Concealment or Masking
Figure 1: noise-modulated jamming, the jammer in 210°
Essentially, Concealment uses ECM to swamp the radar receiver and hide the targets. Concealment (Jamming) usually uses some form of noise as the transmitted ECM signal. In this section, Concealment will be called “noise” or “noise jamming”. An increasing of the noise will decrease the probability of detection and an increasing of the false alarm rate too. A powerful jammer can hide the targets completely.
Noise jamming is either spot jamming or barrage jamming. Spot jamming is simply narrowing the bandwidth of the noise jammer so that as much of jammer power as possible is in the radar receiver the bandwidth. Barrage jamming is using a wide noise bandwidth to cover several radars with one jammer compensate for any uncertainty in the radar frequency. It is also possible to wobble a spot jamming frequency in a wide band. Therefore this is called swept jamming.
Often used in the World War II, chaff is a passive disturbing method which cause high noise-similar clutter on reason of its own reflection.
Figure 2: RR-129 and RR-144: two types of chaff and their canisters
Clutter producing reflective materials (thin staniol stripes or metallic covered glass fibers) are dispensed into the atmosphere. These stay quite long time into the atmosphere and cause impenetrable clouds for the radar signals. For better effect the length of the stripes is tuned with the wavelength of the radar unit.
Special filters and ECCM-Fixes, (Electronic Counter-Counter Measures), sometimes called EPM (Electronic Protection Measures) make an improvement. Against chaff a doppler radar can compare the higher speed of the aircraft with the lower chaffs windspeed (Sub Chaff Visibility).