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GPS signal under threat - A few years of reduced precision might affect scientists worldwide.
Nature News ^ | 21 May 2009 | Lucas Laursen

Posted on 05/23/2009 9:11:58 PM PDT by neverdem

Updated online: 22 May 2009

GPS satelliteOlder satellites will need to be replaced quickly if GPS is to maintain its precision.NASA

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.”

Marek Ziebart
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.

What goes up

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.

Graph showing probability that GPS will have poorer performanceClick for a larger image.GAO

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.

Mitigating risk

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.”

Dave Buckman
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."

Updated:

Richard Langley has created a table(pdf) showing how a shrinking GPS constellation might affect the reliability and strength of the GPS signal.


TOPICS: Culture/Society; Foreign Affairs; News/Current Events; Technical
KEYWORDS: astronomy; geodesy; science; space; spacegeodesy; xplanets
Click for a larger image to open in a new window. GAO
1 posted on 05/23/2009 9:11:59 PM PDT by neverdem
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To: neverdem

>>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?!


2 posted on 05/23/2009 9:18:47 PM PDT by gondramB
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To: neverdem
LOL, posted last week, has more to do with budget negotiations than fact.

Anyway, a back up system using cell phone towers is already in the works.

3 posted on 05/23/2009 9:21:39 PM PDT by org.whodat
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To: neverdem

Well..., my GPS usually shows it’s picking up around six or eight of those buggers in the sky.... LOL...


4 posted on 05/23/2009 9:28:26 PM PDT by Star Traveler
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To: org.whodat

Well that’s good news...0bambis Acorn census workers won’t get a good fix on my front door.


5 posted on 05/23/2009 9:30:15 PM PDT by spokeshave (USA #1; Pirates -3...Voting them all out of office would be a sufficient pay cut)
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To: org.whodat

“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.


6 posted on 05/23/2009 9:37:54 PM PDT by Habibi ("We gladly feast on those who would subdue us". Not just pretty words........)
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To: Habibi

It’s not even new news, http://www.newscientist.com/article/dn11713-tv-and-cellphone-signals-may-provide-gps-backup.html


7 posted on 05/23/2009 9:43:47 PM PDT by org.whodat
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To: Habibi
You know what is really funny, I remember reading the post here back in 2007.
8 posted on 05/23/2009 9:47:09 PM PDT by org.whodat
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To: org.whodat

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.


9 posted on 05/23/2009 11:23:31 PM PDT by Habibi ("We gladly feast on those who would subdue us". Not just pretty words........)
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To: neverdem

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.


10 posted on 05/24/2009 4:12:04 AM PDT by diverteach (http://www.slapobama.com/)
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To: Habibi

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.


11 posted on 05/24/2009 6:45:55 AM PDT by ops33 (Senior Master Sergeant, USAF (Retired))
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To: Habibi
I'm sure the military has a top secret program as a global positioning back up. And for a computer to triangulate is child's play, three positions and bang they know where they are, no problem. There are plane tracking radar stations all over the country and the world. Their positions are fixed.
12 posted on 05/24/2009 7:19:34 AM PDT by org.whodat
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To: org.whodat

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:

http://www.crossrate.com/pdfs/020608.pdf

For a primer on LORAN-E capabilities, see:

http://www.loran.org/ILAArchive/eLoran%20Definition%20Document/eLoran%20Definition%20Document-1.0.pdf

Coast Guard Commandant Allen’s thoughts on shutdown of LORAN-C are here:

http://www.uscg.mil/comdt/blog/2009/04/question-from-a-school-student.asp

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.”

http://madmariner.com/blogs/navagear/28352

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.


13 posted on 05/24/2009 7:54:47 AM PDT by NelsTandberg
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To: NelsTandberg
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.

ROFLOL

14 posted on 05/24/2009 8:04:42 AM PDT by org.whodat
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To: org.whodat
The Lorain E stuff is really interesting.
15 posted on 05/24/2009 8:08:30 AM PDT by org.whodat
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To: org.whodat

“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.


16 posted on 05/24/2009 8:37:32 AM PDT by Habibi ("We gladly feast on those who would subdue us". Not just pretty words........)
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To: Habibi
LOL, I was wondering if it were not possible to have a computer program today that would navigate from the constellation positions. I know in the past it was said a small computer did not have the speed ability to compute all the variables. They may not be true today.
17 posted on 05/24/2009 11:43:48 AM PDT by org.whodat
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To: KevinDavis; annie laurie; garbageseeker; Knitting A Conundrum; Viking2002; Ernest_at_the_Beach; ...
related sidebar, but it's too boring to explain how. :')
 
X-Planets
· join · view topics · view or post blog · bookmark · post new topic ·
Google news searches: exoplanet · exosolar · extrasolar ·

18 posted on 05/24/2009 7:49:32 PM PDT by SunkenCiv (https://secure.freerepublic.com/donate/____________________ Profile updated Monday, January 12, 2009)
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To: SunkenCiv; neverdem; org.whodat; Habibi; gondramB; Star Traveler

A link to the NASA satellite tracking. It’s interactive. Gives you an idea of what is up there.

http://science.nasa.gov/Realtime/jtrack/3d/JTrack3D.html


19 posted on 05/24/2009 8:12:46 PM PDT by bigheadfred (Negromancer !!! RUN for your lives !!!)
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To: bigheadfred

Thanks.

Absolute GPS to better than one meter
C.O. Alley & T. Van Flandern
http://www.metaresearch.org/solar%20system/gps/absolute-gps-1meter.ASP


20 posted on 05/24/2009 8:31:50 PM PDT by SunkenCiv (https://secure.freerepublic.com/donate/____________________ Profile updated Monday, January 12, 2009)
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To: bigheadfred

Thanks for the link.


21 posted on 05/24/2009 8:39:41 PM PDT by neverdem (Xin loi minh oi)
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To: bigheadfred
Great link, have it book marked!!

Thanks!!!

22 posted on 05/24/2009 9:30:38 PM PDT by org.whodat
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