What is Bottom tracking?
by
Paul
—
last modified
Aug 04, 2004 01:58 PM
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| Which correction method is best? |
Later generation VM systems from Nortek allow the user to choose which correction system to use when analysing / viewing their data, either online or in postprocessing. The software offers two choices Bottom track and Auxiliary.
Both methods need a heading input. If this is not available from and external source the VMCP uses the internal compass. This is EM and will be effected by iron in the vicinity - only use the VMCP compass in vessels which do not have a magnetic field, ie wood, fibreglass, aluminium or stainless steel.
Both methods have advantages and disadvantages some of which are mentioned here:
Bottom tracking is a method where the velocity of the bottom is measured together with the water velocity. This allows the system to correct for the movement of the vessel for each ping. This gives better results when the vessel turns. It also means the user can plot the position of the vessel without the aid of external navigation equipment. Some users find that they get good results from interfacing a simple GPS receiver to give a position of the measurement and use bottom tracking to remove any velocity components produced by the boat moving. This allows surveys to be performed from small launches.
The disadvantage of bottom track is that it requires the bottom to be within range. [40m for 1MHz(Water measurement to 30m), 65m for 600KHz(Water measuremnet to 50m)]. Outside this range the correction will not work.
Bottom track allows a couple of extra features. The data can be cut off at the bottom, so that averages over depth only measures water, not below the bottom, also calibration can be verified online.
Auxiliary correction relies on a good DGPS navigation system. This uses the speed over ground and course over ground, to correct for the vessels motion. If the vessel sails a stable course this works very well and does not require the bottom to be within range. The disadvantage is that the DGPS and current meter are not perfectly synchronised and so a rapid change in course or speed will cause errors in the correction.
The ideal is a combination of both, to use bottom tracking when available, and switch to auxiliary when this is not available. The Survey VM software does this automatically if both methods are available, or the user can choose to lock to one particular method.
The beauty of the Nortek Survey systems is that, when logging, all data is logged. This means that if you make a mistake with the correction method, the data can be replayed afterwards with the correct method.
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Note: Bottom track is only available with later hardware versions (VMCP and AWAC) Not with the VM-NDP systems, sorry. |
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