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Ltryptophan's avatar

How do spy satellites get positioned?

Asked by Ltryptophan (9112 points ) May 30th, 2010

When spy sat’s are needed do they have to wait until it hits a preferred spot in orbit before they can use it, or do they just hit the retro rockets and cruise over to where they want it?

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13 Answers

Rarebear's avatar

Depends on its function. Satellites that are designed to map and see the whole globe go in a north south direction so the Earth rotates under them. Otherwise, they’ll be in an orbit that goes over the area in question every 90 minutes or so. They don’t use maneuvering thrusters except to keep in the proper orbit.

Or the satellite can be in geosynchronous orbit—but those aren’t as good as spy satellites as they are must farther out.

timtrueman's avatar

There’s also USA-212 which is currently in orbit on its first flight into space (it’s five weeks in and its return date is classified). Basically it’s an unmanned space shuttle designed to be in orbit up to 270 days that’s capable of maneuvering around a tiny bit. It used to be a NASA project (it was called the Boeing X-37) but it was taken over by the Air Force and became a secret project (speculation says it’s probably only useful for reconnaissance missions). It is reusable just like the space shuttle but unmanned and fully autonomous.

Side note: it’s not the first autonomous reusable spacecraft—the sneaky Russians built the Buran although it was tragically lost when the hangar collapsed (also funding was cut).

Ltryptophan's avatar

@timtrueman i never even heard ‘bout that…usa212…

RocketGuy's avatar

Satellites can be in orbits of various altitudes from ~100 miles to >20,000 miles above the ground. The higher you go, the longer it takes to complete the orbit. The Space Station is at 190 miles altitude – takes 90 min to go around. Communication satellites are in a geosynchronous orbit at 22,300 miles altitude – takes 24 hr to go around. I’m not sure about spy sats.

It helps me visualize an orbit by thinking of Saturn’s rings. If the ring is parallel to the equator, the satellite always flies over the equator. Note that a spot on the ground at the equator goes around Earth’s axis in 24 hours. So if you are in a low orbit, you will go faster than the spot on the ground, and (relative to the ground) move in an easterly direction. That might be good for observing things along the equator.

If the ring is tilted relative to the equator, you will go from NW to SE on one side of the orbit, then SW to NE as you loop around back. Check for yourself using a ball and a ring-shaped object. This is called an inclined orbit. If the axis of this orbit rotates around Earth’s axis, the satellite goes over much of the Earth’s surface. In fact, if the orbit is inclined 90° to Earth’s axis, it covers ALL of the Earth’s surface. Most spy sats are in inclined orbits so that they can cover a lot of area. Since little population is in the polar regions, few spy sats are in 90° inclined orbits.

To shift coverage, one can cause more tilt to view higher latitudes. To change timing, one can cause the axis of the orbit to rotate more or less than it currently is. Bad guys often have a lead on satellite position, so can calculate when the next pass will be. They then hide their stuff until after the sat has gone by. If the axis were suddenly changed, the overfly event will not be as calculated and their attempts at hiding will be in vain. All this is accomplished through clever use of rocket thrusters.

Geosynchronous sats stay over only one patch of ground. To reposition, they use thrusters to go to slightly higher or lower orbits. When they do that, their speed will no longer be 24 hours to get around. They will move E or W relative to their original ground location. When they are above the correct patch of ground, they go back to 22,300 mile altitude and synchronize back to Earth’s rotation.

Rarebear's avatar

You’re not going to get a better answer than @RocketGuy because he is. His avatar is of a satellite he designed. Seriously.

XOIIO's avatar

@Rarebear Really?

I once joined a youth program proposing solar powered thrusters for satelites that could [REDACTED]

Rarebear's avatar

@XOIIO Really. We went on a backpacking trip a couple of years ago and all he talked about was rockets and satellites. Of course I kept peppering him with questions.

XOIIO's avatar

That’s sweet. Is the satelite he designed in use or just an idea?

Rarebear's avatar

@XOIIO Flying over your head even as I type this.

Rarebear's avatar

@XOIIO I think it’s Echostar 4, but that I’m not sure.

RocketGuy's avatar

It is Echostar-XIV. We just handed it over to the customer – works like a champ! All kinds of local HDTV coming down now. They are putting out a lot fewer ads than I expected.

RocketGuy's avatar

Sat pics of African dust heading towards S. America:
http://www.satnews.com/cgi-bin/story.cgi?number=182645346
Amazing that dry dust from barren Africa is needed to fertilize lush S. America.

Note they mention that the satellite orbits from pole to pole. That means the orbit is inclined 90°.

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