Seasat Rediscovered: ASF DAAC Imagery from 1978 Data
Launched by NASA in 1978, the Seasat satellite’s primary mission was to observe oceans using NASA’s first synthetic aperture radar (SAR) sensor. SAR bounces a microwave radar signal off the surface of Earth to detect physical properties. Unlike optical photo technology, SAR can see through darkness, clouds, and rain.
Through the use of SAR, the Seasat satellite collected an enormous amount of data for its time. This data has been processed by the Alaska Satellite Facility, a NASA Distributed Active Archive Center (DAAC), into digital imagery that allows scientists to measure features of the planet’s surface over time.
Seasat was the first NASA satellite with synthetic aperture radar deployed. It operated on L-band in 1978 for 105 days until its catastrophic power failure.
A glossary of terms used in the Seasat project
Citing Seasat Data Cite data in publications such as journal papers, articles, presentations, posters, and websites. Please send copies of, or links to, published works citing data, imagery, or tools accessed
Before and After Images showing the difference between the Seasat images being optically processed shortly after the mission and digitally processed in 2013
Seasat data required cleaning before processing it into imagery. The data product packages were eventually distributed in HDF5 and GeoTIFF format.
During its brief 106-days of lifetime, the Seasat-1 spacecraft, launched on June 28, 1978, by NASA’s Jet Propulsion Laboratory (JPL), collected information on sea-surface winds, sea-surface temperatures, wave heights, internal waves, atmospheric water, sea ice features, ice sheet topography, and ocean topography. This was the first JPL mission to study Earth with the use of imaging radar.
View Seasat technical reports, general references, and publications focused on Seasat data processing, oceans, snow and ice, and land applications.
The Seasat satellite was designed to cover areas up to 75° north latitude. Seasat data was acquired by five ground stations in the Northern Hemisphere.
The Alaska Satellite Facility was tasked by NASA with creating a digital archive of focused synthetic aperture radar (SAR) products from data collected by NASA’s Seasat mission. The basic steps
Seasat was not equipped with an onboard recorder, so in order to collect data during the mission, three U.S. and two international ground stations downlinked data from the satellite in
After the decoding, cleaning and focusing of the Seasat SAR data, many artifacts still exist in the initial ASF Seasat SAR products. Most of the artifacts observed in the images result from system interferences during data acquisition, missing data as a result of multiple transcriptions between media storage since 1978, and processing decisions implemented in the ASF Seasat Processing System. ASF’s intent is to distribute as much of the historic dataset as possible to our users while offering transparency about known quality issues.
This section provides a detailed description of the HDF5 data format used for the final generation of Seasat synthetic aperture radar (SAR) products. Although it does not cover a technical challenge, it is included for completeness of the processing description.
Starting in the summer of 2012, ASF undertook the significant challenge of developing a Seasat telemetry decoder in order to create raw data files suitable for focusing by a synthetic
With the Seasat archives decoded into range line format along with an auxiliary header file full of metadata, the next step is to focus the data into synthetic aperture radar
In order to create a synthetic aperture for a radar system, one must combine many returns over time. For Seasat, a typical azimuth reference function — the number of returns
4.3 Prep_Raw.sh After development of each of the software pieces described previously in this section, the entire data cleaning process was driven by the program prep_raw.sh. This procedure was run
In spite of all of the work done to decode and clean data, many errors remained in the supposedly fixed files that had been decoded and multi-pass filtered. As a result, the current count for swath files able to be processed is 1,346 rather than the 1,399 that first came out of prep_raw.sh. Each of these classes of errors are discussed in this section.
During decoding and cleaning, it was assumed that the time slope of the files would be roughly guided by the Pulse Repetition Interval (PRI) of the satellite, i.e. a Pulse
Seasat synthetic aperture radar (SAR) data holdings at ASF have been converted from their original 29 SONY SD1-1300L tapes into raw swath files with external metadata stored on disk. As detailed elsewhere in these pages, a total of 1,346 decoded, cleaned swath files were created.
In modern systems, synthetic aperture radar (SAR) echoes are sampled in a complex fashion using IQ-demodulation. The I and Q components are samples of the same signal that are taken 90 degrees out of phase. Separating I and Q in this way allows measurement of the relative phase of the components of the signal, and is a requirement for the SAR focusing algorithm. The Seasat platform used an older method to sample echoes, storing real (not complex) returns in offset video format.
At this stage in the development of the ASF Seasat Processing System (ASPS): 1,346 cleaned raw signal swaths were created; ROI was modified to handle Seasat offset video format; New state vectors were selected for use over two-line elements (TLE’s); Caltones were filtered from the range power spectra; Data window position files were created