From: tdj@bbt.com (Todd Johnson)
Subject: Re: CHP can't use radar?(Was: What radar d
Organization: BroadBand Technologies, Inc.,  Research Triangle Park, NC
Date: Tue, 22 Dec 92 13:40:11 GMT

In article <1992Dec19.114320.10336@u.washington.edu> tzs@stein.u.washington.edu (Tim Smith) writes:
>lstowell@pyrnova.pyramid.com (Lon Stowell) writes:
>>    They are also technically ignorant of how small aircraft measure
>>    their own speed...and how difficult it is to pace an auto from
>>    the air.  Also they are ignorant of the difference between air
>>    speed and ground speed...and how impossible it is to measure
>>    ground speed unless you know the exact winds aloft at your
>>    altitude at the specific time of the flight.
>
>Here's how the cops claim they do it:
>
>	They measure how long it takes the aircraft to go between the
>	markers, while they observe the car.
>
>	From this, they determine the aircraft ground speed (65 MPH in
>	the case I was reading).
>
>	They note that they are going the same speed as the car, hence
>	the car is speeding.
>
>The court says (I use the singular rather than the plural, because I could
>only find one California appellate case where someone objected to this on
>the basis of it being a speed trap) that this is NOT a speed trap, because
>the aircraft is using the marker to measure its own speed, rather than
>using them to measure the speed of the car.
>
>I seem to recall a pilot or two saying that the plane could not go this
>slow, but is that really true?  Don't planes of the type the cops use
>have to be able to fly at 71 MPH airspeed in order to be certified?
>(I spent a few minutes with my copy of the FARs, but I couldn't find
>where the requirements for certification are, but I'm pretty sure that
>there is such a speed...anyone have the reference?)
>
>If they can get down to 71 MPH, then it seems that they could catch a
>lot of speeders, depending on the wind direction, because their groundspeed
>will depending on the wind speed and direction.  In at least one direction
>following the freeway, it will be less than 71 MPH (and it may be so in
>both directions if the winds aloft are significantly cross to the freeway).
>
>I also came up with a reference to a law review article on aircraft and
>speed traps.  It's 14 Hastings L.J. 427.  The library here is closed
>right now, so I haven't taken a look at this yet (Westlaw tends to only
>go back to the early 80's or so for law review articles, so I haven't
>bothered to see if its availables there).
>
>--Tim Smith

Sure the planes cops fly are probably capable of 71mph (62KTS). For a
cop to fly at that speed while trying to watch a car would be unsafe. 
The plane is not used to pace the speeder. The cop in the plane just 
times the speeder to go from point A to point B. The speed of the 
airplane has NOTHING to do with it, except that it gives the cop the
best view. There is no rule that says a plane must fly 71MPH to get
certified. Also, the wind speed and direction can very at any point
in time.

	Friendly Pilot,
		Todd

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