🚀 go-pugleaf

In article <1991Feb17.064619.27255@athena.cs.uga.edu> mcovingt@athena.cs.uga.edu (Michael A. Covington) writes:
>Here are some things I learned recently while serving on a jury:
>
>(1) Accuracy to +- 1 mph is definitely _not_ to be expected. +-5 mph is
>believable. The biggest source of errors is from measuring the wrong
>vehicle.

Accuracy to +/1 MPH is REQUIRED (see #2 below).  The +/- 5 MPH due to
multiple vehicles makes no sense.  The radar gun is going to return
the speed of one of the vehicles behing clocked.  The problem is that
you don't know which vehicle it is.  Hence, the accuracy is still within
tolerance, only you don't know which vehicle is the target.  Consider
one car passing another (55 MPH and 75 MPH).  While the passer is 
behind the passee, chances are that the radar gun is going to return
the lead car's speed -- 55 MPH.  When the passer passes the passee, 
the radar will probably read 75 MPH.  Hence a 20 MPH difference.  This
is NOT a +/- 20 MPH error in accuracy.  It's right on the money.  With a
lone target on a road, accuracy will be within +/1 MPH, minus cosine error,
assuming no external interference.  This is theoretically, practically,
and experimentally true.

>(2) Radar guns are calibrated with tuning forks (strange but true).

Not exactly.  They are checked with tuning forks.  Stationary-only
radar guns usually have one or two forks.  Each should return
a particular reading (like 45 and 75 MPH).  If the speed displayed is
off by more than 1 MPH, the radar gun is supposed to be taken out of
use.  For moving-mode radar guns, there are two forks (like 120 MPH
and 55 MPH).  Striking the two and holding them both in front of
the radar gun should yield a "patrol" reading of 55 and a "target"
reading of 65.

When radar guns are calibrated, they are done so with real
test equipment, such as frequency counters, signal generators, etc.
The tuning forks are only used for in-the-field accuracy checks, not
for calibration.  There's a big difference here.

>(3) A radar gun requires an FCC license to operate. In Georgia, the
>police must produce this license, in court, in order for radar evidence
>to be admissible.

True.

>(4) Any other transmitter on radar frequencies would require a license,
>unless there's a loophole in Part 15 after the revision. (Part 15 governs
>license-free transmitters and has just been liberalized.)  In any case,
>the transmitter would be illegal if its purpose were to interfere with
>a licensed user of the radio spectrum (the radar).

There are many many many part 15 devices that operate inside the radar
bands.  Neither X nor K band is a police-only or radar-gun-only allocation.
The X band is shared with part 15 devices, such as microwave door openers,
alarm systems, etc. among others.  The K band is shared with Amateur
Radio among others.  Part 15 devices like door openers don't require a
license.  In any event, jammers would not be considered Part 15 devices,
as their intent is to interfere with another service.



							--- Jeff
--
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 Jeff DePolo  N3HBZ             Twisted Pair: (215) 386-7199                  
 depolo@eniac.seas.upenn.edu    RF: 146.685- 442.70+ 144.455s (Philadelphia)  
 University of Pennsylvania     Carrier Pigeon: 420 S. 42nd St. Phila PA 19104