RADAR EQUATIONS FOR CLUTTER AND JAMMING
3.8 DECEPTIVE JAMMING
In deceptive jamming (also called deceptive ECM or DECM), the emissions are designed to appear as radar echoes at locations where no target actually exists.
The purpose is to introduce confusion into the radar system and the network that uses the radar data, and possibly to saturate the data processor to impede or pre-vent reliable tracking of actual targets. There are two types of deceptive jammers [9, p. 86]:
Transponder jammers generate noncoherent returns that emulate the temporal characteristics of the actual radar return. Repeaters generate coherent returns that attempt to emulate the am-plitude, frequency, and temporal characteristics of the actual radar return.
The repeater jammer has become the preferred method for creating synthetic tar-gets that are realistic enough to pass from the radar processor into the data stream.
This type of jammer includes an intercept receiver, a memory to store features of the radar waveform, and a modulated transmitter in which the radar waveform is regenerated with time delays and Doppler shifts that correspond to false target positions and velocities.
Deceptive jammers are used extensively against tracking radars that are ele-ments of a fire-control system. Their use in this application is designed to prevent the fire control radar from locking on the target, or to break lock if the radar has already locked on. A number of sophisticated techniques are available for this purpose, the success of which generally requires the transmission of the selected jamming waveform at high very jamming-to-signal ratios. The evaluation of the effectiveness of this type of jamming does not generally lend itself to analysis, and testing using the actual radar and jamming equipment (or hardware simulators) is usually required.
The current technology for waveform storage is the digital RF memory (DRFM). The operation of this device is described in [9, Chapter 5], and will not be repeated here. The state of the art has advanced steadily, providing the ability to store and regenerate most radar signals with adequate accuracy and with con-trollable time delays and Doppler shifts. The accompanying intercept receiver is also implemented digitally [10].
A significant challenge to repeater jamming is the presence of multiple radar signals that overlap in time and have large time-bandwidth products. This causes cross-products to appear in the jammer output when overlapping signals are pro-cessed through nonlinear circuits in the repeater.
3.8.1 Range Equations for Deceptive Jamming
Deceptive jammers operate by responding to individual pulses received from the radar, with time delay, Doppler shift, or modulations that prevent proper operation of the radar or interpretation of its data. The applicable range equations are based on single-pulse peak power level rather than energy levels over a CPI.
3.8.1.1 Transponder Equations
A transponder is used to generate false targets in a radar that uses noncoherent integration (as opposed to a pulsed Doppler process). The transponder response is triggered by an incoming pulse from the radar, but is generated by an RF source that does not maintain phase coherence with the received pulse. The transponder is characterized by the following parameters:
Gj = jammer antenna gain;
Fpj = jammer-to-radar polarization factor;
Fj = jammer-to-radar pattern-propagation factor;
Flens j = jammer-to-radar (one-way) lens factor.
Radar Equations for Clutter and Jamming 103 Smin j = transponder sensitivity in W;7
Pj = peak response power in W;
Lj = transmission or reception line loss;
Lj = jammer-to-radar (one-way) atmospheric attenuation;
It is assumed here that the antenna gain and patterns and the line losses are identi-cal for transmitting and receiving, and that the response pulsewidth is the same as that of the radar transmission.
The corresponding range at which the radar detects the response is
where Smin r is the minimum single-pulse signal power at the radar for given detec-tion probability Pd. A probability Pd = 90% should be used for reliable transpond-er jamming. In ttranspond-erms of the radar paramettranspond-ers used in Chapttranspond-er 1:
pulses, and transponder characteristic listed in Table 3.4. The pattern-propagation factor F2j = 30 dB is chosen to allow response in the radar’s sidelobe region. The sensitivity is typical of a receiver using a low-noise RF amplifier followed by a square-law detector and video amplifier [8, p. 429].
7 The usual transponder specification gives the tangential sensitivity, corresponding to a signal-to-noise ratio of +4 dB at the input of a square-law detector [8, p. 427]. For reliable triggering, Smin j
should be several decibels above that level. Receiver sensitivity is commonly specified in dBm (decibels with respect to 1 mW).
Table 3.4 Example Transponder Jammer
Radar frequency band X-band Transmitter peak power Pj 50W
Antenna gain Gj 10 dB Transmit or receive loss Lj 1.0 dB
Polarization factor F2pj 3 dB Pattern-propagation factor F2j 30 dB Atmospheric loss Lj 0.9 dB Lens factor F2lens j 0.1 dB Sensitivity Smin j 70 dBm
Application of (3.80)(3.82) with parameters from Tables 3.1 and 3.4 gives the following results:
Range for triggering response: Rrt = 366 km Radar receiver sensitivity: Smin r = 100.6 dBm Range for detection of response: Rmt = 311 km 3.8.1.2 Repeater Equations
The repeater differs from the transponder in that it accepts the signal received with antenna gain Gj, amplifies it with electronic gain Ge, and retransmits it with anten-na gain Gj, and usually with duty cycle Du < 0.5 to avoid self-oscillation. Design of the repeater is based on responding with a power representing a target with radar cross section e, viewed by the radar mainlobe at range Rj.
Radar cross section may be considered in terms of an equivalent sphere of ra-dius r having projected area r2. Scattering from the sphere is isotropic, so the RCS may be regarded as the product of the projected area and an isotropic gain G = 1:
AG r2
(3.83)
An idealized repeater passes the incoming radar pulse received in aperture area Ae = Gr2/4 directly to the transmitting antenna with gain Gj, producing an equivalent RCS given by
2
4
r t
e e t
A G G G
(3.84)
From this, the idealized repeater gain Grep required for a specified e is
rep 2
4 e
r t
G G G
(3.85)
In practice, repeater antennas are too small to provide this gain, and the re-peater gain is increased by electronic amplification Ge placed between Gr and Gt,
Radar Equations for Clutter and Jamming 105 which must also overcome RF losses Ljr and Ljt within the repeater, in the pattern-propagation factors Fjr and Fjt of the receiving and transmitting paths, and in their polarization factors Fpj r and Fpj t. Assuming equal gains and losses for receiving and transmitting, and use of gating with duty cycle Du to isolate the receiver from the transmitter, the required electronic gain Ge, as derived in [8, p. 424], is
2 the repeater and the two-way polarization and pattern-propagation factors. The factor Fj4 is included on the assumption that a repeater located off the axis of the radar beam, and possibly in the radar sidelobes, is intended to reproduce the signal power that would result from an on-axis target with RCS equal to e.
An escort repeater located in a region of 30 dB sidelobes, with the same pa-rameters as were used for the transponder example, Table 3.4, requires a very (Rj/R)4. Gains of this sort would apply, for example, to escort or stand-off DRFM jammers. The signal power received from the example radar at Rj = 100 km is
58.4 dBm, and the RF receiver gain brings this to a level high enough to drive the DRFM. The DRFM is followed by A/D conversion and sufficient RF transmit-ter gain to raise the analog signal to the required output power of 12.6W = +41 dBm. Introduction of time gating and delay between intercept and transmission of the signal prevents self-oscillation.
For a self-protection repeater (Fj = 1) the required electronic gain is +39.5 dB (input signal 25.8 dBm, output power +13.7 dBm). Additional gain would be required to overcome the duty-cycle loss inherent in a self-protection jammer, since it cannot isolate the output from the input with gating and time delay, and must operate as a straight-through repeater.
Note that these range equations do not describe limits to radar detection range, but rather jammer requirements that allow the radar to trigger and receive the deceptive emission. The effect on the radar depends on factors other than range equations.
3.9 SUMMARY OF DETECTION IN JAMMING