Semi-geostationary orbit?

Zatnikitelman

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The recent business with the Astra 5A going all screwy and possibly threatening two other satellites got me to thinking. Since GEO is becoming increasingly crowded (yes, I know it has a circumference of 226,194Km) would it be possible to put satellites in some sort of near-GEO orbit with the same usefulness?
Consider this scenario, a private spaceflight company wants to put its own TDRS-like satellites into GEO but doesn't want to try to wedge them into a spot that someone else could need. Could they perhaps use a slightly inclined (<= 0.5 degrees) slightly eccentric orbit with the PeA and ApA a few kilometers below and above respectively GEO with the PeA and ApA lying on the descending and ascending nodes to avoid any chance of collision? I don't right off hand know how much a satellite would drift given that orbit so I don't know if it'd be useful for anything that required constant communication.
 

Scarecrow

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The recent business with the Astra 5A going all screwy and possibly threatening two other satellites got me to thinking. Since GEO is becoming increasingly crowded (yes, I know it has a circumference of 226,194Km) would it be possible to put satellites in some sort of near-GEO orbit with the same usefulness?
Consider this scenario, a private spaceflight company wants to put its own TDRS-like satellites into GEO but doesn't want to try to wedge them into a spot that someone else could need. Could they perhaps use a slightly inclined (<= 0.5 degrees) slightly eccentric orbit with the PeA and ApA a few kilometers below and above respectively GEO with the PeA and ApA lying on the descending and ascending nodes to avoid any chance of collision? I don't right off hand know how much a satellite would drift given that orbit so I don't know if it'd be useful for anything that required constant communication.

Well if you've got a slightly inclined, slightly eccentric orbit, with a period of 1 sidereal day, then your ground track should be a small circle. This would seem almost as good, though as you pointed out, without a better background in orbital mechanics, it's difficult to say how the drift would differ. It could be better, worse, or just different, I don't know.
 

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According to the Thales Alenia Space statement, the problem was that they "could not restore the satellite into a stable sun-pointing attitude but consumed a significant amount of propellant and discharged the satellite's battery." I could not find any report that it threatened any other satellites, at least not in the near-term.

Space is big, and it is unlikely that two satellites in geostationary orbit will collide. However, it is standard practice, at the end of a satellite's life, to use the last of the satellite's propellant to push it into a higher orbit so that it will stay out of the way for a very long time.

Doing this with a satellite which no longer has it's solar panels pointed at the sun, is out of battery, and has used most of it's propellant will be a good trick.


-----Post Added-----


A geosynchronous orbit which is inclined will cross the equator twice each orbit. During that crossing there will likely be communications trouble. The reason for this is the beam width of the high-gain antennae used to communicate with the satellites.

Most communications satellites use C-band, Ka-band or Ku-band because these bands suffer little atmospheric absorption. The frequencies in these bands are re-used on many satellites. With the satellites stationed two or three degrees apart, a high-gain satellite with a one or two degree beam width can point at the desired satellite and not interfere with the neighbors.

If two satellites come within a "beam width" of each other, it will not be possible to transmit to one without transmitting to both. For that reason merely inclining the orbit by 1/2 degree will not solve the cogestion problem. It will require that the antennae move north/south (and possibly east/west depending on eccentricty) to follow the satellite. Lots of pain for no gain. For these reasons "almost geostationary" orbits are not used.

A Tundra orbit or Molniya orbit does solve the separation issue (at the cost of requiring the antennae to track the satellites) and these orbits are used for communications purposes.
 

Andy44

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The whole reason a geostationary orbit is desireable is because ground antennas can be bolted in place pointing in one direction and don't need to track anything in the sky. (Like the Direct TV dish on top of your house.)

If you use different orbits with wandering ground tracks, either you have to eqip every ground antenna with expensive and finicky tracking gear, or you have to open up the beamwidth of the satellite's antennae, which means reduced gain and sloppy footprints that wander over the serviced land area.

That said, most geoyncronous satellites are not truly geostationary anyway.
 
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