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Is there any specific reason especially for the remote sensing satellites passing at the same local time over a particular place each day?

I have some experience measuring the radiance received by vegetation at different local times for example, and the variation can be quite large. In the morning it was low, and at noon it was high. I was wondering if there is any specific reason for satellite measurements at the same time each day rather than sampling at different times on different days, in order to better understand daily variations.

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  • $\begingroup$ Since most of these satellites sensing techniques rely on the reflected sunlight from the ground objects, I find quite logical to maximize satellites passes over an area around the noon where radiation is maximum (if local time coordinated with mean sun time) when the sun has its larger elevation angle from the ground object point of view. This applies for mapping applications mostly. $\endgroup$ – Julio Nov 28 '17 at 8:44
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These are called "Sun-synchronous orbits" and extensively used for remote sensing, where satellites are looking down at the Earth's surface. One of the main reasons for these orbits is that the shadows on the ground are always the same (i.e. the illumination angle is constant between each flyby). This is especially useful for imaging since the changes in terrain can be assessed with ease: the shadows are always the same, so if something looks like it has changed, then it is very likely not an abberation but indeed a change in the terrain.

Example of missions in these orbits include spy satellites.

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    $\begingroup$ The answer was in the question. The orbits are designed to pass at the same time because the variation is large based on the time of day, which makes spotting changes harder. $\endgroup$ – JCRM Nov 28 '17 at 10:34
  • $\begingroup$ I think there are examples of other types of non-imaging metrology that is done at a repeatable local time from Sun-synchronous orbits. It's not limited to just imaging and/or spying. However I can't think of one right now, I'll try to look for something. $\endgroup$ – uhoh Nov 28 '17 at 11:35
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    $\begingroup$ @uhoh Some spacecraft are in a polar orbit (which is near-SSO) so that they have longer ground contact times with antennas which has high north, such as Kiruna. I remember being briefed on an ESA mission which had that, but I forget its name (Sentinel 5?). $\endgroup$ – ChrisR Nov 28 '17 at 14:03
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    $\begingroup$ An additional reason is that if the spacecraft is in a sun-synchronous orbit over the terminator, it can guarantee that it is always in sunlight, so it will always have solar power. $\endgroup$ – antlersoft Nov 28 '17 at 16:32
  • $\begingroup$ The satellite would not always be in sunlight because of eclipse region. The earth will come in between sun and satellite. For a 100 minute orbit the eclipse region is about for 30 minutes. The mentioned case is a special one. $\endgroup$ – Sumit Agrawal Nov 29 '17 at 21:12

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