Skip to comments.GPS signal under threat - A few years of reduced precision might affect scientists worldwide.
Posted on 05/23/2009 9:11:58 PM PDT by neverdem
Updated online: 22 May 2009
Concern over the US Global Positioning System (GPS) stepped up a notch today after a senior official from the US government's congressional watchdog warned that the US Department of Defense faced substantial challenges meeting its space-programme commitments.
Scientists use GPS signals for everything from tracking bird migration to making fine-tuned measurements of sea level change. The precision and accuracy of such measurements depends on how many satellites a GPS receiver can detect, and on the satellite's relative positions in the sky.
But on 30 April, the Government Accountability Office (GAO) warned that new satellites might not be launched in time to replace the ageing constellation currently in orbit. And yesterday Cristina Chaplain, director of acquisition and sourcing for the GAO, testified before the Senate Committee on Armed Services that cost over-runs of defence department space programmes are part of the problem.
"It's not that people will suddenly not be able to use the system, but if you start to lose satellites then maybe the performance starts to degrade slightly," says Marek Ziebart, a space geodesy researcher at University College London.
“To measure sea levels reliably you want lots of satellites and you want them working well.”
University College London
Two factors determine whether the GPS service can maintain its precision: the rate at which old satellites wear out and the rate at which the US Air Force can launch replacements. The GAO estimates that the chance that the GPS system will be able to provide full coverage could dip below 95% between 2010 and 2014, when the Air Force plans to begin replacing the current block of satellites with a newer generation.
There are 31 functional GPS satellites in orbit now, with only 24 needed to provide full coverage, but "the more the better" for many science applications, says Ziebart. "To measure sea levels reliably you want lots of satellites and you want them working well," he adds.
The United States has provided a free civilian GPS service since the 1980s, a decade before the completion of its first full 24-satellite constellation. Russia also reached a peak of 24 operational navigation satellites for GLONASS, its own global navigation satellite system, in the 1990s. Currently it has only 20 functional satellites but plans to launch more.
The atomic clocks and reaction control wheels in the satellites are what are most likely to cause failures, says Richard Langley, a geodesy researcher at the University of New Brunswick in Fredericton, Canada. Satellites carry back-up components, says Langley, but time has already taken its toll on all but the last atomic clock on 18 US GPS navigation satellites, according to a recent unclassified US Air Force report.
"The United States has a couple more [older, hibernating satellites] that could be pressed into service if needed," say Langley.
Few satellite specialists think that the US system will cease to provide a useful service. "I don't think [scientists] should worry much about it," says Langley. Already, some GPS receivers can pick up signals from both US and Russian satellites. Scientists who work with GPS should "bear it in mind and look at whether investing in GLONASS is right for them", he says. The smallest receivers, such as those used for wildlife tracking, rarely have the capability to pick up signals from both systems, although larger, more expensive systems do.
“GPS isn't falling out of the sky.”
US Air Force Space Command
And responding to questions on the US Air Force Space Command (AFSPC) Twitter feed, Colonel Dave Buckman, command lead for position, navigation and timing at the AFSPC, wrote that he agreed with the GAO "that there's a potential risk, but GPS isn't falling out of the sky we have plans to mitigate risk and prevent a gap in coverage".
Because many surface-based GPS systems are replaced every few years anyway, any upgrade would be unlikely to disrupt operations much, says Ziebart. But the problem may be more serious for scientists who have to rely on existing equipment, such as low Earth-orbiting satellites including Jason-1 and Jason-2, which use GPS signals to establish their positions, and which cannot be upgraded so easily.
Signals from the GPS system are also used for numerous civilian operations, including financial transactions and aviation navigation. Some have questioned the wisdom of White House plans to deactivate in 2010 a land-based radio navigation system called LORAN, which could serve as a backup to GPS systems.
Although scientists may face a temporary decline in the accuracy of their GPS signals, Washington Ochieng, a navigation researcher at Imperial College London, says, "There's no way the United States is going to let GPS stop."
Richard Langley has created a table(pdf) showing how a shrinking GPS constellation might affect the reliability and strength of the GPS signal.
>>GPS signal under threat - A few years of reduced precision might affect scientists worldwide. <<
To heck with scientists - they can guide be the stars - what about my Lincoln?!
Anyway, a back up system using cell phone towers is already in the works.
Well..., my GPS usually shows it’s picking up around six or eight of those buggers in the sky.... LOL...
Well that’s good news...0bambis Acorn census workers won’t get a good fix on my front door.
“Anyway, a back up system using cell phone towers is already in the works.”
Uh, what?! I guess you’re kidding, aren’t you? If you aren’t, what if you’re not using a cellphone? What if you’re flying a plane, or driving a boat? Not a lot of cell towers available for that sort of environment. Having said that, even with the expecvted degradation of the constellation, it’s still in better shape than it was 25 years ago. I ‘spect we’ll muddle through whatever shape the GPS system is in.
It’s not even new news, http://www.newscientist.com/article/dn11713-tv-and-cellphone-signals-may-provide-gps-backup.html
Thank you for posting the link to that article. It was interesting, but as you can tell, I’m still not convinced. I’ll stipulate the cell towers could be used, and assuming that other terrestrial emitters could be used, it’s obviously possible (theoretically) to come up with some sort of position calculation. What’s less apparent to me after reading the article, what level of accuracy are we talking about using a system that they are describing?
I’ll go back to my previous point since the system described does not really lend itself to marine or aeronautical applications. Flying domestically there would be bazillions of signal sources all yammering at whatever receiver the aircraft was using. You can “line of sight” way too much at 37,000 ft.. In a marine environment, one is operating where there is nothing “line of sight”. Certainly no cell towers, tv stations or vhf/uhf sources available. This goes for aircraft as well. Yes, there are fallback technologies available, but I’ve become spoiled by having a gps receiver in the bottom of my flight case (just in case). NDB, LORAN, laser ring gyros, LNAV systems, they’re just fine and I’ve used them all, but they are not even close to the accuracy of that little box in the bottom of my flight bag that never really had to be used (but it was awfully nice to know it was in there).
Out of curiousity, did the system that was described actually go beyond a theoretical discussion? Just curious, as I truly had never heard of it till you were kind enough to point it out to me. I’m obliged for that.
Our military and thier weapons is so heavily dependant upon the GPS system, do you honestly think the pentagon will allow the system to fail? I don’t think so.
I take it from you comment that you are familiar with flying and instrument procedures. I do not believe that cell towers can give enough accuracy to design an RNAV approach with LPV minimums. Most LPV approaches have a 250’ Height above Touchdown (HAT) but there are some with a 200’ HAT.
The more I dig into this, the more confused I am. The previous administration’s budget for FY 2009 directed the migration of LORAN-C to Dep. Homeland Security from the USCG, and the implementation of an enhanced LORAN system (LORAN-E) as a replacement. A good summary can be found at:
For a primer on LORAN-E capabilities, see:
Coast Guard Commandant Allen’s thoughts on shutdown of LORAN-C are here:
What this administration does probably won’t be settled until the Air Force and the Goast Guard fight over funding, and congress critters pass the FY 2010 budget.
Cmdr Feigenblatt of the Coast Guard’s e-navigation Branch is quoted as stating, “Enhanced Loran (eLoran) does not exist in the United States. There is neither the funding nor the authority to modernize Loran-C to eLoran. Significant additional government investment of hundreds of millions of dollars would be required to transform the Loran-C system into eLoran to potentially serve as a systemic Positioning, Navigation, and Timing (PNT) backup for GPS.”
I’ve seen performance data on the UK LORAN-E prototypes in evaluation. Other references claim an upgrade to the US system would cost $400,000,000. The cynic in me says nothing will happen until broadcast communications and GMD cell phone service is degraded due to poor GPS satellite performance. Or we’ll contract with the Chinese for access to their system.
“I’m sure the military has a top secret program as a global positioning back up.”
Yes, they’re called navigators. Sorry. Couldn’t resist.
I was on a Sacramento overnight once, and the guy I was crewing with had a family friend that was going through Nav school that had supper with us (Mongolian BBQ - excellent). I asked him, “So with all this GPS stuff, what do we need you guys for?”. Of course, the other pilot was horrified that I’d asked his friend such a question, but the young fellow laughed and said, “Ya’ know, we were wondering that too.” Apparently, the contingency plan involves not having the constellation functional at some point.
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A link to the NASA satellite tracking. It’s interactive. Gives you an idea of what is up there.
Absolute GPS to better than one meter
C.O. Alley & T. Van Flandern
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