SA Watch Instructions
Text by Jack Yeazel
Return to GPS General Information Home Page
Revision: 10 June 2001

With SA Watch ($20US shareware here) Ver. 3.75, the Weighted Mean of the points recorded will converge to a stable position after 30 minutes to an hour.  You can set your Reference Point to the Weighted Mean at any time. Then you can compare your present position to the Reference Point and see the current GPS error.

It only takes about an hour to get valid GPS accuracy data with SA turned off.  It's important to test in the clear away from tree to get the best results.  It's been noticed that GPS is most accurate at night and least accurate in the afternoon.  With SA Watch, you can check this out.  With several sessions of averaging, the SA Watch Weighted Mean should provide you with a Reference Point position to within one meter.

SA Watch Operating Screen

The above Plot Screen of a Garmin Vista's cold lockon performance as it approaches the reference point, shows a total run time (All:) of 36:32 minutes with 1,094 points; the Moving Average (MA) of the last 9:58 minutes shows 95% of the errors to be within 2.9m of the reference point, and the 95% errors of the total test (All:) were within 5.2m.  Also shown are the satellite positions.  The 'tails' are where the Sats have moved during 36 minutes. Time is UTC.

The reference point  'Reference UTM' (and the blue '+') coordinates come from the a first-order survey marker.  The weighted mean of the Vista was 1.2m@207° from this reference point.  "WAAS" indicates that the receiver is receiving WAAS differential signals.  If a DGPS receiver is used, the "ID" will indicate which ground station is being received.

SA Watch is now fully compatible with Garmin's new NMEA 0183 Ver 2.3, and if selected, the plot will show the current satellite positions (yellow ones rising and orange ones descending).  A plot of the satellite trails can also be shown, if selected.

The first item in Weighted Mean is altitude weighted by VDOP (if weighting is selected) during a live recording.  When importing a .dat file, the Plot Screen will show altitude weighted by HDOP.  The Statistics Page (F5) will show the simple mean.  In the plot, the Vista altitude mean was within 1m of the reference altitude.

A new feature of SA Watch is a "snapshot" of the Line Graph by pressing F6 or 'clicking' on the Line Graph, itself.  On the left is the legend of the "error lines", and on the right is a legend of the Satellites in View (SV).  The EPE circles (in the first plot) and  error histories are color coded.  The 'times' are UTC with 5-minute tick marks.  Also, the Help menu contains some added internet pointers.

Setting Up SA Watch

Load the program and press:
File > Logging Active: Type a name for your log file, set it to Active (make sure it's checked).
GPS > Setup: Select GPS Type, Port, and Data Rate.  Then set GPS Active.
        Coordinate Format: Set to your preference
        Display Update Interval: Display All Fixes
        Export Interval: Export One Record Every 2 Minutes (or whatever)
                (Sets the interval the log text file is updated)
        Filter: Filter on HDOP
        Weighted Mean Calculation: Inverse Square of HDOP
                (Applies more weight to points recorded with small HDOP)
        Note: Type in a name of the plot (to be shown at the bottom of the plot)
        (The rest can be left as default)

Operating SA Watch

After running for awhile select:
        Options > Reference Point > Set Weighted Mean as Reference
                (This sets the reference point to the best available and recalculates the .dat data based on it)
         Select a unique name for the reference point for future reference.
Do this periodically and at the end of the longest averaging period for the best position reference possible.  There are also options to save the log .txt file and a .bmp plot image file periodically.

PLOT: The center of the magenta circle is the Weighted Mean as compared to the current reference point, and its radius is the average EPE.  (The line graph at the bottom shows a running account of this difference.)  The center of the yellow circle is Present Position, and its radius is current EPE.  The plot of blue/green/red dots are a 'cookie trail' of present positions as compared to the reference point, color coded for HDOP (also shown in the line graph).

LINE GRAPH:  In addition to a plot of current position error, weighted mean error, and the number of sats tracked, it shows UTC time (every 15 minutes).  (The graph can be scrolled left and right) .

TABLE (at left):
           Now: AURA = Average User Range Accuracy; HDOP = Horizontal Dilution of Position; EPE (Garmin only)
           MA = Moving average (per point count set in Options)
           All: Count = Total points collected
                  HDOP = Average HDOP
                  Drift = Average movement speed due to SA and errors
                  Time = Total time of usable points recorded
                  xx% = Percent of data points not exceeding the error shown.
NOTE: The 95% number is important, because it can be compared to the Selective Availability specification of 100m maximum expected error, 95% of the time.

GPS HORIZION:  GPS > Show GPS Horizon (or Ctrl-H) will plot the limits of the satellites' tracks in the sky, and can be used to see what obstructions you have at your local viewing station.  Then pressing "OK" will bring you back to the track-plot page.  See an example (HERE).  It takes about 12 hours to draw a complete outline of the GPS horizion.

STATISTICS:  Press F5 for the Statistics Page which will give you the Mean Altitude plus other data, such as Standard Deviations and Standard Deviation of Means. To see the Statistics Page for the above plot, press (HERE).  When exiting the program, the (.dat) log will be saved automatically, so you can pick up where you left off on a particular test.  Then the next time you run the program, you then can press:
GPS > GPS Active
File > Logging Active > open your logging file, and resume the test.

NOTE:  Logging files (*.dat) can be reloaded and plotted against any reference point that has been saved.

So, after using SA Watch, no one can say you don't know where you are!  I've used it to verify any improvement in accuracy due to the 2-D mode of navigating used by some manufacturers and compare the relative accuracy (and repeatability) of various makes of GPS.  Someday when we will no longer have to "watch" SA, we'll REALLY be able to say who's GPS is the most accurate.

Jack Yeazel