Processing GPS data

GPS data are automatically processed prior to use. The Processing method on the Data page of the Variable Properties dialog box provides two methods for identifying the starting point of your cruise track.

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.

Method 1

Method 1 works as follows:

Stage 1 - Initialization

All available pings are assigned a position status of unknown.
The following GPS fixes are filtered out and are not processed further:
GPS fixes that do not match the expected format
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)
GPS fixes with a latitude value outside the range -90 to 90
GPS fixes with a longitude value outside the range -180 to 180
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.
If Method 1 is selected on the Data page of the Variable Properties dialog box, all remaining GPS fixes are assigned the status unknown.
If Method 2 is selected on the Data page of the Variable Properties dialog box, all unassigned GPS fixes are the assigned the status unknown.

Stage 2 - Determine the fix status for all GPS fixes

If Method 1 is selected on the Data page of the Variable Properties dialog box then assign a status of good to all fixes.
If Method 2 is selected on the Data page of the Variable Properties dialog box then assign a status of good to the unknown fixes.
Assign bad status to fixes which cannot reasonably represent vessel motion.

For each fix from second to last:
Calculate the apparent speed (V) to move from previous fix to current fix.
If V is greater than the Maximum apparent speed assign a status of bad.

Identify the first reliable sequence of fixes, i.e. the first good GPS fix followed by two more good fixes (call this fix G)

Assign bad status to fixes that were received within a given time of the previous good fix.

For each fix from second to last:

Calculate the time gap (T) between it and the previous fix.
If T is less than the Minimum acceptable gap assign a status of bad.

Stage 3 - Determine the position status for all pings

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

Find the time T of the ping
Find the time Tp of the last GPS fix prior to the ping (if such a fix exists)
Find the time Tn of the next GPS fix at or after the ping (if such a fix exists)

Assign a position status to the ping as follows:

If ...	Then assign a 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 fix G (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.

Method 2

Method 2 identifies bad GPS fixes at the start of the cruise track and assigns those fixes a status of bad. All GPS fixes are then processed as for Method 1.