Seasat – Technical Challenges – 6. Slope Issues

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. Based upon this, then, each 1,000 lines of Seasat data should be equivalent to .60716 seconds.

Alternately, in milliseconds, the time slope for these files should always be 0.60716. It was discovered that this is not the case with much of the actual data, as shown in the following table and graphs:

Line Time Time Diff Calculated Slope
1 13851543
500 13851790 247 0.4950
10000 13856399 4856 0.4856
15000 13858818 7275 0.4850
20000 13861260 9717 0.4859
30000 13866139 14596 0.4865
35000 13868569 17026 0.4865
40000 13870998 19455 0.4864
45000 13873447 21904 0.4868

Seasat Times: PRF = 1647 Hz, so PRI is 0.0006071645 msec. In MSEC, the time slope should always be 0.6071645. Yet, for this datatake, the time slope is consistently only 0.486!

Original Data

Original Data: This example shows a dataset that is relatively clean before any filtering is applied. It seems that this file should have been extremely easy to clean.

Filtered Data

Filtered Data: After the dataset went through the prep_raw.sh procedure, this was the resulting time plot. It is, quite obviously, very wrong.

Comparison

Comparison of Original with Filtered: Although the times look fine in the first (unfiltered) plot, they are wrong for this satellite based upon the known PRI. The ASF cleaning software tried to fix these wrong time values using a known slope of 0.607. This introduced a discontinuity into the data and resulted in incorrect times.

These results, wherein the time slope of the raw data does not match the known PRI of the satellite, were incredibly perplexing. At first, it was assumed that these data could not be processed reliably and were simply categorized into the large time-slope error and wrong-fix error categories.

Analysis of the time slopes in the original unfiltered data only pointed out how extreme the problem really was. Well over 100 files showed slopes that were either less than 0.606 or more than 0.608, with the lowest in the 0.48 range. The highest reliable estimate showed a slope of well over 0.62.

6.1 Slope Issues Explained

Eventually, through conversation with original Seasat engineers at the Jet Propulsion Lab (JPL), it was discovered that the Seasat metadata field MSEC of Day actually contains not only the time of imaging but also the time to transmit data from the spacecraft to the ground station. This adds a variable time offset to the metadata field. Once this was understood, it was readily obvious that using the known PRF as a guide for filtering was an incorrect solution.

Thus, the entire cleaning process was revisited, with all of the codes allowing more relaxed slope values during linear regression. This worked considerably better than the previous cleaning attempt. However, it did not solve the problems entirely.

6.2 Final Results of Data Cleaning

The final set of cleaned Seasat raw swaths was assembled using three main passes through the archives with different search parameters, along with a few files that were fixed on a case-by-case basis. Basically, the final version of the code was run and the results examined for remaining time gaps. Any files with large or many time gaps were reprocessed using different parameters. In the end, 1,346 swaths were cleaned, 2 by hand, 14 from the first pass, 25 from the second pass, and the remainder in the final cleaning pass. These then are the final cleaned Seasat archives for the initial release of ASF’s Seasat products.

Date Total Datasets Dataset with Time Gaps Largest Time Gap Largest number of gaps in a file Files with >10 msec gap
1/31/13 1,399 728 54260282 1820
4/9/13 1298 263 180 34
4/9/13 1,299 122 95 33
4/9/13 1,299 55 2113 17
4/10/13 1,299 34 50
FINAL 1,346 49 34 26 28

Final Cleaned Seasat Swaths: Approximately one year after the project started, 1,346 raw Seasat swaths were cleaned and ready to be processed into SAR image products.

Written by Tom Logan, July 2013