OR/15/057 Principles of GPS

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Jones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/057.

The Global Navigation Satellite System (GNSS) is a worldwide, space-based, navigation system consisting of 4 global navigation systems (GPS – United States, GLONASS – Russia, COMPASS – China, Galileo – European Union) with a combined constellation of over 90 satellites (by 2020) orbiting the earth, twice a day, at a height of between 19130 and 23220 km on precisely determined orbital paths. These paths are defined so that at least 5 satellites are visible anywhere on the earth at any time. Each satellite transmits signals which allow the distance between a receiver and the satellite to be calculated. By calculating the distance from at least four satellites whose positions are precisely known then an accurate position for the receiver can be established (Figure 4).

Figure 4    Measurement of position from four satellites.

Positional accuracy with a single receiver, for civilian use, approximately equals 2 to 5 m horizontally and height accuracy is generally 5 to 10 m, for 95% of the time. The positional accuracy is affected by GPS satellite orbit errors, the atmosphere and receiver clock errors. To give better accuracy the known errors must be accounted for. The GPS satellite orbit errors and other errors introduced into the signal travel time due to it travelling through the atmosphere, cannot be computed by a single receiver in real time. The real-time positional accuracy of a single receiver can be greatly improved by using a more accurate technique known as Differential GPS (dGPS).

Initially, dGPS involved the use of twin receivers; the base receiver being installed at an accurately known position and the apparent location error used to correct the remote receiver which was tracking the same satellites (Figure 5). Thus any error which was common to both receivers was accounted for. This can be done in real time with corrections being transmitted from the base to the remote receiver or it can be done by post-processing data from each receiver.

Figure 5    Improved accuracy using dGPS.

Networks of permanent Differential GPS stations have been installed all over the globe with data often available free of charge. Data is available in standard format called the Receiver Independent Exchange (RINEX) Format. This allows for the differential processing of data from a range of different receivers. The great disadvantage of GPS is that as the satellite signal is quite weak, line of sight from the receiver to the satellite is essential. Therefore GPS cannot be used indoors or in areas where a clear view of the sky is not possible such as in forests, next to tall buildings or in deep valleys. Great care should always be taken in positioning GPS receivers and when using twin receivers ensuring each receiver can track the same satellites (Figure 6).

Figure 6    Poor/lack of satellite signal, leading to loss of accuracy.

A Real Time Kinematic (RTK) network is a network of permanent GPS and/or GNSS receivers whose combined data is used to generate RTK corrections for a rover — these network generated RTK corrections are called Network RTK.

RTK Networks can vary in size, from small local networks consisting of only a few reference stations, to dozens of reference stations covering a whole country. A user subscribes to a Network RTK Service to receive RTK corrections with their rover (instead of setting up their own reference/base station). The server generates and sends RTK corrections directly to the rover, which uses these to compute an RTK solution (Figure 7).

Figure 7    Relationship between server and rover in Network RTK.