Re: Radar 101

[Luis Marques <marques@xxxxxxxxxxxx>]

Thi VanAusdal <tvanausdal@domain.elided> wrote:  
> > and?  I am sure you know that the radar will pick up the target
> > moving the fastest.  This changes at great distances, especially
> > when there are other vehicles with considerably more mass, (ie I may
> > not pick up a speeding motorcyclye until it is much closer than the
> > cars it is approaching with, likewise I may not pick up a car at a
> > great distance when it is surrounded by 18 wheelers.
>
> The 8 dB signal reflected off that semi coming at 55 mph
> 800 ft  away?  The 7 db signal off that BMW going 80 mph at 200 ft
> (and apparently ot paying attention!)?  Or the .5 db signal from your
> own heater blower fan spinning the equivalent of 110 mph?  C'mon, it's
> the strongest signal, not the highest speed value! 

First of all, your heater blower fan is inside the car, so no radar gun
will ever detect it.  If you mean the auxiliary radiator fan, the blades
move perpendicular to the car's motion, so it will register a speed very
close to zero unless it is being targeted from the side - and then it
will give both + and - speeds.  And it will be a very weak signal since
plastic doesn't reflect radar nearly as well as metal.

Second, newer radars can resolve between multiple targets and give you
the speed of the fastest one.  While it is true that a strong radar
return at any frequency will tend to mask any weaker frequencies close
to it, if the two signals are far appart (in frequency), the stronger
will not mask the weaker signal.  So if you are driving close to an 18
wheeler going only 10mph faster than he is, you may be "invissible" to
radar.  But if you are flying past him going at 130mph while he's doing
75mph, the radar can lock on you no matter how big the truck is.

Although signal strength (target size and range) does matter, the speed
difference between the vehicles also plays a significant role and John
Fountaine is correct when he says he can lock on a faster car when there
are other bigger cars around it going slower.

Luis Marques
'97 328i

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