Introduction to Synthetic Aperture Radar (SAR) Integrated Systems & Solutions – S & R Systems Introduction to Synthetic Aperture Radar (SAR) Floyd Millet August 2005 Work was done under US Government contract.

RaDAR is an acronym for Radio Detection And Ranging Radar Applications Air Traffic Control Air traffic and weather Ground Control approach landing Aircraft Navigation Altimeter Doppler navigation Weather avoidance Law Enforcement Police speedometers Intrusion alarms Military Surveillance and reconnaissance Weapon guidance and control Proximity fuzes for weapons Bomb damage assessment Remote Sensing the Earth Flood monitoring Crop and forest assessment Location of archeological ruins Remote Sensing the Solar System Planetary rotation rates Range to the moon and planets Meteor tracking Ship Safety Collision avoidance Piloting in restricted waters Space Rendezvousing of spacecraft Spacecraft docking and landing Satellite tracking RaDAR is an acronym for Radio Detection And Ranging

Radar Imaging Individual image points (pixels) must be discriminated in two dimensions, range and azimuth Form a terrain image using a radar in a moving airborne vehicle Problem Simplest approach: Real-Beam Imaging Radar Example: Plan Position Indicator (PPI) PPI Display Range Azimuth

Cell Size: 1/5 Major Dimension Target Not Identified When Coarse Resolution Used Cell Size: 1/5 Major Dimension Corresponding Map

Cell Size: 1/20 Major Dimension Target Identified When Fine Resolution Used Cell Size: 1/20 Major Dimension Corresponding Map

Resolution Required for Various Mapping Applications Features to be resolved Coast lines, large cities, outlines of mountains Major highways, variations in fields “roadmap” details: city streets, large buildings, small air fields Vehicles, houses, small buildings Cell Size 150m 10-20 m 1-3 m 20-35 m Resolution Cell da dr

High-Resolution Mapping Synthetic Aperture Radar (SAR)

What is High Resolution Radar Mapping? HRM involves breaking up real antenna beam into fine resolution cells Footprint of mainlobe on ground The map is made by forming cells and measuring signal intensity in each cell dAZ dr Resolution Cell dAZ = Azimuth Resolution dr = Range Resolution

What Does a Radar Measure? Amplitude versus Time All other SAR parameters are derived: Range (known time relationship) Phase - coherent transmission plus demodulation Doppler frequency Range resolution (pulse compression) Azimuth Resolution

Range Discrimination  2²d ²d The transmitted pulse travels at the speed of light 300,000 km/second 3.3 nanoseconds/meter Round-trip “radar time” 6.7 nanoseconds/meter (²d = 2 meters ² = 13.3 nanoseconds) But target returns overlap if targets are separated by less than c/2

Short Pulse Range Resolution R2 = R1 + ΔR R1 Transmitted Pulse O  Time   Received Pulses 2R1/c 2(R1 + ΔR/c) 2ΔR/c  Pulses Just Resolvable ΔR = c/2 2ΔR/c

Shorter Pulses So for better resolution, just make the transmitted pulse SHORTER However, the shorter pulses must somehow transmit the SAME ENERGY to the target As the pulse gets SHORTER, the peak power gets HIGHER Problem Peak power gets MUCH to high before pulse length even approaches high resolution

Coded Pulses  f1 f2 Solution Transmit a long coded pulse that can be decoded (compressed) after reception into a much shorter pulse Solution  f1 f2 Linear Frequency Modulation (FM) Linear Swept Frequency “Chirp” Note: A typical 200 microsecond pulse extends over 60 km resulting in a range resolution of 30 km

Pulse Compression Frequency Time  2 1 Transmitted/Received Pulse Resolution = c/2 f2 Δf Variable Delay Line “Compression” Filter f1 Delay Time  Time f1 f2 Δf 1/Δf Decoded/”Compressed” Output Resolution = c/2Δf = (/2)(fo/ Δf) F number = fo/ 2Δf Resolution varies as 1/ Δf , that is, it varies with transmitted bandwidth

Linear FM (Chirp) Waveform = c - B W t 2 + n , ( ) o S w e p r q u y h a v i g b d s l T m F P : S ( n , t ) = A c o s ( 2 p f t + pαt2 ) x A - Ý t f c f t t f ( n , t ) = 2 p f t + pαt2 , £ t £ t f ( ) f ( n , t ) = 1 d n , t = fo + αt , £ t £ t t 2 p d t T r a n s m i t b a n d w i d t h g i v e n b y : B W = ατ t o L i n e a r F M h a s d e s i r a b l e p r o p e r t i e s o v e r o t h e r w a v e f o r m t y p e s : • E a s y t o g e n e r a t e • " S t r e t c h m o d e " d e m o d u l a t i o n

Pulse Compression o R e c i v d S g n a l G B y : C m p x ( f t r u / ) * P • , S ( n , t ) = C ( n , t ) c o s [ 2 p f ( t - T ) + pα ( t - T ) 2 ] r R R S ( , t ) x S r , t ( ) P u l s e n = A - t 2 R 2 R + t t g t t g t c c R t g t = R a n g e - t o - T a r g e t S ( n , t ) = S ( n , t ) × e - j w t = [ C ( n , t ) × e - j w 2 T R ] × e x p { j pα ( t - T ) } = D ( n , t ) × e x p { j pα ( t - T ) 2 } v r R R w h i c h i s a n a l o g o u s t o : • M a x i m i z i n g t h e s i g n a l - t o - n o i s e r a t i o * d o e s n t a p l y " S r c h M i g f I F m x - w v B U T : W q u z n t h e i m a g e ( G o o d s i d e l o b e c o n t r o l o n t h e i m p u l s e r e s p o n s e ( I P R ) )

Fine Range Resolution Requires Large Radar Bandwidth Fourier Transform Pair 1/  Time Frequency

Pulse Compression Advantages Range resolution independent of transmit pulse length Transmit long pulses Keep peak power comfortably low Set range resolution with transmitted bandwidth Resolution inversely proportional to bandwidth 150 MHz 1 meter resolution 300 MHz 0.5 meter resolution Resolution independent of slant range

Azimuth Resolution

Ability to Resolve Closely Spaced Targets is Beamwidth Dependent The half-power (3dB) beamwidth is a measure of angular resolution of radar (1/2)3dB 3dB (3/2)3dB A B A B A B

Synthetic-Aperture Radar Azimuth Considerations SAR Synthetic-Aperture Radar Antenna beamwidth is inversely proportional to the number of wavelengths in its length (aperture) Rq L = /L radians = C/f R

Azimuth Discrimination L R/L Δd R As the collection vehicle moves along the flight path, targets are detected as they move in and out of the antenna pattern But target returns overlap if the targets are separated in azimuth by less than the antenna beamwidth So achievable azimuth resolution degrades with range Real-beam imaging radar Flight Path

Azimuth Discrimination So for better azimuth resolution, just make the antenna beam NARROWER! Generate more wavelengths in the antenna aperture by lengthening the antenna or by shorting the wavelength (increasing the frequency) However, very LONG antennas are difficult to carry and position and very HIGH frequencies limit performance in weather and at long ranges Problem Antennas get MUCH too long and frequencies MUCH to high before the beamwidth even approaches high resolution

Synthetic-Aperture Radar Solution Synthesize a long antenna aperture using a physically short antenna SAR Synthetic-Aperture Radar Store the data collected sequentially and coherently across a long aperture and then process the data to synthesize a full aperture collection

Design Options for Improving Resolution Before SAR Range Resolution Decrease pulsewidth, at expense of power and range Operate at short range/decrease power Azimuth Resolution Increase operating frequency to Ku and Ka-bands or higher, with increased atmospheric and weather attenuation, lower available power sources Increase antenna aperture, with attendant installation and stabilization problems

The Resolution Breakthrough Range - Pulse Compression - Increased range resolution without loss of power Azimuth - Synthetic Aperture - Increased azimuth resolution without large antenna installation Note: 1. Both use special waveforms 2. Both use signal processing techniques

Synthetic Aperture Radar

From Hovanessian, “Introduction to Synthetic Array and Imaging Radars” Phase History of a Scatterer From Hovanessian, “Introduction to Synthetic Array and Imaging Radars”

Anamorphic Hologram

SAR Collection Geometry

Resolution Limitation on Sidelooking SAR

Synthetic Aperture -Resolution limitation on sidelooking SAR: Tgt 1 Ro INT Ls -Resolution limitation on sidelooking SAR: Maximum θINT is limited by azimuth antenna beamwidth , Lant is length of linear array -This limitation does not apply to spotlight collection

Point-Target Phase History Compressed in Both Range and Azimuth

First SAR Imagery

ADTS Advanced Detection Technology Sensor Ft. Devens, MA 35 GHZ HH Polarization ± 20 Deg Depression Angle

Where Do You Fit in? Future Topics Implementation of Theory September 6 Algorithm Architecture for SAR Ground Processing Creating an Image September 20 Concepts in Image Processing From Idealization to Realization October 11 PACE: An Autofocus Algorithm for SAR

Texts and Software Texts Software Curlander, John and McDonough, Robert Synthetic-Aperture Radar - Systems and Signal Processing Skolnik, Merrill Introduction to Radar Systems Nathanson, Fred Radar Design Principles Carrara, Walter et al Spotlight Synthetic-Aperture Radar Oppenheim, Alan and Schafer, Ron Discrete Time Signal Processing Skolnik, Merrill Radar Handbook Software Mathcad V6 Matlab