The first three push buttons on the radar from left to right on the top row are "Passive ECM" "Continuous ECM" and "Intermittent ECM" for the MiG-21bis radar.
What is passive ECM? Soda cans. Tin foil. Window. In other words chaff, little strips of metal designed to confuse radar. From WWII through the 80s it was conceivable for bomber formations to dump chaff and lots of it to hide in a cloud of confusion. When we think ECM we think electronic electromagnetic antenna jammers but ECM is just any countermeasure to an electronic. It's passive because it's just out there floating in air being metal and minding its own business until a radar bounces a signal off of it and gets confused.
The other two modes (continuous and intermittent) are anti-jamming modes. They are used in the presence of active ECM. Continuous filter assumes that the jammer output obscures the target at all times. You may notice that the TDC is no longer controllable upon engaging it. Why is that? The answer is that the throttle grip now controls the radar sensitivity instead of TDC. Dialing back the sensitivity the wide splotch of radar return narrows to allow steering in azimuth. If the pattern is reduced by adjusting the sensitivity to the size of the lock on zone (5-6mm width) then normal lock on is possible.
Intermittent is when the jamming source permits the normal target blip to be seen occasionally each jamming cycle. In this condition point the aircraft at the target blip by elevating the nose until the "+" symbol is displayed showing the target neither significantly above or below the radar LOS. Place the TDC around the target favoring the upper gate. Engage the intermittent filter. Press the lock button. The TDC positioning will remain prior to engaging the filter (unlike continuous which sets it to a set value). Once the intermittent filter is engaged the throttle grip adjusts signal sensitivity as in the continuous filter. This requires that the TDC gates be set first and then the filter engaged unlike the continuous filter where the TDC gate position was unimportant.
In summary:
1. Passive ECM filter is for metal chaff but otherwise is normal.
2. Cont/Int ECM filter is for electronic jamming.
3. The throttle grip controls radar sensitivity when either filter is applied.
4. For intermittent preset the TDC and target prior to engaging the filter.
5. Lock within the lock on zone 0-18km, +-5 degrees.
