Chapter 1. Principles of Synthetic Aperture RadarSamuel W. (Walt) McCandless, Jr.
SEASAT Program Manager
User Systems Enterprises, Inc., Denver, CO, USA

Christopher R. Jackson
Radar Imaging Resources, Alexandria, VA USA

1.1

Introduction

Beginning with the launch of SEASAT in 1978, synthetic aperture radar (SAR) satellites
have provided a wealth of information on such diverse ocean phenomena as surface waves,internal waves, currents, upwelling, shoals, sea ice, wind, and rainfall. The influence of man in
the form of offshore facilities, ship transits, and other ocean-related events and artifacts is also
observed from space using fine resolution SAR. Although aircraft-based imaging radar had beenaround since the 1960s, it was unable to provide the wide area global perspective and temporal
coverage necessary to observe many ocean processes. Indeed, SAR is the premier sensor for thedetection of such phenomena because it is sensitive to small surface roughness changes on theorder of the radar wavelength (1 m down to several centimeters). It is also independent of solar
illumination and is generally unaffected by cloud cover. In addition, SAR has the advantage ofproviding control over such factors as power, frequency, phase, polarization, incident angle,
spatial resolution and swath width, all of which are important when designing and operating asystem for the extraction of quantitative information.
SEASAT, launched in June 1978, was the first civilian SAR satellite. Since then,Canada, Europe, Japan, and Russia (the former Soviet Union) have placed SAR satellites in orbit
for investigation and monitoring of both the sea and land surface. The ongoing operation of
ERS-2, RADARSAT-1, and ENVISAT, along with currently planned SAR missions, means thatSAR data will be available for at least the next decade at X- (3 cm), C- (6 cm), and L-band (24cm) wavelengths. While each system has its own configuration, in terms of frequency,
polarization, resolution, swath width, etc., the underlying operating concept for each is the same.
This chapter presents the principles behind the operation of a SAR. It begins with ahistorical look at the development of SAR and the evolution of spaceborne SARs. The concepts
behind radar operation and aperture synthesis are then presented, along with the factors andlimitations that govern SAR performance. A discussion of SAR image formation follows andthe chapter concludes with an examination of some basic SAR image characteristics.
1.2

Imaging From Space

Fine resolution remote sensing of earth surface phenomena was first accomplished fromspace in the 1960s on military and National Aeronautics and Space Administration (NASA)satellites using cameras and camera-like imaging devices (passive radiometers). These early
remote sensors in space were used to collect and discriminate radiated and reflected
electromagnetic energy in the visible or infrared spectra (roughly 0.4-micron to 20-micron
wavelengths). In 1973, the NASA Earth Resource Technology Satellite (ERTS-1), later renamed
LANDSAT, initiated a series of missions featuring fine resolution (10s of meters) optical

1

SAR Marine User’s Manual

imagers with many visible and infrared channels that were thematically associated with specificland applications. The LANDSAT satellites continue operation to this day. Although theseradiometers are capable of providing fine surface spatial resolution and excellent multi-spectral
details, they are inhibited by clouds and depend on solar illumination, and are thus limited todaylight observation. The limitations of clouds and darkness can be overcome by moving to themicrowave portion of the spectrum. Microwaves are capable of passing through the clouds andpermitting unobstructed observations of the earth's surface, including day or night detection.
However, it is virtually impossible for a passive microwave radiometer to achieve the fineresolution produced by devices operating at optical wavelengths.
The diffraction limited angular extent (and subsequent spatial resolution) of an aperture isdirectly proportional to wavelength and inversely proportional to aperture dimension1. From aspacecraft in orbit, a few 10s of meters of resolution at visible and infrared wavelengths can beachieved with a camera aperture of only a few 10s of centimeters. When the wavelength
increases from the visible and infrared to microwaves (microns to centimeters), resolution will
decrease (become coarser) unless the antenna apertures are increased an equivalent amount. To
achieve the same 10s of meters of resolution at microwave frequencies, a spacecraft would needan antenna aperture dimension on the order of 10s of kilometers! In satellite sensor design, thisantenna dimension is ridiculously impractical. Both active and passive microwave instruments
suffer from this resolution problem; however, for an active microwave instrument (i.e., radar), a
technique has been developed to overcome this limitation.
The very limiting factor for a microwave antenna, poor spatial resolution resulting from alarge beam width or angular field of view, can be turned to an advantage for a radar that has theability to precisely measure phase and Doppler. For a radar that has its beam directed orthogonal
to its direction of travel, the large beamwidth will cause an object (or location) on the ground tobe illuminated and linearly traversed by the radar beam for a extended period of time (tenths of asecond to seconds for spacecraft velocities). During this time, the radar collects phase andDoppler measurements that, through signal processing, allow for an aperture to be constructed orsynthesized, equivalent to the distance the physical antenna moves while the location remains in
the beam. This is the basic concept behind SAR, details of which are discussed in other sectionsof this chapter and can be found in greater detail in many of the references cited in Section 1.14
[Curlander and McDonough, 1991; McCandless, 1989; Raney, 1998; Tomiyasu, 1978; andSullivan, 2000]. The new synthesized aperture, which can be several orders of magnitude larger
than the transmit and receive antenna, makes it possible to produce radar imaging with a few
meters of resolution.
Figures 1.1a and 1.1b provide a comparison of a SAR and optical/visible image of thesame area around Nantucket Island off the coast of Massachusetts. Although the images were
not taken simultaneously, they illustrate some of the differences and similarities of these diverse
imaging technologies. Clouds, white beaches and some depth information from the watercoloration are visible in the passive optical image. The active SAR image shows dark beaches,along with internal waves, manifestations of bottom topography and other fine scale ocean
features.
The all weather, day and night, fine resolution capabilities make radar an ideal remotesensing instrument for many applications in target detection mapping, as well as earth resources
1

The Fraunhofer diffraction for a rectangular and circular aperture pattern are worked out in Principles of Optics by
Max Born and Emil Wolf [1975]. The radiation pattern for a circular aperture is worked out in Microwave Radar:
Imaging and Advanced Concepts by Roger Sullivan [2000]

2

Principles of Synthetic Aperture Radar

Figure 1.1a. SEASAT (L-band, HH) SAR image of the ocean south and east of
Nantucket Island collected on 27 August 1978 (Rev 880, 1234 GMT). The
image contains bottom topography, upwelling and internal wave signatures.
Image courtesy John Apel [Evans, 1995]

Figure 1.1b. Optical image southeast of Nantucket and Cape Cod collectedfrom Skylab (1973). The image shows the optical signature of shoals andinternal waves. Image courtesy John Apel [Evans, 1995]

3

SAR Marine User’s Manual

TABLE 1.1: Highlights of SAR History with Space Emphasis
DATE

DEVELOPMENT

1951

Carl Wiley of Goodyear postulates the Doppler beam-sharpening concept.

1952

University of Illinois demonstrates the beam-sharpening concept.

1957

University of Michigan produces the first SAR imagery using an optical correlator.

1964

Analog electronic SAR correlation demonstrated in non-real time (University of Michigan).

1969

Digital electronic SAR correlation demonstrated in non-real time (Hughes, Goodyear, Westinghouse).

1972

Real-time digital SAR demonstrated with motion compensation (for aircraft systems).

1978

First space-borne SAR NASA/JPL SEASAT satellite. Analog downlink; optical and non-real-time digitalprocessing.

1981

Shuttle Imaging Radar series starts - SIR-A. Non-real-time optical processing on ground.

1984

SIR-B. Digital downlink; non-real-time digital processing on ground.

1986

Space-borne SAR Real-time processing demonstration using JPL Advanced Digital SAR processor (ADSP).

1987

Soviet 1870 SAR is placed in earth orbit.

1990

Magellan SAR images Venus.

1990

Evolution of SAR begins in space; Soviet ALMAZ (1991), European ERS-1 (1991), Japanese JERS-1(1992), SIR-C (1994), ERS-2 (1995), Canadian RADARSAT-1 (1995), SRTM (2000), ENVISAT (2002).

management. Oceanic and ice observations are ideally suited to space-based radar because of
the vast geographic scale and the ever-changing nature of the subjects. The ability of radar topenetrate persistent cloud cover over regions important to transport and other commerce haveproduced improvements in nowcasts, forecasts and climatological analyses because diurnal andseasonal variances can be monitored. Application candidates extend well beyond oceanic andice observation to include such things as crop and forest monitoring, land management anddevelopment, hydrology, disaster management, and an ever-growing list of applications
important to science and commerce.
1.3

SAR History

SAR designs and associated applications have grown exponentially since the 1950s when
Carl Wiley, of the Goodyear Aircraft Corporation, observed that a one-to-one correspondence
exists between the along-track coordinate of a reflecting object (being linearly traversed by aradar beam) and the instantaneous Doppler shift of the signal reflected to the radar by that object.
He concluded that a frequency analysis of the reflected signals could enable finer along-track
resolution than that permitted by the along-track width of the physical beam itself, whichgoverned the performance of the real aperture radar (RAR) designs of that era.
4Principles of Synthetic Aperture Radar

Table 1.2. Characteristics of Earth orbital SAR systems.
*Based on 0.1 µs uncoded pulse length.

5

SAR Marine User’s ManualThis "Doppler beam-sharpening" concept was exploited by Goodyear and by a group at
the University of Illinois. One major problem was implementation of a practical data processor
that could accept wide-band signals from a storage device and carry out the necessary Dopplerfrequency analysis at each resolvable picture element (pixel). The Illinois group carried out anexperimental demonstration of the beam-sharpening concept in 19-52 through use of airborne
coherent X-Band pulsed radar, "boxcar" circuitry, a tape recorder, and a frequency analyzer.
Table 1.1 provides a brief overview of SAR development and the timeline of orbital missions.
Industrial and military developments, using airborne platforms, continued at Goodyear,
Hughes, and Westinghouse. The Jet Propulsion Laboratory (JPL), University of Michigan,
Environmental Research Institute of Michigan (ERIM), Sandia Laboratories, and others alsobegan to explore this new technology. In 1974, engineers at JPL formed an alliance with a groupof international ocean scientists led by the National Oceanic and Atmospheric Administration(NOAA) to determine if an ocean application satellite featuring a space-based SAR could beachieved. Up until this time, the major emphasis of space-based remote sensing had been on
land applications using visible and infrared sensors. The resulting NASA/NOAA allianceassembled a multi-agency, interdisciplinary group of engineers and scientists that focused on
ocean and ice applications using active and passive microwave sensors that could collect data
day or night with a general disregard for cloud obscuration. From the very beginning, when
SEASAT was but a future mission study, this group met regularly as the SEASAT User Working
Group chaired by NOAA's Dr. John Apel with JPL's Dr. Alden Loomis serving as his deputy andNASA's coordinator. The SEASAT User Working Group soon expanded to include international
participation and continued seamlessly through the program, working diligently to gain the
support and funding for such a mission, to define and guide the mission and systems
development, and to establish the experiments that would validate the program.
SEASAT [Lame and Born, 1982] operated successfully from late June to early October
1978, when it experienced a massive short circuit in the power system. SEASAT was followedby the Shuttle Imaging Radar-A (SIR-A) and Shuttle Imaging Radar-B (SIR-B) flown in 1981
and 1984, respectively [Elachi et al., 1986; Way and Smith, 1991]. Both the SIR-A and SIR-B
radars were variations on the SEASAT radar operating at L-band and HH (horizontal transmit,
horizontal receive) polarization. SIR-B had the added capability of operating at different
incident angles (the angle of incidence is defined as the angle between the radar line-of-sight andthe local vertical at the point where the radar intersects the earth or ocean surface).
With the exception of the Soviet 1870 SAR (not widely distributed), the 1980s saw only
Space Shuttle based, SEASAT derivative, spaceborne SAR activity. The 1990s witnessed asignificant expansion of SAR missions with the launch of five earth oriented SAR satellites
along with two more Shuttle Imaging Radar missions, as well as the pioneering interplanetary
use of the Magellan SAR to map Venus. The satellite systems ALMAZ [Li and Raney, 1991],
European Remote Sensing [ERS-1, ESA, 1991], the Japanese Earth Resources Satellite (JERS)-1[Nemoto et al., 1991], ERS-2 [ESA, 1995] and RADARSAT-1 [Raney et al., 1991], each
operated at a single frequency and single polarization, like SEASAT. ALMAZ and
RADARSAT-1 had the added ability to operate at different incident angles. RADARSAT-1 also
has a frequently used ScanSAR mode, where the coverage swath extends up to 500 km.
One of the most advanced SAR systems, the Shuttle Imaging Radar-C/X-band SAR(SIR-C/XSAR), was a joint NASA/German Space Agency/Italian Space Agency mission, flown
in April and October 1994 on Endeavor [Jordan et al., 1991]. The system could be operated

6

Principles of Synthetic Aperture Radar

simultaneously at three frequencies (L, C, and X) with the C- and L-band having the ability toalternately transmit and receive at both horizontal and vertical polarization. By collecting a nearsimultaneous and mutually coherent version of the scattered field in a minimum basis set ofpolarizations, this quadrature polarimetry or "fully polarimetric" capability allows for a more
complete characterization of the target’s scattering characteristics within the illuminatedresolution cell area [Zebker and Van Zyl, 1991].
The C- and X-band portions of the SIR-C radar were again flown in 2002 for the Shuttle
Radar Topography Mission (SRTM). During this flight, a second receiving antenna was placedat the end of a 60-m mast, extended perpendicular to the main radar antenna. The purpose of themast antenna was to provide a second receiving point in space for each radar pulse. The slightvariations in phase, between the receipt of the radar pulses at each of the antennas, will beprocessed into a height measurement of the reflecting point on Earth's land surface. [Rosen et al.,2000]
Future SAR missions are expected to provide enhanced capabilities, where the radar canbe operated in several collection modes. ESA's ENVISAT, the follow-on to ERS-1 and ERS-2,
was placed in orbit in March 2002. Like ERS, ENVISAT's ASAR (Advanced SAR) radar
operates at C-Band, and has the added capability to collect data in pairs of four polarimetriccombinations, as well as operate in a wide-swath (> 400 km) mode. The ENVISAT ASAR has
polarimetric diversity, but is not capable of quadrature polarimetry.
The upcoming
RADARSAT-2 and Japan’s ALOS Phased Array L-band SAR (PALSAR) instruments will bothhave fully polarimetric and ScanSAR operating modes. Table 1.2 provides a list of the earth
orbital SAR missions and some of their characteristics. Additional information on these systems
and their image products is given in Appendix A of the Manual.
SEASAT began the evolution of space-based SAR that continues to this day. The active
international participation of the SEASAT User Working Group led to international cooperationin the form of data collection and processing facilities and experiments with ground truth. In
retrospect, the most important result of these cooperative interactions was the zealous expansionof the SAR technology in space. As a result, more than two decades of data collection haveprovided a rich data source, with each system adding unique characteristics in terms ofapplications, radar design, and mission data collection parameters.
1.4

Radar And Synthetic Aperture Radar

Radar self-illuminates an area by transmitting pulses of microwave energy. These pulsesof radar energy are reflected from the illuminated area and collected by the radar receiver. By
precisely measuring the time difference between the transmitted pulse and the receipt of thereflected energy, radar is able to determine the distance of the reflecting object (called range orslant range). The range resolution of a radar system is its ability to distinguish two objects
separated by some minimum distance. If the objects are adequately separated, each will belocated in a different resolution cell and be distinguishable. If not, the radar return will be acomplex combination of the reflected energy from the two objects.
Spatial resolution in the range direction is not range or directly wavelength dependent,
but is instead a function of the effective (processed) pulse-width (τ) multiplied by the speed oflight (c) and divided by two. Range resolution can also be expressed as the reciprocal of theeffective pulse-width (the pulse bandwidth (β)) multiplied by the speed of light7

SAR Marine User’s Manual

Range Resolution = (

cτ
c
)=(
)
2
2β

(1)

As the range resolution becomes finer, the pulse bandwidth and data rate grows accordingly.
Most modern radars (including SARs) transmit a pulse called a linear frequency modulated (FM)"chirp". The transmitter varies the frequency of the radar pulse linearly over a particularfrequency range (an increase in frequency is called an up-chirp). That variation in frequency
determines the radio frequency (RF) bandwidth of the system. The chirp length and slope arebased on the radar hardware capabilities (RF pulse power, pulse repetition frequency (PRF),
Analog-to-Digital (A/D) sampling conversion) and the range resolution requirement. Both realaperture radar (RAR) and SAR achieve their range spatial resolution in this way. Typical spaceborne SAR pulse bandwidths range between 10 and 40 MHz, that produce a slant rangecresolution,, between 15 and 3.7 meters.
2β
In the direction orthogonal to the radar beam (also called cross range, azimuth, or along
track in broadside operation) the SAR is distinctive in its use of aperture synthesis to improve its
spatial resolution. By comparison, optical sensors and RARs obtain their resolution through thephysical dimensions of their aperture, sometimes referred to as diffraction limited performance.
The RAR cross-range spatial resolution is a direct function of radar wavelength (λ) and targetλ 
range (R) and an inverse function of antenna dimension (D); i.e., proportional to  R  . FromD 
space, the RAR range resolution problem can be solved but the poor cross-range or along-track
performance, typically kilometers to tens of kilometers, still has to be contended with. One wayof achieving better along-track performance is to boost frequency; another is to increase alongtrack antenna length; a third is to decrease the target range. None of these options are very
effective from space. However, by using Carl Wiley’s discovery, resolution is determined by the
Doppler bandwidth of the received signal, rather than the along-track width of the radar's antenna
beam pattern. The along-track resolution of side-looking radar can be of the same order ofmagnitude as its range resolution. Table 1.3 summarizes the spatial resolution relationship for
RAR and SAR systems. The key to converting theoretical groundwork into a "full-bodied"system is an appropriate signal-processing scheme. Fundamental to the SAR concept is therealization that SAR is a marriage of radar and signal processing technologies. This is the key
step to understanding the principles upon which SAR is based.
A SAR consists of an end-to-end system that includes conventional radar building blocks
such as an antenna, transmitter, receiver, a high technology data collection system providing
coherent Doppler phase histories, and a similarly advanced signal processor capable of making
an image out of these phase histories. The radar must maintain stringent control of the signal
characteristics and collect coherent phase information to allow the construction of the image. Ifphase-preserving disciplines are rigidly enforced, a SAR can produce an image whosealong-track spatial resolution is largely independent of wavelength and target range.
In the most basic sense, there must be a translation of either the target through the realradar beam, or the real beam through the target, or a combination of both processes that produce
the necessary systematic change of phase in the target’s signal during the observation time of theradar. There are many such embodiments of the SAR concept, (phase history variation can also
be the result of target rotation, among other causative factors) and new variations of radar/target8Principles of Synthetic Aperture Radar

TABLE 1.3: Spatial Resolution Functional Relationships
Spatial Direction

SAR

RAR

Cross-Range(Along - Track)

Along-Track Antenna Length2

Wavelength x Target Range
Along-Track Antenna Length

Range

(

c
)
2β

(

c
)
2β

spatial interaction are still being developed. However, the space-based missions outlined inTable 1.2 generally fall under the category in which a single antenna beam is orientated at rightangles (broadside) to the radar platform velocity vector so that the side-looking beam scans thetarget by means of the translating spacecraft platform. This mode of operation is referred to as
monostatic strip-mapping. All of the radars listed in Table 1.2 are or were monostatic with the
exception of the Shuttle Radar Topography Mission (SRTM). SRTM employed a two-apertureradar with the first antenna transmitting and receiving coupled with a receive-only second
antenna located at a precise baseline distance from the first. This arrangement produces
interferometric signatures, hence the name IFSAR (or InSAR), capable of being converted into
topographic contours and maps.
1.5

SAR Principles

Illustration of a typical space-based, strip-map, monostatic SAR is shown in Figure 1.2[McCandless, 1989]. Consider the string of dots in Figure 1.2 as a set of positions at which the
SAR transmits a pulse. Each pulse travels to the target area where the antenna beam intercepts
the earth and illuminates targets at that location, and the reflected return pulses are in turncollected by the same antenna. SAR "works" because the radar pulse travels to and from thetarget at the speed of light, which is much faster than the speed of the spacecraft [Harger, 1970].
The SAR system saves the phase histories of the responses at each position as the real beam
moves through the scene and then weights, phase shifts, and sums them to focus on one point
target (resolution element) at a time and suppress all others. The SAR image signal processing
system performs the weighting, shifting, and summing to focus on each point target in turn. It
then constructs an image by placing the total energy response obtained in the focusing on aparticular target at the position in the image corresponding to that target. SAR achieves a very
high signal processing gain because of coherent (in-phase) summation of the range-correlated
responses of the radar. All of the signal returns that occur as the real beam moves through eachtarget, as shown in Figure 1.2, can be coherently summed. In many instances, thousands ofpulses are summed for each resolution cell resulting in a tremendous increase of the target signal
compared to that from a single pulse (a coherent benefit of approximately 4000 for SEASAT
SAR). The power from a given scatterer, spread across many pulses (as illuminated) is focused(concentrated) into a single location through processing.
To get some idea of the scale of the data collection and processing challenges faced by
spaceborne SAR system designers and operational planners, consider the general magnitude ofthese subjects for the systems listed in Table 1.2. These SARs transmit more than a thousand
pulses per second; illuminate tens of millions of resolution cells (pixels) in the radar beam at
9

SAR Marine User’s Manual

DISTANCE SATELLITE MOVESTO ILLUMINATE TARGET(LSA = SYNTHETIC APERTURE LENGTH)
t1

t2

VELOCITY (V)DWELL, OR INTEGRATIONLSATARGET RANGE(R)

TIME =

L SA
λR
=
V
VD AT

V
NADIR
h

REAL SAR ANTENNAAPERTURE AZIMUTH
BEAMWIDTH (θA);θA

ALONG-TRACK ANTENNALENGTH (DAT)

θA =

λ
D AT

LSA

SUBTRACK

EARTH TARGETRESOLUTION ELEMENTALONG-TRACK DIMENSION (δAT)

LSA
SATELLITEGROUND SWATH

L SA = θ A R =δ AT =

λR
D AT

D
λ
λR
R=
= AT
2λ R
2L SA
2
D AT

ACHEIVABLE ALONG-TRACK RESOLUTION IS INDEPENDENT OF RANGE AND RADAR FREQUENCY
AND IMPROVES WITH SMALLER REAL ANTENNA APERTURE
Figure 1.2. Basic Principles of Aperture Synthesis

10Principles of Synthetic Aperture Radar

Figure 1.3. SEASAT SAR Imaging Geometryeach pulse time; utilize a spaceborne platform travelling in excess of seven thousand meters per
second; and require thousands of processor operations per cell to resolve an image.
When SAR was first introduced on a space platform by SEASAT, optical processing
techniques were the touchstone of image generation. Digital processors were limited in numbers
and capability and the first digital processors used for SEASAT image generation required 20
hours of processing time to convert an 18-second data collection into a 100-kilometer by 100kilometer image frame. Although this significant time delay has disappeared as digitaltechnologies and processing skills have advanced, SAR data collection and processing stillpresent great challenges to the satellite and mission designers and are one of the salient
yardsticks of program performance and resource allocation.
As indicated, the SAR requires the collection and processing of phase-coherent data and,for an isolated target, the phase history during the integration time follows a complex (higher
order) phase function. For descriptive convenience, it is assumed that the radar antenna beamaxis is oriented at right angles (broadside) to the radar platform velocity vector. This is the basic
orientation of the strip-map SAR systems listed in Table 1.2.
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SAR Marine User’s Manual

In Figure 1.2, the antenna beam illuminates the target when the platform reaches position
t1, but not before. It continues to illuminate the target for a distance LSA (synthetic aperture
length) until it reaches t2. The time required to translate the along-track beam through a point
target is called the integration time or dwell time and is defined in the figure. Figure 1.2
provides a heuristic derivation of the spatial performance result of this process: viz., the spatial
resolution in the along-track direction approaches the physical length of the antenna divided bytwo. This result is, to the first order, independent of the wavelength of the radar and the range
from the radar to the target. Unlike the foregoing RAR along-track spatial resolution, theantenna dimension is now a direct, not an inverse, relationship. As a seeming paradox todiffraction limited performance, smaller apertures produce better spatial resolution. All or part of
the available phase history can be coherently processed. If all of the pulses are used, the result isreferred to as single look, one look, or fully focused processing, achieving a spatial resolution
approaching ½ DAT.
Since the along-track resolution is independent of target range, the question is how far
can we take this for spaceborne SAR? It will become clear in the next section that there areserious application constraints that limit taking along-track spatial performance too far. These
constraints manifest themselves by placing unacceptable limits on important application goals
such as area coverage and illumination geometry. There are also associated technology
limitations that set application limits. Important technology limitations include data collection
rate and volume, and limiting antenna design factors, such as pulse power, phase control andcalibration.
Figure 1.3 further illustrates the imaging geometry of a strip-mapping SAR using theillumination geometry of the first space-based SAR, SEASAT. As shown, the length of thesynthetic aperture is a function of the beamwidth of the real-aperture, 16 km for SEASAT. In
the range direction, the width of the beam as it intercepts the earth is a function of thediffraction-limited beam in the range direction and the illumination geometry. The illuminationgeometry can be described or specified by either the look angle (nadir angle) at the radar, 23
degrees in the case of SEASAT, or by the grazing angle where the radar beam intercepts theearth’s tangent plane. Sometimes the complement of the look-angle, the depression angle, isused for definition or, in the case of the target plane, the complement of the grazing angle(referred to as the incident angle) is used as the reference. Using the grazing angle reference,
Figure 1.3 provides an expression for swath-width first in terms of diffraction limited beamintersection with the earth and second in terms of the differential range of the target space (thefar range minus the near range, Rf - Rn). To image the target space illuminated by the radar, eachradar pulse will transit the differential range distance twice, first as a transmit-transit and then asa return-transit after target(s) interaction.
1.6

The 2-D Sampling Problem: Ambiguity Relationships

A SAR is a two-dimensional (along-track and range) imaging sensor. This produces arequirement to collect data in the along-track direction and in the range direction that can beprocessed into an image that is unambiguous (has one solution). Figures 1.2 and 1.3 provide
illustrations that help visualize the two-dimensional ambiguity relationships that are so important
to SAR applications.
As described, the azimuth resolution of a focused SAR is approximately one half thealong-track radar-antenna length, and this resolution is independent of radar frequency and range

12

Principles of Synthetic Aperture Radar(within limits). It would appear that fine along-track resolution could be obtained by simply
making the real antenna length very small. This is in fact the case, but ambiguity conditions
place certain limits on carrying this too far.
Achieving along-track resolution comes with a requirement that necessitates the radar tosend a pulse each time the radar platform translates half of the along-track antenna length. Thiscondition sets the lower bound of the Pulse Repetition Frequency (PRF). The relationship isgiven by [Tomiyasu, 1978]:2 × Velocity of Radar< PRF
Antenna Length(2)

Another way to express this relationship is:
Velocity of Radar
V
=
< PRF
Azimuth Resolution δ AT

(3)

Thus, as the along-track antenna length is diminished to improve along-track resolution,
the radar must pulse faster. Consequently, there is less time between pulses (the reciprocal of the
PRF known as the Inter-Pulse Period or IPP) to collect data in the cross-track or range direction.
Unfortunately, only one pulse for a given radar frequency can be in the target zone at a time.
Otherwise, range ambiguity occurs, thereby negating two-dimensional imaging. This condition
sets an upper bound on the PRF which is a function of the swath width (S = Rf - Rn) in slant
range direction as shown in Figure 1.3 and the real (uncompressed) pulse duration, T, resulting in
the expression:

PRF <

1
2T + 2( R f − Rn ) / c(4)More spacing between pulses is required as swath width and/or incident angles increase. Thus,
the quest for increased coverage will eventually collide with a desire for improved along-track
spatial resolution. For the space-based examples shown in Table 2, the pulse duration, T, isgenerally much smaller than the swath width time and as a general rule for the single look case[Raney 1998]:
Vc< PRF <
δ AT
2 × Swath Width⇒

2 × Swath Width c
<
δ AT
V

(5)

In practice there are even more complex considerations of transmit and receive event timing andcoordination. For space, several pulses will be in transit simultaneously, and to avoid eclipsing
the reflected signals in the monostatic radar, temporal interlacing of returns with the transmitted
pulses is required. SAR with interlaced pulses of a particular polarization (i.e., SIR-C,
RADARSAT-2), designed to produce polarization specific and unique target signatures, require
even more intricate transmit/receive sequences to prevent ambiguous performance.

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SAR Marine User’s Manual

1.7

Data Rate

All space-based SAR designs have an inherent limitation in the amount of data that canbe acquired. Consequently, range resolution, swath width, and dynamic range all vie forresources within this performance limit. Important design elements, such as the satellite toground link (downlink), analog to digital (A/D) converters, and onboard recorders are still
daunted by the voracious resource needs of contemporary spaceborne SAR. SEASAT posed a
dilemma in 1978 with its 120 Mega-bit per second (Mbps) data rate when available systems ofthat era topped out at 15 Mbps. The solution was an innovative analog downlink coupled with asimilarly unique ground system capable of converting the data to a digital form and recording it
on a SEASAT peculiar 42-channel tape recorder. This end-to-end system was a one-of-a-kindsolution never to be used again. Although considerable effort is underway to improve thelimiting technologies and SAR designers are searching for ways to more efficiently use available
A/D conversion, on-board recording and downlink capabilities, contemporary satellite systems
continue to press the limit with 500 Mbps aspirations. Data collection and subsequent
processing will continue as one of the most important determinants of systems feasibility andcost; influencing data collection, image signal processing, and ultimately the entire applications
process.
The data rate for a full resolution, single look SAR data collection can be described by:
Data Rate = 2 β s N

Swath Width TimeInter − Pulse Period(6)

Four important performance factors are involved in determining the data rate. They are:
1. The range resolution. The pulse bandwidth (β) of the radar determines its rangec). As the pulse bandwidth is increased, and the range resolution
resolution (δR =2βbecomes smaller, the data sampling (A/D conversion) rate (2 βS) grows. The datacollection or system bandwidth (βS) will preserve the pulse bandwidth (βS > β).
2. The number of bits per sample (N) sometimes called the quantization rate. The dynamic
range (number of useful signal levels) requirement determines how many bits are needed
to preserve image data quality.
3. Minimum Inter-Pulse Period. The swath width requirement dictates a minimum length oftime, 2(Rf - Rn)/c, for the Inter-Pulse Period to sustain unambiguous, in range, datacollection.
4. Maximum Inter-Pulse Period. The azimuth (along-track) resolution requirement
determines the maximum length of the Inter-Pulse Period for unambiguous, in Doppler,
data collection.
The SAR designer must select a PRF, or conversely the IPP, consistent with unambiguous
operation in range and azimuth. Factors three and four often provide some needed relief in many
SAR designs because the time required to collect information from the swath (3) is less than theinterpulse period (4) required for azimuth resolution. Positive use of this time advantage reduces
data rate and is referred to as time expansion buffering or stretch processing. This techniqueallowed the SEASAT data rate to be reduced. The SEASAT pulse bandwidth was 19 MHz andthe approximate sampling rate of the ground based A/D converter (2 βS) was 44 mega-samples

14

Principles of Synthetic Aperture Radar

per second which, at 5 bits per sample (N), resulted in a 220 Mbps data rate. Factors 3 and 4
allowed this rate to be reduced to 120 Mbps.
As in the case of range and Doppler ambiguities, the applications conflict between swath
width and along-track resolution reoccurs in the data rate relationship. As along-track resolution
becomes smaller and the swath width grows, the swath width time and interpulse time
approaches 1 and time expansion buffering is no longer available to reduce data rates.
Another technique that is very effective and frequently used to reduce the data rate on
many space-based SAR (i.e., SIR-C, RADARSAT-2 and most notably the Venus mapping SAR
that formed the core of the MAGELLAN mission) is a technique referred to as Block FloatingPoint Quantization (BFPQ) [Johnson, 1991] or Block Adaptive Quantization (BAQ). The
concept of BFPQ is somewhat similar to an Automatic Gain Control (AGC). BFPQ scans a
block of phase history data and estimates an overall scale (called the exponent) for the block ofdata. The block of data is then requantized relative to this scale value (these values are called themantissas). The block of phase history data is replaced by the quantized exponent and mantissas.
For example, the A/D converters on SIR-C produce 8-bits per sample. Blocks of 128, 8-bit
samples are compressed into a single 8-bit exponent and 128, 4-bit mantissas, which is nearly a
2:1 compression. The processed imagery products are virtually indistinguishable from products
produced from the full 8-bit samples.
SAR systems can use additional techniques to reduce the data rate. One such technique isbased on utilizing NA and NR, the number of looks in azimuth and range respectively. These
refer to the percentage (1/N) of the available azimuth or range bandwidth, respectively, that isused to produce the SAR image. If this is done prior to recording the signal history it is often
referred to as presumming. However, most of the systems identified in Table 2 do not use thistechnique to reduce the recorded signal history or raw data rate because of the attendant loss of
spatial resolution that accompanies the reduction of range and/or azimuth bandwidth.
1.8

The SAR Antenna

Antenna technology also paces modern SAR radar design and ambiguity considerations
also influence the sizing of the SAR antenna. There exists a minimum antenna area that willassure simultaneous compliance with range and azimuth ambiguity constraints [Tomiyasu, 1978].
Amin =

4 V λ R tanφ Ic(7)

The antenna area, as shown in Figure 1.3, is the product of antenna dimensions DAT x DR . The
demand for improved along-track spatial resolution is in contention with achieving wide swath
width because it causes the range dimension of the antenna to increase in order to comply with arequired minimum antenna dimension for SAR. Conservative SAR designs pay heed to thislimit and most of the SAR missions tabulated in Table 2 achieved SAR antenna dimensions ofabout twice the minimum required by ambiguity constraints.
In addition to the minimum antenna area constraint, there is another ambiguity constraintthat is crucially important to space-based SAR. The area cover rate (ACR, area mapped per unit
time which equals the product of swath width and platform velocity) is bounded by:
ACR <c
Azimuth Resolution
2

(8)

15

SAR Marine User’s ManualThus, the finer the azimuth resolution, the less is the ACR. As we know from the along-trackambiguity and minimum antenna area constraints, finer azimuth resolution implies a larger PRF,
which in turn implies a narrower swath. As an example, the ultrafine mode of RADARSAT-2
will have 3 m resolution. The above equation says the swath must not exceed 60 km at an orbital
velocity of 7.5 km·s-1. Indeed, the designed swath-width is about 45 km, due to data rate
constraints. To circumvent this constraint, the SAR must be capable of operating two or more
SAR antenna beams simultaneously.
Two basic antenna technologies are currently in use in the SAR systems tabulated inTable 1.2.
1. A constrained (corporate) feed array (planar or modified planar). This system can bepointed electronically although some mechanical redirection may be also incorporated.
Example - SEASAT, Space Shuttle SAR (SIR-A and SIR-B), ERS-1, ERS-2, JERS-1,
ALMAZ and RADARSAT-1.
2. A distributed array using transmit/receive modules (T/R) that is similar in electricalperformance to the corporate feed array. This system has the advantages of better phase
control and beam steering. These systems were first introduced by the Space Shuttle
SIR-C and SRTM designs and are the basis for the ENVISAT and RADARSAT-2
designs.
The first design uses a centralized RF transmitter and receiver, usually not co-located with theantenna, connected to the antenna via waveguides or coaxial cables. The second design moves
the transmit/receive function to the antenna and distributes hundreds to thousands oftransmit/receive modules in a nearly uniform pattern as part of the antenna structure to achieve
the same result.
1.9

SAR Signal Processing and Image Formation

In a SAR signal processor there are specific operations required to convert a raw data set
into an interpretable image. The raw SAR data is not an image since point targets are spread out
in range (because of the long frequency coded pulse) and in the along-track dimension (by thereal beam moving through the point target for the duration of the dwell time), as shown in Figure
1.2. Figure 1.4 illustrates the raw data trace of a typical point-target as the beam moves in thealong-track or azimuth direction. As the radar moves by the target, the radar-to-target range
varies, forming the curved trace shown. This translation also produces an along-trackfrequency/time trace in azimuth, induced by Doppler, and range pulse encoding produces asomewhat similar time/frequency trace in range.
The SAR signal processor compresses this distributed target information in two
dimensions (range and along-track) to create the image. The complexity of this process isrevealed in Figure 1.5. Figure 1.5a is an example of the raw image data recorded by a SAR
system. All of the point targets (δR by δAT resolution elements) that exist in the beam at eachpulse instance are superimposed and create a complex interference pattern that is notinterpretable in terms of targets and target location. This is the situation that a SAR processor
must unravel to produce an image of the sampled scene. Also embedded in the raw data areunwanted distortions and perturbations between the collection platform and the earth that mustbe accounted for. Examples of distortions and perturbations include range curvature, earthrotation during the integration period, orbit eccentricity, spacecraft attitude noise, among others.
As a result, many complex operations must be performed prior to image formation such as
platform motion compensation for roll, pitch and yaw offsets, and target motion during the16Principles of Synthetic Aperture Radar

Figure 1.4. SAR Point Target Returnradar/target dwell times. At present, these perturbations or distortions are removed via a seriesof iterative operations that include attitude/orbit compensations, explicit range-curvaturecorrection algorithms and the option of a variety of applicable auto-focusing techniques. Manyadvanced space-based SAR designs (e.g., ERS-1 and ERS-2, ALMAZ, RADARSAT-2)
normalize the effects of earth rate by using a few degrees (±3 degrees over the course of a typical
90-minute orbit) of programmed yaw steering coordinated with the satellite platform's orbital
location. The objective is to steer the real antenna beam to maintain orthogonality to thesatellites ground track rather than to the orbit plane. Figure 1.5b and 1.5c show the intermediatestep after compressing the distributed target first in range and then the resulting image after
compression in azimuth (along-track). Embedded in this process are all of the complex
compensations discussed.
Several computation techniques may be used to form a SAR image [Curlander and
McDonough, 1991]. One method is to use Fourier spectrum estimation techniques, commonly
implemented via a "Fast Fourier Transform". Time domain matched filters may also be used.
The choice is applications dependent and usually made based on computational efficiencies. In
either case, the result is the same; the along-track resolution of the radar is vastly improved
compared to real aperture radar and is not dependent, to the first order, on target range or radar
wavelength.
In summary, SAR image formation (not to be confused with post image enhancement
processing and feature identification) requires many rote processes and computation intensive
signal processing of coherent radar return phase histories. To provide a scale of the processing

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SAR Marine User’s Manual

(a)

(c)

(b)

Figure 1.5. SIR-C (L-band, HH) images of the tip ofCape Cod, showing the steps in SAR image formation.
(a) is the amplitude of the phase history radar return.
The radar energy is spread out over the SIR-C chirp
length in range (~10 microsecond or ~3 km) and thesynthetic aperture length in azimuth (~1.6 km), so theimpulse response from any point target covers roughly
4.8 km2. (b) is the amplitude image after range
compression. (c) is the fully processed image (after
azimuth compression).
Each imaged area isapproximately 38 km (range) x 43 km (azimuth) andcentered near 41.9o N and 70.0o W. Data were acquired
15 April 1994 (DT 97.2).

burden, consider a single-channel space-based SAR collecting a 100-km swath width image witha space platform collection speed of 7600 meters per second. These values are indicative ofmany of the systems listed in Table 1.2. Suppose that the SAR is capable of producing a 10-m
ground range and 10-m along-track pixel after image processing. The SAR collects over sevenmillion pixels per second and the processor requires more than one thousand operations per pixel
to produce the image. A real time or even near real time image processor must be capable of 109
complex operations per second or greater as a function of radar and platform complexities. Thissituation doesn't get less daunting as modern space-based SAR designs reach for enhanced
performance. Adding more channels to a radar, in the case of a fully polarized SAR such as
RADARSAT-2 (factor of four), or more wavelengths and polarization channels in the case of thespace shuttle SIR-C radar (factor of nine) adds to the toll.
18

Principles of Synthetic Aperture Radar

Figure 1.6. Slant plane and ground plane imaging geometry (flat earth).

Although SAR processing is, and will remain, computationally intensive, it offers theadvantage of fine cross-range (along-track) spatial resolution with a small antenna, which makes
SAR attractive for use on aircraft and satellites. The economic benefits of fine resolution
weather independent imagery are significant. The large area of coverage that is possible fromspace-based radar makes it attractive for many applications, especially for ocean and ice
observations for reasons of scale, synoptic variability, impact of weather and climate, and a hostof known and emerging reasons with scientific and commercial bearing.
1.10 SAR Image Characteristics: Geometry and Speckle
1.10.1 Slant Plane / Ground PlaneA SAR image has several characteristics that make it unique. The position andproportions of objects in the image can appear distorted compared to a photograph and the image
also has a grainy appearance, due to the presence of many black and white pixels randomly
distributed throughout the image. Both characteristics result from SARs method of observation(i.e., transmission of a coherent pulse of energy and measurement of its coherent reflection).
A SAR, like any other radar, very precisely measures the time between a transmitted
pulse and receipt of its reflection. The pulse travels along a straight line between the radar andsurface of the earth. The delay time associated with a particular reflection is converted to
distance, and the distance measured along this line of sight is called slant range. A SAR samples
its signal in the slant range direction at constant intervals (its resolution in this plane is alsoc}). This constant sampling in range, however, does not correspond to a constantconstant {2βsample spacing on the ground (or ground plane). It is this difference in sample spacing between
19

SAR Marine User’s Manual

the slant plane and locations along the ground that produces the unique geometric characteristics
of a radar image.
Figure 1.6 shows the SAR viewing geometry and the difference between the slant planeand ground samples. The sample spacing (and resolution) along the ground is a function of the
SAR's grazing angle where the grazing angle is defined as the angle between the radar line ofsight and the local tangent plane at the point of the reflection on the earth. The resolution in the
ground plane image is coarser then the corresponding slant plane image and the relationshipbetween the two is given by:
δg ~ δr / cos ψ(9)where ψ is the grazing angle. To make a SAR image appear "map-like", the sampling along theslant plane must be converted to the ground plane. Simply converting the slant plane samples to
the ground plane will produce ground range pixels of varying size from one end of the swath tothe other. Additional resampling is required to produce pixels of equal size.
Slant to ground range distortions are often small for the space-based SARs listed in Table
1.2 because of the small variation in incident angle from the near side to the far side of the swath.
For SEASAT, the 6o variation in angle resulted in an approximate 25% variation in pixel size
across the image [Fu and Holt, 1982]. These distortions will increase as the angle variation
increases across the swath.
1.10.2 Speckle
The grainy "salt and pepper" appearance of a SAR image results from constructive anddestructive interference of the coherent SAR pulse by different scatterers contained within aresolution cell. The coherent interaction of electromagnetic radiation with a complicated set of
scatterers is probably the least understood and limiting facet of SAR processing system design
and application. Noncoherent sensors generally employ diffraction limited physical apertures forthe focusing of incident electromagnetic radiation, followed by detectors which are sensitive tothe total intensity of the radiation incident upon them. In contrast, a SAR transmits a very
precise signal toward its target and, when the reflected radiation returns, a SAR records not only
the amplitude of that signal but its phase as well. It is the phase information which allows for the
post-facto coherent summation of the many thousands of recorded signals in the correlator
during the aperture synthesis operation. Coherent signals have properties that are considerably
different from their noncoherent counterparts. The coherent interference between targets
contained within a resolution cell is the basis for much of the scintillation of coherent radar
imagery—an effect often referred to as speckle. Thus, there is a wide variation in the SAR
image, even when given a uniform input. This variation can be considered as a form of noise.
However, speckle is not really noise in the classic sense and there is information to be obtained
from it because speckle is the radar signature of the target under a particular set ofcircumstances. A considerable amount of work has gone into speckle characterization andabatement, details of which can be found in Goodman [1976], Lee [1986], Lopes et al., [1990,
1993], Arsenault and April [1986].
1.11 Multi-Look Processing
Multi-look processing involves creating several independent images of an area by subdividing or partitioning the available bandwidth spectrum (in either range, azimuth, or both) andthen combining those images to produce the final image. The mean Doppler frequency of each20

Principles of Synthetic Aperture Radar

partitioned look is proportional to the time at which the data for that look were collected relative
to the local reference time of the Doppler centroid. This is an important concept because it liesat the heart of cross-spectral techniques for suppressing the 180-degree ambiguity in ocean wave
directional spectra and in developing techniques for mapping sea surface topography. By
partitioning the bandwidth spectrum into N non-overlapping segments, statistically independentimages of the scene can be produced and then combined, reducing image speckle. Dividing the
bandwidth has the effect of breaking up the coherent nature of the SAR signal. Scatterers thatmight combine constructively in one part of the bandwidth spectrum may not in another. The
price for reducing the coherent scintillation in the image is an attendant decrease in spatial
resolution. The benefit lies in the correct interpretation of many dynamic scenes.
A SAR image can be formed using all or part of the signal bandwidth. If the fullbandwidth is used, the result, referred to as single look, one look, or fully focused processing,cachieves a spatial resolution of () in range and approaches ½ DAT in the along-track
2βdimension. The multi-look technique can be applied as three-look, four-look, and so on. Speckle
is reduced with an increasing number of looks. As a rule of thumb, the standard deviation of the
speckle is reduced in proportion to the square root of the number of effective statisticallyindependent looks.
1.12 SAR Imaging and the Ocean Surface
A SAR relies on the precise measurement of phase and Doppler which, through signal
processing, allows for its aperture to be synthesized and fine resolution imaging achieved in thealong-track direction. The discussion of SAR signal processing and image formation (Section
1.8) contained the assumption that the instantaneous Doppler shift of the signal was the result oftransverse movement of the radar beam across the reflecting object. An object’s inherent motion(with a component along the radar line of sight) at the time of observation will also produce a
Doppler shift and affect how it appears in the resulting SAR image.
The interaction between the SAR pulse of microwave energy and the ocean surface iscomplex, dependent on wavelength, polarization, geometry, environmental conditions and theelectrical properties of the ocean surface. (Details can be found in Chapter 2 and Chapter 4 ofthis manual). SAR energy is primarily scattered from the ocean surface by the presence of small(mm- to cm-scale) wind-induced surface waves called Bragg waves. SAR is particularly
sensitive to the Bragg waves in which wavelength is matched to the projection of the SAR
electromagnetic wavelength onto the local ocean surface. A Bragg wave on the ocean surface
will cause all of the electromagnetic wavefronts scattered from its different portions to be inphase (i.e., they travel an integral number of wavelengths) and add constructively.
The sea surface is constantly moving and the mean wave structure will include a variety
of motions with components along the line-of-sight to the radar. These motions will induce(small) Doppler frequency shifts on the reflected signals. These shifts, and the resulting
misregistration of scene scatterers, produce a smearing, or blurring in the azimuth direction.
This smearing is like a low pass filter, removing certain ocean wave components. These shifts
tend to be different for different phases of the dominant (long) waves and the onset andmagnitude of the effect depends primarily on significant wave height, and other parameters. The
term given this phenomenon is velocity bunching. It is a fundamentally limiting factor in a
SAR’s ability to image ocean wave fields (see Chapter 2, Section 2.2.3).

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SAR Marine User’s Manual

The same effect can be observed from larger objects moving in the scene. For example,
ships are observed displaced from the apex of their wake signatures (see Chapter 12, Figure12.9) or trains appear moved from the location of the tracks.
While motion along the radar line-of-sight limits the imaging ability of a monostatic
strip-mapping SAR, a technique called Along Track Interferometry (ATI) takes advantage ofthese small Doppler shifts to measure line-of-sight velocities (details of ATI can be found in
Section 2.4.2). In an ATI SAR system, two antennas are configured such that one is located fore
and the second aft in the platform velocity direction. The motion of scatterers can be determinedby measuring the phase difference between the signals received at two antennas. The technique
has limitations but aircraft-based ATI measurements have been used to examine surface currents
and waves.
1.13 Future Trends
In the 25 years since SEASAT, SAR observations of the world's oceans have progressedfrom a demonstration of capability to near real-time support for operational ice analyses andcharting. ERS 1, ERS-2, and RADARSAT-1 have compiled vast archives covering all of theworld's oceans, providing observations of surface waves, internal waves, currents, eddies,
upwelling, shoals, sea ice, rainfall, the atmospheric boundary layer phenomena, and ships. The
evolution sparked by SEASAT continues with ever more advanced SAR satellites beginningwith the recent launch of ENVISAT (Europe), followed by ALOS (Japan) and RADARSAT-2(Canada) in the 2004/2005 time period. These entries promise finer resolution and increased
polarimetric capability coupled with advanced data collection and processing techniques (e.g.,interferometry) and they will expand SAR capabilities both for the traditional SAR users and foran expanding community of users
1.14 References
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Opt. Eng., 25, 662–666.
Born, M., and E. Wolf, 1975: Principles of Optics. 5th ed. Pergamon, 808 pp.
Curlander, J. C., and R.N. McDonough, 1991: Synthetic Aperture Radar: Systems and Signal
Processing. John Wiley and Sons, 647 pp.
Elachi, C., J. Cimino, and M. Settle, 1986: Overview of the Shuttle Imaging Radar-B preliminaryscientific results. Science, 232, 1511–1516.
ESA, 1991: ERS-1 special issue. ESA Bulletin, 65. [Available from the ESA Publications
Division, 8-10 rue-Mario-Nikis, 75738 Paris Cedex 15.]
ESA, 2001: ENVISAT special issue. ESA Bulletin, 106, 168 pp. [Available from the ESA
Publications Division, 8-10 rue-Mario-Nikis, 75738 Paris Cedex 15.]Evans, D. L., Ed., 1995: Spaceborne Synthetic Aperture Radar: Current status and future
directions. NASA Tech. Memo. 4679, 171 pp.
Fu, L.-L., and B. Holt, 1982: Seasat views oceans and sea ice with synthetic-aperture radar. Jet
Propulsion Laboratory JPL Publ. 81-120, Pasadena, CA, 200 pp.
Goodman, J.W., 1976: Some fundamental properties of speckle. J. Opt. Soc. Amer., 66, 12 1145–
12 1150
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Principles of Synthetic Aperture RadarJohnson, W. T. K., 1991: Magellan imaging radar mission to Venus. Proc. IEEE, 79, 777–790.
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Tomiyasu, K., 1978: Tutorial review of synthetic aperture radar with applications to the imagingof the ocean surface. Proc. IEEE, 66, 563–583.
Way, J., and E. A. Smith, 1991: The evolution of synthetic aperture radar systems and theirprogression to the EOS SAR. IEEE Trans. Geosci. Remote Sens., 29, 962–985.
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