How to calculate Sentinel 3 absolute orbit number from filename?

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#1

I am hoping someone can help me determine the (absolute) orbit number for Sentinel 3 given only the relative orbit and cycle numbers in the filename. (I know that this is stored within the manifest .xml file, but I no longer have those for the files I downloaded.)

From documentation online (e.g. Orbit - Sentinel-3 - Mission - Sentinel Online - Sentinel Online ) my understanding is 385 orbits per cycle. However on downloading a “fresh” file, I can’t reverse-engineer the formula, so must be missing something.

For example, file S3A_OL_1_ERR____20190815T045631_20190815T054045_20190816T083001_2654_048_119______LN1_O_NT_002 metadata says:

<sentinel-safe:orbitNumber type=“start” groundTrackDirection=“ascending”>18187</sentinel-safe:orbitNumber>
<sentinel-safe:relativeOrbitNumber type=“start” groundTrackDirection=“ascending”>119</sentinel-safe:relativeOrbitNumber>
sentinel-safe:cycleNumber48</sentinel-safe:cycleNumber>

The 119 and 48 match what’s in the filename. So from these I should be able to get orbit number 18187.

However, 48*385 + 119 = 18599 which does not match.
I then thought maybe cycles start from 1 not 0, in which case (48-1)*385+119 = 18214 which is closer but also doesn’t match.

Does someone know (and preferably can link to the source) for the correct calculation? I couldn’t find it in the reference documents linked above, or via a Google search.

#2

Hello,

The point is that the early phase of the mission is not regular (in terms of orbit). This was particularly true for S3B which was in tandem phase during 5 months, but with one transition phase to reach it definitive orbit.

I have set up the 2 formulas for S3A and S3B which are:
S3A: AO=(CCC-2)*385+RO+358
S3B: AO=(CCC-20)*385+3352+RO

where CCC is the cycle number and RO the relative orbit (from 1 to 385)

I have an excel file which also computes the inverse (RO and cycle from the AO) that I attach to this post.S3_AO-RO-CCC.xlsx (25.7 KB)

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#3

Thanks so much! This makes sense about the early-on irregularities. I appreciate these formula, and the spreadsheet.