A Grim Look at GPS Health
The Global Positioning System (GPS) satellite constellation consists of 24 satellites (plus spares) in orbit 20,200 km above the earth, in 12-hour sun-synchronous orbits. GPS is becoming synonymous for the method by which position (latitude/longitude) and time is determined. The GPS system has cost US $ 12 billion, and every year the United States Department of Defense spends US $750 million for maintenance and modernization of this system.
GPS satellites have lifetimes on the order of 7-10 years. They must be replenished in order to keep the full set of satellites in their respective positions in orbit.
The GPS constellation places satellites in orbit in very precise ways in order to maximize the number of satellites in view of your GPS receiver. There must be at least 4 healthy satellites in each of 6 orbital planes before the constellation is declared fully operational.
GPS is becoming a cornerstone of the US high techology economy, every bit as important to the economic well being of the US and other high tech countries as the Internet. It is no longer conceivable to imagine a future without GPS.
I've worked at the periphery of the GPS satellite system for the bulk of my career as a satellite and communications systems engineer. For some reason I thought the GPS system was running quite well and was relatively impervious to the entropic forces at work within the DoD and Congressional budgetary cycles, which permit healthy systems to decay into uselessness. It appears I was wrong. The following letter appeared in the most recent issue of Aviation Week & Space Technology, the most authoritative magazine of its kind in the space industry. From the tone of the letter, it is evident that its author is intimately acquainted with the GPS program and its most current state of affairs.
The letter is, quite frankly, a wakeup call. I have shown it to several members of staff at work who work on GPS daily. The reactions ranged from mild bemusement to deep shock. They were unaware of the problem. I therefore have to believe that very few people outside of the satellite community know about this problem, and would like to begin making more technically oriented people aware of it.
I reprint this letter in full in the hopes that the many cognizent supporters of this critical system within the DoD and the government urge the U.S. White House and Congress to make this a high priority item in the next budgetary review cycle.
The language is highly technical. I have attempted to explain a few critical items in footnotes below the letter. However, the letter reads best unadorned, which is why I have left it exactly as written. Space cognizenti will recognize the true seriousness of the health of GPS.
In "New Trajectory" (AW&ST, Sept. 12, p. 55), Col. Allan Ballenger, the GPS program manager, talks a lot about GPS modernization. But I hope he has enough GPS space vehicles (SVs) to provide the "24/7" coverage we have come to expect.
GPS has become a utility for military and civil users that needs four healthy SVs each in six different orbital planes. Late last month, the B plane had only three operational SVs, while SV35A switched to its last clock. For the first time in a long time, there was a hole in the world's GPS coverage. Imagine finally finding Osama bin Laden, calling for a JDAM but being told to wait for the hole to pass over you.
There are few on-orbit spares, many SVs have lost their redundant systems, and the loss of all the GPS II/IIA SVs (18 units, all past their design life) is expected over the next 2-3 years as their aging solar arrays fail. A GPS Joint Program Office official provided enough informationat a forum last March in Munich to forecast the magnitude of this issue. Eighteen known failures are coming, but only eight IIR/R-M replacements are available. Let's hope the IIF SV program stays on schedule.
Instead of making fancy plans for modernization, Ballenger should be launching SVs on a regular basis to replace the SVs with multiple faults before they completely fail and to fill up the orbital planes with robust on-orbit spares. But there has not been a GPS launch since November 2004.
And take a hard look at the near-term "modernization." The IIR-Ms were supposed to have L2 Civil Navigation capability, but have taken exception to the Interface Control Document for those signals, so where is the improvement? Work on versions 6.0 and 6.1 of the control segment has stopped. But that work is necessary for the new civil signals and military M-code. Even if the IIR-Ms work as advertised, the control segment won't support the new signals.
Ballenger needs to get his priorities straight and spend his $750-million budget wisely. A fundamental rule for managing a utility is to keep the service available for the users, then worry about improving it.
Mark Bradshaw
Lake Forest, Calif.
Explanatory Notes
- Design Life Every GPS satellite has a predicted lifetime of about 8 years. In the past, satellites have been designed so carefully that the actual lifetime is almost double the design lifetime.
- Clock Redundancy Each GPS satellite carries triple redundant atomic clocks on board. (A mixture of cesium and rubidium clocks are used.) These offer very stable time standards, with accuracies of parts per billion. Several other very critical portions of the satellite also have redundant backup capabilities.
- Known Failures How can you know that a satellite will fail, and how can you predict its failure date? Sensors on board a satellite keep track of the amount of current coming from solar panels, for example. Downlink telemetry signals tell ground stations the health and status of key satellite subsystems. Extrapolation of decreasing current with time enables SV engineers to predict when the solar panels will no longer provide enough electrical current to keep the satellite fully operational. (Solar panels degrade with constant exposure to the sun's energetic rays by causing silicon crystal lattices to degrade. Lattice imperfections cause subtle current leakages through normally insulative silicon dioxide.) If they can't switch over to a redundant subsystem, the satellite is declared to have failed. Prediction of SV failure is a well known art among satellite engineers.
- Signals GPS satellites broadcast a variety of signals. The most basic sets can be used by any civilian GPS receiver. Military codes have much less clock jitter and therefore give much more accurate lat/long readings,but can only be used by military GPS receivers with the proper decryption mechanisms.
- On Orbit Spares It takes much less fuel to move a replacement satellite to another orbital slot within the same orbital plane than it does for the satellite to be moved to a different orbital plane and then 'walk' through the orbital plane to get to the correct slot. Therefore, it becomes critically important to have each orbital plane populated with a few replacement satellites.
Copyright Notice:
I have requested permission to reprint this letter in its entirety from the editors of AW&ST. If they do not grant permission, I will remove this writeup.
Sources and Helpful Background Information: