DIFFERENTIAL GPS (DGPS)

Differential GPS (DGPS) services have been developed in response to inherent and imposed limitations of GPS. Despite the high order of accuracy available from the Precise Positioning Service, even this may not be sufficiently accurate to meet the stringent requirements of navigation in harbours and their approaches.

The fundamental principle of DGPS is the comparison of the position of a fixed point, referred to as the reference station, with positions obtained from a GPS receiver at that point. The observed difference can either be considered as a 2 or 3 dimensional geographical co-ordinate offset (position differential), or as a series of corrections to the satellite range data (pseudorange differential). Other methods of supplying corrections exists as satellite clock and almanac corrections or pseudorange corrections for a specific location with additional values to correct for user's position with respect to the specific location. The position differential technique is of limited application as it is necessary for the mobile observer to use the same satellites observed at the reference station. The pseudorange technique involves computing a unique correction to the range of each satellite observed at the reference station but does not necessarily require all of the same satellites to be the ones used by the mobile observer. Those pseudorange corrections calculated at the reference station which are not being observed at the mobile will be ignored.

Investigations into ways of transmitting the pseudorange corrections to the user, identified medium frequency marine radiobeacons operating in the 285 - 325 kHz band as the most suitable terrestrial method for use within the coastal zone. An American correction message format known as RTCM SC104 has become the industry standard for encoding DGPS corrections. The correction message is broadcast on the radiobeacon frequency by applying G1D modulation with Minimum Shift Keying (MSK). A Broadcast Standard document describing the broadcast signal structure and format has been produced by the US Coast Guard.

In order to make use of the DGPS corrections, users must have an MSK/radiobeacon receiver and a GPS receiver capable of incorporating DGPS correction data in the RTCM SC104 format.

Several commercial companies provide a DGPS service, but encrypt the RTCM format signal so that either a special receiver or a decoding device is required before the higher accuracy positions can be calculated.

DGPS systems using geostationary satellites have also been developed. A network of reference stations provides the pseudorange corrections, which are transmitted to the satellites for broadcasting to users. The coverage of the satellite broadcast is very much greater than MF radiobeacons and the "range" of the system is only constrained by the distance from the reference station over which the derived corrections remain valid.

<== попередня сторінка	\|	наступна сторінка ==>
Other Electronic Chart Data	\|

Не знайшли потрібну інформацію? Скористайтесь пошуком google:

Генерація сторінки за: 0.024 сек.