1. ## Why radar can't hit you on the side?

I've heard the cop can't get a good read if he hits you on the side. Any idea why radar doesn't work on the side? Thanks!

2. Well if a LEO shoots the side of your car and he is perpendicular to your car then your relative speed to him is zero. It is all about the vector composition of your velocity in order to get your speed in the foward direction the radar needs to be inline with you cars motion.

3. Or in layman's terms, the radar measures the relative speed of objects headed toward, or away from, the radar source. If the officer is radaring the side of your car, you're going across the beam, not toward or away from it, thus your speed (relative to the beam) would be zero.

Traffic radars measure the relative speed a target
is approaching (or receding) the radar. If a target is
traveling directly (collision course) at the radar, the
relative speed is actual target speed. If the target is
not traveling directly toward (or away) the radar but
slightly off to avoid a collision, the relative speed with
respect to the radar is slightly lower than target speed.
The phenomenon is called the Cosine Effect because the
measured speed is directly related to the cosine of the
angle (alpha) between the radar and target direction of
travel.

The cosine effect angle (alpha) is the angle between the
radar and the target direction of travel. Target range from
between radar and the point the target would be closest to
radar if target continues in same direction) determine the
cosine effect angle. Note that the road direction and antenna
direction (direction antenna pointed) are completely irrelevant,
only the angle (alpha) matters (radar stationary).

Antenna direction (alignment to patrol car direction) is important
in moving mode radar. A mis-aligned antenna measures target
speed high if the misalignment is great enough, the target is
approaching the moving radar, and the target is traveling slower

As long as the angle (alpha) remains relatively small, the
error (cosine of alpha) is tolerable. The larger the angle, the
larger the error and the lower the displayed (relative) speed.
and the range of the target determine the angle. The greater
the distance the radar is off the road and/or the closer the
target, the larger the angle (and error). When the target is
even with the radar (alpha equals 90 degrees) the target
speed, with respect (relative) to the radar, is zero.

The Cosine Effect applies to both microwave radars and
laser as well as to targets traveling in any
direction (on-coming or going traffic at any angle). Most
traffic radars do not account for the Cosine Effect; across
exception. These systems point the beam at a known fixed
angle across the road and compensate the measured target
speed for the Cosine Effect.

Cosine Error
The below figure is a graphical representation of the Cosine
Effect for measured speed, as a percentage of true speed
versus angle (alpha) between radar and target -- the larger
the angle the larger the error and the lower the measured
target speed. For example at angles of only a few degrees
the measured speed is 99 to 100 percent of actual; at an
angle of 60 degrees the measured speed is half (50 percent)
the actual target speed.

5. orbital you just nailed it man, thanks for the info.

6. do officers use this to adjust the speeding ticket?

One could get a little too optimistic with angle calculations.

7. Originally Posted by polakatl
do officers use this to adjust the speeding ticket?

One could get a little too optimistic with angle calculations.
No, they only use the parallel component of velocity, which means you get a lucky break

8. Originally Posted by guit123
orbital you just nailed it man, thanks for the info.
No problem, here to help. :wink:

9. Originally Posted by polakatl
do officers use this to adjust the speeding ticket?

One could get a little too optimistic with angle calculations.
I can see the cop doing the trig calculations on the road shoulder right now..."let's see now, I was at about a 14 degree angle to your velocity vector when I first shot you, when I got the lock it was about 16 deg, ummm the component would be the reciprocal (cosine of 16 deg) times your measured speed, errr let's just call it 19 over, sign here please."

10. Orbital is no joke .
That guy is a brainiac.

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