Demo — Standoff Jamming

Demo — Standoff Jamming#

A standoff jammer (SOJ) never enters the threat rings. It holds its own range to the radar while the strike package closes, and it reaches the radar’s receiver through whatever gain the radar happens to point toward it. This demo draws what that does to a radar’s detection contour: a shrunken circle almost everywhere, and a deep, narrow notch — the strobe — pointing straight at the jammer. Drag the jammer and a striker, and find out how narrow the protected corridor really is.

The model#

The standoff jam-to-signal ratio for a target at range \(R_t\) and a jammer at range \(R_j\) is L17’s scorecard with two ranges and two gains:

\[ \frac{J}{S} = \frac{4\pi\,P_j G_j\,R_t^{4}}{P_t G_t\,\sigma\,R_j^{2}}\cdot\frac{G_r'}{G_r}\cdot\frac{B_r}{B_j}, \]

where \(G_r'/G_r\) is 1 (0 dB) when the jammer is within half a beamwidth of the azimuth the radar is looking along, and the sidelobe level otherwise. The radar burns through where \(J/S\) falls to \(+10\) dB, which gives one burn-through range per azimuth, and the jammed detection contour is

\[ R(\phi) = \min\big(R_{det},\ R_{bt}(\phi)\big), \qquad R_{det} = 100\ \text{km}\left(\frac{\sigma}{1\ \text{m}^2}\right)^{1/4}. \]

The radar is notional: \(P_tG_t = 90\) dBW, \(B_r/B_j = -10\) dB, a 35 km missile engagement zone (MEZ), and a keep-out ring 25 km beyond it for the SOJ. The target’s RCS is held constant with aspect. The SOJ is directional: its own transmit beam — and with it its ERP — sits on one victim radar at a time, and a radar outside that beam is treated as unjammed, the same simplification the Final Project makes. At the defaults — a 40 dBW jammer 150 km due west, −30 dB sidelobes, a 5° beam, a −5 dBsm target — the unjammed range is 75.0 km, the sidelobe burn-through 48.8 km, and the mainlobe burn-through 8.7 km.

Interactive demo#

Open in full screen

Walkthrough#

The map is North up, in kilometers, with the radar at the origin. The dashed navy circle is the unjammed detection contour, the red dashed circle the MEZ, and the amber dotted circle the SOJ’s keep-out ring. The solid navy outline is the jammed contour; the pink wedge is the strobe, drawn from the radar out to the jammer; the green sliver inside it is the protected corridor. The stats read the jammed radar; the two lines under them give the striker’s verdict for each radar.

  1. Read the default (the Corridor preset). The jammed contour is a 48.8 km circle with a 5° notch that dives to 8.7 km on the jammer’s bearing. The striker sits 30 km out in that notch: \(J/S = +31.6\) dB, hidden. The corridor readout says the protected lane is only 2.62 km wide at the striker’s range.

  2. Drag the striker a few degrees off the jammer’s line. The moment it leaves the wedge, the jamming comes in through a −30 dB sidelobe and its \(J/S\) collapses by 30 dB. The Striker off-axis preset puts it at 30 km on a bearing of 300°: \(J/S = +1.6\) dB, detected. Same jammer, same range.

  3. Pull the jammer in (the SOJ pulled in preset, 70 km). Both burn-through ranges shrink — but only by the square root of the range ratio, \(\sqrt{70/150} = 0.68\): the mainlobe burn-through goes from 8.7 to 5.9 km and the sidelobe one from 48.8 to 33.3 km. Keep dragging it toward the radar and the SOJ turns amber inside the keep-out ring and red inside the MEZ: the jammer has become a target.

  4. Drive the sliders. The sidelobe level moves the whole shrunken circle — from −30 to −40 dB the sidelobe burn-through grows past \(R_{det}\) and stops shrinking anything, because a good antenna simply ignores an off-axis jammer. The beamwidth widens or narrows the strobe and the corridor with it. Jammer power and target RCS move everything together, by fourth roots.

  5. Add a second radar (the Two radars preset). Radar 2 sits 30 km north and 20 km west of Radar 1, and the Jammer beam on buttons choose which radar the SOJ’s transmit beam holds. With the beam on Radar 1, the striker stays hidden in Radar 1’s corridor, but Radar 2 is unjammed: its contour is its full 75 km, and it sees the striker 31.6 km away — detected. Move the beam to Radar 2 and Radar 2 grows a strobe of its own — pointing at the SOJ from Radar 2’s position, so in a different direction — while Radar 1 goes back to its unjammed 75 km. The striker is now off Radar 2’s strobe (sidelobe, \(J/S = +3.5\) dB, detected) and in plain view of Radar 1. Drag the striker into Radar 2’s green wedge to hide it there instead. One jammer beam, one victim, one corridor.

Key observations#

  • Two ranges, two gains. The jammer’s range stays fixed while the target’s closes, so \(J/S \propto R_t^4/R_j^2\); and the gain the radar points at the jammer is the mainlobe only on the jammer’s azimuth.

  • A strobe, not a sky. The jammed contour is a shrunken circle with one deep notch. The protected corridor is that notch between the two burn-through ranges — about \(R\,\theta_B\) wide, a few kilometers.

  • Geometry buys more than power. \(R_{bt} \propto R_j^{1/2}\), so backing the jammer off to clear the missile zones is cheap; \(R_{bt} \propto (G_r'/G_r)^{-1/4}\), so flying the jammer’s azimuth is worth a factor of 5.6 against −30 dB sidelobes.

  • Two different beams. The radar’s receive lobes decide where the strobe falls around a jammed radar; the jammer’s transmit beam decides which radar is jammed at all. A directional SOJ jams one victim at a time, and the other radars — unjammed here and in the Final Project — see the route out to their full detection range. The fix is a second jammer, SEAD on the other radar, or a route that stays out of its contour — the deconfliction problem of the Final Project’s task T5.

Source#

MATLAB bundle · L36_StandoffJamming.m↓

The companion script uses the demo’s default radar, jammer and target. It tabulates the sidelobe burn-through range against sidelobe levels from −40 to −15 dB, draws the jammed contour around the full circle with the MEZ and the keep-out ring, prints the corridor width at 20, 30, 40 and 50 km, compares the striker’s \(J/S\) in and out of the strobe, and asks whether pulling the SOJ in to its keep-out line is worth it. It covers one radar; the second-radar geometry is the demo’s alone.

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