Of course, it depends on where the nearby transmitter is. The most likely source of the most troubling transmissions would be a customer's Lightsquared mobile phone, not the Lightsquared base station. You don't have to go very far from any base station before the received signal strength is really quite low. 1/r^2 is a powerful attenuator! The mobile phone, being so close to the customer, is almost always the more powerful transmission relatively speaking.
You always *can* build a filter with the required level of isolation, but for very high levels of isolation it is going to be physically quite big and/or expensive. This would not be ideal for a GPS receiver which is by definition a portable device. Military equipment doesn't mind filters being expensive, but large filters can be as much a no-no as for civilian applications.
It is very difficult for the frequency planners to know what to do. They could allocate frequency bands in such a way so that there's adequate guard bands either side to protect against any conceivable transmitter. But that would be enormously wasteful of bandwidth, and it depends on an accurate understanding of what technological developments there may be in decades to come. Not a straight foward task, and something that humans have proven to be very bad at. Remember the 640k RAM limit?
But in this particular case I think that even the original use of the adjacent bands for a satellite mobile service was arguably wrong. Sure, the satellites for that service were a long way away (in orbit, in fact), so their transmissions were never going to be powerful enough on the ground to affect GPS receivers. But those satellite mobile phones themselves would have to be quite powerful to be received by their satellites. It's quite possible that they would have interfered with GPS receivers in much the same way as Lightsquared's transmission apparently does. I suspect that this has been going on all the time. But seeing as the original satellite mobile service is defunct it sounds like it wasn't so popular in the first place. So maybe the problem was always there, just not badly enough for it to come to widespread attention.
So if that is the case, who is to blame for the mess? Lightsquared? Not really, they've acquired the rights to a band; they're not transmitting outside that band, so they're sticking to the rules. However, I do agree with the view that Lightsquared should have known better. In effect they trusted the FCC to know their stuff when they asked for the band. Was that commercially a wise choice? Probably not. And I bet that the receivers on Lightsquared's mobile phones are just as vulnerable to out of band interference [similar bands, similar electronic constraints to achieve significant out of band filtering]. They're getting away with it too, because the adjacent band is GPS, which has no terrestrial based transmitters. I bet that if GPS were replaced with some sort of mobile telephony service, Lightsquared would be complaining just as loudly as the GPS crew are today.
How about the frequency planners at the FCC? Depends on whether they were obliged to consider mass market cheap GPSs when the bands were allocated all those years/decades ago. Back when the bands were first considered (in the 1980s?) it would hardly have been imaginable that we'd have mobile phones, never mind mobiles with GPS in them. And the rules on operating bands are quite clear. If you receiver is open to receiving transmissions from adjacent bands that's your problem, not the FCC's.
How about consumers and their want of cheap, small GPS receivers? No not really, they're the not the designers of the equipment they've bought.
How about the GPS receiver manufacturers? Largely yes - they've got away with ignoring the effects of transmissions in adjacent bands for many years now when they had no right to assume that those bands would be forever quiet. The FCC rules (and the rules from frequency planners everywhere else in the world) are very clear in black and white on paper about that, and always have been. And if the manufacturers had paid strict attention to the FCC rules then we would likely not now have things like GPS in phones.
So what happens now? Personally I don't think that the GPS manufacturers deserve to get away with it. However, the real question is does the paying public deserve the right to access the GPS service in the way they do? Yes, they do! Affordable, convenient and functional GPS does make the lives of the general public better in a very big way, and improving the lives of the public as a whole must be a governmental goal.
I think that the FCC should buy out Lightsquared's band allocation (a pricey proposition), recover the cash through a one off levy on any GPS manufacturer with (currently) non-compliant kit, and place bigger guard bands either side of GPS.