Interferometric Inverse Synthetic Aperture Radar (InISAR)

1. Interferometric Inverse Synthetic Aperture Radar(InISAR) Logan Smith EECS 826 4/30/09

2. Contents • ISAR review • InISAR • Overview • Registration • 3D InISAR • Applications • Automatic Carrier Landing System • Automatic Target Identification

3. ISAR • ISAR measures the rotation of a moving target in two dimensions as it moves in the radars field of view. • This results in an high resolution image that is a 2D plane with the axes of slant range and cross range • Slant range resolution dependent on transmitted signal’s pulse compression characteristics • Cross range resolution dependent on the Doppler bandwidth of the returned signal [2]

4. ISAR • ISAR is an inversion of Spotlight Mode SAR • Typically used for marine or aircraft monitoring • Does not provide height information [1]

5. Contents • ISAR review • InISAR • Overview • Registration • 3D InISAR • Applications • Automatic Carrier Landing System • Automatic Target Identification

6. InISAR • Requirements: • 2 or more antennas, each recording ISAR data • Coherent radar • Pixel registration • Motion Compensation • Large baseline = fine resolution • Baseline too large  Baseline de-correlation • Only used in single-pass mode. No uses yet for a long temporal baseline

7. Terrain vs Vehicle • InISAR is similar to InSAR (also called a phase-comparison monopulse radar) used to find terrain height/elevation • Vehicle heights much smaller than terrain or building height/elevation • No continuous phase. Uses corner reflectors [4] [3]

8. Registration [5] • Goal: To obtain the maximum allowable cross range resolution without de-correlating the two ISAR images • Solution: Use two sets of antenna • Smaller baseline antennas to measure angle • Larger baseline antennas for imaging • Avoids the need for phase unwrapping

9. Registration • Procedure: • Step 1: Remove target’s radial motion and calculate its range profile • Step 2: Find angular value of each range cell • Step 3: Use the smaller baseline data to obtain the scattering-center’s Doppler history • Step 4: Use this result to fit the time-variant curve of the pitching and azimuth angles • These results can be used to calibrate the longer baseline results, removing ambiguity and returning a high resolution phase history

10. Registration Simulation • Center frequency = 10GHz • Bandwidth = 400MHz • Image processing time = 1.17s • Small baseline = 0.5m • Large baseline = 1.0m • Initial slant range = 10km • Pitching angle = 30° • Azimuth angle = 45° • Target speed = 300m/s

11. Registration Simulation Before compensation After compensation

12. Simulation Results

13. 3D InISAR [6] • Uses three antenna configuration • Avoids need for radar to follow target • Can be used to image fast moving space objects (e.g. satellites) • Estimates radial velocity of target • Receiver/Transmitter A: (0,0,0) • Receiver B: (L,0,0) • Receiver C: (0,0,-L)

14. Contents • ISAR review • InISAR • Overview • Registration • 3D InISAR • Applications • Automatic Carrier Landing System • Automatic Target Identification

15. InISAR for Aircraft Carrier AALS [7] • Aug 12th, 1957: 1st successful landing using an automatic carrier landing system (F3D Skyknight on the USS Antietam) • “Look Ma No Hands” Patch from Bell Aerosystems [8]

16. Simulation Scenario • Short-range ISAR approximations fail to adequately measure aircraft motion • ISAR resolution not sufficient to detect small angular changes • InISAR used to detect slight roll or pitch with near 90 deg squint angle. • No contiguous structure so no phase unwrapping methods used for terrain • 13 strong corner reflectors were used to track phase • Imaging done in the ω-k (frequency-wavenumber)

17. Radars offset from runway by 15m (Range, X1) • Start data collection when plane is 500m away (Cross-range, Y1) • Antennas are 1m apart (Height, Z1) • Squint Angle:

18. Simulation Parameters • Two antennas • Antenna one acts as transmitter and receiver • Antenna two acts as receiver • Both aircraft and carrier have GPS information to correct for nonlinear motion • Center frequency: fc = 33 GHz • Bandwidth: BW = 40 MHz • Aircraft veloctiy: v = 100 m/s • Slow-time sampling interval = [-1.5,1.5] (s) • Synthetic aperture length: L = 150 m

19. Phase delays: assuming

20. Aircraft reflectivity function:

21. Resolution Wavenumber: Illuminated aspect angles: Wavenumber bandwidth: Resolution: Δxs = 0.25m Δy = 4m Δz = 0.22m

24. Rotation • Measures the pitch and roll of the aircraft

25. ө = 0 • φ = 0 • Level flight

26. ө = 3 • φ = 0 • Rolling to the side

27. ө = 0 • φ = 5 • Pitched forward

28. ө = 3 • φ = 5 • Pitched and rolling

29. Effects of Rotation • Assuming small ө and φ: Aircraft’s nose: -1.1 rad, y=8m, z=0 Φ = 5.7° Wing reflector: 0.75 rad, (-1,-1,-0.5) ө = 3.25°

30. Conclusions • InISAR can be used to detect irregularities in a landing aircraft’s orientation • Compare measured phases with database of safe landing orientations • Does not use pattern matching since that would be impractical for such variable weather conditions

31. InISAR for Automatic Target Identification [8] • Goal: Airborne or shipborne long range early warning automatic classification of Non-Cooperative and Stealthy Target (NCST) entity. • NCST: 1) Enhanced Information Extraction (EIE) 2) Absolute Decision Formation (ADF)

32. Enhanced Information Extraction • ISAR for resolution in range • Radio Frequency Interferometry (RFI) for resolution in angle • Looking for attributes to assess: • Ship class: abstract information • Ship name: distinct information

33. KMS Bismarck • Inspired by historic case of mistaken identity • HMS Ark Royal attacked HMS Sheffield while looking for KMS Bismarck

34. Radar Specs [#] • fc = 5 GHz • BW = 34.25 Hz • PRF = 1000 Hz • Transmitted waveform • Cross Range Resolution • 26.786m • Slant Range Resolution • 8 m

35. Model Geometry • Radar waves interpolated to a rectangular grid

36. ISAR contribution • ISAR image processing using 2D DFT

37. ISAR results • Strong reflectors at lower heights will be stretched, creating a distorted outline of the ship

38. RFI contribution • Radio Frequency Interferometry angle tracking • Looking for single scatterers in a resolution cell

39. Glint • Multiple scatterers in a resolution cell • Most variation in transversal components with height dimension remaining stable

40. InISAR results • ISAR provides map of dominate scatterers • InSAR provides accurate height values • Results are better when the tracked scatterer is near the highest point of the ship • Can also analyze the absolute Doppler of the dominant scatterer • Higher speeds = higher points on the superstructure

41. Target Identification • Neural network compares ISAR and InSAR results and scales the results based on training rules

42. Moving Ground Targets [9] • Using InISAR to image moving targets against a stationary background • Target and background separated and processed separately (target with InISAR and background with InSAR) • Three-antenna configuration: (0,0,0);(0,ly,0);(0,0,lz)

43. Ground Target Simulation Spotlight SAR • fc, BW, ly, lz, R0 same • Incident angle: α = 10° • Velocity of radar: • Va = 150m/s • Velocity of target: • Vt =20m/s • L = 300m • Angle between Vt and x-axis • Β = 45° • Imaging time T = 2s Stripmap SAR • fc = 10GHz • BW = 300MHz • ly = lz = 0.5m • Antennas: • Planer antennas • antenna length = 1m • Distance from origin to target: R0 = 10km • Incident angle: α = 90° • Velocity: V = 200m/s • Relative velocity of radar to target • L = 150m

44. Simulation Data 2D ISAR images with Conventional processing 2D ISAR images with added clutter Clutter removed using delay-line techniques

45. Ground Target Results Original Stripmap SAR with clutter Spotlight SAR without clutter

46. Summary • InISAR is a combination of ISAR and InSAR • It adds height information and more accurate phase information to the ISAR data • Can be used to extract profile information about moving objects in land, sea, or air • Subject to glint phenomena • Still in research/simulation stage

47. Questions

48. References • [1] SKOLNIK 2001 Introduction to Radar Systems.New York : McGraw Hill. (ISBN 0-07-118189-X) • [2] Kostis, Theodoros G. (2001) Interferometric Inverse Synthetic Aperture Radar. Unpublished master’s thesis, University of London • [3] http://earth.esa.int/workshops/ers97/papers/thiel/Image90.gif • [4] Soumekh, Mehrdad. Automatic Aircraft Landing Using Interferometric inverse synthetic Aperture Radar Imaging. in IEEE Trans. Image Processing, vol. 5, no. 9, Sept. 1996. • [5] Zhang, Q. Novel registration technique of InISAR and InSAR. in IEEE Int. Geoscience and Remote Sensing Symposium. Vol. 1 July 2003. • [6] Zhang, Dongchen. Three-dimensional ISAR Imaging of High Speed Space Target in 9th Intl. Conf. on Signal Processing. 26-29 Oct. 2008. • [7] Kostis T.G., Baker C.J., Griffiths H.D. (2006), An Interferometric ISAR System Model for Automatic Target Identification, Proceedings of the European Conference on Synthetic Aperture Radar 2006, London, England, no. 58. • [8] www.tsretirees.org/memory/Femiano.doc • [9] Zhang, Qun. Three-Dimensional SAR Imaging of a Ground Moving Target Using the InISAR Technique. in IEEE Trans. Geoscience and Remote Sensing. Vol 42. no. 9 Sept. 2004.