GPS Technology
Global Positioning System (GPS) is a free to access Global Navigation Satellite System (GNSS) developed by the United States Department of Defenseand managed by the United States Air Force 50th Space Wing.
A receiver on the ground detects radio signals broadcast from the satellites, and in a similar way to a conventional trilateration, computes its position. A minimum of 5 satellites signals are required and the accuracy will depend upon the way the receiver interrogates the signals.
The position calculated by a single GPS receiver is subject to a range of errors, most significantly distortion to the signals in the ionosphere, which result in an accuracy of only 3-10m. However, instrument manufacturers have developed techniques, or augmentations, which enable the positional accuracy to be greatly improved. The development of differential GPS systems has been the most significant of these to surveyors, which enable sub-centimetre accuracy to be achieved.
Differential systems utilise two GPS receivers working in the same area to remove these error from calculated positions. One receiver remains at a fixed position, the base receiver, and is then able to calculate the ‘differences’ between its known and calculated position. On the assumption that the errors will be the same in a local area this 'differential correction' is applied to the positions calculated by the roving receiver.
Rapid development of this technique has resulted in real–time systems (RTK) broadcasting corrections to the roving GPS via a radio link. The latest developments allow a single GPS receiver to achieve cm accuracies, using network RTK systems which compute ‘virtual’ base stations and deliver corrections via a GSM phone link.
GPS is not the only GNSS system available, but currently the most important to surveyors. However the introduction of the new EU (Galileo) along with the existing Russian (Glonass) navigation satellite systems can be used to compliment the GPS system, and many of the latest receivers are being enabled to receive these systems. Access to more satellites will reduce downtime due to lack of satellite availability and enable surveys to continue in areas where the sky may be partially obscured, such as urban and forest areas.