This information is edited from a repost of an acig.org forum post by mrdetonator:
http://www.secretprojects.co.uk/forum/index.php/topic,25.msg59.html#msg59
Detailed description of the lookdown/shootdown capability of the S-23E:
1. When the MB radar mode has been selected, the returned signal is further processed in the Pulse-doppler channel of the Sapfir-23E.
At first the signal is led to the linear receiver, where it is amplified and sorted out by the amplitude detector.
Then it is routed to the 49 multi-channels Doppler filter (a comb filter), where the selection of moving target takes place.
The doppler shift of the radar is given by:
Fpd= Fd-nxFp =2x(Vr/c)xFo-nxFp
Fpd-doppler shift of pulse radar
Fd-doppler shift cw radar
c- speed of light
Vr-radial velocity component
Fo-source signal frequency of pulses
Fp-pulse repetition frequency
n-integer from 0 to infinite
If the pulse repetition frequency „Fp“and the source signal frequency „Fo“ is constant, the amount of doppler shift depends only by the component of radial velocity.
Considering that the radial velocity can change in a wide range, the doppler shift of the clutter (earth background) is taken as a reference (coherent) signal to process the doppler filtering.
Therefore the filtering technique itself is called the „СДЦ (селекции движущихся целей с внешней когерентностью)“- The moving target selection with external coherence.
Then the doppler shift of the radar is given by:
Fdp=Fdc-Fdt-nxFp=2xFo/cx(Vrc-Vrt))-nxFp
Fdc- doppler shift of the clutter
Fdt- doppler shift of the target
Vrc- radial velocity of the clutter
Vrt- radial velocity of the target
There are operational limitations such as limitations upon altitude of use or so-called blind-speeds. To operate correctly the SDC with external coherence technique needs to synchronize the target signal with the clutter phase. To simplify this task, the clutter signal received by the radar sidelobes in the second/third scan cycle is used to process the doppler filter.
To deal with the “blind speeds” the radar is changing the pulse repetition frequency during each scan line. More than 90% of “blind speeds” are covered, what ensures sufficient target detection.
2. At higher altitudes the SDC with external coherence technique becomes ineffective, the radar uses the BSV-deltaH1 and the BSV-delta H4 mode.
Both the BSV-deltaH modes use a technique called the Single-beam space-time selection.
This filtering technique makes use of the difference between the spatial target location and the earth surface segment illuminated, which distances to the radar are the same.
By utilizing the antenna high spatial selectivity it is possible to separate the target signal from the clutter.
The advantages over the SDC with external coherence technique are no limitations in scanned sector and the target heading.
Its disadvantage is the strong relationship between detection range and the target altitude.
Lower target altitude means smaller detection range.
The detection range equals:
D =k x Hc (km)
D- detection range
k- coefficient given by the antenna directivity pattern, characteristics of the surface background, target RCS and the surplus altitude over the target.
Hc- target altitude.
When illuminating the surface by a high directive antenna, the returned signal from the earth comes a bit later compared to the target signal.
Then it is possible to separate the target signal from the clutter by using common methods of processing.
The power of the clutter signal depends on the distance to the earth by a given antenna bearing as shown on the picture.
There are three specific regions.
-The first sector is evoked by the antenna sidelobes, which receive signals directly from beneath the plane.
It is also here possible to separate the target signal from the clutter, because the target signal coming from the mainlobe uses to be stronger compared to the clutter signal coming from the sidelobes.
-In the second region the ground clutter can be filtered out easily, the power of the target signal greatly exceeds the clutter received by radar mainlobe.
-In the third region the ground clutter exceeds the target signal due to strong ground echoes received by the mainlobe. There is no possibility to detect the target signal.
The radar suffers from the clutter even more by illuminating the surface at very low elevation angles.
Using of the high directive antenna will result in intense backscattering, the ground clutter increase in beam width.
To filter the ground clutter the BSV- delta H1 mode uses the DKP „the differential clutter filter“ which processes the return signal further.
The DKP filter operates only while the target is tracked in the BSV-delta H1 mode.