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GPS Differential Mode and WAAS
Posted by Roy Goyette, Last modified by
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on 16 August 2007 03:29 PM

GPS Differential Mode and WAAS

I have recently become aware of a change in the WAAS satellites numbers that can affect many of our customers that use WAAS system.

Background:

WAAS is a satellite-based differential corrections system. Corrections from a large number of ground stations are sent combined and sent to GPS receivers through one or more satellites using the same signal structure as the GPS satellites. WAAS stands for Wide Area Augmentation Network and is one of a number of similar systems. In general these systems are called SBAS (Space Based Augmentation Systems). Other such systems are: EGNOS - European Geostationary Navigation Overlay Service used in Europe MFSAT - Multi-Functional Transport Satellite used in Japan In order to receive these satellite corrections the GPS receiver needs to know the "name of the satellite" to listen to (similar to knowing the frequency of a radio station you want to listen to). In GPS world the satellites are called by their PRN number. In general the PRN numbers of the different SBAS systems are stored inside the GPS receiver and when the user selects to use the EGNOS system the GPS receiver knows to which PRN numbers to listen to.

Problem:

On July 31, 2007 FAA has decommissioned WAAS satellites with PRN 122 and 134. They have been replaced with satellites with PRN 135 and 138. Hence many GPS receivers may not know to what satellite to listen to for differential corrections and remain in stand-alone mode. In general users should realize that this is a GPS problem, not a HYPACK problem and contact the GPS manufacturer for specific instructions. However I expect a number of them to call us because "HYPACK is showing the wrong boat position" or "HYPACK says my GPS is not in differential mode" (that's pretty much how I found out about the issue).

Solution for CSI receivers:

Although the above solution ("check with the manufacturer") applies also to CSI receivers, it is easy to manually configure the receivers to use the correct PRN numbers. In a terminal window (Hyper-terminal or WCOM32) type the following command: $JWAASPRN,138,135 followed by Enter key. According to CSI tech support the setting is automatically saved.