There are actually more than 24 GPS satellites.

Reilly
If you read the GPS literature, especially books and articles written by people outside our profession, you will read "there are 24 GPS satellites in six orbital planes." There are six orbital planes, but there have been more than 24 satellites for quite some time. In this article I'll show several tables showing the configuration of the GPS constellation.

The original Block I satellites, launched from 1978 through 1985, are no longer operational. All active GPS satellites are in the Block II constellation. Specifically, the satellites are referred to as Block II and Block IIR. Where do you find this information? The easiest way is to go to the U.S. Coast Guard website at http://www.navcen.uscg.gov/gps/geninfo/constell.htm. The information to look for, called SATINFO, is shown in Tables 1 and 2.

Let's look at Table 1. Each satellite has four different numbers: Satellite Vehicle Number (SVN #), Pseudo Random Noise Number (PRN #), Mission Number (Mission #) and IRON number (IRON #).

The Mission Number is obvious; the numbers are sequential, starting from II-1. There have been 28 satellites launched. The PRN Number is the number seen on all civilian GPS receivers. As surveyors and mappers, this is the only number we need to know.

Still looking at Table 1, the Launch Date shows when each satellite was launched and the Operational Date shows when the satellite became operational. As can be seen, the time between launch and operation varies from three weeks to two months.

To understand the column "Slot," refer to Figure 1 below, GPS deployment. This is an "old figure" from a website that no longer exists. The horizontal line through the center represents the equator, and each diagonal line represents an orbital plane. The orbital planes, labeled A, B, C, D, E and F are located 60 degrees apart in right ascension and inclined at an angle of 55 degrees to the equator. Looking at the first orbital plane on the left side of the figure, the four satellites are in slots D1, D2, D3 and D4. Beside each slot number is the satellite PRN Number and the month and year the satellite became operational. (Note: in 1997, PRN Number 15 was in slot D2. Today it is in slot D5 (see Table 1)).

Referring back to Table 1, the symbol (**) appears in five positions in the slot column. These five satellites are no longer operational; this means that 23 Block II satellites are operational.

The plan for the GPS constellation was to launch all Block II satellites. As these satellites began to age, they were to be replaced by another group of satellites referred to as "Replenishment Satellites" and called Block IIR. There are 21 replenishment satellites (don't hold me to that number) and are to be launched as needed. Table 2 shows the status of the Block IIR satellites. As can be seen PRN Number 12, the first satellite, never made it into orbit on Jan. 17, 1997. This was due to a failure of the rocket itself. There were five successful launches after that. This gives a total of 28 Block II and Block IIR satellites that are operational.

Guess what? That number isn't correct. While researching for this article, I came across a GPS constellation table in GPS World magazine. That table, shown in Table 3 below with modifications, shows another Block IIR satellite, PRN Number 18, which was launched Jan. 30, 2001 and became operational Feb. 15, 2001. I have no explanation why that satellite does not appear on the U.S. Coast Guard SATINFO listing.

Let's look again at Table 3. This table only shows operational satellites. Notice that from Nov. 26, 1990 onward, the satellites are referred to as Block IIA. I can't find documentation in the literature, but I believe the letter "A" stands for "Advanced." If my memory is correct, the Block IIA satellites had a computer upgrade. All my technical references use Block IIA, and the U.S. Coast Guard used that notation in earlier SATINFO listings.

So, how many GPS satellites are operational? Twenty nine. But this could change by the time this article is published.