New Seasat Imagery
New Digital Imagery from Historic 1978 Seasat Satellite
by Tom Logan, Alaska Satellite Facility
A new suite of digital imagery featuring historic views of Earth’s oceans, sea ice, volcanoes, forests, glaciers, and more will be available for the first time from ASF in mid-summer 2013. The products will be derived from data collected nearly 35 years ago by NASA’s Seasat satellite — the earliest orbital synthetic aperture radar (SAR) ever gathered. Although it suffered a catastrophic power failure in its fourth month of operation, in 106 days Seasat collected more information about the ocean surface — its original focus — than had been acquired in the previous 100 years of shipboard research. ASF’s suite of products from the mission, available to users under NASA’s open-access data policy, will surely prove invaluable to a range of scientific disciplines.
On June 27 GMT, 1978, NASA undertook a momentous task: To demonstrate the feasibility of orbital remote sensing for ocean observations, it launched the Seasat satellite. This platform hosted an L-band SAR sensor that collected imagery with a resolution of 25 meters, obtained in 100 km-wide swaths covering much of North America, Northern Europe and the adjacent seas. Seasat’s SAR images detected surface waves, internal waves, the Gulf Stream system, the California Current, the North Pacific subtropical front, and various other coastal phenomena. More than 100 passes were recorded over the Beaufort Sea, providing insight into major morphological features, structural changes and drift motion of sea ice. Even with the power failure on October 10 of the launch year, Seasat succeeded in its primary goal of taking oceanography into space.
During operation, Seasat utilized two basic orbital configurations. The primary initial observational phase was a 17-day repeat cycle. On September 10 GMT, the satellite was maneuvered into an exact 3-day repeat. Years later, these repeat-pass data enabled scientists to pioneer the now nearly ubiquitous technique of interferometric SAR analysis, showing that Seasat data are applicable to much more than just ocean observations. Other, non-interferometric applications may include the following:
• Seasat data could be compared to boreal forest maps from the Japanese Earth Resources Satellite 1 (JERS-1) to identify land cover changes between 1978 and 1997,
• the seven orbit cycles of 3-day repeat data may be useful to show whether rates of deformation over known active faults in North America and Pacific Rim volcanoes are consistent with observations today and
• the data acquired in 1978 over Norway and Alaska could be utilized for glacial change observations, establishing a much older baseline than is currently available from other sensors.
Since Seasat was not equipped with an onboard recorder, during the mission three U.S. ground stations (Fairbanks, Alaska; Goldstone, California; and Merritt Island, Florida) collected data in real time. Two international stations also came online about midway through the mission (Shoe Cove, Newfoundland, and Oakhanger, United Kingdom). To ensure that this valuable archive was preserved, the raw data tapes were first duplicated in 1988 and again in 1999.
During the second transcription, the data were transferred from older 39-track tapes to more modern SONY SD1-1300L
19mm tapes. It is from these 13-year-old tapes that ASF’s online Seasat archive was obtained during the fall of 2012. Although some data have degraded beyond usability, data from all five of the ground stations are in ASF’s holdings, and newly created coverage maps match favorably to the older coverage maps obtained from NASA’s Jet Propulsion Laboratory (JPL). Beyond this, the European Space Agency (ESA) has asked NASA to process any additional or higher-quality Seasat data the agency may be able to provide — a task that ASF’s Seasat processing system will be uniquely positioned to accomplish.
While much of the data collected in the U.S. were processed optically, only an estimated 3 to 15 percent of Seasat data was ever digitally processed. In the 1980s, JPL’s technology was able to process only one image a day. Today, ASF, which has the only current processing system for Seasat data, can easily produce an image in less than 30 minutes. For nearly 35 years, the majority of the SAR dataset from this historic mission has been unprocessed, and it has spent the last 14 years residing on tapes in the ASF archives, basically unavailable to scientists in the 21st century.
ASF researchers are processing Seasat SAR archives into imagery products. A telemetry decoding system is in production, ASF’s Seasat holdings have been captured to disk and the data have been filtered into readily processable signal files, in spite of the many metadata problems resulting from multiple transcriptions and 35 years of “bit rot.” In concert with this decoding effort, engineers at ASF are developing a robust processing system to generate a) raw signal swaths, b) processed single look complex and c) detected georeferenced products in HDF5 format with ISO 19115 compliant metadata in XML format. When the beta version products become available in summer 2013, remote sensing scientists will have access to one of the most groundbreaking and unique datasets ever collected — the newest oldest SAR data around.