Re:
There is no reason to believe Iridium and Globalstar sats won't last 10 years.
This needs some clarification. When a satellite's lifetime is quoted, what is usually being reffered to is the amount of fuel on board with respect to its expected future consumption of that fuel. In other words, how long it is going to take for the thing to run out of gas.
In order to be useful as communication devices, a satellite has to remain within a certain tolerance of its specified location. If it drifts too far from its specified location firstly you risk over stepping your licensing restriction, and then ultimately the laws of physics. As an example, A GEO that has thousands of antennas on the ground pointing at it won't do much good if it drifts from its orbit location. Iridium, as another example, needs to have their satellites within a certain position so that their inter-satellite links will work properly.
There are lots of forces that perturb a satellites orbit – Earth's oblateness and Lunar gravity to name two. As a satellites orbit drifts from its orbital slot, as it is commonly referred to, it requires propellant to push it back to where it is supposed to be.
The forces that perturb a satellites orbit are fairly well understood, as is how much propellant it takes to overcome those forces. So, remaining life on a satellite is a fair estimate. Not a perfect estimate, but quite reasonable.
All this is known and calibrated well before any metal gets bent, let alone a satellite is actually launched.
The flip side to satellite life is just plain old mechanical wear out. Much harder to predict, but that doesn't stop people from trying. The probability of a satellite failure due to mechanical wear out is gauged by its expected life due to fuel. For example, It would be dumb to have a satellite with a 99% chance of living to the ripe old age of 20 if it is only going to be launched with, say 2 years of propellant. So, the two design parameters are coordinated.
Using Iridium as an example, they probably have a mechanical design life of 5 years with a Ps (known as probability of success) of somewhere in the low 80% to low 90% range. This is just a guess on my part. They also probably have 5 years of fuel at their respective beginning of life. So, as an extension of the above assumptions, the satellites themselves may last much longer than 5 years, but they may not have the fuel to support themselves. Once fuel is gone, that doesn't mean a satellite is junked immediately if it still producing revenue, but, the writing is on the wall, so to speak. Eventually, it will drift beyond its box, and either licensing laws or physical laws will bring on its demise.
As for Globalstar, the satellites launched in to a 900 km orbit have 7 ½ years of fuel. But for those (like on the Delta) that get launched into a higher orbit, their fuel reserves is much, much greater. That is because the fuel is used not only for orbit keeping once on station, but also orbit raising after launch.
Regards,
Mr A
|
