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The risk to satellites from a solar storm is very different from the risk to power lines and other long conductors on the ground. Ground systems are primarily affected indirectly by the distortions and time-varying fields in the earth's magnetic field. Low altitude satellites, like the International Space Station, are also in this zone. But high-altitude satellites, like the GPS satellites and most communications satellites are well above the primary shielding effects of the Earth's magnetic field. Up there, you've got high-energy charged particles raining down on the satellites during a solar flare. These can create secondary showers in the components of the satellites which can indeed create short circuits. Of course, the satellites are designed with this in mind. Any "space weather" event capable of trashing the GPS satellites would almost certainly kill every other satellite in high orbit. It can happen, yes, but at what probability level? We don't insure against events which are astronomically rare, but in a way we do by robust design.

So what sorts of things could knock out the GPS signal (as opposed to knocking out a single GPS receiver/system, for which the best backup is most definitely another GPS receiver)? I made a list of some possibilities on the back of an index card (the front side had my grocery list on it) over dinner last night.
1) Solar flare or other space weather event. Yes, possible, but as noted above the satellites are designed with this in mind for some wide range of events and events outside that range are very rare.
2) Meteor storm. Yes, possible, but there was only one reported possible satellite failure from the extremely intense Leonid meteor storms around 1999-2002. Nothing bigger than those is believed likely on a time scale of many centuries. "Space is big, really big"... the chances of getting hit even when the sky is filled with meteors just isn't that large.
3) Economic/political distress. Yes. Not only possible, but we already have empirical evidence on this one. The Russian GLONASS system was fully operational around 1995 but it rapidly collapsed because the Russians at that time simply could not afford to maintain it. What is perhaps not widely known is that GLONASS is back. It has been declared effectively operational again (21 out of 24 operational satellites) just within the past few months. But the lesson is plain. A global satellite navigation system can fail, and the easiest way for it to fail is through lack of funding. Another lesson here is that we can no longer say that GPS is the only game in town. You want a backup to GPS? You've got one. It's called GLONASS.
4a) Intentional missile assault against the GPS satellites. This is much harder than it sounds. Lobbing shells at the GPS satellites would be both technically difficult and very obvious. It's not the sort of thing that rogue nation or non-state entity could pull off. One would need a lot of large rockets and sophisticated aiming capabilities. It would be an act of war and obvious well before it happened.
4b) Intentional attack elsewhere in the GPS system. Where is the weakest link in the GPS system? The satellites are the obvious targets, but what about the "control segment" or ground control? GPS satellites require regular, frequent updates from the ground. Now since the US GPS system has been designed originally as a military asset, there are numerous backups in place. They've considered the possibility of a direct military attack on their ground control operations and, supposedly, they have the capability to transfer those capabilities if necessary. But I doubt that this has ever been tested.
5) Intentional energy attack. Blast the satellites with microwaves. I don't believe that this is technically feasible right now without being a blatantly obvious act of war (as with a missile attack), but it would be easier to hide and might be a bigger threat in ten years.
6) Deliberate signal degradation, known as "Selective Availability" in the GPS system. Some folks obsess over this. They're living in the past. It was turned off TEN years ago primarily because it's easy for an enemy to work around it with DGPS, and even the military found it more of a hindrance than a value. The next generation of GPS satellites do not even incorporate it into their design. Those who obsess over this tend to say "but, but, they can still throw the switch if they want!". Sure but ANY signal can be altered. Why don't people worry about the US military or other authorities globally messing with LORAN signals or even radio time signals? Want to throw off celestial navigation by fifty miles?? Then change all the radio time signals you can get your hands on by 200 seconds. This would never be done and was never done historically for all the same reasons that GPS signals would not be degraded. You shoot yourself in the foot without gaining any strategic advantage.
7) Software error. This is always a possibility. As software systems go, the GPS system apparently passes through more quality-control testing than the vast majority of software on Earth. The nice thing about a software problem is that it's temporary if it happens. Some deeply hidden bug might take the GPS system down for a day, even a week, but not longer. Software can be fixed and uploaded.
8) And FINALLY we get to the big one: intentional and unintentional signal jamming. This does occur and will continue to occur. Jammers can be tracked and destroyed, but they cannot be prevented.

-FER

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