May
2009Fears for GPS
Fears for GPS
There are growing fears that
GPS performance will decline significantly next year, as its ageing
satellites begin to fail more quickly than they can be replaced. And an
official US Government report suggests that there is only a 10% chance
of it maintaining full operational capability through the next ten
years.Lt.Gen. Larry James – the USAF General responsible for the system – has warned the Congess subcommittee on National Security and Foreign Affairs that “the GPS constellation includes satellites which have exceeded their design life, operate with partial capability, or are a single key component away from failure”
His comments came in the wake of a report by the US Government Accountability Office (GAO) which expressed serious concern that although there are now 31 GPS satellites in orbit, thirteen of them are old "Block 2A"models, dating back to the days before GPS had been declared fully operational. The youngest of them is nearly fourteen years old, and the oldest is nineteen -- and as their designed life expectancy was only just over seven years, they are now clearly living on borrowed time.
Of course the system has been enlarged and updated since 1995, but the first of the latest batch of replacements (called "Block 2F") is unlikely to be launched until November – three years late and at more that twice its original estimated price.
With seven more satellites than the 24 that its specification requires, there's a reasonable chance that the system will be able to keep going for a few more years, despite fears that the old satellites could break down faster than they can be replaced. GAO gives it an 80% chance of maintaining at least 24 operational satellites between now and 2013.
But there are more serious concerns looming in five years time, when the first of the third generation satellites (Block 3A) should be starting to take over.
GAO points out that work on the Block 3A satellites is off to a late start: it describes the development program as “highly compressed” and says that the 2014 target launch date is “optimistic”.
Whilst achnowledging that many of the administrative and management problems that bugged Block 2F may have been put right, GAO has drawn up its own predictions for the future of the GPS constellation, assuming a two-year delay in the Block 3A launch schedule. They make pretty gloomy reading, suggesting that there is only a 10% chance of maintaining 24 satellites through the next decade, and only a 50:50 chance of keeping a 21-satellite constellation. There is even a slight but real possibility that the number of satellites may drop to 18 or less.
The graph (above) showing the probability of maintaining GPS constellations of 24, 21, or 18 satellites, presented in the GAO report.
Dropping to 21 satellites from 31 will inevitably compromise the accuracy, availability, and reliability of the system, though it’s difficult to predict by how much.
Land users will almost certainly be worst affected. At sea, where our antennas aren't obstructed by trees or tall buildings, we will probably still be able to use GPS, just as we could in the early Nineties. At least this time round we probably won't have to contend with the deliberate jinx of so-called "Selective Availability" building in a deliberate quasi-random error.
So there will probably be no need to rush out and buy a sextant. It might be worth brushing up your eyeball pilotage, but hopes that I might be able to retire on a healthy pension thanks to increased sales of books on traditional navigation seem likely to be torpedoed by those dastardly Russians, whose enlarged and modernised Glonass system is already only a few months away from being able to provide worldwide 24/7 coverage.
