Nautical Studies

about Marine Navigation & Technology

Shall a navigator rely on GPS fixes for anchoring in port?

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The errors in fixing a position with GPS may be grouped into three types. There are errors in the transmission, errors with the propagation and errors in reception. As far as ship stations are concerned, the errors related to the transmission and its propagation can be considered as common errors affecting every station within a small area. It can be corrected using differential technique. Errors in reception are differed from one station to another. The error is local to each station and will not be corrected by differential corrections. The error in GPS position fixing onboard a ship is a compounded one. It consisted of errors related to the transmission, propagation as well as reception. From the following photo, GPS positions were marked on screen for a number of ships as reported by their Automatic Identification Systems. Some of these positions were very close to the echo of corresponding vessels. Some other was rather far away from its echo. It was showing that GPS positioning errors were different on every ship. A radar screen picture was used to illustrate the problem in using GPS for anchoring within confined water. It was taken onboard a containership while approaching an anchorage in Hong Kong. On the southern part of the screen was the northern coast of Lamma Island. Hong Kong Island was along the eastern part of the screen. An assigned anchor position was given and plotted in paper chart to obtain range to landmark readings. The position was then marked by variable range markers and GPS referenced mark on a radar. The anchor position should be 0.92 nm off the northern coast of Lamma Island and 1.5 nm off the coast of Hong Kong Island. The square marker with the letter “i” in the middle was the anchor position using GPS as a reference. The length between the bridge of the ship and its bow was about 0.1 nm. It could be agreed that the ship was proceeding along a course towards its assigned anchor position, though, it was not proceeding towards the GPS referenced marker. Apparently  the discrepancy between the positions fixed by ranges and GPS was more than 100 metres. The proper uses of modern technology in assisting navigation help to improve safety. However, mariners should be cautious with the limitations of each system. GPS is handy. It could be very helpful in more open waters. In a port like Hong Kong, required accuracy of anchoring position could very often down to tenth of metres. The use of GPS fixes in anchoring may be inappropriate. Try again with basic techniques.

Written by Bruce Chun

October 9, 2012 at 12:30

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