Advanced Synthetic Aperture Radars

1. Advanced Synthetic Aperture Radars

2. Objectives • Explain how synthetic aperture radar is able to provide an extremely high-resolution radar image. • Explain how inverse synthetic aperture radar works and how it is able to provide an actual “image” of a contact.

3. Angular Resolution of Radar Systems • Beamwidth is approximated by: • q = kl / L {radians} • If L  q  • At a given range, ability to resolve objects in cross range direction (azimuth) is known as cross range resolution. • D Rcross = R q (q = Arc length swept in radians swept at radius R) • If q  D Rcross (gets better) • Better the D Rcross , better the detail of the image. • For 6o beamwidth (0.1 radians) at 2000 yds • D Rcross = 200 yds

4. Synthetic Aperture Radar (SAR) • Uses motion of transmitter/receiver to generate a large effective (synthetic) aperture. • Creates a very narrow beamwidth & better resolution • Often incorporated in satellite systems • High velocity • Large distance traveled • Exact position known • System stores several returns while SAR system moves. • Reconstructs the returns as if they were taken simultaneously, to create the synthetic aperture. • Used for imaging large stationary objects.

5. Synthetic Aperture Radar (SAR) S R

6. SAR Grid

7. SAR Data Collection • Data collected sequentially and then processed simultaneously.

8. SAR • Advantages • Extremely large effective aperture • Outstanding resolution. • Disadvantages/Limitations • Exact position of transmitter/receiver must be known. • Requires massive storage capacity to accumulate returns while radar (aircraft or satellite) moves. • Requires massive computing power to process returns as if they were received simultaneously. • Flight Path Restrictions

9. Flight Path Restrictions for SAR Mapping • Scan Limit • Mechanically Scanned Array Radars scan limit of ± 60° • AESA Radar scan limit of ± 70° - degraded past ~ ± 55 ° • Forward-mounted and fixed • Modified by installation tilt and aircraft altitude • Some radars have side facing antennas (JSTARS) not limited. • INS/Autofocus Limit • INS supplies data for MOCOMP (motion compensation) to keep the image focused • “Autofocus” used to correct for acceleration errors • Acceleration errors in the INS build up over time, degrading resolution • Accurate INS required for Autofocus

10. SAR • With actual SAR processing, about a thousand calculations are performed for every single pixel. • This image of Washington, D.C., is made up of several million pixels. • Dark areas = low return • White areas = high return

11. SAR Applications • Reconnaissance, Surveillance and Targeting • Treaty Verification and Nonproliferation • Interferometry (3-D SAR) • Navigation and Guidance • Foliage and Ground Penetration • Moving Target Indication • Change Detection • Environmental Monitoring

12. Satellite SAR good for large fixed targets (cities, military bases, TERCOM maps, etc.)

13. UAV’s

14. Lynx SAR M47 Tanks

15. JSTAR

16. Synthetic Aperture Radar (SAR) Operation Crisp Contrast Between Roads and Grass Sharp Radar Shadows from the Trees

17. Very Long Range SAR Decoys Target Target Photo High Resolution SAR Provides clear detail of cracks on runway, targets, & decoys

18. Zoom in on areas of interest. (TARGETS!!) 1 NM Normal Size SAR Image

19. Inverse Synthetic Aperture Radar • Large synthetic aperture can be achieved without moving transmitter/receiver. • If target has motion yaw, pitch, roll, (read ‘ship’), then it has same effect as radar moving equal to corresponding arc length at range R.

20. fD = 2(DY/Dt)L y l l Rcross = 2(DY/Dt)tint ISAR Rcross is independent of range and totally dependent on angular rotation rate Rate of rotation DY / Dt

21. ISAR Principle • Motion of the target causes a Doppler shift that depends on the angular velocity of the target motion • Need a moving (rocking) target • A plot of Doppler shift vs range will give an image • Image will slowly shrink and become inverted as the rocking motion goes from positive to negative producing an inverted image

22. ISAR Resolution • ISAR is used for long-range target imaging and identification. • May not be able to identify target but can certainly identify warship/non-warship. • Can sometimes ID class depending on aspect angle • ISAR platforms can be fixed or moving. • Best targets are ships or submarine scopes.

23. ISAR Example Problem 1 .089m Rcross = 2(.165 rad/s)10secs Find the cross range resolution of an ISAR system operating at 3.35 GHz, that collects data over a yaw angle of 9.5o per sec, with an integration time of 10 secs • = c / f  3 x108 m/s / 3.35 GHz  .089 m DY/Dt = 9.5o /sec = .165 radians/sec T = 10 secs) = .027 m at any range.

24. ISAR 1

25. CRUISE SHIP ISAR IMAGES

26. SAILBOAT ISAR DETAILS Main Mast Main Mast Roller Furling Jib Mizzenmast Mizzenmast Roller Furling Jib Furled Sail Mizzenmast Furled Sail 47 Pixels (15 m) Frame 32 “Flipped”

27. SAILBOAT ISAR IMAGES Doppler 340 Hz Range 032 050 84 m near far 060 088 104

28. TALL SHIP GUYAS ISAR IMAGES

29. Objectives • Explain how synthetic aperture radar is able to provide an extremely high-resolution radar image. • Explain how inverse synthetic aperture radar works and how it is able to provide an actual “image” of a contact.