ERS OPR time tag biases

Last update: Mon 6 October 1997

The time tagging of the ERS altimeter data on the ESA ERS Ocean Product (both Version 3 and Versions 5 and 6) have demonstrated a serious defect. The time tagging of the records is systematically early by as much as a millisecond. This means that the altimeter range measurement is the correct value, but is not taken at the epoch given on the records, but actually about a millisecond later. As a consequence, the orbital altitude given on the records pertains to this early epoch, and not to the right epoch. Because the altitude rate is up to 25 m/s, a millisecond time tag error leads to an altitude error of as much as 2.5 cm.

We can estimate the time tag bias during the orbit determination or after the fact by minimizing crossover height differences and estimating a constant time tag bias per day. It appears that the time tag bias estimates are quite stable and become increasingly stable when the orbit precision improves.

A drawback is that remaining 2-cpr orbit errors may be confused for a similar periodic behaviour due to time tag biases. This explains why the estimated time tag error slightly depends on the gravity model used in the orbit determination.

The table below gives some values for the resolved time tag bias and the variation of the daily estimates (after 3.5-sigma editing) for various orbits and both for ERS-1 and ERS-2. A distinction has been made between ERS-1 Version 3 (Phase C-F) and Version 6 (Phase G) data. Within brackets I have added the suggested value, i.e., the one I use myself.

Time tag biases in milliseconds
Orbits ERS-1 (Phase C-F) ERS-1 (Phase G) ERS-2 (Phase A)
GFZ PGM055 -0.888 ± 1.132
-1.130 ± 0.412
-1.046 ± 0.521
UT/CSR TEG-3 -1.430 ± 0.447

Delft JGM-3 -1.261 ± 0.655
-1.294 ± 0.424
-1.071 ± 0.417
Delft EGM96
-1.494 ± 0.344
-1.248 ± 0.365
Delft DGM-E04 -1.534 ± 0.346
-1.561 ± 0.321
-1.297 ± 0.339

The negative numbers indicate that the time tags are early. To correct the altimeter data, use the following recipe:

The program opr2gdr can handle the time tag bias for you using the tb=TTB option. Fill the appropriate value for TTB in from the table above, and the correction is made for you.

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