The Doppler Principle as applied to velocity measurement
Up to this point, we have been using sound to demonstrate the effects of the Doppler principle. However, as you
may know, radio energy and light also exhibit a waveform and this fact opens several interesting areas to
consideration.
As we have seen earlier, it is possible to determine the existence and the location of an object at great distance by
transmitting a beam of radio energy and then receiving that small portion of the beam that is reflected back. If it
is possible to reflect radio energy from an object, and that object is in motion toward or away from the transmitter,
the reflected radio waves should be altered in accordance with the Doppler principle. More specifically, they will
be compressed to a higher frequency as the object moves nearer to the source and, conversely, stretched as the
object moves away. Furthermore, the faster the object approaches or recedes, the greater the
compression/stretching effect upon the waves.
Therefore, if we are able to transmit a radio wave of a known frequency that travels at a constant speed, and then
construct a device to measure the frequency of the reflected waves, by comparing the two frequencies we will
know how much our beam was altered by motion, the Doppler frequency. From here, it is a straightforward
calculation to determine the velocity of our target object. This is precisely the approach taken in all modern speed
measurement devices.
Practical application of the Doppler Principle in traffic radar
Now that we have an understanding of the Doppler principle as applied to velocity measurement, let us examine
how it is used in MPH traffic radar.
You will recall in the example of the automobile horn that the frequency of the horn tone and its rate of travel
through the air were assumed to be constant, so that the only factor affecting the tone from the observer's
standpoint was the change in position of the automobile. With radio waves, we are able to assume this with much
greater confidence. For a source of radio waves, MPH has selected a sophisticated solid-state device called a
Gunn oscillator that generates radio energy in the microwave region. Specifically, a K-band radar transmits at a
frequency of 24,150 MHz, and a Ka-band radar transmits at a frequency of 33,800 MHz. This high frequency
radio energy is focused into a narrow beam and directed at the target vehicle and travels at the speed of light. A
small portion of the beam is reflected back to a second solid-state device called a mixer diode. The mixer diode
compares the frequency of the reflected beam to the transmitted frequency. The difference between these two
frequencies is called the Doppler frequency. Furthermore, the Doppler frequency is directly proportional to the
sum of the transmitter (patrol) and target velocities. It can be shown mathematically that for a transmitted K-band
frequency of 24,150 MHz, a Doppler frequency of 72.0 Hz will be produced for each mile per hour that the target
is moving. Similarly, a transmitted Ka-band frequency of 33,800 MHz will cause a Doppler frequency of 100.8
Hz to be produced for each mile per hour. For example:
K-band: 72.02 Hz x 60 mph
=
4321.0 Hz Doppler frequency
Ka-band: 100.8 Hz x 60 mph
=
6048.0 Hz Doppler frequency
Knowing this relationship, we are able, by means of modern electronic circuitry, to convert the Doppler frequency
as determined by the mixer diode into a digital presentation of the target's speed in miles per hour.
Some appreciation of the accuracy required of the complete system may be gained by looking at the very small
numerical value of the Doppler frequency as compared to the transmitted and received frequencies.
K-Band
Vehicle
Approaching
at
60
mph
Reflected
Frequency
24,150,004,321 cycles per sec.
24
