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.
GPS Satellite ModernizationThe DoD has to plan many years ahead. Because of the long lead time necessary for a new generation satellite, the DoD contracted with Boeing in 1996 for a fourth-generation spacecraft called Block IIF. The first six of 30 spacecraft are on contract; the remaining 24 are on hold. In addition, a team led by Boeing, Lockheed Martin and Spectrum Astro scientists are conducting a study for the next generation of satellites to be named GPS III. The first GPS III satellite will be launched around 2015.
The GPS satellites currently on orbit are using 1970s technology. Figure 1 shows the GPS signal structure used on the first Block I satellite launched in February 1978 and also the last Block IIR satellite on orbit January 2001. L1 is the civil frequency and L2 the military frequency.
The first step in the modernization process occurred on Jan. 25, 1999. A press release from the White House announced the addition of two new civil frequencies. The first new civil (C/A) frequency (which will be the second civil frequency), at 1227.60 MHz, will be added to the existing L2 frequency, which will be located in the empty box of L2, shown in Figure 1 on page 48. The other civil frequency, at 1176.45 MHz, will be called L5.
In short, GPS modernization is expected to:
- Provide a new L5 frequency
- Add the C/A code on L2
- Implement a military (M) code on the L1 and L2 frequencies
- Add flexible power to the new satellites in the system
The M code (‘M’ for the new military signal) will replace the existing P/Y code. Flexible power, when needed, would allow the DoD to increase power of the M code to prevent jamming. There is a possibility that the C/A codes will be broadcast at a higher power level.
When Will This Begin?This is a good question. The original plans called for implementation on Block IIF satellites. The existing satellites on orbit have performed well beyond their design life, at least three Block IIR satellites have been built, but not launched, using the old technology; it will be several years before any changes are made. There was a change in plans to modernize the remaining Block IIR satellites (those on contract), but they have not been manufactured (those involved say there are 12) making them IIRMs and adding the second civil frequency L5. Because of scheduling problems, the DoD expects to modernize only 10 of the remaining satellites; some people in the industry believe only eight will be upgraded.
So, it will be quite some time before the surveying and mapping community has to upgrade its receivers to accommodate the new L5 signal and the C/A code on L2. GPS receiver manufacturers will be able to produce a superior receiver, one that can pick up three civilian frequencies and avoid the M-code, allowing the civilian surveyor and mapper to use GPS for many more tasks than is possible with the existing constellation.