To measure to 1msec you need to define a virtual barrier width
corresponding to 0.5msec for your measuring system, constant across the
entire road. You could use an optical method I think. At 300kph 0.5msec is
4 cm or so so you will have to use something with a real narrow field of
view. The car id can be done 'later' or 'before', not necessarily at the
same location where the timer is. And optical may not be so good if there
is a cloud of dust from the previous car. I would be tempted to do
something with a microwave beam across for lap time and another 'in front'
to read speed (doppler) and activate a challenge/response type gizmo in
the windshield area of the car. This does not solve the problem of several
cars passing side by side. I have little experience with microwave but
from what I know I think it would work. Each car would have a radar
detector (of commercial manufacture) and this would be wired to activate a
tag transmitter, perhaps on 433 MHz. The beams produced by 'door opener'
type microwave sensors are suitable for making barriers up to 15 meters
wide (using two units).

Peter

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