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. The intended use case for this operator is for dual-head setups (e.g., Kongsberg EM series).

Additional requirements and constraints

  • neither of the operands must have had the multibeam roll at transducer or any other virtual transform operator applied
  • the operands must have the regular-mode 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
  • the operands must overlap, however neither operand can contain the other (ie., Echoview searches for an angle at which only Operand 1 exists, an angle at which Operand 1 and 2 exist, and an angle at which only Operand 2 exists)
  • the variable transducer must not be overridden, or if overridden, must be overridden to the same transducer as Operand 1

If you reassign the output variable to another transducer, you must also reassign Operand 1 to that transducer.

Operand 1

Operand 2

  • Multibeam magnitude
  • Multibeam phase
  • Multibeam Sv
  • Multibeam TS
  • Multibeam unspecified dB
  • Multibeam magnitude
  • Multibeam phase
  • Multibeam Sv
  • Multibeam TS
  • Multibeam unspecified dB

Settings

The Combine Fans Variable Properties dialog box pages include (common) Variable Properties pages and these operator pages:

Operands page

Combine Fans page

Setting

Description

Maximum acceptable time difference (milliseconds)

Specifies the maximum ping time difference in milliseconds between candidate pings.

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

A ping pointer is initialised for each acoustic operand which is designed to point to the first ping then loop to the final ping. For each ping in Operand 1, Echoview identifies the time-matched ping in the next operand (e.g., Operand 2, then Operand 3, etc...)—if the ping time difference between the two is less than the Maximum acceptable time difference (milliseconds).

Essentially, the operator works by finding the first (integer) beam index from both fans which does not contain the 'crossover' direction, and 'shrinking' the operand fans accordingly, then copying those beams across. An important implication of this is that the last included beam on the 'left' fan, and the first beam on the 'right' fan have a different right/left neighbor, respectively, in the output. This means they have different beam widths but the same (copied) value.

In joining the multibeam fans, Echoview first sequences the beam angles from both operands. In overlaying the beams from the inputs, we obtain 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.

The Beam Operand attribute in the details pane displays which operand the beam was taken from for each beam.

See also

About virtual variables
Operator licensing in Echoview