Combine Fans
This operator combines simultaneous overlapping multibeam pings from two operands into a single ping with the same start and stop range and number of datapoints as the first operand.
It requires that both operands have the same transducer geometry and data type.
Additional requirements
- neither of the operands must have had the multibeam roll at transducer operator applied
- the operands must have the multibeam ping mode (i.e., not multibeam H-, V- or S-mode)
- V- or S-mode echograms that have been resampled to change the beam geometry cannot be used
If you reassign the output variable to another transducer, you must also reassign operand 1 to that transducer.
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Operand 1 |
Operand 2 |
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Settings
The Combine Fans Variable Properties dialog box pages include (common) Variable Properties pages and these operator pages:
Operands page
Combine Fans page
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Setting |
Description |
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Maximum acceptable time difference (milliseconds) |
Specifies the maximum ping time difference in milliseconds between candidate pings. |
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From first operand (%) |
Specifies the point—as a percentage—within the overlapping range of angles from which the algorithm keeps beams from operand 1 before using beams from operand 2 for the output ping. |
Algorithm description
For each ping in operand 1, Echoview identifies the matched ping in operand 2—if the ping time difference between the two is less than the Maximum acceptable time difference (milliseconds). Otherwise, there is no matched ping and Echoview does not produce an output ping.
In joining the multibeam fans, Echoview first sequences the beam angles from both operands. The output has three distinct bands
- an exclusive band, containing beam angles from one of the operands (either operand 1 or 2)
- an overlap band, containing interleaved beam angles
- a second exclusive band, containing beam angles from the other operand
From the boundary of operand 1's exclusive band and the overlap band, there is a point β (which you specify with the From first operand (%) setting) along the length of the overlap band toward operand 2's exclusive band. In moving from operand 1's exclusive band, the algorithm discards all operand 2's beams with angles before β, and all operand 1's beams with angles following β, for the output ping.
Each output ping has the same sample thickness as the ping from operand 1, and fan width corresponding to the range of beam angles from operands 1 and 2.
The operator then copies the samples from operand 1 into the output ping. As for operand 2, the joining operation projects the samples onto the output ping, resampling as required in the range dimension using a weighted mean (refer to the resampling algorithm). Hence, there may be distortions, clipping or no data padded values in the ping samples from operand 2.