Scan Position

This operator attempts to remove the effects of local vessel motion and GPS imprecision from position data.

The instrument and target-locked scanning school detection algorithms require that the vessel (transducer) be stationary during a scan. Due to GPS imprecision and movement of a vessel at anchor during a scan the actual measured position may vary over the time of a scan. This operator identifies scanning frames and target-locked ping pairs within which it can move all the pings no more than a specified amount to make them coincident.

Echoview accepts operands of the following data types as input:

The first operand provides the position data and the second operand the ping data for the purposes of detecting frames and target-locked ping pairs. A position variable is produced with adjusted GPS fixes. On the Notes page of the Variable Properties dialog box statistics are reported.

Settings

The Scan Position Variable Properties dialog box pages include (common) Variable Properties pages and these operator pages:

Operands page

Scan position page

Algorithm

The Scan Position operator selectively discards and introduces GPS fixes in order to ensure that the position of each ping within a single scan is the same. This is generally necessary as a preparation for 3D school detection on instrument or target-lock scanned data.

They are used as follows:

1. Each ping in the acoustic variable is grouped into a target-locked ping pair if possible.
2. For each target-locked ping pair determined in Step 1, if any:
   1. Calculate the average position of the two pings.
   2. If the position of either ping is greater than the Allowable movement from the average position discard this target-locked ping pair.
3. Each ping in the acoustic variable is grouped into a frame if possible, ignoring requirement 2.b.i of the frame definition (that the position of each ping is the same).
4. For each frame determined in Step 3, if any:
   1. Calculate the average position of the pings in the frame
   2. If the position of any ping in the frame is greater than the Allowable movement from the average position discard this frame.
5. GPS fixes are then copied from the position variable with the following modifications:
   1. For each target-locked ping pair found, if any:
      1. GPS fixes which occur between the time of first ping and the second ping are discarded
      2. two GPS fixes, both at the average position of the two pings are inserted:
         • one at the time of the first ping and
         • the other at the time of the second ping.
         This ensures that the position of both pings is identical, meeting requirement 2.a of the school detection algorithm.
   2. For each frame found, if any:
      1. GPS fixes which are between the start and end time of the frame are discarded
      2. two GPS fixes, both at the average position of the pings in the frame are inserted:
         • one at the time of the first ping in the frame and
         • the other at the time of the last ping in the frame.
         This ensures that the position of all pings in the frame is identical, meeting requirement 2.b.i of the frame definition.

Notes:

• This ensures that the position of each ping in each detected frame is the same, and hence that requirement 2.b.i will be met when detecting 3D schools using the instrument scanning algorithm.
• It also ensures that the position of each ping in each detected target-locked ping pair is the same, and hence that requirement 2.a will be met when detecting 3D schools using the target-locked scanning algorithm.
• Statistics describing the detected frames are recorded on the Notes page of the Variable Properties dialog box on the resulting variable.
• The WGS84 model is used for all position calculations.

See also

About virtual variables
Operator licensing in Echoview