# Radar

**URL:** <https://boards.straightdope.com/t/radar/327243>\
**Category:** Cecil's Columns/Staff Reports\
**Created:** [October 21, 2005, 12:24pm UTC](https://boards.straightdope.com/t/radar/327243 "2005-10-21T12:24:16Z")\
**Posts on this page:** 3\
**Page:** 3

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [November 4, 2005, 12:53am UTC](https://boards.straightdope.com/t/radar/327243/41 "2005-11-04T00:53:01Z")

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> [@XWalrus2](#):
>
> Or you could paint the car matte black.

Won’t help much. The important reflective parts are the retroflective license plates, the corner cube arrays in the taillights, turn signals, other marker lamps/reflectors and the headlights. Reflections from the body are going to be small in relation to those from the above mentioned sources.

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**Author:** ![eviladam\_part\_2](https://avatars.discourse-cdn.com/v4/letter/e/da6949/32.png) [@eviladam\_part\_2](https://boards.straightdope.com/u/eviladam_part_2)\
**Post date:** [November 4, 2005, 3:31pm UTC](https://boards.straightdope.com/t/radar/327243/42 "2005-11-04T15:31:34Z")

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> [@Kevbo](#):
>
> The Radar unit has to measure the beat frequency between the outgoing and return signal…this is a fairlly low frequency signal. Accuracy of measuring a frequency depends on the time taken to measure it. The time can be reduced by measuring the period, however, due to noise, several averaged measurments are still required.
> 
> LIDAR pulses are pretty easy to pick out of the noise. If the two intervals between three pulses agree on the infered speed, you probably have a good reading, a fourth pulse will confirm it. In human reaction terms, this requires less than an eyeblink.
> 
> The net result is that LIDAR can give an accurate speed reading slightly faster than radar.
> 
> And yes, infrared LEDs and even diode lasers are cheap and compact compared to gunn diodes (or dog forbid magnatrons) needed to generate microwave jamming. Also, the LIDAR units are not too picky about the exact wavelength as long as it is close, whereas jamming radar requires pretty good frequency accuracy and stability (or lots of power).

[this link](http://www.pbelectronics.com/police_lidar_laser.htm) says laser requires ~0.3 seconds to take a speed measurement. the extremely narrow beam of the radar requires more averaging, to eliminate errors from possibility of one pulse hitting the front of the car, and the next pulse reflecting off the windshield. since it’s a calculation based on distance measurements, if the measured signal is coming from a different point it will cause huge errors.  
[this link](http://www.stalkerradar.com/sports_sport.shtml) has a radar gun that requires 0.046 seconds to measure speed.

traffic radar is not a “fairly low frequency”. X band (mostly obsolete) is ~10GHz, and the more popular K and Ka are ~24GHz and ~33GHz.

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [November 6, 2005, 1:57am UTC](https://boards.straightdope.com/t/radar/327243/43 "2005-11-06T01:57:01Z")

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> [@eviladam](#):
>
> traffic radar is not a “fairly low frequency”. X band (mostly obsolete) is ~10GHz, and the more popular K and Ka are ~24GHz and ~33GHz.

He doesn’t mean the carrier signal frequency, he means the beat frequency–the differential between the outgoing carrier and the Doppler-shifted return signal. This can be only a few hertz. That’s what he meant by “fairly low frequency”.

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