Howabout
Point taken - and conceded. I was biased in my thinking by your later comment about him making more sense than other posters.
The OZBUS post of 21:53 yesterday well sums up your query. To expand, when your life depends on it with ADS-B you need FDE and SA. Summed up by these extracts:
Recognising the wide scale civilian application of GPS, the US President issued a directive in the year 2000 turning SA off, thus making higher accuracy GPS available to the world. SA has been turned off now for many years and the USA have committed to it remaining off to the extent that new generation GPS satellites do not have this feature.
Modern GPS receivers can detect whether SA is ON or OFF. These receivers are called SA AWARE receivers.
All TSOC145a/146a certified GPS receivers are SA aware. This is a requirement of the standard. Some modern TSO129c receivers may be SA aware but most assume SA is ON because SA awareness is not required by the standard.
The statistic I mentioned in my previous post can also be expressed as:
99.997% availability is equivalent to a 6 minute outage for each aircraft every
approx 138 days. (Good enuf for me anyway)
FDE on the other hand is best summarised as to identify and exclude the malfunctioning satellite in order to continue navigating using the remaining satellites.