Recognizing of too long echo pulses
Figure 1: in principle mode of operation of the rain-reject
If the radar unit has a transmitting pulse length of e.g. 1 µs, then can be expected that
the echoes of objects of geometrically bounded extension (i.e. aircrafts) then have a length
of about 1 µs also. Even if several airplanes fly in formation, the sum of the lengths of
these echoes of more than 10 µs then is improbably.
Well, therefore you can suspect that echoes with a multiple pulse length of the transmitted
pulse are a kind of jamming. However, it is also unreasonable to switch these echoes off. The example of
Pearl Harbor has this proved, as the operator of the radar unit didn't warn the Marines,
interpreting the heavy Japanese attack formation as a screen fault.
Figure 2: PPI-screen with different clutter.
The Weather-clutter has got a highlighted leading edge owing to the rain-reject operating!
When recognizing a too long echo pulse than the echo isn't switched off therefore, but it is switched
to the coherent channel with the
pulse pair processing.
In Russian radar units this operating mode is called
„автоматический строб”, an automatic switching strobe,
that means in German: „automatischer Bandimpuls”.
In radar units of American origin this device is called „rain reject”.
Right well: this kind of jamming with too long echo pulses usually appears, if heavy rainclouds
produce an area of
diffused reflection.
However, this wiring needs a certain time, before it can recognize that the pulse is too long.
Therefore the front edge of the pulse always remains visible when recognizing such a area of
diffused reflection. Well, the wiring doesn't work only like a differentiating stage,
it is also only a differentiating stage in very simple radar units.
This wiring for example is examined in whether the echo signal is longer than 4 µs.
Note: This function is called
STC (Short Time Constant) in some British publications; and
FTC (Fast Time Constant) in some American publications.
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