Optical methods for data collection on spacecraft and aircraft have been in use for many decades. In the late 1970s, radar became a standard component in space-borne remote sensing systems. Radar imagers are especially useful for space-borne and airborne applications because in the microwave frequency band, they operate independent of atmospheric conditions like clouds, rain, snow, fog, daylight, etc. unlike optical imagers. Synthetic Aperture Radar (SAR) systems are radar imagers that exploit the collection of many independent samples of targets in a particular scene to produce intrinsically highresolution images and have several key advantages over airborne optical imagers

Crockett, Mark T.

Long, David

English

DigitalCommons@USU

1An Introduction to Synthetic Aperture Radar: aHigh-Resolution Alternative to Optical ImagingMark T. CrockettAbstract—Optical methods for data collection on space-craft and aircraft have been in use for many decades.In the late 1970s, radar became a standard componentin space-borne remote sensing systems. Radar imagers areespecially useful for space-borne and airborne applicationsbecause in the microwave frequency band, they operateindependent of atmospheric conditions like clouds, rain,snow, fog, daylight, etc. unlike optical imagers. SyntheticAperture Radar (SAR) systems are radar imagers thatexploit the collection of many independent samples oftargets in a particular scene to produce intrinsically high-resolution images and have several key advantages overairborne optical imagers.I. INTRODUCTIONSynthetic Aperture Radar (SAR) systems are sophis-ticated imaging systems that emit electromagnetic (EM)radiation at microwave frequencies (300 MHz to 300GHz). As opposed to optical imaging systems, whichmeasure the visible spectrum return of solar EM radia-tion in a particular scene, SAR systems illuminate thescene of interest with their own transmit signal. Fig.I shows an example of a high-resolution SAR image.Applications of SAR include intelligence, surveillance,and reconnaissance, interferometry, foliage penetration,moving target indication, and environmental monitoring.SAR is often preferred over optical imagers for theseapplications because its performance is independent ofavailable daylight and visibility.From a high-level perspective, SAR systems emitmicrowaves at a scene and measure the voltage returnedfrom a scene of targets. From these voltages, the radarcross-section (RCS) is found, which is a measure of how“large” a target appears in a radar image. High valuesof RCS show up as bright targets, whereas targets withlow RCS are dim. Targets with rough surfaces tend toscatter EM radiation back towards the radar, so man-made targets like buildings, vehicles, and roads have ahigh RCS. Since the ground is relatively flat, it scattersEM radiation away from the radar and its RCS is low.Because RCS is a measure of how much of a transmittedsignal is scattered back towards the radar, it is also calledbackscatter, which will be used synonymously with RCSin this paper.Fig. 1. Ku-band SAR image of the Pentagon in Washington, D.C.(Courtesy Sandia National Laboratories).This paper takes a high-level approach to discussingSAR systems and their operation, geometry, and imageprocessing. Section II outlines the physical geometrythat enables SAR systems to be such versatile radarimagers. In Section III, the signal properties of SARare discussed, which is followed by an explanation ofSAR signal and image processing in Section IV. Finally,Section V makes a practical comparison between opticalimagers and SAR systems.II. SAR GEOMETRYThis section discusses the basic concept of SARimaging, including its physical geometry and modes ofoperation. Three types of SAR collection are typicallyused, including stripmap, spotlight, and scanSAR modes.In stripmap mode, the antenna boresight is in a fixedposition, and may be orthogonal to the flight path orsquinted slightly forward or backward. Spotlight SARutilizes a gimballed antenna in order to maintain dwelltime on a chosen target for as long as possible. Finally,scanSAR mode scans the antenna pattern outwards fromthe radar platform during each pulse to increase the widthof the imaging swath. This paper deals exclusively withstripmap SAR.2Fig. 2. Stripmap SAR imaging mode. Azimuth is described bythe direction of the aircraft flight path and range is in the directionpointed by the antenna, which is orthogonal to the azimuth direction(Courtesy Evan Zaugg, Brigham Young University).SAR has been in use for radar imaging for morethan 50 years, and its capabilities are consistently be-ing expanded. However, the fundamental concepts thatallow SAR to work so well have been in use since itsconception in the late 1950s. The primary theory that isexploited is that SAR antenna geometry facilitates thecollection of many independent samples for an individ-ual target, thus enabling fine spatial resolution. Fig. 2shows the antenna geometry for a mobile SAR platformoperating in stripmap mode. As the antenna aperturemoves along the flight path, a signal is transmitted ata rate equal to the pulse repetition frequency (PRF). Thelower bound of the PRF is determined by the Dopplerbandwidth of the radar, which is the range of Dopplerfrequencies observed by the radar during the collection.Conversely, the PRF must be low enough that returnsfrom the far edge of the scene for one pulse are measuredbefore the detecting signal returns from the near edgeof the scene of the next pulse. The backscatter return ofeach of these pulses is coherently summed on a pixel-by-pixel basis to attain the fine azimuth resolution desired inradar imagery. Real aperture radar imaging requires anantenna that is long in the azimuth direction to producea narrow beamwidth (and hence fine resolution), whichgives the azimuth resolution ∆yR [1],∆yR = 2R sin(βy2)≈ Rβy = λRly(1)where R is the slant range from the radar platformto the target, βy is the 3dB azimuth beamwidth, λ isthe wavelength of the transmitted pulse, and ly is thelength of the antenna aperture in the azimuth direction.The first approximation is an application of the smallangle approximation, where sin(βy2)≈ βy2 , and thesecond uses the approximation of the beamwidth fromthe physical dimensions of the antenna (β ≈ λly ). Incontrast, SAR azimuth resolution (∆yS) is independentof range [1], and improves with decreasing antenna size(or increasing antenna beamwidth)∆yS =λ4 sin βy2≈ λ2βy≈ ly2. (2)Likewise, the range resolution of SAR is also indepen-dent of the range to the target [1]∆xLFM =c02BW(3)where c0 is the speed of light and BW is the radar band-width, which is fixed for any particular SAR collection.Therefore, regardless of the range of the radar platform tothe target, SAR image resolution is fixed. SAR antennageometry allows for very small and lightweight radarplatforms and very fine resolution, which make it idealfor airborne radar imaging systems.III. SIGNAL PROPERTIESTraditional SAR systems consist of either two co-located antenna apertures (one for transmission and onefor reception) or a single aperture that alternates betweenboth transmit and receive modes. The radar equation fora point target gives the received power (Pr) for a singletarget for a particular pulse, and is given byPr =PtG2λ2(4pi)3R4σ, (4)where Pt is the transmitted power, G is the antenna gain,and σ is the radar cross-section [2]. The radar equationis paramount to determining backscatter values for eachpixel in an image grid and in SAR image processing.The earliest SAR systems employed a transmittedradar signal that consists of a single frequency and avery short pulse duration in order to attain fine imageresolution. These are called interrupted continuous wave(ICW) SARs and have a duty cycle of less than 100%.This short pulse duration results in extreme instantaneouspower requirements for a reasonable signal-to-noise ratio(SNR) (ratio of returned signal energy to noise energy)and is somewhat impractical to implement in hardware.Today, the most common SAR systems utilize a linearfrequency modulated (LFM) signal. An LFM signal isa “chirped” signal in that the its frequency changeslinearly throughout the duration of the pulse. The rangeof frequencies of an LFM signal is what determines thesignal bandwidth. LFM signals also have longer pulse30 100 200 300 400 500 600 700 800 900 1000−1−0.500.51LFM transmit signaltime (t)NormalizedSignalVoltage(V)0 100 200 300 400 500 600 700 800 900 1000−1−0.500.51LFM receive signaltime (t)NormalizedSignalVoltage(V)Fig. 3. LFM transmit and receive signals.lengths than those of ICW systems, so they carry moreenergy per pulse than ICW signals, thus decreasing theinstantaneous power requirement of LFM SAR systems.0 200 400 600 800 1000−1−0.8−0.6−0.4−0.200.20.40.60.81ICW transmit signaltime (t)NormalizedSignalVoltage(V)Fig. 4. A rectangular-windowed ICW transmit pulse.∆xICW =c0T2(5)Note from Eq. (5) that range resolution for ICWsystems is directly proportional to pulse duration T [3].This means that impractically short pulses are needed toattain the equivalent range resolution of LFM systems,which is given in Eq. (3), and can be improved byincreasing the range of frequencies over which the LFMsignal is swept.A very narrow pulse width creates a problem whenseeking reasonable SNR. Since SNR is generally a keymeasure of performance of a radar system, high SNRis desirable. Total energy of the transmit signal must beincreased in order to achieve this, which means increas-ing the instantaneous power of each pulse. It is here thatICW systems become impractical, since narrow pulsewidths are required for fine range resolution, but longpulses are required for high energy and performance.LFM SAR systems overcome these barriers because thetotal transmit energy can be increased by lengthening thepulse duration without sacrificing range resolution.A zero-phase LFM transmit signal can be expressedasst(t) = A(t) exp((2pif0t+ pikrt2))(6)where A(t) is the signal amplitude at time t, f0 is thefrequency at the beginning of the chirp, and kr is thechirp rate [4]. A transmitted signal propagates throughthe atmosphere (which is approximated as free space)and is scattered throughout the target scene. Generally,most of the transmit energy is scattered away from theradar, so a very small amount of energy is received.The received signal for an individual point target canbe expressed assr(t) = A′(t) exp((2pif0(t− τ) + pikr(t− τ)2))(7)where A′(t) is an attenuated version of A(t) and τ isthe two-way time of flight from the radar to the targetat range R, and is given byτ =2Rc0. (8)For simplicity in processing, the receive signal is mixeddown to baseband, meaning that the carrier frequency of4the signal is near zero. This mixed down signal issrmd(t) = A′(t) exp((2pi(f0 − fmd)t− 2pif0τ + pikr(t− τ)2)) (9)IV. IMAGE PROCESSINGIn order to be useful as a high-resolution imagery,SAR data must be processed in steps. These steps includerange compression, range cell migration correction, andazimuth compression. This section gives a brief overviewof this process as well as an algorithm that processes therange-compressed data.Since the return SAR signal is an attenuated, time-shifted copy of the transmit signal, a reference signalis defined to be equal to the transmit signal. The ideabehind range compression is to correlate the receivesignal with the reference signal in order to pull out therange information of the target. This is recognized as amatched filter and is implemented simply by applying thefast Fourier transform (FFT) to each signal, performing acomplex multiply of the two, and performing an inverseFFT on the product. The result of such an procedure isshown in Fig. 5.0 200 400 600 800 100000.20.40.60.81LFM receive signalRange binNormalizedValueFig. 5. LFM range-compressed data.Since most SARs use an antenna with a wide azimuthbeamwidth, targets are imaged for a period of time longenough that their signatures are detected in multiplerange bins (range resolution cells) through the lengthof the synthetic aperture. This effect can be seen inthe second image of Fig. 6. To alleviate this issue, thetrajectories are shifted so that for a single target, theyall fall in the same range bin for the entire syntheticaperture. Finally, azimuth compression is performed onthat data with another matched filter applied in theazimuth direction for each range bin [4]. The result is afocused image similar to the fourth image in Fig. 6.Many SAR image formation algorithms have beendeveloped for processing range-compressed data and arecurrently in use, including the range Doppler, omega-k,and backprojection algorithms (BPA)1. Backprojectionis the most accurate and robust of these, but it is themost computationally expensive. However, recent tech-nologies with massively-parallelized graphics processinghave made the computation expense of backprojectionvery manageable. Fig. 7 and Fig. 8 are examples ofrange-compressed SAR data SR (r[n]), processed us-ing the backprojection algorithm, which, for a pixel at(x0, y0, z0), may be expressed byA(x0, y0, z0) =∑nSR (r[n]) eΦe(r[n]), (10)where A(x0, y0, z0) is the complex power in that pixel,r[n] is the range from the pixel location to the antennaphase center for pulse n, and Φe (r[n]) is the complexconjugate of the expected phase of r[n] [4].V. COMPARISONThe primary differences in SAR and optical imagesare caused by the properties of the targets being imaged.These include dielectric properties, which describe howmuch a material insulates from an electric charge, andsurface roughness, which determines the direction ofEM scattering. Two primary reasons why targets appeardifferently between SAR and optical images are thatdielectrics reflect radiation in a manner dependent on thefrequency of the radiation, and surface roughness on theorder of centimeters cause significant scattering (higherbackscatter) in SAR imagery, whereas this occurs at themicrometer level in optical imagery.Both SAR and optical imagers have benefits andshortcomings. Optical imagers can produce much finerresolution images than SAR and are generally low powerdevices. They can also produce true color images, whichmay sometimes be beneficial in airborne surveillance.SAR, on the other hand, is unaffected by clouds, fog,or other impairments to visibility. Also, since it providesits own target illumination, it works just as effectively atnight as during the daytime. Furthermore, SAR signals1In order to form a focused image, BPA assumes knowledge ofthe physical position of the antenna during each transmit pulse asa reference in determining the location of each target. Many SARsystems employ a Global Positioning System coupled with an inertialmeasurement unit (IMU) to obtain a precise estimate of this position.This measurement of the antenna location allows the precise targetlocation to be determined.5Fig. 6. Simulated SAR data for a point target. From left to right the SAR data is shown as raw data, and after range compression, rangecell migration correction, and azimuth compression (Courtesy Evan Zaugg, Brigham Young University).Fig. 7. SAR image of Spanish Fork, UT compared side-by-side with a corresponding Google Earth image (Courtesy Artemis Inc. andGoogle Inc.).can penetrate foliage and dry ground for useful recon-naissance purposes.Fig. 7 and Fig. 8 make side-by-side comparisons ofa SAR image and an optical image for two identicalscenes. Fig. 7 is an image of a residential area. Note thatalthough the asphalt is optically bright, it shows up blackin the SAR image, and vice versa for the shingles on theroofs of homes. In general, it can be seen that man-madestructures are easily detectable in SAR images. Notice inFig. 8 that while terrain is fairly easily distinguishablein the optical image, it is not so for the SAR image.Also, much of the vegetation is bright in the SAR imagebecause it scatters a significant amount of radiation backto the radar antenna.VI. CONCLUSIONSAR systems utilize simple geometry, precise GPSlocation estimation, and accurate image processing al-gorithms to produce high-resolution images. They canbe lightweight, low power, highly scalable systems andused in a variety of applications, both military andcivilian. Both optical and SAR imagery are used in theseapplications. Each have their advantages and drawbacks,6Fig. 8. SAR image of Payson Canyon in Payson, UT compared side-by-side with a corresponding Google Earth image (Courtesy ArtemisInc. and Google Inc.).and this paper has outlined the fundamental principlesthat make SAR so versatile.REFERENCES[1] M.A. Richards. Fundamentals of Radar Signal Processing.McGraw-Hill, New York, NY, 2005.[2] Fawwaz T. Ulaby, David G. Long, Adrian K. Fung, andRichard K. Moore. Microwave Remote Sensing Modern Edition.Artech House, Norwood, MA, 2013.[3] J.W. Winkler. “An Investigation of Ground Moving Target Indi-cation (GMTI) with a Single-Channel SAR System”. Master’sthesis, Brigham Young University, 2013.[4] E.C. Zaugg. “Generalized Image Formation for Pulsed and LFM-CW Synthetic Aperture Radar”. Master’s thesis, Brigham YoungUniversity, 2010.7Fig. 9. NuSAR-B X-band data collected over agricultural areas and the Bear River near Elwood and Honeyville, Utah. From left to rightthe images show the magnitude of the raw data, the magnitude of the range-compressed data, and the magnitude of the azimuth-compresseddata, respectively (Courtesy Evan Zaugg, Brigham Young University).

An Introduction to Synthetic Aperture Radar: a High-Resolution Alternative to Optical Imaging

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An Introduction to Synthetic Aperture Radar: a High-Resolution Alternative to Optical Imaging

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