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Radar in a Tunnel
Name: Greg S.
Status: Educator
Age: 50s
Location: N/A
Country: N/A
Date: February 2004
Question:
How effective is radar in a tunnel?
Replies:
I think tunnels tend to be a problem for most radars.
Most radars listen carefully for any echo in an otherwise empty space.
In these radars, when too many objects exist the resulting picture looks "cluttered", and it
becomes difficult to discern the targets of interest.
Tunnels often have pillars, and each of these would be a big object.
Even smooth, parallel walls would in themselves act like mirrors, creating multiple view paths
to each radar-reflecting object.
However, bats use their sonar in tunnels. So in principle a radar with sufficiently
sophisticated data analysis could do OK for some things in a tunnel.
For a limited number of large, distinct targets in helpfully designed tunnels, useful if
slightly specialized radars are currently feasible.
How much it would cost to develop, and what it could then do, is unknown enough to be ripe
for scams.
Demonstrations are needed to know.
The relatively long wavelength (inches) of radar waves make it difficult to just "point your
beam only down the tunnel".
Diffraction of long waves means that some of the beam will always leak sideways and touch the
walls.
Signal loss by absorption and scattering by tunnels walls will make it pretty sure that the
range is not very long. Definitely less than miles.
For reasons like this, lidar (light-wave radar)might be better.
The narrowness of a tunnel means that the walls will always be in closer-than average
interaction with the radar waves.
Designing the tunnel to help (or at least not hurt) the waves, is bound to be a
consideration.
Note that asking "How effective is radar?" is like asking "How good are eyes?"
Whose eyes? What situation? What purpose? It is a broad question.
Jim Swenson
PS- if you mean police speed-measuring radar guns, they have about that same chance of
catching the right car as it does on a freeway with crowded lanes on both sides of the
car being aimed at. The narrowness of the beam coming out of the radar may or
may not be good enough to catch only the right car, and that depends on distance too..
In an empty tunnel with only the target car and the radar car, I think it probably works.
If the radar happens to "see" your car in reflection off mirrorlike walls, that will not
result in much speed-measurement error, and most such errors would be in
the driver's favor, making the car look a little slower than it really was. If the reading
were non-repeatable because of multiple reflections, you would be dependent on the radar
and/or the officer to notice and admit it.
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