Like all Decatur units, The GHS is very reliable and very accurate.
The newer units (like this) use DSP circuitry which eliminates most errors, and also shuts down the unit (will not display speed or doppler audio) if it detects an error.
Plus, it is stationary only:
It doesn't have to worry about calculating patrol and target speed, only target speed.
If the cop did an internal circut test and tested the calibration with the fork before he used the unit, and he is RADAR cert, and the RADAR unit and fork have been bench tested at least once within the past, I can't think of any way to fight this ticket.
You could subpoena the cops' RADAR training cert, and the cert of the RADAR unit and the fork he used.
I can not speak for Decatur, but KUSTOM and MPH units and the forks have to be bench tested at least once a year.
It's the only thing I can think of.
Jay