WHAT IS GPS AND HOW DOES IT WORK ? - Courtesy of GARMIN Corporation

Excerpt: The pseudo-random code identifies which satellite is transmitting--in other words, an I.D. code. We refer to satellites by their PRN (pseudo-random number), from 1 through 32, and this is the number displayed on a GPS receiver to indicate which satellite(s) we are receiving. So why there are more than 24 PRN numbers? This simplifies maintenance of the GPS network. A replacement satellite can be launched, turned on, and used before the satellite it was intended to replace actually fails! They simply use a different number (again from 1 through 32) to identify the new satellite


The above is NOT taking into account WAAS or EGNOS
More information covering the higher numbers of Differential GPS

The US West Coast WAAS Satellite is #47
The US East Coast WAAS Satellite is #35

It seems that EGNOS Satellite numbers are #33 and #44


Satellite # Garmin
The NMEA standard WAAS labels are used by Garmin receivers. This system maps the PRN number to the NMEA label by subtracting 87 from the PRN number. (source: EGNOS help desk) The mapping from the Garmin numbers to the standard names and PRN numbers are:

Satellite # ....................Garmin #
AOR-E 120 ......................33
AOR-W 122 ......................35
ARTEMIS 124 ................... 37
IOR-W/F5 126 .................. 39
MTSAT1R 129 ................... 42
IOR 131 ........................... 44
POR 134 ........................... 47
MTSAT2 137 ..................... 50

No information about Satellite #49 here. I guess the system keeps evolving.

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The Garmin unit identifies these geostationary satellites on the satellite status screen by using numbers greater than 32. (This is the NMEA standard for GPS use) The system is still new and will be improved with more satellites in the future (possibly 19 of them world wide), however since they are all geo-stationary you will need a clear view of the southern sky to use them from the northern hemisphere. This means they are very useful for an airplane or perhaps a boat, but less useful to someone on the ground particularly in areas of tree cover or high northern latitudes. While a GPS receiver can possibly receive satellite data from outside the ground coverage area there will be very little correction capability without the correct ground data.
 	  	Satellite 	#	Garmin 	Location

The NMEA standard WAAS labels are used by Garmin receivers and others. This system maps the PRN number to the NMEA label by subtracting 87 from the PRN number. (source: EGNOS help desk) The mapping from the Garmin (NMEA) numbers to the standard names and PRN numbers are (as of 8/23/2006):
AOR-E (3F2)	120 	33	W15.5
Inmarsat 4f2	121	34	W53
Inmarsat 3f4	122	35	W142
ARTEMIS 	124	37	E21.4
IOR-W/F5 	126	39	
MTSAT1R 	129	42	
IOR (3F1)	131	44	E64
POR (3F3)	134	47	E178
PanAm Galaxy 15	135	48	W133
MTSAT2 	137	50	E145
Canadian ANIK-F1R	138	51	W107.3