Hi Deker
What we are trying to do here is to suppress bad data going back to the APRS network which makes A/C appear to 'jump around'. Dont forget this is not a 'real limit' - it is a limit applied to suppress unrealistic data.
Imagine a co-ordinate such as longitude, a single bit flip in the MSB part of the data can make it 'appear' to jump a wild distance, or altitude, again a 1 bit flip in the MSB makes it jump. Bear in mind that a single bit flip in isolation will not cause this behavior, it must be accompanied by a corresponding bit flip in the CRC. This is a very unusual and non-probabilistic event, but when we are recording thousands of transmissions, then the probability is more pronounced.
We saw this on very very few occasions, but enough to introduce the guard.
The downside is that this is destroying the competition I understand that is going on to get the most far reaching detection from the ground stations by their installers
Thx
Lee