The Risk of Degradation to GPS

In April the Government Accountability Office (GAO), the audit and investigative arm of the US Congress, announced the results of their study on sustaining the current GPS service. The main finding was that the GPS service is likely to degrade over the next few years, both in terms of coverage and accuracy, due a decrease in the number of operational satellites. Using data provided by the US Department of Defense (DoD), the GAO ran simulations to determine the likelihood that GPS can be maintained at its agreed performance level of 24 satellites operating at 95% availability. The graph below (double-click to enlarge) shows a 24-strong GPS constellation dipping below 95% availability in the 2010 fiscal year, and dropping as low as 80% before recovering in 2014. The jittery sawtooth nature of the graph is derived from the tussle between the failure of existing satellites and the launching of replacements, with the failure rate dominating for the next few years.

Needless to say the GAO findings have been widely discussed, and were further publicised in a recent televised congressional hearing. The US Air Force, who runs the GPS program for the DoD, has had to assure its military peers, various congressmen and an anxious public that the GPS service is in fact not on the brink of failure – a scenario not even considered by the GAO report. Articles in the popular press such as Worldwide GPS may die in 2010, say US gov from the Register are not helping matters. So how did the GPS service end up in this predicament? According to GAO, the culprit is poor risk management in the execution of the GPS modernisation program.

GPS is a critical service, particularly for the military, as it provides information for the calculation of position, velocity and time. As noted in the GAO report, “GPS has become a ubiquitous infrastructure underpinning major sections of the economy, including telecommunications, electrical power distribution, banking and finance, transportation, environmental and natural resources management, agriculture, and emergency services in addition to the array of military operations it services”. Specifically, GPS is used to guide bombs and missiles to their targets – and we don’t want inaccuracy in those calculations!

There are currently 31 operational satellites, orbiting 12,600 miles (20,200 kilometres) above the Earth, a seemingly safe margin over the required 24. The constellation has grown to this size as the current roster of satellites have performed far beyond their expected operational lifetimes. Even so, according to a DoD report issued last October, 20 satellites are past their design life, and 19 are without redundancy in critical hardware components.

The main threat scenario is that a substantial number of satellites will reach their operational end-of-life before they can be replaced, thus reducing the size of the constellation. Or simply put, the satellite failure rate may exceed the refresh rate. This is not really a question of whether GPS will become extinct (all satellites fail) since GPS will become ineffective long before the number of satellites gets anywhere near zero.

What is the impact of a degraded GPS service? Well the first point is that GPS currently delivers a much better service than committed to, due to the additional satellites above the required 24. So the service impact when dropping below 24 satellites will be quite noticeable. The accuracy of GPS-guided missiles and bombs will decrease, therefore increasing the risk of collateral damage. This leads to a viscous circle where even more missiles or bombs will be required to take out a given target.

Since the current generation of satellites have lasted so long, and GPS still remains at threat from dropping below 24-strong constellation, then there must be some problems with the rate at which the constellation is being replenished. And according to the GAO report, there have indeed been severe problems in executing the GPS program as planned. The current GPS program has experienced cost increases and schedule delays. The launch of the first new satellite is almost 3 years late and the cost to complete the new program will be $870 million over the original estimate.

GAO cites a multitude of reasons for this predicament including multiple contractor mergers, moves and acquisitions, technology over-reach (a common malady for military projects), the short tenure of program leaders, and general “diffuse leadership” (no one group or person is really in charge).

GAO strongly recommends an improved risk management process. In a recent post The Risk Analysis of Risk Analysis I reviewed an article on when to apply a sophisticated risk methodology called Probabilistic Risk Assessment (PRA). The conclusion was that the difficulty, expense and potential inaccuracy of PRA can only be justified when projects are on a grand scale, and the multi-billion dollar GPS program certainly qualifies. And here the risk equation is not merely about technicalities and project management (hard as they are). There is also an overarching directive from the US government to be the premier global provider of GPS services. Europe, Russia and China are creating their own constellations, but relying on these “foreign” constellations does not seem to be an option.

Various representatives from the DoD have responded to the GAO report, stating that action must and will be taken to improve the current GPS constellation. It is likely that the service will experience degradation over the next 5 years, but the DoD claims it be managed and predicted (you can calculate when and where there will be gaps). Let’s hope they’re right.