4--Marine Science and Applications

Synthetic aperture radar images of the ocean surface very often reveal a remarkable range of signatures inscribed on the uppermost layers of the sea. These telltale variations in surface roughness can be interpreted in terms of geophysical and geochemical processes occurring on or near the surface of the sea. When used collectively with (a) adequate theories of radar surface scattering, and (b) an understanding of the underlying hydrospheric, atmospheric or cryospheric dynamics, these images offer unparalleled and indeed otherwise unattainable quantitative data on the marine environment, data that are relevant to important questions in Earth science and marine operations.

This chapter presents a summary of scientific achievements in ocean and ice research, and lays out the potential for further advances based on research to date. It develops a rationale for a vigorous, continuing program that is firmly grounded in past achievements and in new, interesting, and relevant developments in SAR technology, hydrodynamics and scattering theory. The chapter develops the scientific basis for an ongoing NASA SAR program during the next decade, and sets forth a prioritized sequence of space flights with advanced instruments, along with a program of supporting research utilizing data from space-based and aircraft-based SARs. The program keeps governmental agency requirements for space-based data firmly in view.

Synthetic aperture radar was first flown on Seasat; launched in 1978, it carried among other sensors an L-band SAR that yielded stunning day/night, all-weather views of the Earth, its oceans and its ice cover. This mission set into motion a number of follow-on spaceflight programs by the former USSR, the European Space Agency, Japan, and Canada, as well as secondary efforts in airborne SAR by several other nations; in addition, the US has flown short, widely spaced flights of Shuttle Imaging Radars A, B, and C in addition to airborne systems.

These data have resulted in a series of quantitative scientific findings and theoretical advances in upper-ocean and lower atmosphere dynamics and in polar ice studies. Many papers have been published in the scientific and engineering literature, references to the most seminal of which are cited here as examples of the progress. New findings that derive from the ongoing space-based SARs continue to be reported, especially from ERS-1, which provides unparalleled global, continuing data coverage.

The strong view of the scientific community working on these problems is that current single-channel, space-based SARs are yielding only a fraction of the information available from more advanced multifrequency, multipolarization, and multi-aperture configurations. Fascinating glimpses of this have been provided by the SIR-C/X SAR, as well as from aircraft instruments, especially the AIRSAR on the NASA DC-8. An example of prime importance is the potential for measuring ocean surface current velocities on scales ranging from estuarine/coastal to the ocean basins, using the along-track interferometric mode of SAR - the ATISAR. Such a capability would represent a revolutionary advance in the understanding of ocean dynamics, and would provide the missing dimension to large-scale currents derived from radar altimeter measurements.

If properly enhanced capabilities can be realized in a spaced-based configuration, the data on wind stress, current velocities, mesoscale current distributions, sea-ice thickness, air-sea interchange, and similar phenomena entering into the coupled ocean/atmosphere/ice system will be extremely valuable in meeting objectives of Mission to Planet Earth. The data must be blended with other space and surface-derived information and assimilated into numerical models or used to understand physical and chemical processes that can be parametrically incorporated into such models.

Scientific Objectives

A national SAR program for oceans and ice can be formulated considering the concerns of present-day ocean and atmospheric science, the history of past achievements in SAR ocean observations, the state of development of ocean backscatter and hydrodynamic theory, and the recent instrument advances in SAR engineering. The prime objective of a SAR program for the ocean is the determination of surface currents for information on mesoscale phenomena as related to climate modeling, surfactant dispersal and transport, coastal dynamics, marine biology, and lateral fluxes at the ice margin. As will be discussed below, this capability of surface-current determination is still the subject of intense development and experimentation. Secondary objectives for SAR applications include improved surface fluxes of ice-covered seas, determination of fine-scale wind fields (especially in the coastal regime), surveillance of surface films and slicks, and monitoring of surface and internal gravity waves. In the operational community, SAR data over the coastal ocean are additionally seen as a crucially needed tool in three general areas: regulatory functions (compliance monitoring and enforcement); warning of potential threats to life, environmental quality, property and commercial operations; and post-event measurement of impact and responses. The primary applications include search and rescue, fisheries management, monitoring of coastline and bathymetry changes, and ship detection and routing. In most of these areas of science and operations, further development is called for prior to the routine generation of data products.

Observational Results

In some senses, the fractal-like surface of the sea and its overlying cover of ice are simple, especially in terms of topography, composition, and texture. Because of this, it has been possible to arrive at a reasonable understanding of several important oceanic processes, notably internal wave dynamics, surfactant properties, and wave spectra, by using SAR imagery in conjunction with in-situ data and theoretical hydrodynamics. In many cases, considerable success has been achieved; in others, provocative but incomplete results have been obtained; more work is clearly called for.

Beyond the microphysical level, however, SAR has made significant contributions to the disciplines of open-sea and coastal oceanography, marine boundary layer meteorology, and sea ice dynamics. The morphology of the patterns, together with an understanding of backscatter variations, allow one to deduce information on a wide range of oceanic and cryospheric phenomena. For example, extremely valuable data have been obtained on surface and internal waves, fronts, eddies, variations in bottom topography/bathymetry, upwellings, small-scale and mesoscale currents, rainfall variations, surface wind speed and stress, atmospheric convection, sea ice motions, marginal ice-zone processes, ice age, leads, polynyas, vessel wakes, surfactant/oil distributions, and a variety of other processes that give rise to subtle variations in surface roughness detectable by SAR. There are consequent polarization, frequency, and incidence angle sensitivities that must be considered in assessing the specific oceanographic value of SAR data.

Theoretical Developments

Theoretical approaches to SAR interpretation use quantitative analysis from electromagnetics, liquid-state physics, hydrodynamics, scattering from random media, and signal processing to interpret the imagery as two-dimensional field measurements, not simply as patterns on the surface of the sea or the ice. A few examples of theoretical developments will illustrate the advance of oceanographic SAR technology beyond simple geographical image analysis. The transmitted radar signal is characterized by its amplitude, frequency, phase, polarization, and incidence angle, and the received signal will contain information on the properties of the scattering target that are sensitive to those quantities and their modulations or variations. Initially, through the stimulus of surface wave imaging, polarimetric radar scattering theory advanced significantly; early views were that specular-point scatter governs at small incidence angles, while Bragg scatter is the mechanism of scatter at larger angles. Now more complete (but yet not adequate) theories suggest a mixture of both processes occurs. Such studies, together with observational results, suggest that SAR can observe changes in the ocean wave spectrum at the Bragg wavelength as small as a few percent. SAR imaging theory has thus helped to clarify the virtues and shortcomings of SAR as a surface wave spectrum analyzer. Detailed studies have been performed of the SAR Doppler spectrum of waves and the potential of along-track interferometry for measuring surface currents. In surface hydrodynamics, the influences of currents and their gradients on the surface waves have been advanced, as have theories for surface wave/wave interactions and surface wave/internal wave interactions. Models of nonlinear soliton dynamics have been applied to internal solitary waves in the sea with good agreement obtained between theory and data from SAR images and in-situ observations. In ice studies, the application of ice dynamics, rheological theory, and models of multicomponent dielectrics to ice detection and motion have aided the understanding of sea ice.

From the previous discussions, it is clear that SAR has significant capability for observing ocean processes. We will indicate the types of observations that can be made by dividing the discussion along the lines of: Open Ocean, Coastal Ocean and Marine Operations, Marine Boundary Layer Meteorology, and Polar Oceans, together with added comments on Applications. Examples of SAR imagery and analysis are presented to illustrate results; these images are only representative and are not necessarily major developments in and by themselves. The priorities of the NASA Mission to Planet Earth are implicitly assumed here, for which issues of Global and Regional Change are paramount. Many of these relate directly to climate change, since the ocean is a major component of the climate engine.

Open Ocean

(1) Surface Waves. The open ocean process that has been best studied by SAR are long surface waves, their spatial spectra, and their refraction patterns (Pierson, 1965). The SAR image fidelity depends on the altitude and velocity of the platform. A low-altitude spacecraft (300 km or less) such as SIR-C/X-SAR allows direct computation of wave spectra, including those traveling in the direction of spacecraft velocity (Beal et al., 1983; 1986). At altitudes of typical polar-orbiters (such as ERS-1), sophisticated corrections are needed. Wave observations in the Southern Oceans from SIR-C/X-SAR are shown in Figure 4-1, along with a comparison of their speed and direction obtained from (a) an on-board processor/spectrum analyzer, and (b) the US Navy's Wave Analysis Model (WAM) (Monaldo and Beal, 1994). Satisfactory agreement is obtained in most cases, and the remaining differences are likely more due to forecast error than to SAR errors. Long-term observations of ocean wave spectra could allow studies of exchange of momentum between atmosphere and ocean, especially during storm conditions. In addition, wave refraction by large-scale current systems such as the Gulf Stream is readily observed in high-resolution images, and provides estimates of the current speed under some conditions. In the future, a dedicated, low-drag, low-flying SAR could be implemented that would yield valuable operational information on long-wave spectra to forecasting agencies. Such a system is a candidate for a small spacecraft (MacArthur, 1987).

(2) Mesoscale Currents. At the scale of the internal Rossby radius of deformation, 10-40 km, the ocean mesoscale, important interactions between the planetary boundary layer, the ocean mixed layer and quasi-geostrophic oceanic dynamics occur. These interactions control meridional fluxes of heat, global circulation, internal variations in ocean-atmosphere, and other critical processes. The boundaries of oceanic mesoscale systems are imaged by SAR as discontinuities at the current boundaries and as streak lines within the current system. When used in conjunction with infrared imagers such as AVHRR, a synergism is realized that increases the mapping capabilities of both sensors. Mesoscale features include western and eastern boundary currents, rings, eddies, fronts, equatorial waves, upwellings, and similar features. These fluctuations, e.g., Gulf Stream rings, transport large amounts of heat and momentum, and as such are important processes in the general circulation of the ocean. They are also key links of biological ocean processes (Schumacher et al, 1991). Figure 4-2 shows multifrequency AIRSAR images of the California Current, a typical mesoscale boundary current along the eastern edge of the Pacific Ocean. The three images were made at P, L, and C-bands and show significantly different information. As in other applications, the SARs all-weather capability is a major asset in observing regions of the sea covered by persistent clouds or fog, as is often the case in this region of California. The deployment of an along-track interferometric SAR would make possible line-of-sight surface current velocities which will be exceedingly valuable in understanding these phenomena. Such measurements strongly complement and augment the radar altimeter's measurements (e.g., TOPEX/POSEIDON) of time-varying geostrophic currents by providing a quantitative determination of the surface current, not just the geostrophic portion; these current measurements would also resolve small- scale processes as they would operate at much higher resolution than the altimeter. An example is shown on the left of Figure 4-3, which illustrates two SAR current-velocity maps of the Gulf Stream made from the NASA AIRSAR operating in its along-track interferometric mode, taken during the recent ONR High-Res Experiment (Thompson et al., 1994) and color coded as to east (upper) and north (lower) components of velocity. Superimposed on these are red current vectors determined by a Doppler hf radar system on shore. A comparison of the current-velocities measured by the well-established Doppler hf system and the SAR velocities (scatter diagrams on the right side of Figure 4-3), argues that the SAR is measuring open-ocean surface currents with good fidelity, although problems remain with establishing the zeroes of Doppler shift (Thompson and Jensen, 1993). In the deep sea, much spatial and temporal averaging is possible, which would enhance the accuracy of the data. Assuming that open-ocean surface current velocities can ultimately be determined satisfactorily from space-based SAR, the combination of SAR and IR imagers, radar altimeters, and in-situ observations beneath the surface will allow the oceanic component of fluxes of heat and momentum and their variations to be well determined. Such measurements would contribute strongly to the objectives of Mission to Planet Earth.

(3) Surface Wind Speed/Stress. The SAR is actually an imaging scatterometer, and one that is radiometrically calibrated can be used to determine scalar wind speed via absolute backscatter measurements. However, such measurements are contaminated by surfactants, upwellings, etc., (as are wind-scatterometer measurements) so that ancillary information is required. Surface wind stress, waves, and surface currents are related, but in a complicated, nonlocal fashion. Low resolution and wide swath observations, similar to those for currents in the open ocean, are required for the winds, with the required resolutions being finer than the scales of frontal systems and storm gradients at sea. These would complement the coarse-scale winds (10-50 km) obtained from wind scatterometers.

(4) Open-Ocean Internal Waves. Large solitary waves are generated by flows over topographic features, generally but not exclusively near shore, and are known to propagate for hundreds of kilometers in the open ocean. They contribute unknown amounts to the global internal wave spectrum. High-resolution SAR images of such waves in the deep sea have been obtained, but are rare, and more data are needed for survey purposes (Rufenach et al., 1985). If the ATISAR is implemented in a high-resolution mode, the surface currents can be used in a model to obtain the underwater amplitude and current fields. In addition, deep bathymetric features such as mid-ocean ridges and seamounts are suspected to be sources of open-ocean signatures, including internal waves. Russian work suggests that topography as deep as 5000 m may have surface expression through formation of a "Taylor-cone;" similar features are speculated to occur in convective regions of the ocean (Carsey and Garwood, 1993)

Coastal Oceans and Marine Operations

Although coastal oceans and estuaries comprise about 10% of the Earth's surface, they produce about 25% of global biomass production. In the coming decades, the effects of global change and human activities will be especially pronounced in the coastal zone "where measurements are difficult to make and processes difficult to model because of the many more factors that influence the system, compared to the open ocean." (OSB, 1992, p.107), and cross-shelf exchange processes are poorly understood. Here the spatial scales vary from tens of meters to kilometers and temporal scales from tens of minutes to a few days (Liu et al., 1994a). Surface forcing by winds and waves drive vertical mixing throughout much of the water column, depending on season and geography, and ice can play a unique role (Liu et al., 1994b). High-resolution surface current and wind speed observations from the ATISAR would be especially valuable here; in a recent review (Flemming, 1995) operational users listed surface currents and sea surface temperature fields to be the two most valuable ocean data products of those discussed for development. Figures 4-4 (a) and 4-4 (b) respectively show a Seasat L-band SAR scene taken southeast of Nantucket, Mass., in July 1978, and a Skylab Ektachrome photograph of the same region taken several years earlier; both of these display many signatures of features that typify those in coastal oceans; the image will be used as an example for this discussion.

(1) Underwater Topography. The medium and fine-scale bathymetry of the region is mirrored in surface roughness variations caused by tidal flow and associated wave spectral changes over shallow areas. A comparison of the lighter patterns from bottom-reflected light in the photograph with the roughness variations in the SAR image demonstrates that underwater shoals having depths from perhaps 5 to 50 m are imaged with SAR via surface roughness and radar backscatter variations. There is a successful quantitative theory for this process that uses a Boltzmann-like equation to describe the changes to the surface wave spectrum that lead to the signatures.

(2) Upwelling and Air-Sea Interaction. The pumping up of colder subsurface water to the surface shows up in this image as a smoother (lower backscatter) area east of Nantucket, the "Nantucket Upwelling," well known to local fishermen and sailors. Reduced sensible and latent fluxes over the colder water because of lowered air-sea temperature differences is accompanied by reduced atmospheric turbulence levels, and thus less roughness in the regions of the cooler ocean contacting the atmosphere. In VV polarization, such a pattern would appear similar to this HH-image under stable air/sea conditions; however, under unstable conditions, the polarizations would differ significantly, thereby identifying areas of strong evaporation.

(3) Internal Waves. To the east of Nantucket is a region of quasiperiodic oscillations that are surface signatures of internal waves, or propagating underwater motions supported by density gradients. They are generated by tidal flow over Georges Bank to the east through complex nonlinear processes. Such a phenomenon is better illustrated by Figure 4-5, which is an ERS-1 C-band/VV SAR image of the ocean near the Hudson Canyon southeast of New York; the bathymetric contours are shown in red. The packets of quasiperiodic oscillations are internal solitary waves, or solitons, generated by tidal flow near the edge of the continental shelf break (the region of highest gradient of bathymetry). Each semidiurnal tidal cycle (approximately 12 h) during the spring, summer, and fall, the tidal currents flowing offshore produce a depression of the upper layers of the sea that quickly develop into nonlinear oscillations. As these solitons propagate, the larger waves travel faster than the smaller ones, and there is a sorting of the waves by amplitude and hence wavelength. Figure 4-5 presents a history of seven packets generated by the previous 3 days of tidal action. The lifetimes of the solitons are of order 1 to 2 days. When they break inshore, they resuspend bottom sediments, thus adding nutrients to the water column, greatly enhancing biological productivity, and altering the optical opacity of the water column significantly.

(4) Soloy. This Russian word describes combined upwelling and mixing caused by flow against a topographic feature. Figure 4-6 (a) is an S-band/HH image from the Russian spacecraft Almaz-1, made on July 5, 1991 off the west coast of Ireland. The soloy is occurring at an indentation in the continental shelf and is the source of internal waves seen at the upper left. Figure 4-6 (b) is a simultaneous NASA/AIRSAR image at P, L, and C-bands/VV, taken along the swath shown in Figure 4-6 (a); the lowest frame is a color-composite of the three frequencies.

(5) Coastal Wave Refraction. Surface waves in water of approximately one-half their wavelength feel the effects of the bottom and undergo refraction, focusing/ amplification, and defocusing/diminution. For example, in the New York Bight, storm waves arriving from the east impact the shorelines of New Jersey and Long Island quite differently. The wave refraction patterns must be observed during the storm in order to understand the effects correctly, and this only can be provided by SAR on aircraft or spacecraft.

(6) Shoreline Changes. Storm damage to coastlines and loss of land due to sea level rise are significant issues in a milieu of global change. The SARs ability to image through clouds and storms makes it a valuable instrument for observations during storms (Zhang et al., 1994).

(7) Coastal Watch. Repeated SAR observations on the scale of Figures 4-4 a and 4-5 will allow monitoring of coastal and estuarine waters for a variety of information. Essentially any process leading to a change in radar backscatter of a few percent can be observed under normal wind conditions. Here multiparameter SARs may be of even more importance for detecting subtle changes in the coastal environment.

(8) Surfactant Monitoring. Detection of natural and man-made slicks on the sea is crucial to the protection of the environment and the enforcement of regulations. In order to demonstrate the signatures of various types of oils and other surfactants such as oleic acids, a controlled release of oils and surfactants in the North Sea was made recently by German scientists and imaged by SIR-C/X; the slicks are readily detectable and appear as regions of variously reduced backscatter (Wahl et al., 1993; True et al., 1994; Wismann et al., 1993).

(9) Monitoring of Shipping and Search and Rescue. Ships and ship wakes are often detectable in SAR images (Griffin et al, 1992; Fu and Holt, 1992), and the line-of-sight velocity of the vessels can be estimated from their Doppler shifts. Wakes include both the invariant 39 degrees Kelvin wake, and in stratified water, occasionally an internal wave wake. Figure 4-7 is a summertime ERS-1 image of a large vessel in the Skagaarak off Sweden, and shows a long, asymmetrical internal wave wake. Monitoring of vessels in the US Exclusive Economic Zone (EEZ) via hard echoes and wake detection is a possibility, although 100% detection probability is not assured. Similarly, support of search-and-rescue operations is feasible via SAR or real-aperture radar (RAR) on aircraft.

(10) Fisheries Support. Wind-driven offshore transport and turbulent mixing have important effects on the larval survival and subsequent recruitment of many fish species (Bakun and Parish, 1982). Space-based SAR can locate current boundaries, fronts (Grabak et al., 1994), convergence zones, eddies, and ice fields, all of which influence commercially-important fish populations (Burns et al., 1981).

The addition of a current-measuring mode to a future SAR system would enlarge the suite of environmental data significantly and greatly enhance the information available for change management and regulatory enforcement as well as an increased value to science. Prediction of coastal currents via numerical models that are tuned and initialized by current measurements of all types (especially including SAR) is a capability that is highly desirable. Obtaining a two-dimensional realization of surface current velocities would add greatly to the efficacy of marine forecasts, fish stock assessment (Fiedler et al., 1984), vessel drift, search-and-rescue, and similar items requiring a knowledge of surface drifts.

Information from the combination of an advanced SAR, an ocean color scanner, and a thermal-infrared scanner will be of much relevance to science and applications in coastal waters. There, spatial scales vary from tens of meters to tens of kilometers, and, in shallow depths, the surface signatures visible in imaging devices are connected more closely with subsurface conditions than is the case in the deep ocean. Thus the SAR for coastal oceanography needs high resolution (25 m) and a narrower swath (100 km) than its open-ocean equivalent. It also needs more frequent revisit times. These requirements argue for a US receiving station in the contiguous 48 states having real-time processing and analysis capabilities for the present-day SAR systems.

Marine Boundary Layer Meteorology

In both the open and coastal oceans, various atmospheric signatures, usually due to wind stress and its gradients, are seen with varying sensitivities. It should be cautioned that these investigations are recent and are not yet fully established capabilities. The realization that SAR shows atmospheric effects is only now being appreciated and still ill-understood. The subject is an important area of investigation in the SAR program proposed ahead.

(1) Surface Wind Stress. One prospect is the quantitative measurement of surface wind stress by the SAR (see Gerling, 1986; Shuchman et al., 1994; Wackerman et al., 1994), where backscatter observations may be interpreted as with a wind scatterometer. The development of a scatterometric SAR is in its infancy but the promise clearly exists. An example of such effects is shown in Figure 4-8 (Alpers, pri. comm., 1994), which is an ERS-1 C-band image of the Straits of Messina between Italy and Sicily made late at night. One observes katabatic or drainage winds flowing down mountain valleys and out to the sea. Also visible are long, periodic signatures paralleling the coast that are thought to be eddies, internal waves, and atmospheric roll vortices of some type. These atmospheric processes are being imaged via differential roughness at the short Bragg wavelengths of the ocean surface wave spectrum. A number of similar atmospheric processes have been investigated in SAR data (Vachon, 1994; Pri. Comm., W. Alpers and G. Stilke paper submitted to J. Geophys. Res., "Observation of an undular bore in the marine atmosphere by the synthetic aperture radar aboard the ERS-1 satellite.")

(2) Atmospheric Convection. In real aperture radar (RAR) imagery taken at Ku/VV (15 GHz) frequencies, atmospheric convective cells are made manifest via the variations of surface wind stress that they cause at the surface. Figure 4-9 shows simultaneous HH (Panels 1 and 3) and VV (Panels 2 and 4) RAR images from the Russian Toros aircraft radar; vertically polarized images show atmospheric stress variations due to the convective cells, while horizontally polarized images show internal wave strain rates. Larger incidence angles favor the imaging of such features. The differences between such dual polarized images allow one to identify regions of atmospheric convection over the ocean. It may even be possible, using data from multiparameter SARs, to make estimates of the evaporative flux occurring in such unstable regions. Such measurements would be of first-order importance to atmospheric dynamics on the time scales of both weather and climate. Direct space-derived estimates of evaporative flux observations are not currently part of Mission to Planet Earth.

(3) Rain Rates. The X-band system on the SIR-C/X-SAR radar has recently observed what appear to be rainfall patterns. While not yet quantified, the promise of high-resolution measurement of tropical rain rates is clear. In studies of the ENSO (El Niño-Southern Oscillation) phenomenon and short-term climate variability, the determination of realistic rainfall rates (which are measures of the release of the latent heat of evaporation to the atmosphere) is paramount to the parameterization of numerical models of the coupled ocean-atmosphere system. Cloudiness and rain in the tropics are sub-grid processes about which much remains unknown. Their importance was reaffirmed in the recent Tropical Ocean Global Atmosphere/Coupled Ocean-Atmosphere Response Experiment (TOGA/COARE) campaign.

(4) Rain Patterns. In addition to rain rate measurements, it is known that SAR can image rainfall patterns on the sea surface via (a) damping of Bragg scatterers in the region of heavy rain, which leads to a "rain-free hole" and (b) the increased stress of atmospheric winds in the region of downdrafts and outflows from local rain storms, which shows up as a larger area of increased backscatter. It is possible that local wind speeds can also be inferred from the backscatter variations within the storm footprint. Again, these observations are of small scale but of large importance to climate studies.

The realization that SAR shows atmospheric effects is recent and still not well-understood. The area is an important one to investigate in future SAR programs.

Polar Oceans

In the polar oceans, the fluxes of heat, momentum, mass, and salt that result from air-sea-ice interactions play fundamental roles in the global climate system (Aagaard and Carmack, 1995; Maykut, 1986; Stouffer et al., 1989). The dynamics and thickness of the sea ice cover are the key elements of these fluxes. Ice motion controls the distribution of thin ice and therefore surface exchange processes dependent on ice thickness, including heat flux to the atmosphere, ice production, and the associated salinity production in the polar oceans. As ice thickens by growth and deformation, the turbulent heat exchange between the ice-covered ocean and the atmosphere is reduced. Detailed time-series measurements of ice motion and thickness from satellite sensors are essential in providing estimates of these fluxes.

(1) Fluxes. At present, heat and salt fluxes may be estimated using repeated SAR data for ice motion, radiative fluxes estimated by other means, and modeled air temperatures and winds. These approaches parameterize ice thickness changes, which are notoriously difficult to measure (Bourke and McLaren, 1992) and essentially assume that the sea is at its freezing temperature. While such an assumption is valid in the wintertime Arctic, it is known to be significantly in error in the marginal seas and in much of the Southern Ocean surrounding Antarctica. The estimation of surface fluxes and the response of the upper ocean is also important at the ice margin (Carsey and Roach, 1994)

For areas in which there are significant oceanic fluxes, a direct measurement of ice thickness over time is the optimal approach for deriving heat and salinity transport. There are good indications that ice thicknesses of less than 100 cm can be estimated using the like-channels (HH and VV) of a polarimetric L-Band SAR (see Drinkwater et al., 1992), as shown in Figure 4-10 (Winebrenner et al., 1995). Copolar phases ranging from -50 degrees to +30 degrees have been observed in ice of differing apparent thickness; such values span both lower-than- expected values for thick ice and higher-than-expected values for open water. A model has been developed that explains these variations, and it has considerable potential for robustly estimating sea ice thickness using magnitudes and phases of SAR data; cross-polarized data are not required. Field validation and time-series data are needed to diagnose both the polarimetric and the ice growth models, in order to estimate ice growth over time. This work is of great importance for accurate estimates of surface fluxes in polar regions, where intermediate and deep water-mass formation takes place. For the short-term, surface fluxes relying on repeated coverage of single-channel SAR is the only solution; in future, a polarimetric L-band SAR system to directly measure ice thickness changes is highly desirable.

(3) Ice Dynamics. Ice motion is a key variable for predicting ocean- ice fluxes. During the last several years, maps of the vector motions of Arctic sea ice have been automatically generated from SAR data by the ASF (Kwok et al., 1992; Holt et al., 1992). The ice-motion algorithm identifies the same sea ice features seen in pairs of SAR images taken three or more days apart and then derives their absolute motion relative to a fixed geographical grid. Ice motion vector maps have been made of several regions of the Arctic and Antarctic Oceans, with the densest coverage being over the Beaufort/Chukchi Seas extending over a period of greater than two years. Ice motion maps have been used to examine lead dynamics (Stern et al., 1994), oceanic circulation in the Arctic Ocean and the Weddell Sea (Kwok and Colony, 1994), and ice fluxes through the Fram Strait (Schweiger and Rothrock, 1994). The RADARSAT Geophysical Processor System (RGPS) is being developed to monitor ice motion, to determine ice age, and to infer ice growth rates and surface fluxes for the Arctic Ocean, the marginal ice zone, and its adjoining seas (Kwok et al., 1994). This technology is useful and well-developed, and should be continued and enhanced with SAR data to come.

(5) Monitoring and Prediction of Iceberg Motion. Airborne and spaceborne SAR systems are useful for monitoring icebergs, but the detectability of icebergs depends on many factors such as the SAR system characteristics (e.g., frequency, signal-to-noise ratio, incidence angle, and polarization), as well as the physical characteristics of the iceberg (i.e., size and shape) and environmental characteristics (e.g., sea state). Studies have shown that higher-frequency SAR instruments provide greater radar return from icebergs. Additionally, a cross-polarized SAR system can be used to enhance the contrast between icebergs and the oceanic background.

(6) Sea and Lake Ice Coverage. Experience with ERS-1 has demonstrated the utility of SAR for analysis and forecasting of sea and lake ice. Ice parameters obtainable from this SAR include concentration, age, location of leads and polynyas, and ridge location and density. Research into SAR imaging suggests that in the future, multipolarization/ multifrequency SAR measurements will allow acquisition of (1) improved ice-type and ice concentration data during the spring, when melt processes complicate the interpretation of simple SAR imagery (Onstott, 1992; Fetterer et al., 1994), and (2) improved discrimination between open water, new ice, and first-year ice (Steffen and Heinrichs, 1994).

From the viewpoint of either a research or an operational program, the existence of ongoing, continuous flights is essential. However, the other side of such a commitment is that technical capabilities, especially sensors, advance only slowly in such programs. Almost all of the SARs on recent spacecraft (excepting SIR and the two Russian SARs) are single polarization, single channel devices. Therefore the opportunity to perform the kinds of sophisticated analyses of SAR imagery that have been suggested above will be quite limited. Thus, while the extra-U.S. programs will provide useful, continuing data, the diversity of the channels, and hence the information derivable from them, is quite limited. Repeat data at 3 to 5-day intervals from near-term systems are required to track and monitor ocean mesoscale and ice features for operational programs such as Coastal Watch, for tracking of storms and icebergs, and for fisheries monitoring.

The analysis of SAR capabilities and ongoing programs above clearly points to an important role that NASA should play in the future--as developer and operator of advanced, multiparameter synthetic aperture radars that use the data as measurements, rather than just as backscatter maps of the oceans and sea ice. Furthermore, it will be important for both oceanic and polar science to have the sensors in high-inclination orbits, preferably near-polar, so that high-latitude processes may be observed. The orbit should be prograde so as to precess through the day/night cycle as rapidly as possible, to avoid aliasing diurnal effects into seasonal ones.

A review of the requirements given earlier suggest that the following future SAR configurations would serve a large percentage of the research and operational needs of the Nation (Table 4-1).

Thus this future program posits two distinct types of missions, one having modes with both high and low resolution, each with an along-track interferometric SAR capability; and one with a single channel looking both left and right.

A significant amount of research and development needs to be undertaken before the ATISAR can be exploited with confidence; on the other hand, the SpectraSat technology has been proven, and a program could be launched with little further development.

It is clear from the above discussion that SAR observations are highly valuable to the ocean and ice sciences, as well as serving numerous operational requirements. Very good data have been obtained on surface and internal waves, coastal ocean processes, variations in bottom topography/bathymetry, upwellings, small-scale and mesoscale currents, rainfall variations, surface wind speed and stress, atmospheric convection, sea ice cover, marginal ice- zone processes, ice age, leads, polynyas, vessel wakes, surfactant/oil distributions, and a variety of other processes leading to subtle variations in surface roughness. Many of these observations are unique to SAR; others take advantage of the operation of SAR under conditions of cloud cover and nighttime. SAR data to support scientific analysis, algorithm development, and process modeling are now acquired from international satellites, airborne systems, and SIR- C/X-SAR. However, additional exciting scientific prospects are in the early stages of development; of special scientific and applications interest is the determination of surface currents in the open sea and in the coastal zone using along-track interferometry. Such a capability would be of utmost importance in understanding ocean circulation. Commercial and applications interests that may be served include: wave and marine weather forecasting, Gulf Stream nowcasting, ship routing, ship and surfactant and slick surveillance in the EEZ, fish stock assessment, coastal monitoring, iceberg warning, and sea ice cover.

Specific recommendations are:

(1) Continue with and significantly strengthen the U.S. research and development program in SAR oceanic/atmospheric/polar science and applications. The promise of SAR in these areas is considerable and needs proper support to yield a return to the nation that is appropriate to its investment in SAR programs to date. A vigorous airborne SAR program with the AIRSAR is needed to support multiparameter SAR sensor development investigations using multifrequency, polarimetric, interferometric data, and to serve as a test-bed for advanced concepts in SAR observations.

(2) Undertake a three-satellite program with a polar orbiting ATISAR, a SIR-C/X-SAR free-flyer, and a SpectraSat being the major program elements. The AIRSAR is needed throughout the program as a developmental platform and as a source of scientific verification and studies. These programs will ultimately produce along-track interferometric data over the oceans of first-order importance to Mission to Planet Earth, to Global Change objectives, and to operational agencies.

(3) Provide improved access by U.S. scientists to the SAR available from international programs. These data sets are known to be quite good in topics such as sea ice and mesoscale features. In addition, cooperation with foreign space agencies in science activities and even in joint flight programs should be sought; for example, Russian Federation boosters are capable of injecting the SIR-C/X-SAR free-flyer into the medium altitude polar orbit desired.

(4) Maintain close cooperation with NASA and other government agencies having appreciable marine responsibilities. These include NOAA, Navy, NSF, USGS, EPA, Coast Guard, and Army Corps of Engineers. Other-agency support was highly valuable during the evolution of Landsat and Seasat.

(5) Develop, in concert with a larger community of scientists, a more detailed scientific program plan for use of SAR, in a fashion similar to the NASA space physics and astronomy/astrophysics programs several years ago.

(6) Blend the requirements for use of SAR in oceans/atmospheres/ice with those from the other Earth science disciplines to arrive at a specification for a spacecraft program that serves a broad range of needs in Earth science and applications. Reports to date show significant commonality of both sensors and spacecraft.