same way but the corrections are broadcast from satellites. The service, called EGNOS in Europe, is now integrated in some GPS Tablet PCs. For EGNOS, just one satellite in geostationary orbit covers the whole of Europe, Middle East and Africa. So the signal is weak and low in the sky for northern latitudes. While this limits accuracy and availability, SBAS has the advantage of covering wide areas and being free. Precision of two to five metres is reported for SBAS enabled GPS.

Where real-time accuracy is not needed, Post-processed DGPS works by storing all your measurements in the unit’s memory and relying on computer software to later correct the positions. Accuracies up to ten centimetres are possible this way but you cannot see or correct readings in the field. More in part three.