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SEL1 and SEL2 are about 0.01 AU away, 10 seconds light travel time back and forth, 4 times Lunar distance, 45 times the distance to GEO. From those two regions combined, all surface on Earth and the Moon and all of cislunar space are always in line of sight.

SEL1&2 are already used for scientific applications. But wouldn't they be useful also for commercial and military "inward looking" applications? Isn't it great real estate for communication and surveillance of military spacecrafts and of debris and impacting asteroids? Or are they too far away?

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  • $\begingroup$ GEO is much more practical, because it's much more closer (about 42 times closer to the Earth's surface, more if we consider just its orbital space) and you'd also require just two spots coverage (per longitudinal hemisphere, just like SEL1&2) since all orbits have to cross the equator. You don't get complete surface coverage (you'd need four for that), but you do complete orbital. GEO is also already used for such purposes. $\endgroup$ – TildalWave Jul 26 '15 at 8:46
  • $\begingroup$ @TildalWave But you miss the far side of the Moon, and some assets in EML2. (Who knows what Dr. Evil is up to there?) $\endgroup$ – LocalFluff Jul 26 '15 at 8:53
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    $\begingroup$ You ask about space surveillance, not remote sensing. Difference being, that the former refers to surveillance of orbital space, and the latter surface observations (and surveillance) from space or high altitudes. If there's anything in lunar orbit, you'll also be able to infer that from GEO at a minimum of 1.5 times closer distance than from SEL1&2 (and on average a whole lot closer than that). Lunar libration from such high altitude is also substantial, so you'd "see" some of its far side. $\endgroup$ – TildalWave Jul 26 '15 at 8:58
  • $\begingroup$ @TildalWave So it is hard to motivate SEL commercially or even militarily, in competition with other orbits. The geometry is interesting, but maybe only useful for shipping and science instruments. $\endgroup$ – LocalFluff Jul 26 '15 at 9:54
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For voice and video communications, a 1-second delay is annoying. 10 seconds is way too much to be useful. Current data networks don't tolerate 10-second delays either so you'd need new communications technology. You'd also need large dishes on the ground instead of the small rod antenna now used in satphones.

Military surveillance of space can be done from the ground for now. It's much easier to build a megawatt radar on Earth than to cram the Cobra Dane installation into a satellite.

Cobra Dane

If militarization of space starts in earnest, that's early enough to start thinking about monitoring locations that are hidden from Earth's view.

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