Can you shed some more light on point 3, how distance affect jamming or lock on?
It pretty simple really ,let talk about how a radar work first: How can a radar detect a target ?
Radar send out pulses and analyze reflection to detect and track targets. The reflection signal power is competing with some interfering signal in order to be detected or recognized. Interfering signal sources may be ground or sea returns, meteorological clutter returns, atmospheric reflections, or more likely, random noise generated within the receiving circuitry. The latter source is always present to some degrees, while the other sources are variable and can be zero.The maximum detection range performance is determined by a very small
signal to noise ratio when the signal begins to fade and become indistinguishable from the noise. As you can easily imagine , the further target stays from the radar , the less reflection you will receive , while background noise really doesnt change that much
Now on to jamming.There are 2 class of jamming in general :
The first one is noise jamming : Noise jamming is the form of electronic countermeasure where jammer transmit an interference signal ( white noise) in enemy’s radar direction so that the aircraft reflection is completely submerged by interference.This type of jamming is also called ‘denial jamming’ or ‘obscuration jamming’. The primary advantage of noise jamming is that only minimal details about the enemy equipment need be known. This class of jamming is rather simple and can be used not only as self protection but also support ( cover )jamming , disadvantages is that it often require much higher jamming power than deceptive jamming
The second class of jamming is deceptive jamming : Deception jammers carry receiving devices on board in order to analyze the radar transmission and then send back false target-like signals in order to confuse the radar.This is in contrast to noise type of jamming,whose objective is to obscure the real signal by injecting a suitable level of noise-like interference into the victim system.Techniques like “noise jamming” are useful for taking a radar installation out of commission, but more sophisticated deception jamming can make the enemy think their radar is still working when it is actually reporting incorrect target range and velocity information With deception jamming, an exact knowledge of not only the enemy radar’s frequency, but all other transmission parameters is required. Deceptive jamming, in a way , is spot or point jamming of a more intelligent nature, HoJ mode of missiles are often less effective again deception jamming because missiles often do not know they are being jammed ( It important to note that , if jamming is detected then HoJ can still be used ).
In recent years capability of radar deceptive jamming has been enhanced significantly with the development of Digital Radio Frequency Memory (DRFM) techniques .Jammers with DRFM technology are widely reported in literature , for example ALQ-187(v)2 , ALQ-131 EA PUP , Falcon edge , ALQ-211(V)9 , ALQ-214(V)3 , Spectra , ASQ-239.DRFM is a technology in which a high-speed sampling digital memory is used for storage and recreation of radio frequency signals.The most significant aspect of DRFM is that as a digital “duplicate” of the received signal, it is coherent with the source of the received signal. As opposed to analog ‘memory loops’, there is no signal degradation caused by continuously cycling the energy through a front-end amplifier which allows for greater range errors for reactive jamming and allows for predictive jamming.
.Deceptive jamming require much less transmitting power and often less vulnerable to HoJ , however ,unlike noise jamming , radar can counter deceptive jamming by various method such as frequency hopping , PRF jittering , pulse compression or leading edge tracking ..etc ( more sophisticated jamming method will require more sophisticated ECCM method ).
But the most important factor that enthusiasts always overlooked when talking about jamming is the jamming-signal ratio , that can be seen as the deciding factor whether jamming will work or not.
As a matter of fact when Jamming is factored into the radar equation,the quantities of greatest interest are Jamming to signal ratio (J/S) and Burn-Through Range.”J-to-S” is the ratio of the signal strength of the jamming signal (J) to the signal strength of the target return signal (S) [ it kinda similar to signal-noise ratio , the only different thing here is that background noise consist of jamming signal too ]. It is expressed as “J/S” and often measured in dB.Apart from their unique requirements of each specific jamming technique, for jamming to be effective J must exceed S by some amount , therefore , the desired result of a J/S calculation in dB is a positive number .It is a common mistaken that J/S ratio required to jam any radar is a fixed value.But in reality the required J/S varied significantly depending on jamming techniques and radar type.For example :
Burn-through range is the radar to target distance where the target return signal can first be detected through the ECM and is usually slightly farther than crossover range where J=S. It is usually the range where the J/S just equals the minimum J/S requirement.
ERPs = Effective radiated power of radar
ERPj =Effective radiated power of jammer
G = Antenna gain
RCS = Target radar cross section
As can be seen from the equation above , for jammer a longer distance and smaller RCS is desirable. Why lower RCS is more desirable has been explained before , so i will not say it again ,but the effect is very dramatic to say the least
About distance : because jamming signal only has to travel one way, as the range get further , the jammer has more advantage than the radar because jamming power decrease at slower rate ,so the further you are from a radar , the easier it would be for you to jam it