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Curious lay person here...

I download and analyze Sentinel-2 imagery for a variety of mapping tasks. According to the Sentinel mission description webpage:

The Sentinel-2 mission orbit is sun-synchronous. Sun-synchronous orbits are used to ensure the angle of sunlight upon the Earth's surface is consistently maintained. Apart from small seasonal variations, anchoring of the satellites orbit to the angle of the sun minimises the potential impact of shadows and levels of illumination on the ground. This ensures consistency over time and is critical in assessing time-series data.

If I understand the quote, the satellites (there are two of them, orbitally offset by 180 degrees) will pass overhead at my area of interest throughout the year when the sun angle is the same (plus or minus a few minutes). Since the sun angle varies from day-to-day, this also means that the clock time that the satellites pass overhead will shift from day-to-day.

Am I correct so far?

But here's where I'm confused: My area of interest is in Washington State, USA, at approximately 48 degrees north, 121 degrees west, utilizing imagery tile 10UGU. I download tiles that were acquired on a variety of dates throughout the year, yet each downloaded file contains the same acquisition time, which is always 19:00Z, +/- 10 minutes, regardless of the day of the year.

If the satellite's orbital mechanics are engineered to pass overhead at a different clock time each day, how can the acquisition time (Zulu) be the same each day throughout the year?

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  • $\begingroup$ I'm not an exepert, but I'm not sure the premise "If the satellite's orbital mechanics are engineered to pass overhead at a different clock time each day..." is necessarily correct exactly as written. The orbit chosen certainly "minimises the potential impact of shadows" but I think the quote goes overboard with "anchoring of the satellites orbit to the angle of the sun". Orbits don't have anchors. I think if you plot your +/-10 minute deviations they may fairly well track the difference between local solar time and clock time. $\endgroup$
    – uhoh
    Nov 9, 2022 at 23:58
  • $\begingroup$ See for example Reason for satellite passes at the same local time each day? $\endgroup$
    – uhoh
    Nov 10, 2022 at 0:00
  • $\begingroup$ Also see how to better understand how dawn dusk orbits work? There are many other related questions. $\endgroup$ Nov 10, 2022 at 6:16

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I think the step that has caused you problems is "Since the sun angle varies from day-to-day, this also means that the clock time that the satellites pass overhead will shift from day-to-day.". Its hard to know what your sticking point is but I suspect it is because there is more than one angle you could be thinking of and that one changes and another doesn't. i.e. that statement is incorrect as far as your question is concerned.

See page 22 on this course, shown below The diagram is the important bit, helping to visualise in 2D the Earth's movement around the sun and the plane of the satellite orbit around the Earth

NB whilst the plane of the satellite is shown as a slanted line going through the middle of the circle representing the Earth, remember this is a 2D view and it doesn't mean the satellite passes over the Earth's poles. Such satellites are usually near about 98 degrees inclination from the equatorial plane.

enter image description here

The image is from https://www.slideserve.com/gafna/aro309-astronautics-and-spacecraft-design and is credited to Try Lam at CalPoly Pomona Aerospace Engineering, 2014.

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