User Manual

33
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: 72.0 Hz x 60 mph = 4320.0 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.
Transmitted Frequency 24,150,000,000 cycles per sec.
+ 4,321 cycles per sec.
Vehicle Receding at 60 mph
Reflected Frequency 24,149,995,679 cycles per sec.
Transmitted Frequency 24,150,000,000 cycles per sec.
– 4,321 cycles per sec.
Note again how the reflected frequency is greater than the transmitted as the vehicle approaches
and less than the transmitted as it recedes, yet the difference, the Doppler shift, remains constant
for this particular vehicle speed.
Stationary radar theory and Automatic Same Direction
™
(ASD
™)
technology
Most stationary radars cannot detect what direction a target is moving. In both of the previous
examples, a “normal” stationary radar would detect a Doppler shift of 4312 cycles per second and
convert that to 60 mph. They cannot tell if the true Doppler shift was +4312 or -4312 cycles per
second.