Just a minor correction. When you say "paid for by the Pentagon", what you really mean is "paid for by the U.S. taxpayers". As Jesse Ventura pointed out in his book "I Ain't Got Time To Bleed", the government doesn't earn any of its money, it simply takes it. Therefore, when the Pentagon pays for something, it's not something where you can just say "well it's their money, so they can do what they want with it", because it's my money, and the money from the rest of the U.S. taxpayers. Not that that has ever made a difference to the government in the past.

It's, like, built-in to the system. You can promise not to use it, but who's gonna believe that? Comes time of war, it would get turned off in a heartbeat. Long as DoD owns the birds, they're gonna have SA. And who else has the budget to pay for 'em.

GPS is the indispensible capability for budding ballistic missile builders. Inertial guidance systems are way harder than rocket motors to design. GPS gives away guidance for free. The problem with GPS is it's a really helpful system. Great for navigating whatever payload you want to whatever destination you desire. And no way to sort out the good payloads from the bad.

"Removing SA from GPS might, in fact, largely remove the case for Galileo, as many foreign users of GPS would then have more confidence that they could rely on the American system."

And that is probably the main reasoning for the change of heart, the loss of business to a rival system which delivers all the time and not just at the whim of someone with their finger on a switch which can turn it off whenever it suits them.

And rather than getting into a conflab about whether there is any confidence in relying on the American system, surely they would be the first to admit that having two or even more systems will keep them all honest and accurate..... Failsafe, with abuses/malfunctions/oversights/call them what you will then easily highlighted for investigation.

And heaven forbid that we would think that being able to abuse the system for advantage was the purpose of the switch.

Forget SA- the boys at Schriever can just switch off the civillian feed.

Okay, let's assume Terry Wrist manages to build an ICBM (well he can turn bottled water into a binary explosive, I trick even Jesus might fail to better)... Terry then uses GPS to guide his ICBM across the globe towards the US.

Now exactly how much error does SA introduce? Let's face it, it's gonna have to be several thousand miles for Terry's ICBM to miss the USA, it's kinda big! And I bet Canada would be dead chuffed to end up on the receiving end instead!

Given the recent demonstration of Terry's bomb making abilities over here in blighty, I imagine more death and and destruction would be caused by Randy turning right when his confused GPS tells him, and ploughing down the isle at Walmart.

If they turned on SA, can you imaging all the tom-tom users who switch off all common sense, and blindly follow it's advice ? They be driving into supermarkets, and peoples driveways!

Finally a general that has some intelligence. I took a class years ago from one of the original GPS designers and he had told the military at that time that they shouldn’t suppress SA or a workaround would be found that would make it moot and it was. Differential GPS eliminates the need for SA and provides a higher level of accuracy. Depending upon your local satellite geometry and the quality of your receiver, (survey grade in this case) you can get down to centimeter accuracy on your measurement. To achieve this you only need to have a reference station that could provide the corrections for a missile or other delivery vehicle. This reference station could be broadcasting on just about any frequency and using spread spectrum technology make itself immune to jamming. There are other ways to get this accuracy such as using differential measurements and calculations on a multi-channel, multi-antenna GPS receiver, but this requires you to be in control of the receiver’s firmware. The only people that really want/need SA are those with in-car or hand-held navigation devices since it gives them a lot better location accuracy. These days with the right equipment you can get better quality GPS measurements without SA, just not as cheap.

Not the panel, the Pentagon (okay, it's a rhetorical question). Every commercial aircraft built after 2000 not only uses GPS, but depends on it. LAX mandates Cat 3C landings for anything less than perfect conditions. Used to be that only DC10s and L1011s had that capability. Now everybody does, thanks to GPS. I've even seen a Super King Air autoland.

And that's only one application.

In 1990, SA was turned off for Desert Storm because the military didn't have enough CA capable receivers.

SA works by introducing artificial clock drift. If you want to visualise the propagation time from the SV (fancy name for flying garbage can) to the observer, then SA just means wiggling the string so that it cannot be measured as accurately.

SA degrades the positioning from approx 5 or so metres up to 100m. If Mr T Wrist just want to make a big bang in a certain city then SA will not save the day. If he wants to hit a particular spot with some precisions then that 100m might make a difference.

DGPS (differential GPS) removes SA and is widely available. Mr Wrist's collegues can set up their own for a few k or they could just use WAAS or beacon kindly provided by the FAA and coastguard respectively. This should easily give sub-3m accuracy with no firmware dabbling.

In short, turning on SA would be rather pointless from a defense perspective.

I am kind of hoping for a triple system, American, European, Russian – with “modified” firmware that would use all three systems at the same time, negating “protection” methods of each system, just a little bit. Then it would give cm accuracy, with near-instantaneous position fixes. Eeh, the dreams.

Alas, I seen a report about the Russian system, let’s just say that the receiver has cathode display (yep, like in the fifties) – looks like a pair of brinks... though, it is most probably EMP resistant, and will survive a nuclear blast...

One feels that a lot of the design ideas in selective availability came from cold war thinking rather than addressing the current threats. Not that this means that in the lifetime of the next generation of satellites the issues won't re-emerge. SA simply meant that a Soviet tank commander wasn't able to navigate usefully with the hatches shut, whilst a NATO tank could. P channel accuracy of 3 metres is useful. But as has been noted, differential GPS with only the CA channel is actually superior to the military only P channel. In general the specific question of using SA or not has been overtaken by technology.

Commercial GPS receiver chips are required to limit their use to essentially subsonic velocities. They are required to stop providing data if they detect their velocity is above a given threshold - specifically to prevent their use as a cheap missile guidance system. It is assumed that anyone with the technical competence to build a GPS receiver from scratch, one without such a limitation is already much too sophisticated for such restrictions to matter. Although this logic is now flawed.

Further, totally turning off the CA channel reduces the P channel's accuracy to that of the CA channel - it is the use of both together that provides the P channel's accuracy (by allowing for real time estimation of ionospheric propagation delay.)

Use of GPS signals abounds. For instance the protocol timing of GSM and other mobile phone networks is commonly provided by a fixed GPS receiver at each cell. Total loss of GPS would probably result in the failure of most mobile phone networks.

The issues are now clearly much wider, and strangely the reverse of what had been intended. The extraordinary and growing reliance on GPS in the modern world means that reduction of civilian availability probably presents a greater security threat than use of GPS by an enemy. Here is where thinking must move. For the Europeans the spectre of having security critical technology controlled by the US is not pleasing. Worse, it has becomes single point of failure for technological rather than political failures. Every country on the planet needs to recognise that the loss of GPS would represent a varying, but very real, economic and security risk. So, rather than reside in cold war thinking about the military control of GPS, thinking needs to move to the converse, the military's role in protecting a nation's security by ensuring GPS availability.

Eventually the only viable solution is an additional loosely tied second constellation. Not because of any mistrust of the US government, but simply from an argument of reliability of what is becoming a mission critical resource for the day to day life of a modern nation.

There are other reasons that the US military might want to keep GPS to themselves. In a conventional conflict, it would be nice to be the only one with accurate positioning, and it would be especially galling to know that your enemy is using *your* satellites to help him wage war on you.

So it's about more than just guiding ballistic missiles from North Korea to the US.

Of course, the arguments above re: civilian disruption and SA counter-measures still hold up.

US generals will just have to get used to the idea that, if they want to deny GPS to the enemy, the only thing they can do is turn off the civilian channels completely, disrupting civilian life, and probably killing quite a few of them, too!

I don't have any proof of this, so I'll just toss it out here as a conspiracy theory:

Are we sure that GPS is all that the US has as far as navigation systems? It seems to me that, given that China has developed satellite busting technology, and no doubt the US and former USSR were already developing space-based weapons, is it likely that the entire US military forces is dependent on just 30 birds that absolutely everyone knows there exact position at any given time?

Plain logic suggests that there has to be a backup.......

@Chris - so, governments don't "earn" the tax paid ? If you can do without healthcare, education, transport infrastructure, police ... need I go on ?

And who voted them in anyhow ? (OK, I guess having Jeb Bush onside helps ...)

"Alas, I seen a report about the Russian system, let’s just say that the receiver has cathode display (yep, like in the fifties) – looks like a pair of brinks... though, it is most probably EMP resistant, and will survive a nuclear blast..."

Reminds me of the story about spending a fortune on a biro that could write in Space [the pen that saved the Apollo 11 Space Mission] whilst the Russian used pencils.

Nowadays anyone can simply just buy one ..... http://www.spacepen.com/Public/Home/index.cfm

I'm not military genius and I have only a basic understanding of the underlying technologies involved in GPS but I have a somewhere simplistic theory. Since the signal from the GPS network of satellites is being provided from many miles away from a device drawing it's power from a presumably low-powered source, solar power, then the signal must be pretty weak. If you were in a war zone and you wanted to stop the local populous from using your signal, could you not just jam it? That would, of course, render it useless to you, too, I suppose...

"improve capabilities to deny hostile use of [satnav], without unduly disrupting civil and commercial access to [GPS] outside an area of military operations, or for homeland security purposes..."

Oh so your average, fundamentalist nutter can't read a map if his £200 Curry's SatNav goes dead in certain areas? Once again the ever-so insular US administration has a hold on something so useful to global infrastructure and they'd like to keep it that way and if you don't like it, tough!

Well to quote Bender from Futurama, I'm gonna put up my own GPS sats, "with blackjack and hookers! Oh screw it, forget the GPS and blackjack!"

@ammanfrommars ...using pencils creates carbon dust, which over time builds up, possibly on vital electronics. As carbon is conductive, this is not deemed to be a good thing.

@Richard. It is quite easy to jam GPS with relatively low-powered, low-tech transmitters, but it is not a good military tactic. Not only do you deny positioning to your own forces you instantly become a 'hot' target. It is a then simple for the enemy to locate (and target) you. They simply fire a missile at, or bomb, the beacon you have conveniently provided. As such 'jammers' tend to be short-lived. If you want to jam, and live till lunchtime, you have to keep your transmitter constantly moving (preferably at high speed/altitude).

Standard military tactics often include not broadcasting your exact position to your enemy.

The space pen/pencil thing is an urban myth. One that people who should know better don't bother to think about before recanting.

What actually happened was that both US and USSR used pencils for space writing. The abandoned this as soon as possilbe because the thing with pencils is that they are made from graphite, bits of graphite chip off the tip. The thing with space is that it has no gravity. The upshot is that you have tiny little bits of conductor floating round and getting into your equipment. This is a really bad thing. Oh yes, you also have wood, a fuel, again very bad in space.

A separate company, Fischer, identified this as a problem and developed the space pen, which they sold to both the US and USSR.

As it turns out a normal Biro would work in space as it uses capilary action to pull the ink down with the ball. Although there is the slight matter of extremes of temperature...

Actually Everyone used pencils in space originally (US include) and every used the Fischer Space pen for the last 30 years because no one likes getting floaty bits of pencil sharpenings in your eyes. Ironically the twist is that the Russians use the fischer pen.

As for the GLONASS the recievers are not CRT. A fact that a quick wikipeadia will also remedy. Also there are only 11 of the 24 birds required. As such it is great if you are planning on using your Rom Tom visiting an Auntie in Chechnya but it is not much use if she lives in Chepping Norton.

And therein lies why I work in the relative safety of an office building and not some hostile war zone where it's plain my suicidal military tactics would ensure my immediate and messy demise.

Since the sats are not geostationary, they will cover Chechnya *and* Chipping Norton. Just don't always expect a 3D fix within the first few minutes.

I can't see the European "me too" GPS system being any more than a large scale willy-waving exercise. Let's face it, if you were a Western organisation looking for security of provision of GPS services would you go into partership with China? Ha bloody ha.

I'll bet a stack of fivers a foot thick that this one will have a unilateral off switch in Beijing, whether or not the design specs allow for it.

If the Yank taxpayers want to gripe about the Pentagon lavishing their hard-earned dosh on something useful, they could spare a thought for the poor European taxpayers who are having a huge quantity of wonga hosed at an alternative we don't need purely to make our politicians feel more important than they really are.

TeeCee

"Removing SA from GPS might, in fact, largely remove the case for Galileo"

and Galileo would provide both an alternative and a method of checking. No wonder they don't want it!

GPS satellites are placed in a very high orbit, about 11,000 miles up. This is a very far cry from the few hundred miles of low earth orbit, used by the various spysats and the like. Killer satellites would have a vastly harder time finding their way to a GPS satellite. The energy requirements become huge, and are not the stuff of the existing killer sats. Boosters capable of reaching the needed orbit are not launch on demand missiles, but major spacecraft in their own right.

Since the constellation is multiply redundant any enemy force would need to knock out at least 7 satellites before any reduction of service occurs at all. This would be a major feat for even the US, let alone any lesser space capable country.

Killer sats are a very dubious military device. It is impossible to deny their origin, tracking of every launch on the planet is closely watched, and it would take quite some time (many hours to a day) to effect a kill. In the meantime you have provided perhaps the most emphatic declaration of war possible short of launching a nuclear strike.

In all it reenforces the point. GPS is ceasing to be primarily a military technology. Killing it for military purposes (either turning it off if you own it - trying to to destroy it if you don't) makes little to no sense. The current hand wringing over funding Galileo is probably only a first round in what will eventually become a global question about who should pay for the long term upkeep of a critical service. And implicitly, who should control it.

Providing they've out-paid the "panel" that wants civilian GPS switched off or degraded.

This isn't a partisan statement, or anti-US or any such thing. All western governments seem to operate on a pay-per-bill basis these days - so if business asks for something it will be done, then dressed up and made to look as though it's of benefit to your average joe.

This panel don't want the big switch thrown - all they have to do is grease the correct palms.

We may even get a "the terrorists will win if we don't" line twisted in some way to make it match the decision. Jobs going overseas is another good one (maybe a Chinese $0.05c per hour GPS system promises not to throw its degrade switch), giving Minutemen GPS to help them catch Mexicans is another (so they can re-find that 3 ft piece of crap they call a fence on the 60 trillion mile border).

I can think of a few things to do when you notice an enemy using your GPS sats against you.

Instead of jamming the signal how about broadcasting localized (other a couple 100 miles) alternative data. This could be done by a higher power single drowning out the real single (ok, maybe not that simple, but some method). Do it from a lower orbit sat over the area or some other safe area to avoid being targeted, also as it's not on all the time, only when enemy are moving which should also reduce targeting and make the enemy less trusting of the real signal. The friendlies use the redundant signal (the ones only they now about) for their GPS when the alt is active.

Steer that missile back to where it came from, or lead tank commanders into some bad terrain.

Muhahaha

"Removing SA from GPS might, in fact, largely remove the case for Galileo, as many foreign users of GPS would then have more confidence that they could rely on the American system."

For once the BBC managed to write a better more accurate article on the system than el reg http://news.bbc.co.uk/1/hi/sci/tech/4555276.stm