ISAR is useful in Beaufort Sea State 2 and higher and near radial direction of target movements, between 5 and 80 NM. Additionally, what is the recommended antenna-to-target aspect?

• A. 5 to 30 degrees off target track
• B. 0 to 5 degrees off target track
• C. 60 to 90 degrees off target track
• D. 30 to 60 degrees off target track

Answer: (A)

ISAR images are built from the Doppler history created by the target’s motion relative to the radar. To obtain useful cross‑range information, you need an aspect angle that gives the target a significant motion component across the line of sight. If you look almost directly along the target’s path (tiny offset), Doppler variation is small and the ISAR image quality suffers. If you look from a much larger angle (toward broadside), Doppler rates become large and can introduce other imaging complications, especially in real-world sea conditions. A modest offset—about 5 to 30 degrees off the target track—strikes the right balance: there’s enough Doppler modulation to resolve cross-range features while avoiding excessive foreshortening or other issues, which is why this is the recommended antenna-to-target aspect for targets moving roughly radially within the 5 to 80 NM range in Beaufort Sea State 2 and higher.