Technical Note / Note technique

RADARSAT-2 SAR modes development andutilization

Peter A. Fox, Anthony P. Luscombe, and Alan A. Thompson

Abstract. RADARSAT-2 is required to generate a much wider range of data products than any other preceding civiliansatellite Synthetic Aperture Radar (SAR). The RADARSAT-2 mission will support the so-called heritage beams ofRADARSAT-1: single-polarization standard, fine resolution, wide swath, ScanSAR, and extended coverage beams.However, the new mission requirements additionally include Quad-Polarization, Multilook Fine and Ultrafine resolutionmodes, and selectable single- and dual-polarization options for the heritage beams. The radar is also required to operate inexperimental modes to provide data for detection of moving objects. To support this mission, a technologically advancedinstrument has been designed, with a number of extra degrees of freedom in its operation. This paper describes some keyfeatures in the design of the instrument and explains how they will be utilized in generating the new types of data.

Résumé. RADARSAT-2 devra générer une beaucoup plus grande diversité de produits de données que tout autre satellitecivil précédent doté d’un capteur radar à synthèse d’ouverture. La mission maintiendra les faisceaux légués en héritage pourainsi dire de RADARSAT-1 : les faisceaux standard à polarisation simple, à résolution fine, à faisceau large, ScanSAR et àcouverture étendue. Toutefois, les nouvelles exigences de la mission incluent de plus les modes à polarisation quadruple,multivisées à résolution fine et ultra-fine et les options de sélection de polarisation simple et double pour les faisceauxlégués en héritage. Le radar devra aussi opérer dans des modes expérimentaux afin de fournir des données pour la détectiond’objets mobiles. En soutien à cette mission, un instrument performant au plan technologique a été conçu, avec un nombresupérieur de degrés de liberté prévus pour son opération. Cet article décrit certaines des caractéristiques principales de laconception de l’instrument et explique comment elles seront utilisées pour générer des nouveaux types de données.[Traduit par la Rédaction]

264Mission overview

RADARSAT-2 is a Canadian spacecraft carrying a C-bandSynthetic Aperture Radar (SAR) (5.405 GHz). It will provideusers with advanced, commercially available space-borne SARimagery having fully polarimetric modes and resolution as fineas 3 m. This increased capability will provide a high level ofdetail for research, analysis, and commercial operations in awide variety of applications, including agriculture, forestry,mapping, surveillance, environmental monitoring, naturalresource exploration and management, and many dynamicocean and sea-ice processes.

The RADARSAT-2 mission is designed for a duration of7 years. MacDonald, Dettwiler and Associates Ltd. (MDA) ofRichmond, British Columbia, is developing the RADARSAT-2mission in partnership with the Canadian Space Agency.

RADARSAT-2 imaging capabilitiesThe RADARSAT-2 imaging modes are depicted in Figure 1

and listed in Table 1. RADARSAT-2 will continue to supportall RADARSAT-1 beam modes, namely Single-PolarizationStandard, Fine resolution, Wide Swath, ScanSAR, andExtended Coverage beams. For these modes, all RADARSAT-1image-quality specifications will be met or exceeded.

New modes include Quad-Polarization imaging capabilitiesand Multilook Fine (which has the same resolution as fine butuses four looks) and Ultrafine 3 m resolution modes. Allmodes, new and heritage, are available in both left- and right-looking orientations.

RADARSAT-2 SAR sensorThe RADARSAT-2 payload can be broadly partitioned into

the Antenna and the Sensor Electronics (SE). The mostfundamental change in the instrument design fromRADARSAT-1 is the introduction of an active phased arrayantenna on RADARSAT-2. This replaces the passive slottedwaveguide antenna with one-dimensional elevation beam-forming of the earlier satellite. A block diagram of the SARsensor is shown in Figure 2.

The 512 TR modules (TRMs) in the RADARSAT-2 two-dimensional active phased array are organized as 16 columnswith 32 TRMs per column. All TRMs have independent

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Can. J. Remote Sensing, Vol. 30, No. 3, pp. 258–264, 2004

Received 19 August 2003. Accepted 25 January 2004.

P.A. Fox,1 A.P. Luscombe, and A.A. Thompson. MacDonaldDettwiler and Associates Ltd., 13800 Commerce Parkway,Richmond, BC V6V 2J3, Canada.

1Corresponding author (e-mail: pfox@mda.ca).

control of transmit phase and receiver phase and amplitude forboth vertical and horizontal polarizations.

Transmitter and receiver phase and amplitude control in theelevation dimension allow for the formation and steering of allbeams. Transmitter phase control in the azimuth dimensionallows the formation of the wider beams required for theUltrafine resolution mode. This is accomplished by thedeliberate defocussing of the beam. The provision of selectablehorizontal (H) or vertical (V) polarization for transmission andof simultaneous H and V polarization on reception provides thecapabilities needed for dual-polarization imaging in mostmodes, and for the quad-polarization imaging in thepolarimetric modes.

The SAR Antenna is partitioned in the flight direction intotwo wings. At the antenna output there are four radio frequency(RF) channels: vertical and horizontal polarizations for theleading wing, and similar polarizations for the trailing wing. ASwitch Matrix is implemented in the antenna receive pathbetween the antenna and the dual-channel SE.

The Switch Matrix allows the four signals receivedsimultaneously by the two wings of the antenna to be combinedin various ways to support the various modes. Signals of thesame polarization from the two wings can be added, reducingthe number of RF channels to two to support the full aperturesingle-, dual-, and quad-polarization modes. Alternatively, asingle-polarization signal from each wing may be routed to one

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Beam mode

Nominalswath width(km)

Swath coverage toleft or right ofground track (km)

Approximateresolution (m;range × azimuth)

RADARSAT-1 modes with single and dual polarization (transmit H or V; receive H or V or H and V)Standard 100 250–750 25×26Wide 150 250–650 30×26Low incidence 170 125–300 40×26High incidence 70 750–1000 18×26Fine 50 400–750 8×8ScanSAR wide 500 250–750 100×100ScanSAR narrow 300 250–720 50×50

Fully polarimetric modes (transmit H and V on alternate pulses; receive H and V on every pulse)Standard quad-polarization 25 250–600 25×8Fine quad-polarization 25 400–600 9×8

Selective single polarization (transmit H or V; receive H or V)Multilook fine 50 400–750 8×8Ultrafine 20 400–550 3×3

Table 1. RADARSAT-2 beams and modes.

Figure 1. RADARSAT-2 imaging modes.

channel of the SE, allowing for a dual aperture mode ofoperation.

In the Ultrafine resolution mode, these two receptions areused as separate elements in the formation of the SyntheticAperture to increase the sampling of the Doppler signal withoutincreasing the transmit pulse repetition rate. In theexperimental moving object detection mode, referred to asMODEX, the signals from the two channels provide inputs fordual phase centre antenna (DPCA) processing or along-trackinterferometry.

The SE allows selection of 14 precompensated transmitpulses with bandwidths from 11.6 to 100 MHz. The set ofavailable pulse bandwidths and corresponding signal samplingfrom RADARSAT-1 have been augmented to includebandwidths up to 100 MHz, as required to achieve the rangeresolution of approximately 3 m in the Ultrafine mode. The SEhas two independent receiver channels, with in-phase (I) andquadrature (Q) downconversion and block adaptive quantization(BAQ) for data compression. Flexible control is provided bymeans of Event Control Code (ECC), which supports allmodes.

Mode generationAt the Payload command level an image is defined by its

start time, duration, and beam–mode properties. Theseattributes are realized by the RADARSAT-2 SAR Payloadthrough the use of a 1553 command interface to the bus. The SEmacrocommands (MCMDs) and antenna commands togetherenable the loading of beams to the antenna and modes to theSE. A top-level diagram of the payload architecture is given inFigure 3 and shows the primary data paths.

Macrocommands are executed by the SE and are used forsetting and controlling SE parameters and functions. For everyimage, the following MCMDs are the primary command subsetthat is used to define timing and hardware parameters and areexecuted in advance of the imaging time: (i) Set Transmit (Tx)Pulse (sets up the transmit pulse samples, duration, and pulsestitching as required), (ii) Set Swath (sets up filter bandwidth,sample rate, BAQ compression, pulse repetition interval,sample window length, etc.), (iii) Set Sample Window StartTime (SWST) (sets SWST values and their application times),and (iv) Set Operate Sequence (sets up the ECC code to beexecuted by an operate MCMD). The parameters set in theseMCMDs are stored in a database within the SE according totheir Swath Data Set Number (SDSN). Together theseparameters define the beam and mode parameters known to the

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Figure 2. SAR payload block diagram.

SE, such as bandwidth (Set Swath), Sample Window StartTime (SWST) and variations (set SWST) parameters, and thesequence of events (Set Operate Sequence).

The operate MCMD is then used to control the antenna forimaging by referencing the appropriate parameters with anSDSN. The various modes are defined through the ECC, whichis embedded in the “Set Operate Sequence” MCMD. Someparameters, such as polarization, which are stored within the setoperate MCMD ECC code, are also used to control componentswithin the antenna, through the Setting Selector Bus (SSB).

The ECC is the primary means by which patterns of radarevents are created. The ECC is extremely flexible to allow forall the modes described here and to support a wide variety ofexperimental modes. An ECC program consists of severalEntries (up to 2000) and Instructions (up to 1000) storedtogether in an ECC program. Up to 32 ECC programs can bedefined at any one time and are created using the Set OperateSequence MCMD. The ECC allows modes to be built up withsequences of pulse repetition interval (PRI) types. The PRItypes available are shown in Table 2.

The ECC code generally consists of a lead-in sequence, animaging sequence that lasts for the duration of the image, and astopping condition to halt the ECC program when the imagingis complete. The lead-in sequence of events configures theantenna to support a particular mode. It is here that the antennais configured as a single or dual aperture. The transmit andreceive beams are always selected at the start of the loop, butmay be changed repeatedly within the loop so as to supportrepetitive beam changes such as those found within the

ScanSAR modes. When imaging, regular replicas of thetransmitted beam are taken.

In a simple standard beam mode, the imaging loop consistsof one Tx + Cal + Echo (transmit + calibration + echo) PRIfollowed by seven Tx + Echo PRIs. The loop repeats for aduration equal to the image length (in PRIs) divided by 8(PRIs). The stop instruction is sent when the loop is finished.

An example of a single beam image is given in Figure 4,which shows that the entire image consists of a sequence ofevents, each associated with one or more PRIs. To start theantenna a “Go Image” command is sent through the SSB bus tothe antenna. The beam type and overall receive window are setup, with a delay to allow the antenna to load up the TRMs withthe required phases and amplitudes to effect that beam, andthen the imaging loop is executed and controlled with a repeat-until structure.

In the case of a four-beam ScanSAR mode, the imaging loopconsists of an outer repetitive loop containing a sub-sequenceof images, each dwelling on a particular beam for a number ofPRIs. In the case of ScanSAR, the ECC allows for thegeneration or suppression of receive echo activities in the gapsbetween beams, simply by changing the type of PRI generatedat these times.

Alternating up/down slope pulse Quad-Polarized modes aresupported by the inclusion of “Dual Tx” PRI types that allow asingle Tx pulse to be set up, with the first half pulse containinga linear frequency-modulated (FM) up slope and the secondhalf containing the down-slope linear FM pulse. This allows thegeneration of FM chirps with up slopes on the vertically

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Figure 3. Payload top level architecture.

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No. PRI type Activity during PRI

0 Tx + Echo Pulse transmitted and echo received1 Noise No pulse transmitted; noise received2 TX + uncompressed Echo Echo contains non-BAQ (8-bit) data3 — Not used4 — Not used5 Tx only Pulse transmitted; Rx enabled but no sampling6 Echo only No pulse transmitted; echo received7 Idle No Tx; no Rx; no Cal8 Cal + Echo Pulse transmitted, echo received plus Cal1 measurement9 Dual Tx + Echo Two PRIs long; first half of Tx pulse transmitted during first PRI;

second half of Tx pulse transmitted during second PRI; two echos10 Dual Tx + uncompressed Echo Two PRIs long; as with No. 9; non-BAQ data11 Dual Tx – no Rx As with No. 9, but no Rx window12 Dual Cal + Echo As with No. 9; replica data for first half and second half13 Uncompressed Echo only As No. 6, with data uncompressed; echo contains non-BAQ data

Table 2. Pulse repetition interval (PRI) types.

Figure 4. Single beam ECC program.

polarized transmissions and FM pulses with down slope on thehorizontally polarized transmissions, switching back and forthon a pulse-to-pulse basis.

Special modes

Ultrafine imaging mode

Ultrafine mode is designed to provide approximately 3 m ×3 m resolution images covering swaths of 20 km at incidenceangles from 30° to beyond 40°.

The mode of operation defined to achieve the 3 m resolutionwith the RADARSAT-2 system is referred to as dual-receivemode and involves the simultaneous reception of all pulsereturns by separate apertures in each wing of the antenna.

A pulse bandwidth of 100 MHz is used to achieve therequired resolution in range. This wide bandwidth isaccomplished with “pulse stitching”, a technique whereby two50 MHz chirps are “stitched” together by repeating the chirptwice and switching their carrier by 50 MHz at the start of thesecond pulse.

In the dual-receive mode of operation, two echoes, one fromeach wing, are recorded for every pulse transmitted. Becausethe phase centres of the two receive antennas are in differentpositions, the two-way path lengths for the two simultaneousreturns are different and so the two returns effectively provideseparate samples along the synthetic aperture. With anappropriate choice of pulse repetition frequency (PRF), knownas the “ideal PRF”, the resulting samples will be equally spacedand standard SAR processing techniques can be employed. The

azimuth ambiguity ratio is minimized if the PRF is set at orvery near to an ideal value equal to the satellite velocity dividedby the distance between the receive aperture phase centres. Ifthe PRF deviates from this ideal value, the azimuth ambiguitylevel rises sharply. To achieve greater flexibility in the choiceof Ultrafine PRF, the number of active columns is limited in thereceive apertures to adjust the positions of the phase centres.

To provide the Doppler bandwidth required to achieve 3 mresolution, an azimuth beam width of approximately 0.5° isneeded. This is achieved by using a combination of adefocussed beam on transmit and the shorter aperture providedby a single wing on receive. The defocussing of the Tx beam isaccomplished by applying an approximately quadratic phaseacross the transmit beam, making the wavefront appear to havebeen generated by a smaller aperture behind the actual aperture.Since the TRMs are quantized in phase, this phase taper is astepwise approximation, and the resultant beam shape is similarto that of two equal subapertures, each steered very slightlyaway from broadside.

To improve sensitivity, the TRMs can be operated at highertransmit power than in the other modes.

Figure 5 illustrates the dual-receive mode of operation. Thesolid lines indicate one pulse transmission with two echoes; thebroken lines indicate the pulse and echoes from the next pulserepetition interval.

Quad-polarization modes

The Quad-polarization modes provide fully polarimetricdata. This means that complex images with each of the fourcombinations of H and V on transmit and H and V on receiveare provided (HH, HV, VH, and VV). From the compleximages, the relative phase between these channels will beknown. The simultaneous availability of H and V on receive isachieved using TRMs and receive chain capable of receivingboth polarizations at the same time. The availability of both Hand V on transmit is obtained by transmitting H- and V-polarized pulses alternately. This requires an overall PRF that isapproximately twice as high as that for the RADARSAT-1heritage modes.

Since cross-polarization returns are generally lower thancopolarization returns, it is important to optimize the noise-equivalent sigma zero (backscattering coefficient σ0) in theQuad-polarization modes. This is done by using focussedbeams and relatively narrow swaths.

Ground moving target indicator (GMTI) capability

RADARSAT-2 includes an experimental ground movingtarget indicator (GMTI) capability known as the MovingObject Detection Experiment (MODEX). This experimentalmode allows a wide variety of operating modes and parametersto be tried. In one version of the experiment, similar toUltrafine mode, MODEX makes use of the dual-receivecapability of the RADARSAT-2 antenna. This dual-receivecapability provides two apertures aligned in the along-trackdirection, which is suitable for detecting moving objects. By

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Figure 5. Dual-receive mode operation. Each pulse is received withtwo apertures on the two antenna wings. If one pulse is transmittedeach time the satellite moves by a distance equal to the separationof receive phase centres, the set of receive pulses is equivalent toconventional operation with a doubled PRF, referred to as the “idealPRF”.

processing the received echo data using along-trackinterferometric and DPCA techniques, objects with nonzeroradial velocities can be detected and their radial velocities canbe estimated.

Parameters that can be varied for MODEX include thetransmit and receive beam patterns, the columns of the antennathat are used, the PRF, the transmitted pulses, and the level ofBAQ (8, 4, 3, or 2 bits).

In addition to the dual-receive mode of operation,RADARSAT-2 will also support an alternating-transmit modewhere pulses are transmitted alternately from each wing andreceived alternately on each wing. This mode allows greaterseparation of the two-way phase centres in the along-trackdirection. To support MODEX-2 there is a requirement that theantenna provide the capability to alternate transmissionbetween the +X and –X wings on a pulse-by-pulse basis. Thismode is implemented with a combination of the ECC code andthe use of the polarization control bit (which is transmitted overthe SSB bus on a pulse-by-pulse basis). The antenna recognizesa command to enter the MODEX operating mode for all futureimaging operations, and likewise recognizes a command tocease MODEX operation. During MODEX operations, eachColumn Drive Unit (CDU) modifies its response to the V–Hpolarization signal from the antenna interface unit (AIU) toeither transmit or not transmit, depending on the programmingin each particular CDU (e.g., one wing would transmit on the Vsignal, and the other wing would transmit only on the H signal).From an ECC point of view it is much like operating theantenna for a quad-polarization image, but with the Ultrafineconfiguration selected.

ConclusionThe advanced hardware and software technologies

implemented in the RADARSAT-2 mission have provided thedegrees of freedom that enable the generation of a much widerrange of data products than any other preceding civiliansatellite SAR. The functionality of the radar and the mechanismwhereby the radar events are determined to implement themodes have been described. The ECC that sequences the radarevents is shown to be a flexible command mechanism, whichallows a common structure for the implementation of allmodes, including the imaging, diagnostic, and experimentalmodes. For updated information on RADARSAT-2, refer to theRADARSAT-2 Web site at http://www.mda.ca/radarsat-2.

ReferencesLuscombe, A.P., and Thompson, A. 2001. RADARSAT-2 calibration:

proposed targets and techniques. In IGARSS‘01, Proceedings of theInternational Geoscience and Remote Sensing Symposium, July 2001,Sydney, Australia. IEEE, New York.

Thompson, A., and Livingstone, C. 2000. Moving target performance forRADARSAT-2. In IGARSS’00, Proceedings of the InternationalGeoscience and Remote Sensing Symposium, 24–28 July 2001, Honolulu,Hawaii. IEEE, New York.

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