Skip to content

Expert Advice: GPS Constellation Maxed Out at 30

November 1, 2009  - By
photo: Kristine Larson
Figure 2. The number satellites set healthy since 1998 (courtesy of Analytical Graphics, Inc.)

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.

Figure 1. Locations of the 35 current GPS satellites: green squares denote satellites marked healthy in the broadcast almanac, satellites marked by red squares transmit L-band signals but are curerently set unhealthy, and blue squares indicate reserve satellites. Bunched pairs show satellites being flown in tandem.

Figure 1. Locations of the 35 current GPS satellites: green squares denote satellites marked healthy in the broadcast almanac, satellites marked by red squares transmit L-band signals but are curerently set unhealthy, and blue squares indicate reserve satellites. Bunched pairs show satellites being flown in tandem.

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.

Figure 2. The number satellites set healthy since 1998 (courtesy of Analytical Graphics, Inc.)

Figure 2. The number satellites set healthy since 1998 (courtesy of Analytical Graphics, Inc.)

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.

This article is tagged with , and posted in From the Magazine, Opinions

About the Author: Richard B. Langley

Richard B. Langley is a professor in the Department of Geodesy and Geomatics Engineering at the University of New Brunswick (UNB) in Fredericton, Canada, where he has been teaching and conducting research since 1981. He has a B.Sc. in applied physics from the University of Waterloo and a Ph.D. in experimental space science from York University, Toronto. He spent two years at MIT as a postdoctoral fellow, researching geodetic applications of lunar laser ranging and VLBI. For work in VLBI, he shared two NASA Group Achievement Awards. Professor Langley has worked extensively with the Global Positioning System. He has been active in the development of GPS error models since the early 1980s and is a co-author of the venerable “Guide to GPS Positioning” and a columnist and contributing editor of GPS World magazine. His research team is currently working on a number of GPS-related projects, including the study of atmospheric effects on wide-area augmentation systems, the adaptation of techniques for spaceborne GPS, and the development of GPS-based systems for machine control and deformation monitoring. Professor Langley is a collaborator in UNB’s Canadian High Arctic Ionospheric Network project and is the principal investigator for the GPS instrument on the Canadian CASSIOPE research satellite now in orbit. Professor Langley is a fellow of The Institute of Navigation (ION), the Royal Institute of Navigation, and the International Association of Geodesy. He shared the ION 2003 Burka Award with Don Kim and received the ION’s Johannes Kepler Award in 2007.