lesson 2 - active jamming ( stealth technology )

Active jamming is an electronic countermeasure (ECM) that can have one or three or 'hidden' objectives :

• To mask the presence of a target by introducing random interference
• To generate false targets by simulating radar echoes at different times during the pulse repetition interval or he scan interval.
• To cause erroneous target location by introducing modulation on the echo or jamming pulse. Radar equations can be written for the first two cases.

Equivalent Temperature of Stand-Off Noise Jamming

One type of masking ECM is random noise radiated over a bandwidth BJ wider than radar receiver bandwidth Bn, either continuously or over time intervals chosen to mask the actual echo signal without giving useful range delay data. The effect is to raise the total spectral density of the background noise in the receiver from No to N0 + J0, where the received jamming spectral density is

                                                                 J0 = Pj Gj Gr λ² F ²pj F ²j

                                                                (4 π) ² Bj R ²j Lαj

Here. PjGj represents the ERP of the jammer, PjGj/Bj is the effective radiated power density (ERPD) at the frequency to which the radar is tuned, Fpj ² is the jamming polarization factor, F ²j is the one-way pattern-propagation factor from the jammer into the radar antenna, Rj is the jammer range, and Lαj is the one-way atmospheric attenuation. For a stand-off jammer ( SOJ ) that maintains essentially constant component of noise temperature added to Ts at he receiver input :

                                                          Tj = Jo = Pj Gj Gr λ² F ²pj F ²j

                                                                         k(4 π) ² kBj R ²j Lαj

                                                                 T’s = Ts + Tj

For two or more jammers, individual values Tji are calculated, and the total input temperature is found as:

                                                              T’s = Ts + Tj1 + Tj2 + …..

The target detection range of the radar is an SOJ   noise  jamming environment can be calculated by replacing Ts by T’s or he Blake chart. The value of Tj for jammer within the main lobe of the radar antenna ( Fj ≈ 1 ) is normally so great that the detection range is negligible, but in search factors whrer he jammer is in a sidelobe ( Fj << 1 ) the jammer contribution Tj can be comparable to Ts, permitting a useful detection range to be obtained.