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A handheld device is much the same as a GATSO.
It is laser beam operated.
They use a narrow beam of light which bounces from the target vehicle and provides a reading.
the basic premise is that radio waves target the vehicle and they are reflected back at the radar which allows it to work out the speed a vehicle is travelling at.
They don't photograph number plates twice like GATSOs but they do take a shot of the number plate so that a S172 form can be sent out with a NIP.
They are thought broadly as reliable as GATSOs if they are used in the manner recommended by ACPO.