Perspectives on SAR Processing Ian Cumming Professor Emeritus Radar Remote Sensing Group Dept. of Electrical and Computer Engineering University of British

Perspectives on SAR Processing

Ian CummingIan Cumming

Professor EmeritusProfessor Emeritus

Radar Remote Sensing GroupRadar Remote Sensing Group

Dept. of Electrical and Computer EngineeringDept. of Electrical and Computer Engineering

University of British ColumbiaUniversity of British Columbia

Perspectives on SAR Processing

1.1. Some SAR processing historySome SAR processing history

2.2. Review of current SAR proc. algorithmsReview of current SAR proc. algorithms

3.3. Which algorithms are used today?Which algorithms are used today?

4.4. Some image examplesSome image examples

5.5. Summary and final thoughtsSummary and final thoughts

Sept. 11, 2007Sept. 11, 2007 22Cumming – ASAR 2007Cumming – ASAR 2007

Some Processing History

In 1976, SAR data were processed by coherent opticsIn 1976, SAR data were processed by coherent optics only one book available only one book available very hard to understand for a DSP engineervery hard to understand for a DSP engineer used laser beams and lenses to focus imageused laser beams and lenses to focus image fast, but limited dynamic rangefast, but limited dynamic range must be a better way – digital processing!must be a better way – digital processing!

Challenge of digital processingChallenge of digital processing modest computing resources (memory & speed)modest computing resources (memory & speed) processing algorithms not knownprocessing algorithms not known received signal model was needed (geometry)received signal model was needed (geometry) limited satellite orbit knowledgelimited satellite orbit knowledge


Early Airborne Processing In 1970’s, CCRS obtained an airborne SAR from ERIMIn 1970’s, CCRS obtained an airborne SAR from ERIM

Installed on a Convair-580Installed on a Convair-580 Previous processor was opticalPrevious processor was optical MDA contracted to built an on-board real-time processorMDA contracted to built an on-board real-time processor First RTP delivered in 1979 (additional units in 1985)First RTP delivered in 1979 (additional units in 1985)


Early Airborne Processing

Processor characteristicsProcessor characteristics time-domain correlation time-domain correlation X-band, no RCMC neededX-band, no RCMC needed built from discrete built from discrete

componentscomponents multiplier-accumulator multiplier-accumulator

chips, small memory chips, small memory chipschips

4 giga ops per second in 4 giga ops per second in one chassisone chassis

performed azimuth performed azimuth compression first, then compression first, then range compressionrange compression

real-time waterfall display real-time waterfall display on-boardon-board


On-board real-time processor On-board real-time processor on CCRS Convair-580on CCRS Convair-580

Early Satellite Processing

SEASAT (October 1978)SEASAT (October 1978) developed detailed model of satellite motiondeveloped detailed model of satellite motion led to received signal modelled to received signal model matched filtering done by FD correlation processmatched filtering done by FD correlation process had to separate range and azimuth processinghad to separate range and azimuth processing

to fit in to computer resourcesto fit in to computer resources range cell migration correction a challengerange cell migration correction a challenge

needed interpolator to eliminate paired echosneeded interpolator to eliminate paired echos


First Digital SEASAT Image


Featured in Aviation Week, Feb. 26, 1979

Typical product 1979-1981


• Niagara Falls

• 40 x 40 km scene

• 25 m resolution

• 4 looks

• 40 hours to process

• “big” minicomputer

• corner turning on disc

• needed autofocus for L-band

Review of SAR Processing Algorithms

Developed primarily for satellite SAR processingDeveloped primarily for satellite SAR processing Range/Doppler (1978)Range/Doppler (1978) JPL & MDAJPL & MDA Time-domain Time-domain (1978)(1978) SPECAN SPECAN (1980)(1980) MDAMDA Omega-K Omega-K (1988)(1988) Polimi, ItalyPolimi, Italy Chirp scaling Chirp scaling (1992)(1992) CCRS, DLR , MDACCRS, DLR , MDA


SAR Signal Model


• Range and azimuth extent of signal received from a point target (shown in red)

• The signal in the two axes is not orthogonal because of the curvature – if it were, the processing would be quite simple

•The processing is achieved by a matched filtering (correlation) operation, but what to do about the curvature, if we are restricted to 1-D operations?

Range/Doppler Algorithm – 1

Developed at MDA and JPLDeveloped at MDA and JPL 1977-79 plus later refinements1977-79 plus later refinements

Used 1-D frequency domain correlationUsed 1-D frequency domain correlation processing separated in range and azimuth dimensionsprocessing separated in range and azimuth dimensions for computing efficiency & memory limitationsfor computing efficiency & memory limitations

Had to deal with range migrationHad to deal with range migration MDA recognized importance of MDA recognized importance of accurate interpolationaccurate interpolation JPL developed JPL developed secondary range compressionsecondary range compression to deal with to deal with

range-azimuth coupling when migration large – 1984 range-azimuth coupling when migration large – 1984




So named because the key operations of RCMC and azimuth compression are performed in the range time/azimuth frequency (i.e., Doppler) domain.

SRC not SRC not shown, as shown, as it can be it can be applied in applied in different different placesplaces



A one-dimensional interpolator can correct multiple targets in the range/Doppler domain.

After the interpolation, the locus of energy is corrected, but a phase error persists, which can defocus the image. SRC was implemented to correct the phase error.



When the squint is high or the aperture wide, SRC is needed to ensure accurate focusing. SRC is mainly a function of azimuth frequency, then range frequency.

Therefore, it is best to apply SRC in the 2-D frequency domain.

It is applied efficiently by a phase multiply after an azimuth FFT is done.



Processing with SRC can be done by modifying the range FM rate in rangcomp.

This is the approximate approach.



This slide shows how the target focus deteriorates when the squint angle is increased.

With no SRC, the focus deteriorates quite fast with squint.

Approx SRC allows a fair amount of squint, but the accurate SRC (not shown) allows a large squint.



This slide shows the RDA with1) No SRC2) Accurate SRC3) Approximate SRC

The Option 3 is very simple to apply, and is most frequently used.

Which one is needed depends on the squint angle and the width of the aperture.

Option 3 can be applied when needed.

1818October 2007October 2007 Cumming – Perscectives on SAR ProcessingCumming – Perscectives on SAR Processing


Easy to understand & programEasy to understand & program uses only 1-D operationsuses only 1-D operations

Accommodates range-variant Accommodates range-variant parameters easily (except SRC)parameters easily (except SRC)

Doppler centroidDoppler centroid azimuth FM rateazimuth FM rate

Interpolator introduces a small Interpolator introduces a small but controllable error but controllable error

Cannot handle very wide Cannot handle very wide apertures or high squintapertures or high squint

unless SRC is applied as unless SRC is applied as neededneeded

Advantages Disadvantages


• City of Vancouver

• RDA processing

• RADARSAT-1 FINE mode

• 8 m resolution


Convair-580 Data processed using RDA


Chirp Scaling Algorithm – 1

Want to improve accuracy by Want to improve accuracy by replacing the RCMC interpolatorreplacing the RCMC interpolator with with a more accurate DSP operatora more accurate DSP operator

The chirp scaling concept The chirp scaling concept if a linear FM signal is if a linear FM signal is shifted in shifted in frequencyfrequency, it will be , it will be shifted in timeshifted in time after pulse compression after pulse compression

same concept that causes a same concept that causes a railway trainrailway train to be imaged off the tracks to be imaged off the tracks the train’s Doppler shift moves the train in azimuth after azimuth the train’s Doppler shift moves the train in azimuth after azimuth compressioncompression

Chirp scaling is used to perform Chirp scaling is used to perform differential RCMCdifferential RCMC in the in the range/Doppler domain, to equalize the curvature over rangerange/Doppler domain, to equalize the curvature over range


Chirp Scaling Algorithm – 2Illustrates how a chirp (Panel 1) multiplied by a sine wave (Panel 2) causes a registration shift in the compressed pulse (Panel 4).

Because the matched filter (not shown) has a zero centre frequency, the compressed pulse is registered at the zero frequency point of the scaled signal (Panel 3) .

This shift performs RCMC in the range/Doppler domain, assuming range compression is not done yet (because the amount of shift is limited by the range oversampling, only differential RCMC is done with chirp scaling).

The sine wave scaling is applied in the range direction, with a different frequency at each azimuth frequency.

Note that the resulting shift is constant with range.



But the shift needs to be varied with range (as well as with azimuth frequency).

As the change with range is small, this can be achieved by making the frequency of the scaling function change slowly with range.

A linear FM scaling function makes the range shift vary linear with range (called linear chirp scaling).

This linear form is adequate for most satellite SAR parameters.



The 2-D freq domain can be used to perform The 2-D freq domain can be used to perform the the remaining RCMCremaining RCMC without an interpolator. without an interpolator.Range compression, SRC also done. SRC is Range compression, SRC also done. SRC is accurate as it is accurate as it is range and az freq dependentrange and az freq dependent..

All operations are FFTs and phase multiplies

Chirp scalingChirp scaling is applied in the RD domain is applied in the RD domain

Subsequent ops are the same as the RDASubsequent ops are the same as the RDA


SRTM Image Processed by CSASRTM Image Processed by CSA



More accurate RCMC More accurate RCMC no interpolatorno interpolator

The 2-D frequency domain is The 2-D frequency domain is available to apply a more available to apply a more accurate form of SRCaccurate form of SRC

better phase accuracybetter phase accuracy

Can handle slightly wider Can handle slightly wider apertures and squint anglesapertures and squint angles

Requires 2-D processingRequires 2-D processing

Additional complexity if the Additional complexity if the Doppler centroid changes Doppler centroid changes quickly with rangequickly with range

Assumes SRC is independent Assumes SRC is independent of rangeof range

Inefficient if range Inefficient if range compression already donecompression already done


For many satellite SAR parameters, the advantages over the RDA tend to outweigh the disadvantages, but RDA preferred for zero Doppler cases.


Omega-K Algorithm – 1

If the range equation is hyperbolic (straight line sensor motion), and If the range equation is hyperbolic (straight line sensor motion), and the effective radar velocity is independent of range, an the effective radar velocity is independent of range, an “exact” “exact” solutionsolution can be obtained can be obtained

this assumption is excellent for airborne radars (after mocomp) and this assumption is excellent for airborne radars (after mocomp) and quite good for satellite SARsquite good for satellite SARs

Engineers at the Polytechnic of Milano discovered this, adapting an Engineers at the Polytechnic of Milano discovered this, adapting an algorithm known as Stolt interpolation from the seismic fieldalgorithm known as Stolt interpolation from the seismic field

All processing is done in the 2-D frequency domainAll processing is done in the 2-D frequency domain



The Stolt interpolation consists of The Stolt interpolation consists of scaling and shifting operationsscaling and shifting operations, as , as illustrated by a point target illustrated by a point target simulation.simulation.

Col 1: phase plot in 2-D FDCol 1: phase plot in 2-D FDCol 2: range slices in 2-D FDCol 2: range slices in 2-D FDCol 3: data after range IFFTCol 3: data after range IFFT(2-D energy & 1-D azimuth slice)(2-D energy & 1-D azimuth slice)

The simulation starts with data The simulation starts with data after the bulk compression, and after the bulk compression, and shows the effects of the scaling shows the effects of the scaling and shifting separately.and shifting separately.

SRC is done exactly during the SRC is done exactly during the bulk compression – bulk compression – the Stolt the Stolt interpolation effectively makes the interpolation effectively makes the signal stationary in range and signal stationary in range and azimuth.azimuth.


Omega-K Algorithm – 3• SIVAM X-band airborne radar built by MDA

• Spotlight operation – 1.8 m resolution



Most accurate algorithm if Most accurate algorithm if constant-velocity assumption constant-velocity assumption valid (SRC exact)valid (SRC exact)

Accuracy not affected by wide Accuracy not affected by wide aperture and high squintaperture and high squint

Fairly easy to program if Fairly easy to program if Doppler centroid does not vary Doppler centroid does not vary much with rangemuch with range

An interpolation is needed in An interpolation is needed in the 2-D FDthe 2-D FD

Cannot handle large changes Cannot handle large changes of Doppler frequency with of Doppler frequency with range efficientlyrange efficiently

need range block processingneed range block processing



SPECAN Algorithm

When the signal is a chirp, it can be deramped using a When the signal is a chirp, it can be deramped using a phase multiply, making it a sine wavephase multiply, making it a sine wave

a single FFT then focuses the dataa single FFT then focuses the data

Single-look version was developed many years agoSingle-look version was developed many years ago multilook version developed in 1979 under an ESTEC contract multilook version developed in 1979 under an ESTEC contract

for on-board processingfor on-board processing

Has the most efficient computing, but the worst image Has the most efficient computing, but the worst image quality, notably phase propertiesquality, notably phase properties

particularly suited to burst-mode data, such as ScanSARparticularly suited to burst-mode data, such as ScanSAR


• RADARSAT-1

• ScanSAR Narrow

• 300 km wide

• SPECAN processing

• Vancouver Island


Doppler Centroid Estimation

Considered a mature technologyConsidered a mature technology yet processing mistakes are still madeyet processing mistakes are still made

Recommend a new approachRecommend a new approach take a global viewtake a global view use spatial diversity over a wide areause spatial diversity over a wide area use quality checks and filtering to eliminate bad regionsuse quality checks and filtering to eliminate bad regions use a physical geometry model for overall estimateuse a physical geometry model for overall estimate use phase increments for primary estimatoruse phase increments for primary estimator

Can be used for specialized applicationsCan be used for specialized applications such as ocean current estimationsuch as ocean current estimation

More details can be More details can be found in our “SAR found in our “SAR Processing” bookProcessing” book

Published by Published by Artech HouseArtech House

January 2005January 2005

Also to be published in Also to be published in

Chinese, November 2007Chinese, November 2007

3434

Summary – 1

RDARDA a well-known algorithm for general use (30 years experience)a well-known algorithm for general use (30 years experience) except when the aperture is wide and the squint angle is highexcept when the aperture is wide and the squint angle is high

CSACSA a little more accurate than the RDA, as long as the Doppler a little more accurate than the RDA, as long as the Doppler

parameters do not change too quickly with rangeparameters do not change too quickly with range

WKAWKA the most accurate as long as the flight path is linear and the velocity the most accurate as long as the flight path is linear and the velocity

does not change with range (well suited to airborne applications)does not change with range (well suited to airborne applications)

SPECANSPECAN needs the least memory and fewest DSP operationsneeds the least memory and fewest DSP operations ideal for low-resolution imagingideal for low-resolution imaging


Summary – 2

Many choices of algorithm are availableMany choices of algorithm are available

Best algorithm for each application depends onBest algorithm for each application depends on geometry and radar parametersgeometry and radar parameters accuracy required accuracy required efficiency requiredefficiency required

Most commonly-used algorithm for general Most commonly-used algorithm for general precision processing seems to be the RDA, precision processing seems to be the RDA, followed by WKA and the CSAfollowed by WKA and the CSA

but a systems study needs to examine the pros and but a systems study needs to examine the pros and cons for each radar situationcons for each radar situation

SPECAN is commonly used for ScanSAR and SPECAN is commonly used for ScanSAR and quicklook processingquicklook processing


Current & Future Research

Many people consider SAR processing for Many people consider SAR processing for conventional (remote sensing) SARs to be a conventional (remote sensing) SARs to be a mature subject, not requiring significant further mature subject, not requiring significant further developmentdevelopment

Current work seems to be directed towards Current work seems to be directed towards advanced items such as:advanced items such as: bistatic SAR processingbistatic SAR processing

ground moving target detection (GMTI)ground moving target detection (GMTI)

information extraction, e.g.information extraction, e.g. classification of polarimetric SAR dataclassification of polarimetric SAR data

change detection through interferometrychange detection through interferometry

polarimetric interferometrypolarimetric interferometry


Final Word

Ever since working with ESTEC in 1979, on-Ever since working with ESTEC in 1979, on-board, real-time, digital SAR processing on a board, real-time, digital SAR processing on a satellite has been a dreamsatellite has been a dream

For various technical, financial and strategic For various technical, financial and strategic reasons, it has not been implemented yetreasons, it has not been implemented yet it is becoming feasible now, although its necessity is it is becoming feasible now, although its necessity is

not justifiednot justified

As a compromise, I would love to build a As a compromise, I would love to build a real-real-time Doppler estimatortime Doppler estimator would give the most accurate antenna steering would give the most accurate antenna steering

possible, andpossible, and

be a technical showcase !be a technical showcase !


Documents

Perspectives on SAR Processing Ian Cumming Professor Emeritus Radar Remote Sensing Group Dept. of Electrical and Computer Engineering University of British