Self Protection Jammer
Figure 1: A self-protection jammer on the left wing of a Tornado
Self Protection Jammer
A self protection jammer is a jammer, usually housed in an external load (POD), carried by an aircraft intended to perform other tactical tasks. The function of jamming enemy radars is only a secondary function to self-protection. The jamming source and the attack aircraft to be protected have the same coordinates in space. The jamming thus takes place in the main lobe of the antenna radiation pattern and is therefore also called “main lobe jamming”. The jamming strobe at the periphery of the radar screen thus has the same direction as the attacking aircraft and provides the radar with side angle and, if necessary, elevation angle information. Only a range measurement is made difficult by the jamming.
A range determination can also be made by bi- or multilateration by two or more radars from different directions. The tactical advantage for the jammer is then only a time delay in the determination of coordinates. The disadvantage is that the jamming source itself can be used as a target for defensive missiles (“home on jam mode”). To prevent this, the jamming source can be towed as a glider, but this can affect the maneuverability of the aircraft.
The ratio of jamming power to echo signal power is the ratio of the magnitude of the echo signal (from the radar range equation) and the magnitude of the received jamming power (free-space path loss of simple range only). All other parameters relevant to a range truncate away in this ratio because they are identical for both RF sources. This leaves the ratio of the linear (not in decibels) effective radiated powers of the radar transmitter and the jammer multiplied by 4πR² (as the remainder from the free-space path loss) and divided by the radar cross section σ of the jamming carrier.
- ERPJam = effective radiated power of the jammer;
- ERPRadar = effective radiated power of the radar transmitter;
- R = distance radar to jammer [m].
Burn through range
The received power of the echo signal is proportional to the 4th root of the distance, and the received power of the jamming signal is only from the 2nd root of the distance. Thus, as the jammer approaches the radar, the magnitude of the echo signal increases faster than the magnitude of the jamming signal. At a certain distance, the echo signal can become larger than the jamming signal and the radar can detect the aircraft and make a range measurement despite the interferences. (Provided the jamming signal does not drive the receiver into saturation.) The distance between the radar and the aircraft at which detecting and ranging is possible is called burn through range RBT, the target sign “burns through” the jamming power.
This equation is derived when the ratio J/S = 1 is assumed and equation (1) is rearranged according to the range R.
In escort jamming (ESJ), an escort jammer is located within, or close to, the attack aircraft group at only a slightly deviated angle and/or range. Therefore, the jammer is also located in the main lobe of the radar's antenna pattern. The calculations of the rate jamming power to received power J/S and the burn through range RBT are calculated according to the same rules as for a self-protection jammer.