Figure 1: PPI-Scope with typical sea clutter
Wind compensation is an electronic protective circuit against passive interference.
The chaff perturbations (known from the Second World War) are also fixed targets like sea clutter (see adjacent picture), but chaff can move with the wind speed. With normal MTI systems based on the Doppler principle, this clutter can no longer be suppressed at a certain wind speed.
A technical solution to this problem is to simply modulate an additional frequency, which corresponds to the Doppler frequency of the wind speed, onto the coherent oscillator of the MTI system. Thus, the targets moving at wind speed are suppressed.
As long as this wind speed is constant (as imaginable with weapon guidance radar, which only accompanies a single target), this is no problem. But even if the antenna rotates (as it is usual with airborne reconnaissance radars), the radial speed of the wind also changes depending on the current lateral angle according to a sinus function.
Already very early radar sets, like the Russian P-12, provided a sine/cosine voltage generated by a resolver, which as tuning voltage influenced the voltage controlled coherent oscillator depending on the actual azimuth angle of the antenna according to the sine function more or less in the reference frequency.
When this procedure is applied to land-based radar equipment, however, the fixed targets unfortunately appear as interferences now, since these do not exhibit any wind speed. However, if this method is used against sea clutter, then this disadvantage is not significant, since ground clutter is quite rare on the high seas. But also on land this disadvantage was gladly accepted, because it was to be assumed that exactly in the chaff cloud very important targets wanted to hide.