Following our reader responses the other day, I'll throw in one last post on the JDAM issue and then we're done. It's another individual who prefers to remain anonymous - let's just refer to "Reader Y":

"I no longer have the reference at my fingertips, but the same 1995 law that decreed that Selective Availability (the feature that degraded civilian GPS signals, worldwide, to a militarily acceptable level) be reduced to zero by 2005 tied that to efforts to deny GPS to our enemies while preserving it for ourselves in a given area, without screwing up the rest of the presumably non-belligerent world. And, of course, the law did not spring out of thin air, there were many studies and projects in and out of the military prior to that."

This is an interesting angle that no-one had brought up.

Quick Background: the Clinton administration saw that other nations and commercial projects were looking at putting a rival to the U.S. Global Positioning System (GPS) into orbit. That way, the ability to know exactly where one is would not be dependent on the USA's whims. So Clinton took a leaf from the Microsoft playbook, and opened access to the GPS system... but on the USA's terms. The idea was that this would be "good enough" for most, and so remove the business case for multi-billion dollar investments by others. Like Microsoft, the USA also wanted to make sure it had the hooks it needed to make life difficult for people if that became necessary.

With me so far? OK, now for the payoff paragraph...

"By presidential directive, the DoD eliminated Selective Availability from the civilian signal in 2000, five years early, which presumably means the DoD felt sufficiently comfortable with its other efforts."

Wow. To do this way ahead of schedule... yes, I'd say they're very confident. Those efforts to selectively deny the signal to enemies and preserve it for ourselves would require them to address jamming and encoding issues. Reader Y continues:

"Here is a reference to some seminars at one of the annual civilian Institute Of Navigation meetings that deal with all kinds of GPS jamming issues - this should give you an idea that people have been working the problem."

Interestingly, here's a reference from an Indian source that claims the GPS signals over Afghanistan are in fact degraded during our military ops.

In short, while like any other system GPS has its weaknesses, there are numerous technical, procedural, and operational techniques used to make it work for us and not for the bad guys - and as you discovered with the link emailed by Jay Manifold, and the comments by Sgt Stryker, GPS is not used in isolation but along with a lot of other systems, making any one jamming technique or effort much less likely to succeed."

One would certainly hope so.

UPDATE: Steven Den Beste weighs in today with some more technical thoughts, drawing on his own career in electronics engineering and telecom. Some of the points were already covered in yesterday's update, but a lot of it is new and adds to the discussions. Have a look.