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Clairifications on EZ-Guide 500 with WAAS
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alange
Posted 3/7/2008 11:34 (#328147 - in reply to #327904)
Subject: Re: Clairifications on EZ-Guide 500 with WAAS


Sunnyvale, CA
WAAS SVs 122 and 134 are no longer in service and must be turned OFF to avoid confusing the receiver. SVs 135 and 138 are normally set to ON (or Heed Health). A few months ago, there was a different message coming from 135 compared to 138. To the best of my knowledge, the same message is nowcoming from both of the active SVs, so they should give comparable results, depending on their visibility.

When you turn on the receiver in the morning after it has been off for more than about 2 to 4 hours, the Ionosphere (Iono) data in the receiver becomes stale and is not used by the receiver, and a new Iono message is collected, which takes about 12 minutes to downoad from the WAAS SV. This data covers the entire WAAS operating area (much of North America) and is a large file, thus the 12 minute download time. The total WAAS download bit rate is 500 bits per second, and only about half of the bandwidth is used for the WAAS Iono model. The other important data in the WAAS message is the fast time correction, which was much more important in ancient times when Selective Availability was enabled.

Until the current WAAS Iono model has been downloaded, the GPS receiver is set to use the GPS Iono model. Because the GPS Iono model is much simpler and coarser, and not as accurate as a current WAAS Iono model. A receiver using the GPS Iono model will exhibit a less accurate position using WAAS corrections until the current WAAS Iono model is downloaded. During the day, every few hours a new IONO model is downloaded and used automatically in the background. You only notice the startup delay in the WAAS Iono model when you turn it on after it has been off for a few hours. The Az-Guide 500 and 250 both display the state of the WAAS Iono model on the Info tab. When you first turn on the receiver selest the GPS info tab and time the changes in the WAAS Iono status. You will see with a warm start, it takes about 12 minutes to become useable. On a Hot start, where the receiver has been off for only a few minutes, the current Iono model is recalled from memory and it is then used, and the info tab will show this on the WAAS status screen.

Since the beacon DGPS corrections, as used by the Ag132, are based on the direct measurements of time of arrival of the GPS SV signals in at a local reference station, there is no Iono startup delay as seen with a Beacon DGPS correction as is seen with the WAAS system.
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