The first non-military operation of radar from a spacecraft was the Seasat mission developed by JPL and launched (on a satellite) in 1976. This L-band instrument had a high depression angle which helped to reduce shadowing but heightened foreshortening. Much of the world was imaged until Seasat’s radar failed on the 99th day. Although the intended use was primarily to monitor "sea state", Seasat obtained many fine examples of land imagery. However, the ocean observations, coupled with a radar altimeter onboard, produced a view of the marine surface which had surprising implications about the ocean floor.
Military aircraft were the first to fly radar units during World War II. These units used very long (1 to 10 m) wavelengths that extended into the radio region of the EM spectrum. Airborne SLAR systems were developed soon thereafter. The first radar systems in space had military applications, so the resulting imagery was classified. The first civilian use of spaceborne radar was the Seasat system operated for NASA by the Jet Propulsion Laboratory (JPL). Launched on June 26, 1978, the onboard SAR radar lasted only 99 days before a circuit failed, causing it and other sensors to quit. During normal operations, its L-band (23.5 cm) transmitter produced a focused 1 x 6 degree HH-polarized beam pointed starboard (right) at 20 degree off nadir (vertical). From a slightly elliptical, nearly polar orbit at a nominal altitude of 790 km (491 mi), Seasat's radar had an outward swath width of 100 km (62 mi), giving an image that was printed as four individual strips of 25 km each and then combined to make a full-width image. Along any track (retraced every 24 days), this swath had near and far boundaries that lay between 24 and 240 km (15-150 mi) off nadir. The high depression angles (between 67 and 73 degrees), reduced shadow effects in rugged terrains but induced notable layover. The resolutions achieved by the radar depended on the method by which the synthetic aperture data was processed. With an optical correlator, image resolutions were as low as 70-80 m (230-262 ft), but a digital correlator improved the resolution to about 25 m (82 ft).
Seasat's principal mission was to study various properties at and near the ocean surface, including sea surface temperature, wind speeds, and wave heights. The SAR, with its low look angle, was designed to measure directions and wavelengths of ocean waves exceeding 50 m (164 ft) in fetch (distance between crests), and to look at sea ice. Below is an image made with the digital correlator of waves off Alaska's southern coastline near Yakutat (note the glaciers on land).
This Seasat image strip below shows sea ice off Banks Island, Canada.
8-14: Look carefully at the upper scene, focusing on the mountains. Do you see any examples of layover (extreme foreshortening, cause one topographic feature to overlap another? ANSWER
8-15 In the Banks Island strip, there is both first year and multiyear ice. How do they differ tonally? Hint: think what can happen to ice year after year. ANSWER
We already showed two Seasat images of land surfaces - the Pine Mountain and Harrisburg scenes. As another example, examine this Seasat image of central Jamaica in the Carribean obtained through cloud cover on August 8, 1978.
The image shows a variety of landforms: some of mountains that are foreshortened, areas of land use, and roughened offshore waters. Limestone beds govern much of the island's underlying geology. In this area of heavy rainfall, these beds readily dissolve to form typical karst topography. Doline depressions (solution pits) occupy large sections of the scene. These pits are obvious in the aerial photo below, but in the Seasat image, they form a fine texture that may not show well on a monitor. If it does, look also for a faint but visible lineation caused by fractures that have a preferred orientation because of the look direction.
Seasat was adept at showing both onland and submerged landforms. This next image covers much of southern Florida which, like Jamaica, is a limestone platform. Most of the Everglades shows here as dark (low signal returns) where water flow has spread out but the hardwood tree hammocks (streamlined elongate forms) are bright (white). The edge of urban Miami is shown in lighter tones at center right. The Florida Keys form a line of connected reefs at the bottom leading to Key West and in Florida Bay to the north the reefs (some submerged) are revealed by radar to be an interlocking network.
Similar to aerial radar imagery, space radar imagery is well suited to mosaics, as strikingly depicted below in this JPL mosaic of southern California. Use the Digital Elevation Map mosaic of nearly the same region that we presented in Section 7 on Mosaics, as a locator guide (i.e., find Los Angeles).
8-16: Why is the Mojave Desert (the great arrowhead-like salient) shown as dark in this Seasat image? ANSWER
We can also co-register Seasat radar imagery with Landsat imagery. Below, we superimposed part of a Landsat image of dissected Allegheny Plateau in West Virginia (alone in the upper right), on a Seasat image giving a new impression of apparent relief by virtue of the light tones in the foreshortened foreslopes.
Seasat included a second instrument, a radar altimeter, designed to measure height variations of the ocean surface and large waves imposed on the water. The generated pulses are sent directly downward (as close to vertical, or nadir, as possible [slight variations as the spacecraft wobbled]) and a large fraction of the energy is then reflected straight up. This is timed, so knowing pulse speed (that of light), the total duration or transit time is a measure of distance from Seasat to surface and back). Slight variations in transit time along track (orbital path) represent differences in elevation over the distance traversed.
This led to a remarkable result which proved a sensation among geoscientists and oceanographers. Look at this image, produced at the Lamont-Doherty Geological Observatory of Columbia University, New York:
The map shows the morphological features of the ocean floor at a gross scale. These features are largely the mid-ocean ridges, the trenches, and the many transverse faults (those that join the ridges at high angles) that emanate from either side of the ridges as the sea floor spreads in both directions away from these ridges. Amazingly, both the faults, which have cliff-like expressions. and the ridges have differential gravitational effects (being topographic highs and lows) that cause the surface waters to mirror these differences. This is what is picked up by the radar altimeter. The map has been color-manipulated to highlight (using blues and yellows) these differences. This altimeter map is very similar to the (integrated) maps produced over the years by depth soundings and other geophysical measurements of the ocean floors.
