There are two GPS signals: P and C/A.

There are two GPS frequencies: L1 and L2. Don't remember the numbers -- a gigahertz and a half or so.

Both L1 and L2 carry the P signal. The P signal is encrypted, and only military and military-related users can get the code for it. The P-code is accurate to roughly one meter, partly because it can use both frequencies. The frequencies are different enough that they can be used to determine atmospheric distortions.

The L1 frequency also carries the C/A signal. C/A stands for "coarse/acquisition", and was originally designed as a first approximation for receivers using the P-code. Any GPS receiver will start up more quickly if it has an approximate location to start with; finding out where you are in the world is the slow part. The C/A signal is not encrypted, and is accurate to about 10 meters if the Air Force allows it. At Falcon AFB near Colorado Springs is a tiltup building; somewhere in that building is a computer. Typing the right strings into that computer engages "selective availability", which induces errors in the C/A signal. Selective Availability can be set from zero (as now) to "somewhere on the planet". When it's used, it's normally set for 100 meters accuracy with 95% confidence.

When civilian users started using GPS in earnest the U.S. Government started calling the C/A code the "Standard Positioning Service" or SPS. That's what your Garmin uses; it's also what any civilian GPS receiver uses. SA was in effect from the late 80s up until 1999, except for during Desert Storm; as noted above, military receivers were in short supply, so the military bought thousands of civilian receivers [assuring the early profitability of Trimble and Garmin :-)] and turned SA off for that period. In the mid-eighties, the Europeans started campaigning for their own GPS system, because they didn't trust the United States not to turn the system off on them. In response, President Clinton ordered SA turned off, and promised [I think it's an EO] that it won't be turned back on again except in the case of a declared war.

The original reason for SA was to deny, e.g., Saddam Hussein the opportunity of stuffing your $300 Garmin in the nose of a SCUD missile and nuking the White House. That point was made moot by the invention of Real Time Correction Messages, which I won't go into without a specific request. :-) But with SA off, the military worries. There are several methods of jamming GPS; the most sophisticated is to set up a "pseudolite", a radio transmitter that sends simulated GPS signals with wrong data on them. That's what this sounds like.