Seasat

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.

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 on all of the swaths that were output from SyncPrep to create the first version of the ASF online Seasat raw data archive (the fixed_ …

Seasat – Technical Challenges – 4. Data Cleaning (Part 2) Read More »

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 aperture radar (SAR) correlator. In this case, that means processible by ROI, the Repeat Orbit Interferometry package developed at Jet Propulsion Laboratory. In addition to …

Seasat – Technical Challenges – 2. Decoder Development Read More »

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

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 combined into a single focused range line — is 5,600 samples. Each of these samples is actually a range line of radar echoes from the …

Seasat – Technical Challenges – 4. Data Cleaning (Part 1) Read More »

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 Repetition Frequency (PRF) of 1647 Hz means that 1,647 lines are being transmitted and received per second. This means that the PRI is 0.00060716 msec. …

Seasat – Technical Challenges – 6. Slope Issues Read More »

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 (SAR) imagery. Focusing is the transformation of raw signal data into a spatial image. Unfortunately, pervasive bit errors, data drop outs, partial lines, discontinuities and …

Seasat – Technical Challenges – 3. Decoded Data Analysis Read More »

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.

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.

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.