Wind compensation
Figure 1: PPI-Scope with typical sea clutter
How does a sea state suppression work?
Wind compensation
Wind compensation is a classic interference suppression circuit against passive interferences.
The chaff known from the Second World War as well as sea clutter (see adjacent picture) are fixed clutter which, however, can move at a speed dependent on the wind direction. Simple MTI systems based on the Doppler principle can no longer suppress these interferences above a certain wind speed.
A technical solution to this problem is to modulate the coherent oscillator of the MTI system with an additional frequency that corresponds to the Doppler frequency of the speed of the wind. Thus, the targets moving at wind speed are suppressed.
As long as this wind speed is constant (as with weapon control radar tracking only a single target), this is not a problem. But if the antenna rotates (as it is common with air surveillance radars), then the radial speed of the wind also changes depending on the current bearing angle according to a sinusoidal function.
oscillator
transmitter
reference
excitation voltage
adjustment
Figure 2: Principle diagram of wind compensation
transmitter
reference
excitation voltage
adjustment
Figure 2: Principle diagram of wind compensation
Analog solutions
Already very ancient coherent-on-receive radars, like the Russian P–12, had resolvers, which generated a control voltage depending on the actual bearing angle of the antenna position. This corresponds in the voltage curve to a sine/cosine function, in which a complete sine curve of 360° also corresponds to the full circle of the respective antenna position. This resolver can be rotated to the respective maximum of the interference and thus generates a voltage which, after rectification, is used as a measure of the current bearing angle of the antenna. This additional tuning voltage influences the coherent oscillator according to the sine function sometimes more or sometimes less strongly, depending on the current lateral angle of the antenna.
When using this method on land, however, the fixed ground clutter now, unfortunately, appears as interference, since it does not have any wind speed. However, if this method is used against sea state interferences, then this disadvantage is of no importance since fixed targets are also to be displayed as navigation aids. However, this disadvantage was also accepted on land, since it could be assumed that very important targets would be hidden behind or in the chaff cloud.
Digital solutions
To be able to digitally calculate this component in a Doppler frequency, the radiation direction of the radar in relation to the wind direction must be known. The signal processor can easily determine this employing the maximum of a Doppler frequency occurring over a large area. This maximum is also a measure of wind strength. Magnitude and direction are weighted for each bearing angle according to a sine function and subtracted from the measured Doppler frequency. Thus all echo signals moving with wind speed get a Doppler frequency of zero and are suppressed like a fixed target.
At the same time, even the radar’s platform speed is taken into account, because the radar signal processor can only measure the superposition of wind direction and its course, and wind speed and its cruising speed.
But be careful! Unfortunately, cheap navigation radars for ships and boats often also sell a simple manual control of threshold values as “sea state suppression”. Such a primitive solution worsens the probability of detection of smaller objects to the same extent.