In fewer than twenty years, GPS has become the primary technology for geographic location on Earth. GPS, which stands for Global Positioning System, consists of a network of thirty satellites that circle the earth in such a geometry that it is always possible to receive signals from at least four of them. With these four signals, a GPS receiver uses geometry to triangulate its location on earth in three dimensions- latitude, longitude and altitude.
GPS History
The GPS satellite system is owned and managed by the US military. The system was originally designed for military purposes and, to maintain security, a filter called Selective Ability prevented GPS receivers from taking precision readings. Selective Ability was disabled on May 1, 2000, and since that time, increased accuracy has expanded the market for GPS applications considerably.
GPS Technology
In technical terms, GPS uses time and speed to calculate geographic location. Each satellite sends out a constant stream of signals with a fixed speed. A GPS receiver collects these signals and calculates their transmission time. Because the signals’ speed is constant, the GPS receiver can then calculate its distance to each of the available satellites. With that data, the receiver’s location can be triangulated with a series of calculations that are not unlike those used to conduct traditional triangulation surveys.
Technically, three satellites would be enough to triangulate a GPS receiver’s position. However, because the time factor in location calculations must be precise, and the satellite’s and receiver’s clocks are not synchronised, a fourth satellite is necessary for ensuring accurate readings. Since Selective Ability was switched off, it has become possible to take GPS readings with an accuracy of 5-20 metres in latitude and longitude and an accuracy of 10-40 metres in altitude.
The launch of GALILEO
In 2010, a consortium of EU and European Space Agency specialists will unveil a new global satellite positioning system. GALILEO, which is currently in a test phase, will consist of thirty satellites circling the earth at an altitude of 23,000 km. Based on a new geometry and new technology, GALILEO is expected to vastly improve the accuracy of global satellite positioning, especially at high latitudes where GPS is less efective.
GALILEO vs. GPS
When the system is made public, GALILEO is expected to work with GPS on a complementary basis and is designed to be integrateted into existing GPS technology.
The most significant difference between GALILEO and GPS is their ownership. The US military fundamentally determines the openness of the GPS system and the availability- and accuracy- of the satellites’ signals. GALILEO, on the other hand, will be managed by a private European consortium and will function as a common European good.
The GALILEO satellites present the benefits of newer technology and consequently more robust functionality. GALILEO positioning is expected to be significantly more accurate that GPS and greater signal strength will also improve the precision of readings taken in cities, indoors and under foliage cover. The system will include an integrity signal that provides alerts about system problems or other interference to accurate readings.
Ultimately, the level of accuracy available from GALILEO will be a question of commercial enterprise. The basic service will be free, as GPS is today. Paying customers will have access to increased signal strength, accuracy and the ability to transmit small amounts of data via the GALILEO satellites. An exclusive service will also provide high security signals that are safe for confidential transmissions.
Satellite Technology and the
National Survey and Cadastre
At present, GPS plays a fundamental role in the National Survey and Cadastre’s surveying and mapping activities. When GALILEO comes online in 2010, our instruments and programmes will be prepared to make use of the new technology and to gain the benefits that the new positioning system presents.