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Updated:
28 June 2005
Global Positioning System (GPS) & GLONASS
IntroductionThe Global Positioning System (GPS) is a satellite based navigation system developed by the United States Department of Defense. It is widely used for civilian navigation and positioning, surveying and scientific applications. GLObal NAvigation Satellite System (GLONASS) is operated by the Russian Federation Ministry of Defense. An overview of GPS Positioning Techniques
Point positioning and navigationPrior to 1 May 2000 the GPS specifications stated that under conditions of 'Selective Availability' a single horizontal GPS position will have an uncertainty of 100 metres at 95% confidence. However, on 1 May 2000, 'Selective Availability' was turned off by the United States Government, so that best practice is observed, a single GPS position will now have an uncertainty of about 10-20 metres. This type of position can be obtained from simple hand-held GPS receivers costing a few hundred dollars, or from more sophisticated (and expensive) surveying receivers operating in a simple point position mode. However, the consistency of the result can be affected by the hardware (eg clock, number of channels) and the firmware (ie the filtering and processing used). As this type of positioning is not dependent on another GPS base station, it is particularly important that the correct datum is selected. By default, GPS uses the World Geodetic System 1984 (WGS84), but many GPS receivers include a menu of local datums that you may select. Before starting to collect data with your GPS receiver, you should select the datum that is compatible with the map, chart or digital data you are using, or you may be able to collect the data in terms of WGS84 and transform it later, using a separate software package. The first strategy is preferable if you wish to immediately locate your position on a map or chart, but the second method may be better if you are uncertain of the local datum, or if you wish to later use a better transformation method. Heights are a particular problem as the GPS usually gives the height in terms of the GPS's WGS84 system (an ellipsoidal height) rather than the more usual sea level. The difference between these two reference surfaces is the geoid-ellipsoid separation. In Australia this correction may range between -40 and +75 metres and globally it can be in the range of -100 to +100 metres. Differential navigation (DGPS)To improve the accuracy of positions obtained from GPS, differential corrections can be applied to the observed position at a remote site. These corrections are calculated at one or more base stations by comparing the observed position with the known position at those base stations. The base stations may be a permanent service, or be setup specifically for a project. When applied at the remote site these corrections greatly enhance its positional accuracy. The correction can be in terms of position, or more often in terms of the observed satellite-receiver distance (the pseudo-range). The corrections may be collected and applied at a later time, or they may be broadcast immediately to the remote site by mobile phone, radio or satellite communications. Two commonly used formats for this type of differential correction are the NMEA and RTCM formats. DGPS positioning can be carried out with some simple hand held receivers over a few 10s of kilometres, or it may be done with sophisticated multi-basestation systems integrated with satellite communications, to cover a region of thousands of kilometres (wide area differential navigation). The accuracy of DGPS, of the order of a metre, generally degrades with increased distance from the nearest base station. Static differentialStatic Differential GPS, typically used for accurate surveying measurements, determines the difference in position between a GPS receiver on a known position and another GPS receiver at an unknown position. This technique uses the phase of the GPS signal, rather than the observed satellite-receiver distance (the pseudo-range), giving at least an order of magnitude increase in accuracy compared to the DGPS technique. By observing simultaneously for a few hours and with appropriate post-processing of the data, this technique can be used over distances up to several hundred kilometres to produce a relative accuracy of about a part per million (1 part per million = 1 mm per kilometre). This requires sophisticated GPS receivers, typically costing $20,000 or more. Pseudo-kinematicKinematic GPS uses the phase of the signal, with sophisticated post-processing, to determine the position of a moving platform (car, boat, plane etc) relative to a fixed base station. Knowledge gained from the kinematic technique has been used to develop techniques that allow the difference in position, between a GPS receiver on a known base station and another GPS receiver at an unknown position, with an accuracy of a few centimetres, over distances of 10 or 20 kilometres, with only a couple of minutes of observation. Variations on this type of technique are known by a variety of names (rapid static, fast static, stop and go kinematic, pseudo-kinematic etc) but they all require sophisticated GPS receivers typically costing $20,000 or more. Regional processingThe International GPS Service (IGS) maintains a global network of permanently operating GPS base stations, which contribute to a database of GPS observations. The Australian Regional GPS Network (ARGN) contributes to this network. By observing for a long period (several days) with a single geodetic GPS receiver and using sophisticated software together with data from the global network, it is possible to obtain a position with an accuracy of a few centimetres, in the global reference frame. This type of processing is typically only available from academic or national geodetic organisations. The Geoscience Australia Online GPS Processing Service - AUSPOS provides users with the facility to submit dual frequency, geodetic quality, GPS RINEX data to the Geoscience Australia GPS processing system and receive rapid turn-around International Terrestrial Reference Frame (ITRF) coordinates and Geocentric Datum of Australia (GDA) for Australian users. This service works with data collected anywhere on Earth. It is a FREE service. Best Practice for GPS Point Positioning and NavigationThe quality of a GPS position can be affected by the number and position of the satellites at the time the position was determined. Although a 2-dimensional position can be obtained with signal from just 3 satellites, 4 or more are preferable, and they should be evenly distributed across the sky. Typical DOP pattern for eastern Australia The geometry of the visible satellites is indicated by a quality indicator known as the "Dilution of Precision (DOP)". (This is displayed in the image for eastern Australia, with the green line as Positional Dilution of Precision (PDOP), the red line as Horizontal Dilution of Precision (HDOP) and the blue line as Vertical Dilution of Precision (VDOP)). The smaller the DOP the better, with less than 5 being quite good, 5 to 10 marginal and greater than 10 poor. Many GPS receivers provide this or similar information (eg figure of merit (FOM), root mean square (RMS)) based on the satellites actually observed, but the theoretical value can be calculated for a given time and place prior to observation. Pre-calculation may be useful to determine times to avoid, but it does not take into account masking of the satellite signal when actually observing. You should always take note of the receiver's indication of the quality of the fix (however it is provided) and be wary of any position obtained when there is a sudden change in the quality indicator. The number of visible satellites may be limited by surrounding objects (eg buildings, hills, trees) or even the observer's body. It is therefore necessary to take care about where you use the GPS, how you hold the receiver, or where you mount the antenna on a vehicle. DatumA datum is the reference framework for the coordinate system, generally consisting of one or more known positions. A geometric model for the Earth's shape (an ellipsoid, or spheroid) is usually associated with the reference framework, to allow the computation of geographic coordinates (latitude and longitude). In the past each country established a datum that best fitted its region, based on astronomic positions. However positions in terms of these local datums may be up to many hundred metres different from positions in terms of the global coordinate system used by GPS (the World Geodetic System 1984 - WGS84). Many GPS receivers include an option to transform the GPS's WGS84 position to a selected local datum, using parameters developed by the United States Department of Defense. These transformations are generally only accurate to a few metres, but this is generally adequate for a single point position. More accurate transformation techniques may be available from the GDA Technical Manual. These transformations should not be confused with the conversion between two ways of representing positions on the same datum - geographical coordinates (latitude and longitude in degrees, minutes and seconds) and grid coordinates (eastings and northing in metres). In Australia the local coordinate system established in 1966 was the Australian Geodetic Datum (AGD). Positions in this system are about 200 metres different from the default WGS84 positions produced by GPS. However, Australia is changing to a new coordinate system, known as the Geocentric Datum of Australia (GDA), which is compatible with the GPS coordinate system. Selective Availability'Selective Availability (SA)' was the deliberate degradation of the broadcast GPS signal, by the United States Department of Defense. On the 1st May 2000, the United States Government turned off this deliberate degradation. This degradation affected either the satellite positions broadcast by the GPS satellites (the broadcast ephemerides) or the timing used to determine the satellite-receiver distance (which in turn is used to determine the receiver's position). This 'Selective Availability' may be varied at times, within the design specifications. For more information contact: geodesy@ga.gov.au
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