The following image shows Earth and the trajectory of the ISS. A green line indicates which part of the earth can be observed from the ISS simultaneously.

What is the name of this line or this area?

enter image description here


2 Answers 2


What is the name of this line or this area?


A term for the line that's perfectly usable for this purpose is "horizon".

The horizon, the line line separating the land from the sky, would be the green line in your image. Anything closer than the horizon will be visible to the spacecraft.


Note also that even though the area appears to have an irregular shape in your image, it's just distorted due to the map projection. It's otherwise circular.

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    $\begingroup$ you are right....it "is" perfectly usable here :D now this question feels even more silly $\endgroup$
    – rul30
    Commented Oct 5, 2020 at 10:11
  • $\begingroup$ Note that to see circular horizon from ISS in one single picture, you need one ultra wide angle lens (almost fisheye) since required aperture of lens is 140° (6.5mm focal lenght for 36mm sensor) (human eye is roughly 50mm equivalent for 36mm sensor) $\endgroup$
    – user19132
    Commented Oct 5, 2020 at 18:22
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    $\begingroup$ Although "horizon" is the boundary of the region, not the region itself. $\endgroup$ Commented Oct 5, 2020 at 21:15
  • $\begingroup$ @Acccumulation indeed. Wouldn't it be weird to say that the point right below the ISS is the horizon? $\endgroup$ Commented Oct 6, 2020 at 2:02
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    $\begingroup$ Technically it is "Line of sight". $\endgroup$
    – A. Rumlin
    Commented Oct 6, 2020 at 9:05

What is the name of this line or this area?

Typically, the part of the earth's surface that a satellite can view at any moment is known as its footprint, a term frequently used for remote sensing satellites and communications satellites.

SE's answer about the horizon applies if the satellite is viewing the entire visible portion of the globe at once. As somebody pointed out in a comment, that would require a camera with an extremely wide-angle lens.

Your question is asking about ANY satellite, but then you go on to ask about the ISS. Different satellites have different orbits. A remote sensing satellite will probably have a polar orbit, where it covers the entire surface of the earth after a number of orbits.

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    $\begingroup$ "the satellite is viewing the entire visible portion of the globe at once" What other part would it be viewing, than the visible part? The green area in the question literally is the visible part to the satellite. $\endgroup$ Commented Oct 5, 2020 at 20:27
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    $\begingroup$ Obviously the satellite couldn't observe a portion that wasn't visible, but most remote sensing satellites only view a narrow band of the earth at any one time. They assemble a view of the whole planet by taking multiple passes during multiple orbits. $\endgroup$
    – Duncan C
    Commented Oct 5, 2020 at 20:44
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    $\begingroup$ Sure, but that's not what the question is asking about. The definition of "horizon" is literally "where the earth appears to meet the sky" and in this case, it bounds the area that is visible to the satellite. $\endgroup$ Commented Oct 5, 2020 at 21:00
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    $\begingroup$ Semantics, but: I have mostly seen footprint or in particular field of view used to mean the extent of a single measurement (be it pixel or sounding). But then again, the area that can be observed from a satellite is not of interest to most users, what is of interest is the area that is observed. So there are three different things: the area it could observe at a given time; the area it does observe at a given time, and the extent of an individual measurement. $\endgroup$
    – gerrit
    Commented Oct 6, 2020 at 8:16
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    $\begingroup$ @gerrit there are also "field of regard" (FOR) and "instantaneous field of view" (IFOV), and I concur with your semantics, assigning FOR = could observe, FOV = does observe, IFOV = single measurement. In my radio communication experience, we used "footprint" to mean the area inside a contour of X decibels down from peak (3, 10, other), at some frequency F, for each aperture, real or synthetic, on each of the antennas. That means there are hundreds or thousands in use simultaneously for big comsats trying to cover the geosynch FOR at Ka-band. $\endgroup$
    – Ryan C
    Commented Oct 6, 2020 at 17:29

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