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Can a sun-synchronous satellite (let's consider Sentinel-1) go over the same place, one time in an ascending pass and the next time (or few times after) in a descending pass?

  • "The constellation will cover the entire world’s land masses on a bi-weekly basis" sentinel.esa.int/web/sentinel/missions/sentinel-1/… – Mapperz Jul 13 '15 at 20:01
  • look at 'stripmap' details sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar/… – Mapperz Jul 13 '15 at 20:05
  • @Mapperz Thanks for your answer. I guess that covering the land each 12 days means that the satellite will return to the same place in the same pass direction (in all the following cycles the same way it was in the first pass). – Hakim Jul 13 '15 at 23:52
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    I don't know about whether or not it's possible, but I don't know of any satellites that do it. Generally speaking, a satellite with a polar orbit is attempting to cover the earth during daytime, making descending passes at about the same time of day for each local spot/pass in order to minimize shadow, keep consistency, and get clearer data. This means the ascending passes happen on the dark side in these cases. Maybe there are satellites that behave differently, but I know landsat, at least, behaves in the manner I described. – nicksan Jul 14 '15 at 21:20
  • Thanks @nicksan this should be the accepted answer. In the case of a SAR satellite, I think that since the absence of sun light doesn't affect the images, it will capture active radiations both in the ascending and descending phases. – Hakim Jul 15 '15 at 11:47
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Sentinel-1 definitely takes images of the same point on Earth in both ascending and descending passes. Just search for all images intersecting a point (or tiny area) on any service that provides Sentinel-1 image access (SciHub, Google Earth Engine, AWS, etc.) and you'll see that images from both descending and ascending passes are returned.

Descending and ascending images will differ quite a bit though. In the case of SAR imagers such as Sentinel-1 that is because the radar waves come from a different direction and hence radar shadows, foreshortening, etc. will look differently. For optical imagers, there will be huge differences too because either of ascending and descending passes will be at night and the other one at day, as explained in nicksan's comment.

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It seems to me that many satellites are programmed to go over a certain area at the same time and in the same pass.

This increase coherence between each pass's images, which facilitate operations such as change detection on the image series generated from the set of images collected from the different passes.

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For a sun-synchronous system with two opposite passes it seems that you need more than one satellite. For the consideration of Sentinel-1 it actually consists of two satellites, Sentinel-1A and Sentinel-1B, which share the same orbital plane with a phase difference of 180°. Some services package the constellation into a single data source. When both are considered, than one will pass a particular location every 12 days in ascending orbit, while the other satellite will pass that same location every 12 days in descending orbit. Because of the phase difference of 180° and the sun-synchronicity, the time interval to revisit that same location in both passes is maximum 6 days.

This image from ESA Sentinel website visualises the orbits of the Sentinel-1 constellation with sunlight locked on the left side:

enter image description here

Image source and additional info: https://sentinel.esa.int/web/sentinel/missions/sentinel-1/satellite-description/orbit

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