Test Drift calculation
How do we execute a test drift?
To execute a test drift, we need to observe
•An object that is as similar to the Casualty as is possible
•Drifting as close to the LKP as is possible
•As soon after the casualty was known to be at the LKP as possible
•The conditions must not have changed much during the casualty drift
What information do we need for the test drift and where does it come from?
a.The exact amount of time the object was left to drift – the crew executing the test drift will record the exact start and end times.
b.The coordinates of the position the object started drifting – the crew will send these coordinates when the object starts to drift.
c.The coordinates of the location of the object at the end of the test drift – the crew will send these at the end of the test drift.
How and where do we input this information for the Test Drift?
•A : The test drift option has been selected
•B : You must enter the exact coordinates of where the test drift began
•C : You must enter the exact coordinates of where the test drift ended
•D : you must record the number of minutes between the start and end of the test drift
•E : Speed Factor - this is a guide to how much faster the casualty will drift when compared to the object drifted for the test drift. For example, the tracked drift buoys we use drift at pretty much the same speed as a person in the water, so the Speed Factor equals 1 (the default value), but a RIB is likely to drift faster than the drift buoy (possibly between 1.5 and 2.5 faster), so if you make the "Speed Factor" = 2, the calculated Datum position will consider the speed factor and consider a drift speed twice as fast as that of the buoy.
•F : These items are defaulted
oSearch Error % - this is used to calculate the search area. The default is 20% which means that we go back 20% of the drift distance and search a square this distance away from the Datum Position.
oPosition Error nm - if you are not confident with the LKP of the casualty, you can add a distance which will be included when calculating the search area eg, if your LKP comes from a GPS (eg SafeTrx) then you can be confident with the coordinates (IAMSAR suggest adding 0.1nm), but if it was a visual sighting, IAMSAR recommends adding 1nm to the radius of the circle (we measure 20% of the drift distance and then we add the Position Error to that distance). This will make the search area bigger.
oLand Check - toggling this on/off allows you to define whether or not you want the system to check if the casualty or random points are on land. The datum calculation is substantially faster if the land check is not done - not selected is the default state. The option will be "grayed out" and unavailable if the file containing land information is unavailable.
oProbability Run - toggling this on/off allows you to define whether random points should be calculated. The default is ticked .
oConfidence level for the random points - the options being "High", "Moderate" and "Low" (see Drift Confidence)
oNumber of random points that should be calculated
•G : SearchWell displays the results of the test drift after the calculation has been performed
•H : click on the Calculate button to perform the calculation
It is important to note that SearchWell can also calculate a "reverse" Test Drift Datum calculation - if the time at LKP is after the time at Datum, then SearchWell reverses the drift bearing by 180 degrees indicating where the casualty might have come from using the results of the test drift
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