Movement operators

Settings

Description

Offset (nmi)

The distance calibration offset in nautical miles.

Scaling

The distance calibration scaling parameter.

Setting

Description

Fix Specification

Enter an expression, for a filter, containing zero or more groups of GPS-fix letters.

1. A space is used to separate each letter group.
2. The ? symbol represents any letter.
3. The groups of letters can include: "undefined" and NMEA GPS-fix letter groups identified by Echoview in the Fix Availability list on the GPS filter page.
4. Restore Default will create a virtual variable that contains GPS fixes matching the default filter expression. The default filter expression produces a pre-Echoview 4.20 GPS variable.

Fix Retention

This setting specifies what to do with GPS fixes in the operand that match the filter expression set in Fix Specification.

You can choose to Retain matching fixes or Discard matching fixes.

Fix Availability

Lists the available GPS-fix letter groups in the operand.

Fix Selection

Lists the GPS-fix letter groups selected for the virtual variable. This list updates when OK or Apply is used.

Setting

Description

Allowable movement (m)

Sets the maximum acceptable alteration to ping position during an instrument scan or target-locked scan. Will modify the GPS data to ensure that the position during each identified scan is constant, effectively removing any undesired wander in the position measurement during the scan. This makes schools detection possible on such scanned data.

Settings

Description

Smoothing slider

Selects degree of smoothing that is applied.

Hint: To view the cruise track that is created from a specific smoothing setting: arrange the cruise track window and Variable Properties dialog box so both windows are visible; move the slider to the required position; then click Apply and observe the changes in the cruise track.

Notes:

• A Kalman filter is applied to produce the smoothing effect.
• The value that you set with the slider has the meaning of 'Expected standard deviation of GPS measurements'. It is a measure of how different you expect a measured GPS fix to be from the estimated (smoothed) one. An estimate is made based upon the history of vessel motion up to that point.
• If you expect a small difference, then the Kalman filter will put a lot of credibility in the estimate and less in the measurement, resulting in a smoother track. Conversely, if you expect a large difference, you are basically saying that the measurements can vary widely, suggesting less confidence in the estimate and the Kalman filter will rely more on the actual measurements, which has the effect of smoothing less.

Expected standard deviation of GPS error

Shows the standard deviation of GPS error value currently being used by the smoothing algorithm. The value is determined by the position of the slider and cannot be altered in this box.