RE: More laser stuff

["Mike Miller" <JoMi@msn.com>]

The laser guns operate at a frequency of 125 hz - thats 125 readings per 
second. They make the reading over a .3 sec interval so they have 40+ readings 
to average. At 70mph you travel about 34 feet in .3 sec. Assuming a 400 foot 
range to the laser gun initially and shooting 30 feet off center, the angle 
will change about .4 degrees in .3 sec. and the speed reading error will 
increase by about .04 mph. Insignificant in other words..As the speed error is 
the cosine of the measured angletimes the actualk speed, any angle less than 
about 20 degrees is really insignificant and 20 degrees will only produce 
about a 4 mph error at 70 - ie about 66..

mike miller

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From: 	owner-quattro@coimbra.ans.net on behalf of Jack Rich
Sent: 	Wednesday, September 18, 1996 8:29 PM
To: 	quattro@coimbra.ans.net
Subject: 	Re: More laser stuff

>... since my theory was that it takes multiple samples to get the speed 
estimate you would need to be driving in a circle centered on the
ossifer 
to actually get the zero speed reading.<

The angle will affect the reading regardless of whether it is phase
shift or return time differences.  The difference in time only changes
by how much closer the car being measured is to the measuring unit,
which is the actual speed of the car only if it is on a collision
course.  If more than two readings are being taken for the timing
difference, the angle keeps changing, but not very fast compared to the
rate the measurements are being made.  The car will appear to be slowing
down as it drives by.  Indicated speed will be zero as the car goes
past, which it must as the oncoming speed becomes negative, meaning the
car is moving away from you.
-- 
Jack Rich
84 4000S Quattro
88 Corvette convertible
90 V8 Quattro