A Pulse – Doppler – Radar sends out a series of short microwave pulses.
The radar works at a optimum frequency in the C-Band. At this frequency, signals
are well-reflected by standing or moving snow masses and are less influenced
by atmospheric disturbances.
The following types of measurement can be done by the radar system:
Position of the target (from the echo delay)
Velocity of the target (from the frequency doppler shift)
Reflectivity or “size” of the target (from the echo intensity)
Range Gating:
The range discrimination of the radar is regulated by so called range gates.
This means that the received echo signal is sampled at well known points in
time and each of these points in time corresponds to a range gate. The range
gate length is normally 50 m. The principle of range gating is shown in Fig.
5.
The area up to the first range gate is called “Blind – Range”. The radar
is not able to measure movements in this area. The minimum blind range correlates
with the range gate length and is normally 50 m.
Velocity Measurement:
The velocity is calculated from the radar data via the doppler effect. The
frequency of the reflected signal (frefl) is shifted against the frequency
of the transmitted pulse if the target is moving. frefl = f0 + df This change
in frequency of the echo signal is called doppler frequency shift. f0 is the
frequency of the transmit pulse. For velocities which are small in comparison
to the speed of light, the velocity can be calculated as follows: df = fDoppler
= 2*vrad*f0 / c. vrad is the unknown radial velocity (velocity component in
the direction of the antenna beam) of the moving target. This velocity component
is positive if the moving target comes closer and negative if the objects
gets away from the radar. Any movement orthogonal to the antenna beam cannot
be detected. (see Fig. 6)
