bobcat4:
Come on... Going from +1950 to -8 feet in say one second is 117480 feet per minute. Well, I would say that is a very impossible sink rate. Isn't that supersonic? Anyway, it not a normal condition. And this sink rate is based on the assumption that the RA outputs its measurements once every second. The real output rate is probably a lot faster, which makes the sink rate even greater.
The simple solution is to have the AT remeber the last reading, and the timestamp this was read. Then it should calculate the sink rate. If it's close to supersonic or some other far out speed it will simply flag the RA as unusable/faulty. What is the aircrafts envelope regarding sink rate? That would be a good limit to set for a sanity check.
The change in radio altitude is not equal to the sink rate! Think of terrain - tall buildings, cliffs, hills, etc. If steep enough, this could lead to very high "sink rates" according to your calculation.
Guys, you essentially want aeroplanes to fly without pilots. I am not saying that it will be impossible forever, but consider this: There are only a handful of underground/tram/light railway systems worldwide that are driverless. These systems are magnitudes less complex (especially in the case of a failure) than an aeroplane, and still, they are not in use everywhere.