Example of a Garmin Vista Unit Acquiring WAAS for the First Time
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(15 Jan. 2002) Jack Yeazel

 Master Reset: While holding down the Page, Find, and Clickstick buttons, press the ON button.   (One must hold them all down for over 10 seconds).
 Below, geosynchronous WAAS satellites are referred to as Geo Sats.

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After the Master Reset of the Vista (1), WAAS is disabled, so I Enabled it (and Battery Save must be OFF).  With the unit under some trees, it took only about two minutes to gain position lock (2).  It still was searching for GPS Sats after position lock (3).  Apparently the GPS Sat being searched is at the 12:00 o'clock position.  But it took about 10 minutes before it began to search for WAAS Geo Sats (4).  You'll know it's searching when you see two Sats at the bottom of the satellite page with PRN numbers greater than 32 (4) thru (7).

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 The unit will reserve the two sats to the right for these PRN numbers, 33 thru 51, as it searches for them -two at a time.  It looks for each Sat for about two minutes, thus you see a new number once a minute.  The 19-possible WAAS PRN codes then take about 20 minutes to cycle through.  In these tests, Sat  #35 was detected on the second cycle (8).

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It seems that the highest Geo Sat number being searched is displayed at the 12:00 position.  Once #35 was detected, it was locked into the 12th sat position and the 11th position continued to search for the other Geo Sats up through sat  #51.  It's interesting that Geo Sat  #35 started to give differential corrections to the other GPS Sats, even though it had not locked on, and wasn't displayed in the sky-position graphics (10).

Once  #35 was locked on (11&12), it was correctly displayed at an elevation of about 29° to the south-east, and no more Sats were searched or shown at the 12:00 position.   At which point the unit had downloaded almanacs of all the WAAS geo sats from Sat  #35.  From the downloaded Geo satellite almanacs, it concluded that only  #35 was visible from this location, thus freeing up the 11th sat position for regular GPS Sats -or another Geo Sat.  The only other Geo Sat visible in the US is  #47, but not at this location.  Even though  #47 was never detected, its almanac is included in the #35 transmission.  The whole operation took about 30 minutes, but could take up to an hour under difficult conditions.

The Accuracy numbers don't represent actual accuracy, because the test was done under trees, and also it looks like the installed MetroGuide map prevents an Accuracy number less than about 20 feet.  Upon taking the unit to a surveyed spot, a short run indicated that it was accurate to within 3m 95% of the time.  Thus a loss of calibrations due to the master reset wasn't indicated.

Some other interesting WAAS information sites are: http://gpsinformation.net/waasgps.htm and  http://www.gpsinformation.org/dale/dgps.htm

Jack Yeazel