*Running out of gas* Mr A, I have good reason to believe you sent me on a wild goose chase with that post, ending up with random orbits with instantaneous licensing to save orbit control fuel with the intent of increasing the life of satellites.
Now, after a lot of beating around the bush, it seems that your post was not strictly accurate - or at least emphasized fuel rather than wear and tear. There is heaps of fuel in Globalstar satellites and carting fuel around is not a significant issue, even if they are dropped off in a low orbit and have to heave themselves up to 1414km.
I still quite like the idea of random orbits, but that can wait until there are a couple of hundred satellites up there. Nothing to be gained but a bit of fuel saving by the look of it, which is of minor importance. Even the cost of satellite jockeys will be reduced if the satellites are put on autopilot to hold orbits.
A few puzzles came out of the explanations but they don't really matter for Globalstar performance.
For example, if two satellites crash, the debris will spread out in 3D, not in a plane. But all the bits of it will go slower, so will fall down; conservation of momentum would see to that. Some will spread sideways in both directions contaminating other planes. If there is fuel on board and it ignited, which I suppose it would do in many collisions, there would be an explosion so plenty of debris would be speeded up and would shoot up into higher orbits and sideways out into other planes and into eccentric orbits and all over the place. I can't really see that collisions would end up destroying whole planes of satellites because too much debris would be headed earthwards - it wouldn't be like a nuclear chain reaction where each collision increases the probability of further collisions.
Anyway, it isn't really an issue because there is no shortage of fuel and the dead satellites are parked up in a satellite cemetery orbit.
Another puzzle is that since perturbations due to the moon and oblateness of earth put satellites out of orbit, the orbit would be likely to become eccentric and that eccentricity could increase for uncontrolled satellites. The uncontrolled cemetery-orbit satellites would bump into each other and the debris would fall down. But the rings of Saturn seem happy to hold position in distinct rings, though asteroids in the asteroid belt relocate often enough. Sometimes with serious consequences for reptiles and causing an Iridium layer in the geological strata, which presumably resulted from previous constellations of prehistoric LEO satellites which didn't work. [Just kidding - I think sometimes people miss the less serious comments]
Anyway, I heard that there's lots of fuel on the Globalstar satellites, especially those from the Delta rocket. There being enough to last 20 years [not that wear and tear will allow that].
I also heard stationkeeping is taking less fuel than expected where stationkeeping denotes moving a satellite around within the orbital plane. This is called changing the satellite's 'argument of latitude'. Everyone agrees attitude control takes negligible fuel, thanks to the use of momentum wheels.
The only real constraint on fuel is that about 10-15% of the total fuel is needed to decommission the satellite, which is done by raising the satellite to about 1500 km - the internationally established "Space Cemetery" orbit. They just turn the satellite off and it drifts around with all the other relics. There isn't nearly enough fuel on board to have the satellite deorbit and crash back to earth.
Mr A, contrary to the 'running out of gas' idea you put forward, what will ultimately determine the life of the satellite is the performance of the electrical power subsystem. This includes the batteries, the battery charge controller, and the solar arrays. In an earlier post, someone noted that the duty cycling of the batteries is what establishes the satellite lifetime. However, the failure of the batteries or the charge controller is something that may or may not happen; whereas the degradation of the photovoltaics is certain. So the solar arrays are guaranteed to establish the lifetime of the satellites, assuming no failures of the batteries. The photovoltaics should degrade enough to end operations after about 10 years.
Here are some more satellite jockey activities: orbit raising, planning launches, constellation evaluation, coverage analysis, stationkeeping [fixing inclination, eccentricity, and RAAN deviations, as well as argument of latitude], supporting over-the-air testing, yaw-steering transitions, software development...it goes on and on [to coin a phrase]. By the time they get it all working correctly, it'll be time for Globalstar2.
Maurice
PS: Phew, I'm glad to see the end of that one. No more wild geese please. Now, back to the regular programming of "CURRENT PRICE IS..." to maximize the use and profitability of those critical photovoltaics, batteries and circuits and terrorize ICO into cancelling their damn system!
Executive Summary: Fuel? No worries mate. We'll get about 10 years out of these satellites by the look of it unless the batteries go on the blink. Performance will drop off over those years, but she'll be right. |
