Differential Global Positioning Systems work with a second, stationary GPS unit at a known location. The stationary unit calculates its position from the GPS signal and compares this to its known coordinates. The difference is then applied to the mobile unit as an error correction.

The correction can be transmitted directly to the mobile unit and applied in real time or applied at the end of a session using accurate time records from both the mobile and stationary units (post processing).

The closer the base station is to the mobile unit the more accurate the error correction will be. This is because a close base station will see the same set of satellites as the mobile unit and the signal will travel a similar path through the atmosphere.

There are three common base station systems available at present.

1. Marine beacons.

These base stations use AM radio transmitters to send the error correction to the mobile unit. Beacons are located in coastal areas and were originally designed to assist marine navigation. However, the signal can be used up to about 300km inland.

The marine beacon signals are a free service and are available in coastal areas of most developed countries. The user only requires an additional radio receiver and software to handle the error correction. The beacons are most common near difficult or dangerous waters. The Queensland coast has excellent coverage due to the presence of the Great Barrier Reef. For beacon location information in Australia click Here.

Maximum errors using the marine beacon systems are generally <3m. Radio interference (e.g. from overhead power lines) or loss of signal in very steep topography results in a fallback to the standard GPS system with errors of up to 10m. Data collected in these areas can be flagged as lower accuracy or discarded.

GEOMAP has a system of this type available as a service option.

2. Satellite Signal.

These systems transmit the error correction using a separate satellite signal and receiver attached to the mobile unit. The errors are calculated from an array of ground stations using arithmetic weighting for the distance between each station and the mobile unit. Because the error correction is transmitted via satellite, coverage is available over most land areas. However, density of ground stations may be low in some countries.

Ground stations for this system are provided by commercial organizations and the error correction signal must be purchased for defined periods. Typical rates are approximately AUD$2500 per year. Monthly rates may be available from some vendors.

Maximum errors using the satellite signal error correction are typically <1m in Australia. Exceptions can occur in areas where the main GPS signal is reflected off a large metal object or very steep topography (multipath errors).

An alternative set of ground stations and satellites with a free public signal is now available in the US (WAAS system) and Europe (EGNOS system), but the nearest station to Australia is in Hawaii which results in errors similar to uncorrected GPS.

3. Local Base Stations.

These systems use a base station set up in the near vicinity of the mobile unit. Error correction is sent to the mobile unit via an FM radio transmitter or collected for post processing. Because the base station is very close to the mobile unit, these systems provide the highest accuracy.

This requires purchase of the complete base station, transmitter and receiver system and accurate surveying of the location of the base station.

There are three flavours of base station linked GPS systems; DGPS sends a simple distance correction for each satellite and generally achieves maximum xy errors of about 2m. RTK L1 measures the distance to each satellite in wavelengths of the carrier frequeny and achieves maximum xy errors of about 0.5m. RTK L2 is similar to RTK L1 but also monitors a second frequency transmitted by each satellite to calculate an ionospheric delay. This reduces maximum xy errors to about 5cm. All of these numbers can be reduced by about half by integrating a point for a few minutes or selecting times with good satellite geometry.

It is a fundamental rule of GPS geometry that vertical (z) errors will always be about double the xy error.

The accuracy of DGPS and RTK systems is generally beyond that required for geological mapping, but they can provide a very rapid and cost effective means for surveying large numbers of drill collars or picking up infrastructure and surface topography such as waste dumps. These systems are still subject to multipath errors. Radio dead spots can also be a problem if using real time correction.

GEOMAP has an RTK system available as a service option.