There’s been some upset in the blogosphere over the shutdown of the LORAN system of radionavigation beacons. This post at Chicago Boyz is representative (hat tip to Instapundit). The author worries “Iâ€™m not totally sure that this was a good decision.” and various commenters are much more emphatic, bemoaning the lack of a backup for GPS.
In my capacity as lead of the GPSD project I’ve been required to become a topic expert on the strengths and weaknesses of GPS and various competitors to it, including LORAN. So here’s the straight scoop: yes, a backup for GPS would be a really good idea. No, LORAN was never plausible as that backup, so angsting about its passing on those grounds is just silly.
GPS is locally vulnerable to ground-based jamming, and globally vulnerable to attack by satellite-killers. Other than the U.S. and Russia, only the Communist Chinese are thought to have anywhere near the capacity for the latter, but that’s enough reason to worry; taking out GPS would be continuous with their strategic doctrine, which seems to puts high priority on unconventional warfare against enemy technological infrastructure. See for confirmation the recent rash of crack attacks against the Pentagon apparently originating in China.
The biggest problem with justifying a life extension for LORAN on this basis is that its area coverage was totally inadequate. The entire half of the world south of the Equator was a dead zone as far as LORAN-C was concerned; in fact coverage was never really good outside the land area and near coastal waters of U.S, Europe, and the Pacific Rim. A secondary issue was that LORAN accuracy was very poor – 60 to 300 feet, with significant multipath problems in night operation. The accuracy issue could have been addressed with a relatively cheap technology upgrade, but fixing the coverage issue would have required an expensive station-building program.
So, what would make sense as a backup for GPS?
For starters, multiple satellite constellations at different orbital altitudes; Galileo and GLONASS would be a good beginning, but only that. If I were the Pentagon I’d actually be putting money on the table to get the Europeans off their lazy bureaucratic butts about Galileo. (The first of those birds was supposed to go operational two years ago.) GLONASS doesn’t look like a good bet; it’s kind of a shambles, and Russia’s accelerating descent into the status of demographically-collapsed third-world pesthole doesn’t inspire confidence that the problems will ever be fixed, especially since more than half the engineers who used to run GLONASS are probably now living in Israel.
If I were designing a backup, it would be a fleet of solar-powered high-altitude aerostats. They’d use laser ranging to fix their position with respect to ground stations at known locations and each other, and broadcast microsecond-timed here-I-am signals that could be used the way GPS signals are. If a bad guy shot some down, launching more would be cheap.
The reason this wouldn’t have made sense even twenty years ago is that the local calculations to deduce a position from a network like this are a chrome-plated revolving bitch. Your aerostats will be moving unpredictably, remember, so they have to be recomputing their own position from observations in real time. It’s not like the satellite case where knowing your orbital elements and the time gives you your position to high accuracy; the aerostat net would need very powerful but dirt-cheap onboard computers, which was not a capability we could take for granted even as recently as the early 1990s.
Then, of course, your navigation receiver would have to do trigonometry to deduce its position from the positions and times shipped by the aerostats in view. Humans couldn’t perform these fast enough to be useful, but receiver firmware can; this is well tested in GPS receivers.
Actually, an aerostat-based system would have one significant advantage over GPS — no accuracy loss due to variable signal delay in the ionosphere. It might be worth building for that reason alone.
How many aerostats would be needed in order to make a reasonable alternative to GPS?
Would it even make sense to for the aerostats to be a fully independent system, or could you just integrate them into the GPS network as additional redundancy?
If you take the US Coast Guard Auxiliary boating safety course, they will (repeatedly) tell you that you need paper charts, and the knowledge of how to use them. Your electronics might fail, or your electricity might fail. If you have paper charts, you’ll have a backup.
Yeah, it’s Old School. That’s kind of the point, I guess.
I’ve always got my Silva & a ToPo ;)
>How many aerostats would be needed in order to make a reasonable alternative to GPS?
That depends strongly on how high you could deploy them. Let’s do a back-of-the-envelope calculation. Let’s say we put them 15km up, high enough not to be a hazard to airliners. Horizon distance would then be 441km. Computing the area of that circle, each aerostat would be line of sight for about 611000 square kilometers. Dividing the Earth’s surface area of 510,072,000 by this gives about 834 aerostats.
That’s not too bad. Of course that’s with zero redundancy and perfectly equal spacing, and the things are going to drift like crazy. But it suggests that if we were willing to keep maybe 3000 of them in the air it would do the job. That should be well within the capability of a single midsized industrial nation, let alone a worldwide consortium.
Indeed. This goes for pilots of aircraft, as well. There are lots of pilots out there trusting in their $10K panel-mounted IFR GPS receivers and forgetting how to use other radio navaids, much less how to locate themselves on a chart. They’re going to get themselves in serious trouble some day, and all it takes is something as simple as the connector falling off the back of their GPS antenna (as happened to me one day). I knew what to do when my GNS430W said NO GPS SIGNAL. Too many pilots don’t.
Eric, 15 km is 50,000 feet. They’d need to be higher than that, as civil jet aircraft routinely operate at FL510 (51,000 feet, more or less), and you need to get them enough higher to provide good geometry to users at those altitudes. This is good, in that it gives each one greater coverage, but since atmospheric density is pretty low up there, it makes lifting with lighter-than-air aircraft harder. Not only that…but how do you maintain them?
> Not only thatâ€¦but how do you maintain them?
You can always maintain them the same way you do satellites: launch new ones when the old ones crap out. However, since they’re theoretically steerable, you do have some options for recovery and repair/salvage, which is not an option for GEO sats. They’d be faster turnover than satellites, probably, but the launch costs (and probably design and manufacture costs) would be tremendously less.
I don’t remember the show, but I’ve seen a similar idea for making a worldwide communications backbone. They were testing a laser based communications system that was communicating with a satellite, but acknolwedged that it would bee too expensive. They mentioned using UAV’s flying racetrack patterns to provide the relays instead. That would also help with the drift and altitude issues.
@Jay – For any of these ideas, you would have 2 – 2.5 the number of aircraft needed and they would be cycled out periodically for maintenance, upgrades, etc.
>They mentioned using UAVâ€™s flying racetrack patterns to provide the relays instead.
Come to think of it, that might be a better solution than aerostats. Something like the solar-powered Zephyr would be capable of better station-keeping than a dirigible. And aircraft in this class can fly really high; one of NASA’s solar UAVs holds the altitude record for a non-rocket-powered vehicle. 70,000 to 80,000-foot operating altitudes should be readily achievable.
Being an internet SUAVE herder (Solar Unmanned Aerial VEhicle) would be one of the more science-fiction-meets-reality jobs I’ve heard of in a while; managing a herd of semi-autonomous robots scattered across the country, calling them in for repairs when they complain or for maintenance at which point they fly hundreds or thousands of miles to come home, for the purpose of providing internet connectivity to the masses. Need better coverage over Tulsa? Hit a few keys on your keyboard, look out the window, and watch some of your backup SUAVEs automatically roll down the runway and take off. There’s a job you could barely conceive of existing even in the late 20th century. (Yes, I know a few people had the idea back then, but only a few.)
I’m not capturing the spirit here very well; there’s something oddly poetic about all this.
Borepatch and Jay Maynard are right. Having been offshore on a commercial fishing boat when the GPS had a nervous breakdown, I can assure you that some knowledge of dead reckoning and charts is essential. The skipper, who was young, did not and was literally nauseous until I did a bit of figuring with the heading and speed and told him where we were. There are entirely too many people in boats and airplanes who do not have a clue and are screwed if the electronics crap out. The same was true even when loran was the way to go. Even something as simple as noon-sight navigation can get you just about anywhere using a sextant and a good watch if you know what you are doing.
Some random thoughts:
Would the Wide Area Augmentation System address these issues significantly?
One of the issues that is always lost in discussions like this is the question: who needs what level of accuracy where? GPS is accurate with within 4-12 inches or so (source: wikipedia – *cough*, *cough*). Where is this needed? Well, munitions drops are a good case. But that’s a military application and I think the DoD can handle it’s concerns itself. Possibly for complete hands-free auto-landing of aircraft (Jay, care to comment?), but that’s only really needed at runways, which can be provided though the Local Area Augmentation System. Basically we’re left with civilian uses at that point.
As a practical matter, if you are in the middle of the ocean, you only need to be accurate to within maybe a couple of miles. After all, unless you are going to run into shore or a reef, one piece of ocean pretty much looks like another. I know that for trans-oceanic flights they’ve used inertial guidance for decades. Of course, this ran into a few planes being shot down by the soviet union but that was a political issue and resulted from improper use to boot.
If you assume that you need far less accuracy in the middle of the ocean than on land, you end up needing far fewer aerosats or drones from Eric’s proposal for that application.
Here’s another thought. Most people need the greatest accuracy for terrestrial navigation within cities – you have a higher population density, higher road density and shorter time in which to make decisions. If you’re on a rural freeway you know you are supposed to be on for another 100 miles and your GPS device things you’re on a side road, you can pretty much ignore it. If you’re in downtown New York and your GPS receiver thinks that you are 1 street over, the directions you start getting can be very unhelpful (turn onto a 1-way street the wrong way). There are already radio broadcasts in highly populated areas. FM radio comes to mind. Would it not be possible to use some of the 200kHz for an FM radio station to include what would effectively be another GPS signal internal to that? Transmit an exact location and time and use that as part of synchronization data? It becomes a public service provided by the station, and from a practical standpoint could be required as a part of station licensing by the FCC. Alternatively, unused AM radio frequency slots might make an interesting use.
Attempting to provide super-accurate navigation support everywhere as a backup to a military navigation aid strikes me as a waste of resources. High accuracy in limited locations and “good enough” accuracy everywhere else – maybe a better choice.
WAAS is what lets an aircraft use GPS as a replacement for such things as ILS. It’s only useful with functioning GPS. It’s also only suitable for an ILS replacement, not for autolanding; for that, you’re correct that LAAS is needed. ILS is normally only good to 200 or 250 feet above ground level and about a mile from the runway; GPS/WAAS LPV approaches have the same design criteria.
I’ve flown a GPS/WAAS approach a few times (always in VMC (visual meteorological conditions), since I’m not instrument-rated). To a pilot, it acts just like an ILS.
I’m not sure that any replacement for LORAN will be necessary. GPS 3 will have about 500 times the transmitter power and about 30 satellites. This will allow for much greater accuracy along with signals that are much more resistant to signal jammings and interference.
That being said, I am nearly 100 percent certain that no matter what that article says, LORAN did not entirely go away (they probably changed frequencies or something and simply didn’t publish the new ones), nor will it at least until GPS 3 is up and running. (I can’t say how I know this other than to say that my non-public sources for this information are very, very reliable.)
>GPS is accurate with within 4-12 inches or so
Not usually. 2 meters is the usual error budget with consumer-grade GPS and differential correction, 8 meters without DGPS; Wikipedia says 5 meters, which conveniently is the average of these. You can get more accuracy with postprocessing or with very expensive survey-grade or military GPSes. 4-12 inch accuracy is close to the limit of accuracy you can get in real time under ideal conditions.
>If you assume that you need far less accuracy in the middle of the ocean than on land, you end up needing far fewer aerosats or drones from Ericâ€™s proposal for that application.
Alas, no. The way the trigonometry works, you need lock on three birds with known locations to have a horizontal fix, four for altitude. Doesn’t matter whether the birds are sats, aerostats, or UAVs. Accuracy doesn’t increase a whole lot with more and you get no fix at all with fewer.
The main thing you get from a 12-channel receiver that can lock on every bird visible is the option to ignore timings from birds that are closest to the horizon and thus most subject to variable ionospheric delays.
>Would it not be possible to use some of the 200kHz for an FM radio station to include what would effectively be another GPS signal internal to that?
Something like this is already being done with cellphone towers. It’s called AGPS, Assisted GPS. But it’s kind of a swamp; the protocols are proprietarty and undocumented or poorly documented.
Per AGPS, GPS, Cell-phone towers, and cities…
I was testing a smart-phone-based application that used GPS data as part of my job.
When testing indoors, I would get AGPS (cell-phone-tower) data, which would usually be reports as having 1500 meters or so of accuracy.
Trouble was, the service had an incomplete list of tower-locations, and when it didn’t know the location of a particular tower, it defaulted to a location in the West Coast. Walking outside would cause a huge correction, as the smart-phone suddenly reported our true location (in Michigan).
That was eventually fixed by an automated-update of cell-tower-data listing on the phone. But it was both amusing and annoying.
And it’s likely an example of the kind of swamp that AGPS can be.
(We didn’t deal directly with AGPS, the OS of the smart-phone had that data all abstracted away and hidden behind a single data-structure and a Location service.)
This would be a J2ME phone?
Aren’t all these systems vulnerable to the EMP from one nuke going off 300 miles above Kansas? It doesn’t matter if all the GPS satellites are still there and you’ve got lots of backup aerostats if every piece of non-military electronics from coast to coast is fried. (That includes all the heavy-duty power grid transformers that would take years to replace.)
And the power to do that is a-l-m-o-s-t within reach of Iran….
>Arenâ€™t all these systems vulnerable to the EMP from one nuke going off 300 miles above Kansas?
Yes, but there’s basically no way to harden against that. You deal with the threats you can deal with.
I would personally love to see more work done with Simultaneous Localization And Mapping (S.L.A.M.) or a similar technology. The basic concept here is to make the device responsible for finding its position with only the use of a camera or similar sensor.
The wikipedia article.
And a practical example.
A tangential question: I read somewhere that GPS satellites are updated DAILY with clock signals from the Naval Observatory, obviating any relativistic physics corrections for GPS satellite clocks. Is daily updating a true fact?
ESR says: Yes, though it’s not clear to me from the descriptions that the update has anything to do with relativistic corrections.
It actually wouldn’t matter from GPSD’s point-of-view as GPS does no correction itself (nor should it). Some correction is done by the GPS receiver for leap seconds, and there’s actually a FAQ about that:
I think you want the aerostads to be 20-25km up. That’s the spot where the atmosphere is normally the calmest – see http://www.classzone.com/books/earth_science/terc/content/investigations/es1702/es1702page09.cfm for an illustration. Obviously it’s different in different lattitudes, and does vary depending on various weather events, but ISTR it’s around the 25hPa level.
>I think you want the aerostads to be 20-25km up.
Good point. Let’s say 23km height; that yields a radius to the horizon of 542 km, for a line-of-sight disk about 923,226 km**2 in area. Then you get single-ply coverage with about 553 aerostats or UAVs, reducing the count needed for three in sight from anywhere to about 1159 birds.
I thought the Russians already had a GPS alternative flying.
I think this is a very good idea, but that we better test different things to know what works better before.
esr, had you thought about making a company of UAVS to provide internet to rural areas?
>I thought the Russians already had a GPS alternative flying.
That’s GLONASS. As Wikipedia says:
Following completion, the system rapidly fell into disrepair with the collapse of the Russian economy. Beginning in 2001, Russia committed to restoring the system, and in recent years has diversified, introducing the Indian government as a partner, and accelerated the program with a goal of restoring global coverage by 2009.
They haven’t re-achieved global coverage yet.
Jeremy Bowers comment would also work ( in a way ) for disaster relief. The need for extra cell phone / wireless links could be handled by a few extra units sent over head to the site.
My friend next to me also made the rude comment ” oh like the thunderbirds”, prat
Between Loran and GPS, there WAS an intermediate electronic navaid; does no one remember “Omega”? As a VLF radio system, the antennas tended to be relatively large, and could NEVER have been pocket-sized like GPS, but it also works underwater or underground to a limited extent.
Of course, if you have a view of the sky, celestial navigation isn’t THAT difficult.
What’s interesting about this is that you don’t need that many to get a useful capability. As soon as you have no more than a squadron (16) aircraft or aerostats – 4 for continuous precision nav over their common footprint plus airborne and on-ground spares and relief vehicles (depending on how long it takes them to get to and from station). Off the top of my head, 6-such units (under 100 vehicles) could completely cover CONUS or a similar size region anywhere. They don’t have to be based locally (again, depending on transit time and tradeoffs between time on station and range) and once at a high enough altitude can provide a useful service en-route. Not just nav, but comms (as noted above), also remote sensing/surveillance, and whatever else you have space and power to carry. In a military scenario, you have to worry about someone taking a shot at your system, that’s been true since the Soviets shot down Powers’ U-2 and high altitude UAVs are still useful.
For the powered (or put up enough and let them just drift) aerostats, if they are cheap enough, recover the payload capsule by a wing-style parachute (powered or gliding) and don’t worry about the lift section – just replace it.
Has anyone done the paper engineering to define the accuracy limits of an electronic sextant? A quick web search didn’t yield an answer.
With maybe a smartphone’s computational ability. And an (unfiltered) cmos camera as found in the high-end phones w/ a mechanical, perhaps approximately positioned by humans optics to match. – i.e. a camera with broader spectrum than the eye can see, ability to resolve/average the sun center-point-of-energy thru most clouds, power sufficient to process horizon, sun / starfield, time, device, at a fast enough sample rate to eliminate movement issues on rough seas (though I suspect samples could be taken at a fast rate and post processed over minutes).
How about the old Omega. About 10 ground stations world wide. Was about +/- 5 miles in initial form and never got further off the ground. But lane mixing was in it’s future uintil Transit came along. With the possible triple lane count system it should be +/- 1 mile. Thuis is good enough to get you there. GPS in autos teaches people how to get lost when it fails. Nobody NAVIGATES anymore!