...Air Force report on difficulties replacing aging GPS satelites. They are behind schedule and predict the probability of keeping a 24-satellite constellation in orbit drops below 95 percent in 2010, and could drop as low as 80 percent in 2011 and 2012.
Translation: if the AF doesn't quit screwing around, tinkering with the specs and just *build* *what* *they* *already* *know* *they* *need*, then some satellites already in-orbit might be beyond their design service life before a replacement is launched.
The AF is distracted by systems from Europe, Russia, etc, and trying to make sure theirs is "better", forgetting that nothing on overhead slides is ever as good as something in orbit and operational.
Accuracy is already great. If a lot of money is to be spent it ought to go into jamming-resistance.
You need 4 satellites for a fix, so having 24 in the aid includes a large safety margin.
You need 4 *over your location* for a fix. You need more in-orbit to guarantee that 4 are visible from every point on Earth at all times.
At little more than a flip of a switch they can turn off all non-military use. They can, and often do, insert various amounts of random error, referred to as 'dither', into any or all parts of the system.
Selective Availability was turned off by Presidential Order during the Clinton administration.
Simply turning the system off for civilian users in time of war turns out not to be an option. In the Iraq war DoD was woefully short on the military grade GPS units, which can decode the encrypted signals, and had to rely heavily on civilian models, which can't decode the encrypted signals. That meant no turning off civilian units or even significant degradation.
High end GPS units, which use more satellites and fold in a considerable amount of computational power and multiple readings over time, can avoid a good bit of the error and give more consistent readings.
The key to accuracy is a stable oscillator. Better units shield their oscillator better, with better temperature insulation & compensation, etc. Propeller-heads that want to locate their antenna within a millimeter will use pairs of rubidium oscillators.
The Indian government is cooperating with the Russians on Glonass. I'm not sure if they actually went through with it, but I remember reading a while back that the Indians were even going to launch some of the satellites for the Russians.
The Russians that the best launch systems in the world: I cannot imagine why they'd spend money to build the birds and then launch on an Indian rocket.
More likely the Indians will build the satellites and the Russian contribution is launch services.
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