Since I started using Sentinel-2 data, I’ve always relied on the first date and hour provided within the Sentinel-2 filename as the actual sensing time of the data.
Yet, in one of our recent applications, one of our partner compared the Sentinel-2 sensing time with the navigation logs of his boat captured in an image and according to him, there are a few minutes gaps between both times.
Looking deeper in the metadata, it found that the ‘SENSING_TIME’ value provided in the MTD_TL.xml file is different than the one appearing in the filename and indeed more consistent with the navigation logs of our partner.
Two questions:
what’s the difference between the sensing time in the filename and the sensing time in the MTD_TL.xml file ?
Who is right ? The date and hour in the Sentinel-2 filename or the navigation logs ?
Anyone has an idea on how to lift the doubt on this one ?
Many thanks for your help
EDIT:
Reading this topic, I understand that the sensing time given in the S2 filename is the datatake sensing time.
I found this post on the ACOLITE forum that explains it. Datatake sensing time is not the same as the granule sensing time.
So in the end, this means the navigation logs are right.
The S2 User Guide identifies that – for the Compact naming Convention – the End User Product (EUP) contain two dates. So in the Tile S2B_MSIL1C_20220127T081109_N0400_R078_T36PTR_20220127T100649.SAFE , the first Date/Time sequence is the Datatake (not necessarily the Datastrip – because see below ) sensing time.
A Datatake is:
The continuous acquisition of an image from one SENTINEL-2 satellite in a given MSI imaging mode. The maximum length of an imaging datatake is 15,000 km (continuous observation from northern Russia to southern Africa).
A Datastrip is:
A portion of a Datatake downlinked during a pass to a given station. If a particular orbit is acquired by more than one station, a datatake is composed of one or more datastrips. It is expected that the maximum length of a datastrip downlinked to a ground station is approximately 5,000 km.
So, a Datatake can contain several Datastrips.
More information on the Compact Naming Convention (used since December 2016) can be found in the Product Specification Document [PSD] available from the S2 User Guide Document Library (Sentinel-2 MSI Document Library - User Guides - Sentinel Online - Sentinel Online). The average sensing time of the Tile can be found in the MTD_TL.xml. This value is set to the average sensing time over the whole Tile.
) sensing time.