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The sensitivity of synthetic aperture radar (SAR) to surface and, in some cases, subsurface geometry and roughness, and to electrical properties can provide information about land, ocean surfaces and vegetation cover that is complementary to measurements made by sensors operating in the visible, near infrared, and thermal infrared portions of the electromagnetic spectrum. SAR provides its own illumination and can therefore produce reliable multitemporal data independent of weather or solar illumination, through all seasons, and at any latitude. Radar waves penetrate clouds and, under certain conditions, vegetation canopies, ice, and very dry sand or soil, making it possible to explore near-surface zones not accessible with other remote sensing techniques. It should be noted that 3-dimensional features (e.g. mountains) are enhanced, in comparison with optical imaging sensors, due to the side-looking illumination and imaging geometry.
The SIR-C/X-SAR missions, in April and October 1994, extended the capability of an aircraft campaign by providing regional scale data over a short time period. The mission design also enabled areas to be imaged at different incidence angles on subsequent days, an important parameter for studying many land and ocean processes. The extensive ground truth measurement campaigns provided critical data to be used in development of algorithms to produce data products for studying global change issues. By having multiple flights, insights on seasonal variations for the key science issues is also provided. Such long-term development studies are critical for developing the requirements and mission design for future radar missions.
The two flights of SIR-C/X-SAR in 1994 differ from the earlier missions, SIR-A and SIR-B, in that (1) SIR-C/X-SAR was the primary payload, (2) the two flights provided a unique look at seasonal change in radar signatures and (3) the astronauts provided round-the-clock observations of the Earth to coincide with radar data collection. The crew onboard the SIR-C/X-SAR flights had an opportunity to help explore the relationships between the radar data and weather conditions beyond what is possible by the investigators on the ground.
The first Shuttle Imaging Radar, SIR-A, was flown in November 1981; this was the second flight of the shuttle and the first scientific payload ever flown. There were two crew members onboard. For the early flights, experiments on board the Shuttle did not involve any crew interaction. The handheld photographs (HHPs) acquired on the mission were assessed along with the SIR-A radar data. In several cases, coincident photographs and radar data were coincidentally obtained which prompted the Science Team for the next SIR experiment, SIR-B in 1984, to pursue the use of shuttle-based observations obtained in parallel with the radar data.
Pre-mission planning and crew training prepared the astronauts for acquiring photographs of the SIR-B sites during the mission, but only as time permitted. Observations made by the crew had a very significant effect on the results of some the SIR-B investigations. A photograph of the southern ocean ice was used to determine the location and concentration of thin first-year ice and open water which was critical in the interpretation of the radar data. A map of geologic structure, generated from a photographic stereo pair obtained over the Andes mountains, was used in the interpretation of the radar data over this area.
Science observations by the shuttle crew included photographic documentation of the sites on a routine basis and visual observations of features of interest which are recorded as notes, voiced down to the operations center during the mission, and discussed with the science investigators after the mission. These observations may or may not be backed up with photographic documentation. The observations describe interactions of the atmosphere, oceans and land surface, and identify unpredicted or transient phenomena for potential imaging.
The primary camera used for Earth observations is a Hasselblad 70 mm camera. Accompanying equipment includes three lenses (50, 100, and 250 mm), a data link to record time, filters, film magazines and various types of film. The 100 mm lens offers spatial resolution similar to the Landsat Multi-Spectral Scanner (MSS) (80 m) and the 250 mm lens offers Landsat Thematic Mapper (TM) resolution (30 m). With the 250 mm lens, the Hasselblad is capable of obtaining photographs at the same resolution as the SAR images but with a much larger field of view.
A Linhof Aero Technika and a Nikon F4 35 mm camera are also available. The Linhof uses 5 inch film and is useful for photographing large areas with resolutions similar to the Hasselblad. Lenses include a 90 and a 250 mm. The Nikon F4 is provided with an interchangeable 35-70 mm zoom lens, and 28, 200, and 300 mm autofocus lenses.
The shuttle provides a number of unique optical perspectives. A non-polar shuttle orbit provides an opportunity to obtain variable sun-angle photography over the duration of the mission. The current polar orbiting platforms (SPOT, Landsat, AVHRR, etc.) are all in sun-synchronous orbits therefore preventing acquisition of variable sun angle data. From the shuttle's lower inclination orbits, the complete range of sun angles from dawn to dusk are available; all are useful for observations, although low sun angles are particularly useful for highlighting subtle topographic or roughness features.
Sun glint is the reflection of the sun on water surfaces; it represents scattering in the forward direction and is a function of the sun angle and the amount of small scale surface roughness. Wind stress, waves, and currents control ocean patterns that may be observed in sun glint. When the ocean is calm, the sun glint is bright and the area of bright ocean is small. When the ocean is rougher, the scattered light is more diffuse and the bright area is enlarged by wave facets that produce reflections from many different directions. Thus the sun glint can be related to physical phenomena that roughen and calm the ocean's surface such as wind stress, wave-current interactions, and biological or chemical properties of the surface of the ocean which can create surface slicks. In a similar fashion, radar energy is scattered off the ocean surface, in this case in the backscattered direction. The rougher the ocean the greater the radar return. During the SIR-C/ X-SAR mission, the crew was on the look out for sun glint and took photographs of this phenomenon. Some of these photographs can be found in the Oceans directory.
Although the radar can penetrate clouds and "SEE" the Earth's surface day or night and in all seasons, the radar is very sensitive to the seasonal and meteorological (weather) state of the surface at the time of imaging. Changes in the seasonal state of the surface can change the radar backscatter by up to 10 dB. Clouds not only affect the state of the surface as viewed by the radar, but may attenuate the radar beam by several dB, especially at the shorter X- and C-band wavelengths. Observations of cloud location and type are readily made from space. For SIR-C/X-SAR, observations of the atmosphere and surface at the same time are very important in understanding the radar observations for a site, especially when comparing results obtained at different times during the mission. Radar data interpretation must be done in the context of the Earth's surface state at the time of imaging.
The central science themes of the SIR-C/X-SAR missions are examined here in more detail.
i) Oceans
How waves move through the oceans and how the air and sea interact with each other play a major role in determining the earth's climate. The ocean stores heat and energy and air-sea interactions move this heat and energy around the earth. The Gulf Stream, off the east coast of North America is a good example of how heat is moved around the globe; this major current moves heat from the equatorial region into the northern Atlantic allowing tropical plants, such as palm trees, to grow along the southern coast of Ireland.
SIR-C/X-SAR images of oceans are used to study large surface and internal waves, wind motion at the ocean surface, and ocean current motion. These data assist scientists in understanding how the Earth's climate is moderated by the ocean. In shallow areas, radar images can be related to the topography of the ocean bottom. Natural and man-induced oil spills can also be imaged and monitored using imaging radar.
The distribution of sea ice largely determines the heat and water balance near the Earth's poles. Imaging radar can be used to study the seasonal distribution of sea ice. Although SIR-C/X-SAR's was not in a polar orbit (the highest latitude it reached was 57deg. north and south), sea ice images were collected over the Sea of Okhotsk in the eastern Soviet Union, and the Labrador sea off the coast of Newfoundland.
At high latitudes, sea ice is in constant motion, particularly along the ice pack margins, the regions viewed by SIR-C/X-SAR in its 57deg. orbit inclination. The concentrations of open water, first year (thin) ice and multiyear (thicker) ice, as well as the location of the ice margin and the location and concentration of ice bergs changed from day-to-day throughout the mission. The interaction of the open ocean and the ice at the ice margins is of particular interest; these regions often contain extensive spiral eddies.
The ocean is a dynamic system, strongly influenced by the atmosphere. The ocean surface temperature also has a significant effect on clouds. Radar is sensitive to the manifestations of this dynamic air-sea system, specifically to capillary and gravity waves, internal waves, mesoscale and sub-mesoscale (spiral) eddies, current boundaries, bathymetric features (and tides), ocean fronts, sediment fluxes, island wakes, currents in tidal inlets and shallow areas, and convergent surface currents in upwelling regions. In addition, the radar return is sensitive to oil slicks and natural algal blooms which influence the roughness of the ocean's surface.
Our current understanding of the geophysical information contained in radar imagery of the ocean surface is often limited due to the lack of other data describing the state of the ocean at the time of data collection. Documentation of ocean state in parallel with SIR-C/X-SAR radar data may provide key information needed to more fully evaluate the radar ocean imagery. In addition, shuttle-based photography of the ocean experiment sites shows the investigators' ships involved in surface truth data collection relative to the radar swath and ocean features. Observations and photography of regional ocean systems and clouds provides the regional context within which the radar swath is located. This is more important to radar studies of the oceans than of land sites; the regional context of land sites does not change as rapidly during an 11-day mission (at least we hope not!).
ii) Ecosystems
Ecologists study life on earth and how different life forms interact with each other and their local environment. SIR-C/X-SAR collected ecology data over tropical forests including the Amazon basin in South America, and over temperate forests in North Carolina and Michigan, and in Central Europe. SIR-C/X-SAR images are used to study:
The earth's vegetated surface as viewed by SAR varies significantly with weather conditions and the surface cover. Recent results of experiments to understand the day-night variations in the radar backscatter of forests indicate there is a strong diurnal (day/night) signature related to the dielectric constant (electrical property) which in turn is related to plant water status. When clouds pass over vegetation and cut off solar energy, the photosynthetic process slows down or stops, water potential rises and the dielectric constant changes.
On a longer term basis, changes in the weather conditions and forest vegetation state over the duration of each mission and between missions produced significant changes in the radar backscatter.
Specific phenomena which may be documented through visual observations include snow existence and extent, flood existence and extent (through sun glint photography), leaf on/off and/or leaf color (green or yellow/red), deforestation extent and vegetation vigor or greenness which is related to water status. In addition, acquisition of radar imagery of forests during and after forest fires would provide a valuable "TARGET OF OPPORTUNITY" data set. Depending on the season, the probability of fire occurrence in particular regions determined specific areas to monitor intensively.
iii) Hydrology
Hydrologists study the global water cycle focusing on processes that occur on land. (As opposed to Oceanographers who focus on the worlds oceans.) In addition to water in swamps, lakes, rivers, and mud puddles, large amounts of water are stored as soil moisture and in vegetation; this is an important part of the global water cycle and plays a major role in surface moisture and global energy fluxes. The amount of water a surface or material contains in part determines its electrical properties. Since radar is sensitive to the electrical properties of a surface it is useful for measuring soil and vegetative moisture over large areas and how these vary between seasons. SIR-C/X-SAR hydrology investigations are focused on Brazil, Oklahoma, Pennsylvania, and Italy. Radar data from these sites are being used to determine soil moisture patterns. These studies will help develop ways to estimate soil moisture and evaporation rate over large areas. This information will be valuable input to large-scale, regional hydrologic models.
Likewise, in mountainous and high latitude regions, seasonal snow cover is a major storage component in the hydrologic cycle. Spring snow melt often dominates the annual runoff cycle and resulting water supply, ground water and reservoir recharge rates. For many areas, long-term or ground-based snow cover data do not exist and remotely sensed data provide the only way to acquire this information. SIR-C/X-SAR acquired data on snow cover over Mammoth Lakes, California; radar data on snow and glacial cover were acquired over the Austrian Alps, the Himalayas and the Patagonian district in Southern Chile which contains the largest modern glaciers and ice fields in South America. X-band data are useful for determining snow type, while L- and C-band data are be used for estimating snow volume.
Wetlands are sources of many trace gases that are important parts of the global atmospheric cycle. Wetlands are also especially vulnerable to human alteration. SIR-C/X-SAR radar data are used to determine the extent and limits of selected wetlands areas, as well as their changing conditions.
The hydrologic state of the earth's surface varied significantly over the duration of the mission and from mission to mission due to precipitation (including snow) and the ensuing dry-down. Although it is not possible to observe either rain or soil moisture visually from the shuttle, it is possible to observe clouds which could potentially be raining by identifying cloud type, and lightning, which is directly correlated to rain. This knowledge is important for rain and snow experiments. It is also important for other experimenters requiring calibrated radar data as the existence of snow and/or rain within the experimental area may influence the radar backscatter.
iv) Geology
Geologists study the present surface of the Earth and by looking at older rocks, how it came to be and how it may have looked in the past. SIR-C/X-SAR data are useful for mapping geologic structures and variations in rock types over large areas. These data are especially useful in areas of heavy vegetation and continuous cloud cover where field work is often difficult.
Long radar wavelengths (24 cm: L-band on SIR-C) can penetrate below the surface in extremely dry desert areas. This allows scientists to map geologic structures buried under the sand and to identify ancient stream systems. Discovery of ancient stream systems in the Sahara have had important implications for past climate histories and also for present possible sources of water.
The SAR is sensitive to scatterers (sand grains, rocks, etc.) that are approximately the size of SIR-C/X-SAR's radar wavelengths (3, 6 and 24 cm). Thus a variety of surfaces, from sand to rough lava flows, can be mapped with imaging radar. These different wavelengths allow various geologic processes to be studied including soil/sediment erosion, transportation, deposition, and degradation. These processes have an impact on sedimentation in rivers, streams, river deltas and coastal environments and affect the amount of land available for food production.
The radar's sensitivity to surface roughness allows scientists to study the history of past climate change and the relative age of surfaces because as land surfaces age and are exposed to weathering, they generally change their roughness characteristics.
Although the geologic state of the surface was unlikely to change during the SIR-C/X-SAR mission or even between the two missions, the state of the surface in terms of vegetation and snow cover did change and do strongly influence the interpretation of the radar imagery for geologic purposes.
In addition to monitoring meteorological conditions, shuttle-based photographs for geology experiments with SIR-C/X-SAR provide information on the geologic setting and regional context of the radar imagery. Low sun angle photography not available through SPOT or Landsat provide a unique opportunity for viewing subtle surface features to which the radar is sensitive; these photographs are particularly valuable in understanding the mechanisms of subsurface imaging of ancient river systems in northeastern Africa as they highlight surface roughness patterns which may be confused with subsurface radar signatures. Stereo photography provides a three-dimensional perspective of a region. Monitoring of active volcanoes during the mission provides an opportunity to obtain radar imagery of erupting volcanoes and/or fresh lava flows. Active volcanoes are observed on approximately 50% of all shuttle flights and this was the case for the two SIR-C missions. Indeed, Kliuchevskoi volcano, on the Kamchatka peninsula started an eruptive episode the day the second SIR-C mission was launched!
v) Rain and Clouds
Throughout the history of radar, one of the main selling points has been its ability to "SEE THROUGH CLOUDS". Recently, however, clouds have become an important factor in the future analysis of SIR-C/X-SAR data due to three factors:
Two SIR-C/X-SAR experiments are evaluating the ability of radar to image rain. These investigations require imaging of rain systems and therefore decisions on whether or not to take data were made during the mission. The scientists responsible for those experiments identified areas in the Western Pacific ocean, the "RAINIEST PLACE ON EARTH", as having the best chance for imaging rain.
| Radar Images: An Overview | Guided Tour |
Converted to the IBM-PC by Al Wong, sirced03@southport.jpl.nasa.gov
Jet Propulsion Laboratory
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Pasadena, CA 91109