From: jgd@dixie.com (John De Armond)
Subject: Re: LIDAR workings (was Re: Laser Jammer Sighting?)
Date: Wed, 07 Oct 92 03:39:02 GMT
Organization: Dixie Communications Public Access.  The Mouth of the South.

ptrei@bistromath.mitre.org (Peter Trei) writes:

>(1) The laser is in the milliwatt range. It can't get higher, for
>reasons of size, weight, power supply, and last but not least, safety.

>(2) The receiver has to pick up the laser pulses in the microwatt
>range - only a fraction of the light will be reflected back to the
>gun. A receiver sensitive to such low levels probably (note
>qualification) has a limited upper range, and can be swamped.

>(3) The receiver is probably as inexpensive as possible, and relies
>more on time domain intensity variations to identify it's returned
>signal than on precise frequency discrimination.

The laser is low power but as best I can tell, the unit uses interferometry
which provides a high degree of discrimination to other sources of light.
Plus the gun has what looks like a dichroic bandpass filter on the 
receiver.  More selectivity.

Note that at a retail price of more than $5000, the technology does not
have to be cheap and does not seem to be.

>(4) Therefore a jamming device does not need to be exactly the same
>frequency as the laser source - it just needs to return enough energy
>in the entire frequency range in which the detector is sensitive to
>overwhelm it. (Note that this can be countered with a frequency
>specific filter on the detector).

You need to realize what you're saying.  The range of frequencies of light
that falls within the nominal range of IR is massive, comprising many 
octaves.  A radio analogy would be to attempt to jam a highly selective
VHF receiver using a wideband noise source that covers from AM broadcast
to microwave.  Even with massive total power from the jammer, the power
concentrated in the band of interest is minimal.  You simply will have 
to know more about the IR used in order to jam.  Once you DO know something
about it, generating the necessary power is easy.

>     John, when you get your hands on a lidar gun, a lot of us will be
>interested in what you find. For a start:

I have already had my hands on one for a few minutes.  Because of the 
source, no more details regarding the incident are available.

>*     What warnings vs, say, pointing it at the sun, does it carry?

No markings on the gun.

>*     What happens when you point it at a very HOT car - say a black paint
>      job that's been sitting in the Georgia sun all day?

Probably nothing, as noted above.

>*     If we can't do a brute force intensity jam, how about an intense,
>      rapidly flickering IR source (eg, hot object behind fan blades)?

My guess is an array of laser diodes shining through a suitable diffuser 
would be the most effective jammer.  Given the low price of laser diodes
these days, this would be very inexpensive to construct.

>*     How well does a license plate cover reduce IR return? Glass?
>      Plastic? Should we rely on absorption or reflection away from
>      the lidar? What can we do about rear (and front) reflectors?

This is light. You can easily test your hypothesis by simply shining a 
light on your car and observing the reflection.  The LIDAR is not as
sensitive as your eyes so filter your view appropriately.

I spent my time with the gun doing what cops would do - shooting at cars.
My experience was quite similar to _Car & Driver_ in that it is a difficult
unit to operate and requires steady aim.  I only had about 5 minutes with
it so I have only first impressions.  I believe a good defence will be to
minimize the reflective surfaces on the car and to use a detector.  Though
the gun can acquire a reading in under a second under ideal conditions,
under real world conditions it will require several seconds.  I'd not 
be surprised to hear someone has developed a clearcoat with IR absorbance
properties.  The market is certainly pregnant.

John
-- 
John De Armond, WD4OQC               |Interested in high performance mobility?  
Performance Engineering Magazine (TM) |
Marietta, Ga                         |Interested in high tech and computers? 
jgd@dixie.com                        |Write me about PE Magazine
Need Usenet public Access in Atlanta?  Write Me for info on Dixie.com.
Newsgroups: rec.autos.driving