X-SAR) missions

ELSEVIER

Overview of the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) Missions Diane L. Evans,* Jeffrey j. Plaut,* and Ellen R. Stofan* T h e Spacebome Imaging Radar-C/X-band Synthetic Apemlre Radar (SIII-C/X-SAII), the most advanced imaging radar system to have flown in Earth orbit, was cartied in the cargo bay of the Space Shuttle Endeavour in April and October 1994. SIR-C/X-SAR simultaneously r~'corded data at three wavelengths (L-, C-, and X-bands; 23.5 cm, .5.8 cm, and 3.1 cm, respectively). In addition, the fidl polarinwtric scattering matrix was obtained at Land C-band over a variety of terrain and vegetation types. Scientists are using multifrequency, polarimetric SIR-C/X-SAIl data in studies of geology, hydrology, ecology, oceanography, and radar remote sensing techniques. The October SIR-C/X-SAII flight also included acquisition of experimental repeat-pass interferometr~j data which hat~e been used to generate digital elevation models and to detect surface nu)tions in volcanic, tectonic, and glacial terrains. Results' from SIII-C/X-SAR clearly show the increased value ( ( using multiparameter and interferonwtric capabilities to characterize Earth's' surface and vegetation cover and to generate geophysical products compared with optical sensor,s" or single-channel raclans' aboe. © Elsevier Science Inc., 1997 INTRODUCTION The Spaceborne hnaging Radar-C/X-band Synthetic Aperture Radar (SIll-C/X-SAIl) is the most advanced imaging radar system to have flown in Earth orbit. Carried in the cargo bay of the Space Shuttle Endeavour in April and October [994, SIll-C/X-SAIl simultaneously re°Jet Propulsion Laboratory, California Institute of Technology,', Pasadena Address correspondenceto Diane L. Evans, Jet Propulsion Lab,, California Inst. of" Technolo~'. 480t) Oak Grove Dr., Pasadena, CA 91109. Receiccd 29 March 1996; revised 14 June 1996. REMOTE SENS, ENVIRON. 59:135-140 (1997) ©Elsevier Science Inc., 1997 655 Avemlcof the Americas,New York, NY 10010

corded SAt/ data at three wavelengths (L-, C-, and X-bands; 24 cm, 6 era, and 3 cm, respectively). In addition, the fidl polarimetric scattering matrix was obtained by the SIII-C instalment at L- and C-band over a varieb, of terrain and vegetation types. The integrated system is steerable in look angle (electronically ill the case of Sill-C, mechanically in the case of X-SAIl) to obtain data in the angular range of 15-60 °. hnaging resolution varies from about 10 m to 50 m, depending on the geometry and data taking configuration. Over the two flights, a total of 143 h, (93 terabits) of SAIl data were digitally recorded on tape for subsequent processing ill the United States, Germany, and Italy (e.g., Evans et al., 1994a: Stof:an et al., 1995; Jordan et al., 1995). SIll-C/X-SAIl is a cooperative experiment between the National Aeronautics and Space Administration (NASA), the German space agency, l)eutsche Agentur far Ilamnfahrtangelegenheiten (DAIlA), and the Italian Space Agency, Agenzia Spaziale ltaliana (ASI). SIII-C was developed by NASA's Jet Propulsion Laboratory. X-SAt/was developed by the Dornier and Alenia Spazio companies, with the Deutsche Forschungsanstalt fiir Lui} und Ilaumfahrt (DLIl), the major partner in science, operations, and data processing. The experiment provides an evolutionary step in NASA's Spaeeborne hnaging Radar (SIR) program that hegan with the Seasat SAIl in 1978, and continued x~th SIR-A in 1981 and SIII-B in 1984. It also represents a continuation of Germany's imaging radar program which started with the Microwave Remote Sensing Experiment (MIlSE) flown aboard the Shuttle on the first SPACELAB mission in 1983 (e.g., Evans et al., 199:3). Data from the April and October SIll-C/X-SAIl flights provide a synoptic view of changes on a short temporal scale and a baseline from which longer-term changes can be assessed in the filture. High qnali~' data

0034-4257/97/$17.00 PI1 S0034-4257(96)00152-6

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Shuttle hand-held photo taken by astronaut crewmember (left) and coincident SIRC image of volcanic eruption of Kliuchevskoi volcano on the Kamchatka peninsula which began 30 September 1994. The image shows an area approximately 30 kmX60 km that is centered at 56.18° N latitude and 160.78° E longitude, North is toward the top of the image. Red= L-band, horizontally transmitted and received (LHH); green= L-band, horizontally' transmitted and vertically received (LHV); blue=C-band, horizontally transmitted and vertically received (CHV).

Figure i.

were acquired over all planned targets, and in addition, the mission afforded unique opportunities to observe flooding in Illinois and Germany, three different views of tropical cyclone Odille, and a significant volcanic eruption of Kliuehevskoi volcano on the Kamchatka peninsula (Fig. 1). SIR-C data were downlinked, processed, and released via Interuet within 24 h. after launch. X-SAR data were processed in survey mode and displayed in real time (Evans et ',d., 1994a; Stofan et al., 1995). SUMMARY OF R E S U L T S

The SIR-C/X-SAR Science Team consisting of 52 investigator teams from more than a dozen countries are using SIR-C/X-SAR data in studies of ecology, hydrology, geology, and oceanography. Other investigations are focused on topics in SAR calibration and eleetrornagnetic theo,T. Interferometric data from SIR-C/X-SAR are also being used fbr topographic mapping and surface change monitoring connected with natural hazards. In addition, SIRC/X-SAR measurements of" rain storms demonstrated for the first time, the capability of a multifrequency, multipolarization spaceborue radar system to quantify precipitation rates and to classify rain type (Jameson et al., 1996). Since the flights in 1994, hundreds of additional investigators have accessed SIR-C/X-SAR data for studies as diverse as monitoring habitats of endangered species, archeologg~ and cultural resource management, and landuse mapping (e.g., Evans et al., 1994b) indicating that new findings and discoveries can be expected from this data set tbr many years to come. Kasischke et al. (1996) provide a general overview

of use of imaging radars fbr ecological applications. The p r i m a l applications of multiparameter SAR data such as those obtained by SIR-C/X-SAR to problems in ecology, are: land-cover classification and forest growth estimates (e.g., Dobson et al., 1995; Ilanson et ~d., 1995; Ilanson and Sun, 1996; Saatchi et al., 1996; Rignot et al., 1996a), biomass estimation (e.g., Dobson et al., 1995; Ranson et al., 1995; Harrell et al., 1996), and mapping of wetland inundation (e.g., Hess et al., 199,5; Pope et al., 1996). SIII-C/X-SAR data have been used in sew;ral hydrology studies to improve our understanding of tile role of water in the global climate and for management of water. Examples include snow wetness mapping (Shi and Dozier, 1995), soil moisture estimation (Dubois et al., 199,5; Wang et al., 1996), and flood monitoring and damage assessment (Izenberg et al., 1996). In each of these studies, multiparameter SAt/ data were shown to improve characterization of Earth's surface or vegetation cover, or to generate geophysical products compared with optical sensors or single-channel SAR sensors alone. SIR-C/X-SAR data over geologic targets are being used to address a broad range of topics, including volcanic terrain mapping and monitoring (Mouginis-Mark, 199,5; MacKay and Mouginis-Mark, 1996)_ aeolian processes (Greeley and Blumberg, 1995), studies of elimate change (Farr and Chadwick, 1996; Forster et al., 1996), and lithologic and structural ,napping of exposed (Kruse, 1996) and subsurface features (e.g., Abdelsalam et al., 1996; Stern and Abdelsalam, 1996; Dabbagh et al., 1996; Sehaber et al., 1996). Finally, SIll-C/X-SAIl data are being used fbr studying a broad range of oceanographic phenomena, including internal waves, oil slicks, fronts,

Overview of Spaceborne Ima~in~ Radar Missions 137

!

Figure 2. SIR-C image of an offshore drilling field about 150 km west of Bombay, India showing oil slicks (dark streaks), internal waves (upper center), and ocean swell (blue areas adjacent to internal waves). Drilling platforms appear as bright white spots. Red=L-band, vertically transmitted, vertically received (LVV) polarization, green=average of L-band (VV) and C-band (W), and blue=C-band (\.A:). eddies, and currents (Fig, 2) (e.g., Stofan, et al., 1995; Monaldo and Beal, 1995). During the October 1994 flight of SIR-C/X-SAR, over a million square kilometers of repeat-pass SAR interferometry data were obtained. Several orbits were designed to closely duplicate orbits from the first flight, giving a time separation of approximately 6 months. Most observations during the last four days of the second flight were devoted to i-day repeat-pass data takes, over several dozen targets around the world. The Shuttle navigation was extremely accurate, and most interferometrie pairs were acquired with baseline separations of no more than a few hundred meters. Formation of interferometrie fringes was accomplished tbr all three wavelengths. Although repeat-pass interferometry is probably not the ideal method for generation of digital elevation models

(DEMs), the SIR-C/X-SAR interferometry data set has produced good results for many sites (e.g., Moreira et al., 1995; Goldstein, 1995). A DEM was generated from SIR-C L-band interferometric data that extends from the California-Oregon border to the Mexican border (Fig. 3). The statistical uncertainty of the DEMs heights for this traverse is as good as 1-2 m and typically ranges from 5-15 m rms (Rosen et al., 1995). Repeat-pass interferometry data have also been used in studies of Earth-surface change. Zebker et al. (1996) used decorrelation measurements among one-day repeat passes to track advancing lava flows at Kilauea volcano on the Big Island of Hawaii and to estimate, rates of lava extrusion. Rosen et al. (1996) analyzed a 6-month repeatpass pair in an effort to quantify deformation of the Kilauea edifice. Rapidly advancing glaciers in the Northern Patagonian lcefield, Chile were observed with 1-day repeat-pass data (Rignot et al., 1996b,c). Maps of ice motion along the radar line-of-sight and maps of glacier topography were derived simultaneously. In addition to collecting the first multifrequency and multipolarimetric data from space, thereby allowing scientists to explore the planet in a way that has never befbre been possible, SIR-C/X-SAR represents several other engineering "firsts." In addition to selectable transmit bandwidths of 10 MHz, '20 MHz, and 40 MHz, SIRC also has seleetable pulse lengths of 8.5 #s, 17 #s, or 33 #s to increase signal-to-noise ratio over low-baekscatter targets. The SIR-C L- and C-band active phased array antennas 'also provide the capabili b, to steer and shape the antenna pattern to more optimally illuminate the ground track, and enable operation in modes known as SCANSAR and SPOLIGHT (Jordan et al., 1995). For SCANSAR, the antenna pattern coverage on the gromld is stepped across traek during the synthetic aperture period to allow coverage over a wider swath. This mode extends the swath to a width of 225 km, but at a reduced azimuth resolution of 100 m (Chang et al., 1996). For SPOTLIGHT, the boresight is positioned in azimuth to dwell on an area as the system {ties by. This mode allows a higher resolution in azimuth (as good as 6 m at L-band and 3 m at C-band) for the selected area, at the expense of the along-track swath. FUTURE OPPORTUNITIES

The suite of spaeeborne SAR systems and programs currently flying and em4sioned by the international torontonit,/provide an important framework for addressing key science issues and applications. However, additional interferometric and multiparameter measurement capabilities are required for long-term environment monitoring and commercial applications (e.g., Evans et al., 1995; Dixon, 1995; Winokur et al., 1995). Based on these emerging requirements, the SIR-C/X-SAR hardware will be modified by adding a 60 m boom for single-pass inter-

138 Evans et al.

3. DEM generated fiom SIB-C L-band interferometric data extending from the CaliforniaOregon border to the Mexican border. The statistical uncertain~ of the heights for this traverse is typic'ally 5-15 m rms (I/osen et al., 1995) Color depicts topographic height, ranging from deep blue at -100 m relative to a reference surt:ace to white at 3800 m. Figure

ferometry (Fig. 4) and will be flown in the 1998-2000 timeframe to generate a global topographic data base. This mission will provide an unprecedented digital topographic map of the world equator-ward of 60 ° latitude, which will serve as the reference data for future higher resolution topographic and topographic change studies. In a single l 1-day Shuttle flight a digital topographic map of 80% of Earth's land surface will be produced with data points spaced every 30 m and with 8 m relative vertical accuracy. Data sufficient to produce a rectified, terrain-corrected C-band (5.6 cm wavelength) mosaic at 30 m resolution and a limited amount of L-band data will also be acquired. A "LightSAtt" concept is also being considered which would make use of advances in materials science Figure 4. Artist's conception of SIt/-C/X-SAR hardware for

single pass interferometry hy adding additional antennae on a 60 m booin.

and hybrid structures (low mass, low launch volume antenna) and microelectronics (low power) to design a lower cost SAt/. New inflatable, deployable antenna technology is also being investigated to further reduce mass and size (pre-launeh), and hence reduce mission costs. Low-cost X-band distributed ground terminal stations would also be wdidated as part of this mission. Demonstrably lower SAIl mission costs will enable a varieb: ()i" opportunities for scientific, operational, and eommercM systems in the future.

CONCLUSIONS

SAt/ data provide unique information about the health of" the planet and its biodiversity, as well as critical data tbr natural hazards and resource assessment. Interfbrometric measureinent capabilities uniquely provided by SAIl are required to generate global topographic maps, to monitor surface topographic change, and to monitor glacier ice veloci~" and ocean lbatures. Multiparameter SAt/data have been shown to improve land cover classifications, measurements of above-ground woody plant biomass, delineation of wetland inundation, and measurements of snow and soil moisture over optieal sensors or single-channel SAR sensors alone. Although the suite of spaceborne SAR systems currently fl~ng and envisioned by the international community provide an importaut framework for addressing key science issues and applications, additional interferometric and multiparameter measurement capabilities are required tbr long-term environmental monitoring and commercial applications.

Ovarview of Spaceborna I,utginJ, Radar Missions

This work was pe@mnwd by the Jet Propulsion Laboratonj, California Institute of Technology, under contract from the National Aeronautics and Space Administration. Additional i~)rmarion about imaging radar, sample data sets and softtcare to display them are available on the World Wide Web, at URL: http :/Zsouthport jpl. nasa.gov. REFERENCES Abdelsalam, M. G., and Stern, R. J. (19961, Mapping Preeambrian structures in the Sahara Desert with SIR-C/X-SAB radar: the Neoproterozoie Keraf zone, NE Sudan. J. Geophys. Bes. Planets" 101(E10):23,063-23,076. Chang, C. Y., Jin, M. Y., Lou, Y., and Holt, B. (1996), First SIR-C SCANSAR results. IEEE Trans. Ceosci. Remote Sans. 34(51:1278-1281. Dabbagh, A., M-Hinai, K., and Kban, A. (19961, Detection of sand covered geologic features in the Arabian Pennisula using SIR-C/X-SAIl data. Remote Sens. Environ. 59: 375-382. Dixon, T. II. (1994), SAIl interferomet~ and surface change detection: workshop held, Eos. Trans. Am. Geophys. Union 75:269-270. Dixon, T. t t (19951, SAR interferomet~, and surface change detection, Report of a Workshop held in Colorado, BSMAS Technical Report TR 95-003, University of Miami t/osenstiel School of Marine and Atmospheric Sciences. Dobson, M. C., Ulaby, F. T., Pierce, L. E., et al. (19951, Estimation of {brest biophysical characteristics in Northern Michigan ~4th SIR-C/X-SAIl. IEEE Trans. Geosci. Remote Sans. 33(4):877-895. l)ubois, P. (k, van Zyl, J., and Engman, T. (19951, Measuring soil moisture with i)nagmg radars. IEEE Trans. Geosci. Remote Sans. 33(41:914-926. Evans, D. L., Elachi, C., StoLm, E. R., et al. (19931, The Shuttle Imaging Radar-C and X-Band S511thetie Aperture Radar (SIR-C/X-SAIl) mission, Eos Trans. AGU 74(13): 145-15h. Evans~ 1). 1~., Stot~nl, E. B,., Farr, T., et al. (1994a), Mission employs synthetic aperture radar to study global environment. t':os Trans. AGU 75(36):415-420. Evans, I). I,, Stofan, E. R., Jones, T. D., and Godwin, L. M. (1994b!, Earth from slg'. Sci. Ant. 271(61: 7(1-75. Evans, I)_ Kasischke, E., Melaek, J,, et al. (1995), Spaceborne s)nthetic aperture radar: current status and future directions, NASA Technical Memorandum 4679, 171 pp. Farr, T. (;.. and Chadwick, O. (1996), Geomorpbic processes and remote sensing signatures o{' alluvial fans in the Kun lain Mountains, China. j, Gaophys. Res, Planets 101(El0): 23,091 23,100. Forstcr, Il. (19961, Spaceborne imaging radar reveals South Patagonian icefield responses to seasonal and synoptic weather changes..]. Gaophys. Bes. Planets 101(El0): 23,169 23,180. Goldstein, It. (19951, Atmospheric limitations to repeat-track radar interferometrv. Gaophys. Res. Lett. 22:2517-2520. Greele',, It., aim Blund)erg, 1). G. (1995), Preliminai T analysis ot: Shuttle Radar Laborato)T (SRL-I) data to study aeolian features and processes. IEEE Trans. Geosci. Remote Sans. 33(4):927-933.

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Harrell, P., Kasisehke, E., Bourgeau-Chavez, L., Haney, E., and Christensen, N. (19961, Ev~duation of approaehes to estimate aboveground biomass in southern pine forests using SIII-C data. Renmte Sans. Ent:iron, 59:22:3-233. ttess, L. L., Melack, J. M. Filoso, S., and \Van~,, Y. (19951, Delineation of immdated area and vegetation along the Amazon floodplain with the SIR-C Synthetic Aperture Radar. IEEE Trans, Geosci. tlenlote Sans. 33(41:8917~904. Izenberg, N. R., Arvidson, R. E., Brackett, R. A., Saatehi, S. S., Osb(lrn, G. If., and Dohrenwend, j. (1996), Damage assessment from the 199:3 Missouri River floods usillg LANDSAT, SPOT, SIIl-C and TOPSAR data. J. Geophys, Ras. Planets, in press. Jameson, A., Li, F., Durden, S., etal. (19961, SIR-C/X-SAIl Observations of rail/ storms. Nmu~ta Sans. Environ. 59(2): 267-279. Jordan, t/, L., Huneycntt, B. L,, and Werner, M. (19951, The SIII-C/X-SAR syaathetic aperture radar system. IEEE Trans. Gaosci. Remote Sans. 33(4):829-839. Kasischke, E. Melaek, J., and Dobson, C. (19961, The use of imaging radars for ecological applications -a review. Bemola Sans. Environ, 59:141-156. Kruse, F. A. (19961, Geologic mapping using combined Landsat TM, Thermal Infrared Mulitspeetral Scanner (TIMS), alnt SIR-C/X-SAR data, IEEE Trans. Gaosai. tlamota Sens. in press. MacKay, M., and Mouginis-Mark, P. (19961, The effect of wuTing acquisition parameters on the interpretation of SIR(; radar data: The Virunga volcanic chain, l{amola Sans. Era:item. 59::321-336. Monaldo, F. M., and Beal, R. (2. (19951 Ileal-time observations of southern ocean wave fields tbrm the Shuttle hnaging Radar. IEEE Trans. Gaosci. ttemote &'ns, :3:3(4):942-949. Moreira, j., Schwabisch, M., Fornaro, (;., Lanari, 1t, Bander, R., and Just, 1). (1995), X-SAR interlbrometrv: first results. IEEE Trans, Geo.s'ei. lte,u~te Sans. 33(4):950-956. Mouginis-Mark, P. J. (19951, Preliminarx obse~x:ations of vok:anoes with the SIR-C radar. IEEE Trans. Geosci. Remote Sans. :33(41:934-941. Pope, K., Bejmankova, E., Paris, J., and Woodruff, 1t. (19961, Monitoring seasonal flooding cycles in marshes of the Yucatan Peninsula with SIR-C polarimetric radar imagery. Ramola Sans. Encimm. 59:157-166. Ilanson, J., Saatchi, S., and Sun, G. (19951, Boreal fi)rest ecosystem characterization with SIR-C/X-SAt/. IEEE Trans. Gaosci. Remote Sens. 33(4):867-876. Ila)lson, K. J., and Sun, G. (19961. An exaluation of AIIISAB and SIR-C/X-SAt/ images of northern Iorest attributes in Maine, USA. Remote Sans. Environ. 59:203 222. Rignot, E., Salas, VC A., and Skolc, I). L. (1996a), 1)eforestation and secondaD' growth in Ilondonia, l!;razil from SIBC SAIl and Landsat/SPOT data. Remote Sensin(d of b'ntqronment 59:167-169. Rignot, E., Forster, R., and lsaeks, B. (1996|)), hlterferometrk: radar observations of (;lacier San Rafitel, Chile. J. Glaciol. 42(141 ):279-291. Ilignot, E., Forster, If., and Isacks, B. (1996c), Mapping of glacial motion and surface topography of t lielo Patagonieo Notre, Chile, using satellite SAt/ L-band interferometr~ data. Ami. Glaciol. 23: in press. Ilosen, P., Henslev, Lou, Y., Shaffer, S., and Fielding, E.

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(1995), A digital elevation model from Oregon to Mexico derived from SIR-C radar interferometry. 1995 Fall meeting. Eos Trans. AGU 76(46):64. Rosen, P., Hensley, S., Zebker, H., Webb, F., and Fielding, E. (1996), Surface deformation and coherence measurements of Kilauea Volcano, Hawaii from SIR-C radar interferometry. ], Geophgs. Res. Planets 101(E10):23,109-23,126. Saatchi, S,, Soares, J., and Alves, S. (1996), Mapping deforestation and landuse in Amazon raintbrest using SIR-C Imagery. Re~u)te Sens. Environ. 59:191-202. Schaber, G., McCauley, J., and Breed, C. (1996), The use of multiwavelength and polarimetrie SIR-C/X-SAR data in geologic studies of Bir Safsaf, Egypt. Remote Sens. Environ. 59:337-363. Shi, J., and Dozier, J. (1995), Inferring snow wetness using C-band data from SIR-C's polarimetric synthetic aperture radar. IEEE Trans. Geosci. Remote Sens. 33(4):905-914.

Stern, t/. J. and Abdelsalam, M. G. (1996), Tile origin of the Great Bend of the Nile from SIR-C/X-SAR imageD~. Science 274(5293):1696-1698. Sto{)n, E. R., Evans, D. L. Schmullius, C., et al. (1995), Overview of results of Spacebome Imaging Radar-(;, X-Band Synthetic Aperture Radar (SIR-C/X-SAR). IEEE Trans. Geosci. Remote Sens. 33(4):817-828. Wang, J., Hsu, A., Shi, J. C., O'Neil, P., and Engman, T. (1996), Estimating surt~aee soil moisture from SIR-C measurements over the Little Washita River Watershed. Remote Sens. Environ. 59:308-320. Winokur, R. S. (1995), Operational use of civil space-based Synthetic Aperture Radar (SAR), Interim Report of the Interageney Ad Hoe Working Group, 34 pp. Zebker, H. A., Rosen, P., Hensley, S., and Mouginis-Mark, P. (1996), Analysis of active lava flows on Kilauea Volcano, Hawaii, using SIR-C radar correlation measurements. Geolo(,y 24:495-498.