The GPS Observer
When one hears the expression GPS modernization one often thinks of improvements in GPS receiver technology. But there is another side: modernization of GPS satellites in the NAVSTAR constellation.
Where Are We Today?The first GPS satellite was launched on Feb. 22, 1978; it was the first of the Block I constellation. A total of 11 launches were made between 1978 and 1985 with one launch failure (see Figure 1). These satellites were labeled experimental-the U.S. Department of Defense (DoD) wanted to be certain the system would work.
The first of the operational satellites, Block II/IIA, was launched on Feb. 14, 1989. There were a total of 28 launched with no launch failures; the last Block II/IIA satellite was launched Nov. 6, 1997 (see Figure 2). Of the 28 satellites 18 were still operational as of Dec. 20, 2004.
As of Dec. 20, 2004, 50 GPS satellites have been successfully launched, and 30 are operational. All 50 satellites had/have the same signal structure, which is shown in Figure 4.
Let's discuss Figure 4. The fundamental frequency is the atomic clock, which can be either Cesium or Rubidium. The frequency 10.23 Mhz is multiplied by 154 to generate the L1 carrier, and multiplied by 120 to generate the L2 carrier. These two frequencies are in the L-band ultra-high radio frequencies spectrum. On L1, two codes are modulated: the C/A-code and the P-code. C/A is course acquisition and P can be either protected or precise. On L2, only the P-code is modulated. On all Block II/IIA and IIR satellites the P-code is encrypted and available only to the DoD. The navigation message is modulated on both L1 and L2; this contains, among other things, the orbital data needed by the GPS receiver to calculate the positions of the satellites.
If you are a hobbyist and own a small handheld receiver purchased at a sporting goods store, your receiver uses only the C/A-code on L1 and, with four or more satellites visible, the receiver can determine the pseudorange to each satellite and calculate a position accurate to about Â± 10 meters.
If you are a surveyor using L1-only receivers, you must have at least two receivers. Each receiver, observing the same satellites at the same time, records into memory the pseudorange and the phase delay of the last carrier wave that reaches the receiver. The pseudorange and phase data from each receiver is post-processed in a computer to determine the difference in position between the two receivers.
If you are a surveyor using dual-frequency receivers (L1 and L2), you gather the same data as with L1-only receivers, but the manufacturers have developed semi-codeless tracking techniques on the encrypted P-code (called the Y-code) to get L1 and L2 measurements. Semi-codeless works, but the signal margins are slim; L2 is much slower and the receiver is expensive to build.
GPS ModernizationThe GPS civil community has been complaining to the DoD that the existing GPS system is using 20th century technology in the 21st century. The discussion has been going on for many years. From what I read in the literature, DoD has agreed to a modernization program with three specific goals:
- implementing military (M) codes on the L1 and L2 for DoD use
- providing a new L5 frequency in an aeronautical radio navigation service (ARNS) band with a signal structure designed to enhance aviation applications
- adding the C/A-code to L2
The last two items are years away and apply to newly designed satellites. Block IIF satellites, to be launched in 2007 or later, will have all three of the items mentioned above; there are only 12 Block IIF satellites planned. The GPS III constellation is many years away.
The L2 Civil SignalA White House press release on March 30, 1998, announced that a civil signal would be added to the GPS L2 frequency. Instead of replicating the C/A-code, a modern signal structure will be used. The signal will be called L2C, and will be modulated on L2 as shown in Figure 5.
In 2001, the plans were to include L2C on the remaining 12 Block IIR satellites that had not been launched. The modernized satellites were to be referred to as Block IIR-Ms. It's now 2005, and as of Nov. 6, 2004, the 13th Block IIR satellite was launched; there are now only eight IIR satellites on the ground. The U.S. Naval Observatory lists the new satellite as IIR-13. To the best of my knowledge, L2C was not on IIR-13.
When L2C is available for GPS users, when will it be practical for GPS receiver manufacturers to produce receivers to pick up the signal? An excellent article appeared in GPS World magazine in September 2001  that discussed this topic. The authors stated that manufacturers "can't sell single-frequency L2 products until there are at least 24 satellites in good orbital positions. Until then, even with a better signal, we can't overcome the geometry advantage a 30-satellite constellation gives L1-only products."
The authors of the article expected the first IIR-M satellite to be in orbit by 2003 and the first Block IIF satellite in orbit by 2005. Even with those predicted dates, which are wrong, they didn't expect 24 satellites with the L2C signal to be operational until 2009.
When L2C becomes available for the surveying community, manufacturers can reduce costs significantly because, on dual-frequency receivers, semi-codeless technology will not be necessary. However, waiting for GPS modernization is like watching concrete harden. We will be surveying with GPS in its present configuration for many years.