COTS GPS receivers have upper limits on altitude and velocity and possbly
acceleration.

This is to stop them being used for "other purposes [tm]" (eg in Southern 
Israel at present)
You can get GPS's with these limitations removed but they are probably ITAR
items and you probably need to have suitable clearances.
The Arocket comminity will know the limits but Gargoyle probably does too.

AFAIK the receivers generally stay in lock if they are able and start
outputting data again when they next exit the forbidden zone.

Here's some possibly useful commentary [:-)]

    http://showcase.netins.net/web/wallio/GPSrcvrsvs60kft.htm

See "Dynamic condition" here These may all be artificially imposed limits as
the 18,000 metres is one of the set limits.
http://showcase.netins.net/web/wallio/GPSrcvrsvs60kft.htm
Max alt 18000m
Max V 515 m/s
Max acc 4g
Max jerk 20 m/s^3

          R



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