It appears that the GPS satellite constellation has a glass ceiling, so to speak.
GPS was designed as a 24-satellite constellation, with four satellites in six orbital planes arranged to provide maximum observability around the globe. According to the government’s Space-Based Positioning, Navigation, and Timing website, “The U.S. government is committed to provide a minimum of 24 operational GPS satellites on orbit, 95 percent of the time. The U.S. Air Force launches additional satellites that function as active spares to accommodate periodic satellite maintenance downtime and assure the availability of at least 24 operating satellites. As of August 28, 2009, there were 35 satellites in the GPS constellation, with 30 set ‘healthy’ to users.”
Figure 1 shows the locations of the 35 satellites. Green squares indicate satellites marked healthy in the broadcast almanac. The numbers displayed are the satellites’ pseudorandom noise (PRN) codes. Red squares, with PRN codes, indicate satellites transmitting L-band signals but currently set unhealthy. Note that SVN24/PRN24, although active, is not included in almanacs. Blue squares indicate reserve satellites with space vehicle numbers (SVNs) in parentheses. Notice the bunching together of certain pairs of satellites. The constellation of 30 healthy satellites is not configured to maximize geometrical performance. Rather it is to help guarantee a minimal level of performance considering that many of the spare satellites are one component away from failure. Basically, the 30-satellite constellation is actually being flown as a 24-satellite constellation.
But with 35 satellites in working condition, why are only 30 set healthy? Modern GPS receivers can handle all 32 PRN codes, and many studies have shown the more satellites the better as far as position accuracy and reliability are concerned. In fact, a recent Air Force Space Command article stated, “One additional GPS satellite can make a difference between getting a degraded GPS signal and getting an accurate GPS-based location, whether it is for warfighters in Baghdad or firefighters in Boston.” The current control system should, in principle, be able to handle 32 healthy satellites.
Ground Control System. But, according to the GPS Wing, the de facto limit is 31 satellites. We don’t know if this is a problem related to 2nd Space Operations Squadron (2SOPS) functions or if there is some military system or equipment platform that cannot tolerate 32 healthy satellites.
Further, if the de facto limit is 31 satellites, why have we had only a maximum of 30 satellites set healthy since early this year? After all, 2SOPS rightly crowed about having 31 satellites set healthy for the first time on February 27, 2008, when SVN23 was reintroduced into the healthy constellation as PRN32.
Figure 2, courtesy of Ted Driver at Analytical Graphics, Inc., shows the number of satellites set healthy from 1998 onward, according to the Notice Advisories to Navstar Users (NANUs) issued by 2SOPS and the almanacs broadcast by the GPS satellites. Often, a satellite is actually set unhealthy for only a portion of the day, but this plot tallies only the number of satellites healthy for a full day. As we can see, off and on through most of 2008 and into 2009, we had 31 satellites set healthy on orbit. With 31 satellites, users benefited from better availability and accuracy and were slightly better able to handle the occasional three-satellite outages due to SVN35/PRN5 and SVN25/PRN25 being set unhealthy for extended overlapping periods.
But since March 26, 2009, when SVN35/PRN5 was decommissioned from active service, we have not seen a return to 31 healthy satellites.
Why is that?
Ground Testing. I asked this question to Col. Dave Madden, the GPS Wing commander, during a panel discussion on GNSS program updates at The Institute of Navigation’s GNSS 2009 meeting in Savannah, Georgia, in September. Apparently, the reason why 31 satellites cannot currently be set healthy simultaneously has do with ground testing of Block IIF satellites. One PRN code is needed for the test satellite on the ground. Presumably this means that testing involves tracking the constellation in space and a IIF test satellite simultaneously.
So, although everyone acknowledges that more GPS satellites are better, we have hit a 30-satellite ceiling. As IIF satellites are launched and further improvements are made to control system operations, and any incompatible old military systems are replaced or updated, perhaps we can break through this glass ceiling and have 31 or even 32 healthy GPS satellites available to users.
We can hope.