Page 20 - Laser Technology, Inc. LTI 20/20 TruSpeed Metric User’s Manual 3rd Edition
Copyright (c)[2013] Laser Technology, Inc. All rights reserved. Unauthorized duplication, in whole or in part, is strictly prohibited
The cosine effect decreases as the range to the target vehicle increases.
•
At the maximum range of the instrument, the vehicle is so far away that
the angle between it and the instrument is very small indeed. The
instrument's perception of the target's speed is identical to its true
speed.
•
As the vehicle approaches, the angle increases until it becomes large
enough to affect the measurement.
The table below shows acceptable parameters for minimizing the cosine effect.
The chart indicates the percentage of true speed measured, given the distance
from the roadway and the distance from the target vehicle. To find a target's
measured speed, multiply the true speed by the number in the chart.
•
The crosshatch indicates impractical angles, i.e. distance off the road is
greater than range to the target.
•
The diagonal created by the boldface numbers indicates the boundary
between acceptable and unacceptable parameters.
•
Numbers
above
the diagonal are acceptable margins of error.
•
Numbers
below
the diagonal are unacceptable margins of error.
Remember that the cosine effect is always in the motorist's favor.
To minimize the cosine effect, keep the angle small. Set up the
instrument as close to the road as possible without creating safety
risks, and target down the road at ranges sufficient to keep the angular
difference small.
Distance
off the
roadway
(meters)
Range to Target Vehicle
30 m
100 m
150 m
300 m
600 m
fraction of the True Speed that will be measured
3
.9950
.9995
.9998
.9999
1.0000
10
.9428
.9950
.9978
.9994
.9999
15
.8660
.9887
.9950
.9987
.9997
30
.9539
.9798
.9950
.9987
60
.8000
.9165
.9798
.9950
As a general rule, do not exceed 1 meter off the road for every
10 meters shooting down range to the targets. If target vehicles
will be 150 meters down the road, set up no more than 15 meters
off the road.
