How to calculate the position of datum in sar

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FOLLOW

Datum means a geographic point, line, or area used as a
reference in search planning.

The following factors should be considered

* reported position and time of the SAR incident
* any supplementary information such as DF bearings or sightings
* time interval between the incident and the arrival of SAR facilities
* estimated surface movements of the distressed craft or survival craft, depending on drift  The datum position for the search is found as follows:
 drift has two components: leeway and total water current
 leeway direction is downwind &  leeway speed depends on wind speed


* the observed wind speed when approaching the scene may be
used for estimating leeway speed of liferafts by using the graph given in vol 3 of IAMSAR
(Persons in the water (PIW) have no leeway while liferaft stability and speed vary with or without drogue or ballast.)
* total water current may be estimated by computing set and
drift when approaching the scene
* drift direction and speed is the vector sum of leeway and total water current
* drift distance is drift speed multiplied by the time interval between the incident time, or time of the last computed datum, and the commence search time
(drift distance = drift speed x drift time)
* datum position is found by moving from the incident position, or last computed datum position, the drift distance in the drift direction and plotting the resulting position on a suitable chart.