Determining GPS Fix Status

Echoview processes GPS data to assign quality statuses to each GPS fix. This ensures accurate cruise track representation by evaluating data based on validity checks, platform speed, and time gaps.

The processing algorithm proceeds in three main stages:

Stage 1 - Filter invalid GPS fixes

GPS fixes are removed if they:

1. Have an incorrect format.
2. Contain latitude or longitude values outside valid ranges (-90 to 90 for latitude, -180 to 180 for longitude).
3. Have both latitude and longitude equal to 0 (this rule does not apply to file types like SM2000 files or RESON sbi).

Stage 2 - Identify the first good fix (Fix G)

The algorithm determines Fix G, the first good fix, which serves as the starting point for the cruise track. Echoview provides two Processing methods to accommodate varying data quality. These methods are found on the Data page of the Variable Properties dialog box:

1. Method 1:
   1. Fix G is identified as the first fix in a sequence of three consecutive fixes meeting the following criteria:
      1. Speed between consecutive fixes is below Maximum apparent speed.
      2. Gap between consecutive fixes is within the range of Minimum acceptable gap and Maximum acceptable gap.
2. Method 2:
   1. Fix G is identified as the first fix in any sequence of three fixes (not necessarily consecutive) that meet the same criteria as Method 1.
   2. Note: Method 2 is particularly useful for datasets with inconsistent or missing early fixes.

Stage 3 - Assign status to GPS fixes

Once Fix G is determined, the status of each GPS fix is evaluated as follows:

1. For fixes before Fix G:
   1. Method 1:
      1. If Speed > Maximum apparent speed OR Gap < Minimum acceptable gap, assign status bad.
      2. Otherwise, assign status uncertain.
   2. Method 2:
      1. Assign status bad to all fixes before Fix G.
2. For fixes after Fix G:
   1. If Speed > Maximum apparent speed OR Gap < Minimum acceptable gap, assign status bad.
   2. If Gap > Maximum acceptable gap, assign status uncertain.
   3. Otherwise, assign status as good.

Notes on GPS fix status:

1. A Good (uncertain) status is assigned when fixes partially meet gap criteria but they are not definitively good or bad.
2. A Bad fix status is assigned when fixes fail to meet speed or gap criteria.
3. An Unknown GPS fix status is assigned when no valid GPS data exists to confirm status.
4. Echoview automatically parses GPS data into different NMEA sentences, which may reduce the need for manual filtering. For legacy data formats or unexpected issues, the GPS filter operator can still be used to refine cruise track data. For more information refer to Processing and smoothing cruise tracks.
5. For details on how GPS fix status affect pings, see Determining Ping Position Status.

See also

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