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Drawing illustrating GPS technique

Global Positioning System, GPS, is a satellite-based radio navigation system run by the United States Department of Defense. The system is based on 24 satellites that circle the earth in six different orbit paths. The satellites transmit signals containing information your receiver uses to determine its position. The receiver must receive signals from at least three satellites to perform the calculation for a 2D position and from four satellites for a 3 D position. Positional accuracy is better than 9 metres (95%).

DGPS improves accuracy

The position could be more accurate if it not for that the signal from the satellite is affected by the ionosphere and troposphere, as well as errors in the satellite clocks and satellite ephemeris. You can install a DGPS receiver to obtain even better precision. The positional accuracy is then 1-2 metres in practice, but only 8 metres is indicated for legal reasons.

In cooperation with our neighbouring countries and in line with the recommendations of the IALA, the Swedish Maritime Administration has established a reference station network for GPS. The reference station calculates the distance error to every satellite and transmits a correction via LF, along with information about the reliability of the station, the quality of the correction, and notice of whether any particular satellite should not be used. Corrections can be sent for a maximum of 12 satellites whose elevation angles exceed 7 degrees. The transmitted signal is monitored by a control receiver. If the calculated position deviates more than 8 metres from the measured position of the station, it sends an error message to your DGPS receiver.

Positional accuracy can be as good as around one metre if you have a good DGPS receiver. Resistance to interference increases if the correction receiver’s antenna is of the H-field type (loop) and if the receiver has special built-in technology for removing impulse interference.

The transmissions take place according to ITU-RM.823. The message types used comply with RTCM SC-104, numbers 3, 6, 7, 9 and 16. The transmission datarate was changed (as of 18 September 2001) from 200 to 100 bps.

The Swedish Maritime Administration monitors the reference stations. The transmissions are logged and stored for a certain time. The service is intended for the shipping/boating industry and is provided free.

GPS uses the geodetic reference system WGS-84. The corrections transmitted by the Swedish Maritime Administration refer to EUREF 89 and deviate less than one metre from WGS-84. All Swedish navigational charts have been based on WGS-84 since 1996.

Mail: dgnss@sjofartsverket.se

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