Instruction manual Wav WaveGuide Server
Appendix 4
Measuring principle
The basic principle of a FMCW radar to measure distances is travel time measurement. This is done by
transmitting a signal burst that increases in frequency in a linear way. The signal reflects on the water surface and
is received by the antenna. Both the transmit and receive signals are mixed. This results in a signal with a
frequency that is equal to the frequency difference of transmit and receive signals (the beat frequency). The travel
time is determined by this beat frequency. The higher the frequency the larger the distance.
Processing
The basis of processing is simple. The beat signal is Fourier transformed and in this spectrum (also called
reflection diagram) the reflection peak is determined, see figure 1 below.
Automatic Doppler correction
If the water level rises the frequency of the return signal increases due to the Doppler effect. The effect is that the
frequency difference becomes smaller and the distance therefore also smaller. (see fig. 2)
To correct for this effect the 973 makes an up sweep followed immediately followed by a down sweep. For the
down sweeping part of the signal the return signal has at the antenna a lower frequency than the frequency of the
transmit signal. In the up sweep the measured distance is the real distance minus the Doppler distance (a-d) and in
the up sweep the real distance is measured plus the Doppler distance (a+d). By addition and dividing by 2 the real
distance a is obtained.
Version 4.1
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Oct 2013
Fig. 21 Reflection diagram of WaveGuide 973 up
