What is the minimum and maximum distance from the aircraft for ISAR?

• A. 5 NM and 80 NM
• B. 1 NM and 10 NM
• C. 10 NM and 60 NM
• D. 20 NM and 100 NM

Answer: (A)

ISAR imaging relies on the motion of the target and the platform to build a Doppler history that focuses the cross-range detail. You need a standoff that’s large enough to keep the wavefront effectively usable as the target’s aspect changes, but not so large that Doppler information becomes too weak and the target’s details blur.

At a minimum distance around 5 nautical miles, you stay out of near-field distortions and ensure safe, reliable geometry for focusing the image while still maintaining adequate angular change during the recording interval. If you get much closer, the assumptions used in ISAR processing break down and the phase history becomes unreliable for sharp focusing.

At a maximum distance around 80 nautical miles, the target still subtends a sufficient angle from the radar’s viewpoint over the imaging period, so the Doppler spread remains detectable with the available bandwidth and integration time. Beyond this range, the Doppler variation shrinks and the cross-range resolution degrades, making it hard to resolve detail.

So, a practical ISAR standoff range is about 5 to 80 nautical miles.