I have the facts of decrease in accuracy with WAAS on in Russia. Accuracy on satellite page was be greater - even 2 meters. But real position was so far of the truth - even about a few ten of meter. It was not surprise. The closest ground station was in thousand of km in absolutely different day time zone, atmospheric and weather conditions. Nevertheless GPS catched signal from nearest WAAS broadcasts sat zone (stationary orbit - Inmarsat probably).
So we was strong recommended to disable WAAS in our locations!
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