As background, let’s define the three different blocks of GPS satellites that have been launched up to this point. Table 1 on page 49 shows the Block I satellites launched from 1978 to 1985, none of which are operational today. Table 2 on page 49 shows the Block II satellites launched from 1989 to 1997; 28 were launched and 23 are operational today. (As a point of interest, modified Block II satellites, which provided additional onboard data memory in the satellite computers, were launched after the ninth Block II. Although not shown in Table 2, all satellites from SVN/PRN 23 onward should be referred to as Block IIA (‘A’ for advanced).) Table 3 on page 49 shows the Block IIR (‘R’ for replenishment) satellites. Five Block IIR satellites have been launched according to this table, but at least one more has been launched. Block IIR-1 failed to achieve orbit because of a booster failure on the Delta II rocket. The U.S. Department of Defense (DoD) has contracted for 21 IIR satellites, which will be launched as needed when a Block II or IIA ceases operation.
As surveyors and mappers, we can look forward to 15 additional Block IIR satellites; since these are launched as needed, many years will pass before all Block IIR satellites are on orbit.