Processing GPS data

GPS data are automatically processed prior to use. The default Method for initial processing is Method 1. The Processing method on the Data page of the Variable Properties dialog box offers two methods for identifying the starting fix of the cruise track. The processing algorithm assigns GPS fix status and ping position status (shown on the integram, the Echoview Status bar and the cruise track window) by testing against specified position variable Data page settings. The settings represent the maximum apparent vessel speed, and the maximum and minimum acceptable time gaps between GPS fixes.

• Method 1 can be used when the first GPS fix on the cruise track is good.
• Method 2 can be used when your first fix or fixes are bad, and you have confidence in the other GPS fixes.

Algorithm with processing for Method 1 and Method 2

Stage 1 - Initialize

1. All available pings are assigned a ping position status of unknown.
2. The following GPS fixes are filtered out and are not included in the processed GPS variable:
   1. GPS fixes that do not match the expected format.
   2. GPS fixes with a latitude of 0 and a longitude of 0 (filter not applied to SM2000 files, RESON sbi files, and .gps.csv files).
   3. GPS fixes with a latitude value outside the range -90 to 90.
   4. GPS fixes with a longitude value outside the range -180 to 180.
   5. A GPS fix is used if the gap between the fix and the ping time is thirty seconds or less. If the fix is already used by a ping, the next nearest valid fix is considered.
3. If Method 1 is selected on the Data page of the Variable Properties dialog box, the remaining GPS fixes are assigned a ping position status of unknown.
4. If Method 2 is selected on the Data page of the Variable Properties dialog box, the remaining GPS fixes are assigned a ping position status of unknown.

Stage 2 - Identify the FirstGoodFix

1. Scan the GPS fixes from start to end, and look for 3 consecutive fixes which fall within both the Minimum acceptable gap (s) and Maximum acceptable gap (s). In the identified set of consecutive fixes, the first fix is the FirstGoodFix and its GPS fix status is set to good. For Method 1, the FirstGoodFix is expected to be the first fix of the cruise track.
2. If Method 2 is selected on the Data page of the Variable Properties dialog box, all GPS fixes before the FirstGoodFix are assigned to GPS fix status of bad.

Stage 3 - Determine the fix status for all GPS fixes

With respect to the FirstGoodFix and starting at the next fix, assess GPS fixes against the Maximum apparent speed and the Minimum acceptable gap.

1. If Method 1 is selected on the Data page of the Variable Properties dialog box then assign a GPS fix status of good to all fixes.
2. If Method 2 is selected on the Data page of the Variable Properties dialog box then assign a GPS fix status of good to fixes after the FirstGoodFix.
3. Assign bad status to GPS fixes which cannot reasonably represent vessel motion.
   1. For each fix from second to last:
      1. Calculate the apparent speed (V) to move from previous fix to current fix.
      2. If V is greater than the Maximum apparent speed assign a GPS fix status of bad.
4. Assign bad status to fixes that were received within a given time of the previous good fix.
   1. For each fix from second to last:
      1. Calculate the time gap (T) between it and the previous fix.
      2. If T is less than the Minimum acceptable gap assign a GPS fix status of bad.

Stage 4 - Determine the ping position status for all pings

For each ping from first to last, ignoring all bad fixes:

1. Find the time T of the ping.
2. Find the time Tp of the last GPS fix prior to the ping (if such a fix exists).
3. Find the time Tn of the next GPS fix at or after the ping (if such a fix exists).
4. Assign a position status to the ping as follows:

   If ...	Then assign a ping position status of ...
   the ping is earlier than the first GPS fix	unknown
   the ping is later than the last GPS fix	unknown
   the ping is before the FirstGoodFix (identified earlier)	uncertain
   (Tn - Tp) > Maximum acceptable gap	uncertain
   otherwise ...	good

   Note: You can use the GPS filter operator to filter position data, containing unexpected GPS fix formats. For more information refer to Processing and smoothing cruise tracks.

See also

About GPS fix status
About Ping Position Status
About Cruise Tracks
Position status area on Echograms
Processing and smoothing cruise tracks