Hmm, yes kind of like how a new GPS costs $5K, because the first .mil model did cost that much. Or all computers currently cost millions of dollars because the first ones did.
As somebody who's primary field of study is GPS / Navigation / Positioning, I'd like to remark that survey grade receivers such as in the eLoran stations can cost much, much more than £2,000.
Their price isn't so much the hardware itself, in fact, most of the receivers seen in cell phones today cost less than $1USD. However, when dealing with more accurate receivers, the receivers typically need extensive calibration and testing. I mean, it's possible to get measurements from GPS accurate to 1mm, but you need to have both a careful setup and very good knowledge about the errors associated with your receiver. So yeah, these receivers are actually quite expensive, and most of it is because the calibration on repeatability and stability of these sensors is of the utmost importance.
On a secondary note, survey-grade receivers are typically not as prone to jamming as consumer-grade receivers, mostly because they can use multiple GPS frequencies / can collect over a wider bandwidth. There's a lot more to it than that, but effectively the point I'm making is that the receivers spoken of in the article are not the same as everyday GPS receivers.
If you want to see how crazy the prices on these receivers can get, look no further than http://www.surveyorsmart.com/product.sc?productId=548. I doubt many surveyors would go for that particular model by Leica, as it is definitely expensive, even for survey equipment standards, but it's not unreal in terms of pricing.
So going in the other direction, are there cheap eLoran receivers available? Would one be any use as a backup to, say, someone with a cheap second-hand yacht who occasionally goes sailing?
I don't know of any personally, but I'm going to say probably not. If you're looking for dual band / GLONASS + Beidou support high grade receiver, you're typically looking into the $1000-$10,000 area.
The best I've seen in consumer space so far is (surprisingly) the iPhone 5. Full AGPS and GLONASS support, which is better than most Android phones.
Ultimately, I don't know if I can give you explicit advice on the matter, but I would suggest you search for some way to integrate something such as an IMU into your navigation system, if possible. A properly implemented dead reckoning plus GPS is usually pretty hard to fool, as one of the systems will drop errors as soon as something starts to go wrong. However, getting the proper integration might cost you more than you're willing to pay, so it may just be in poor taste for me to even bring it up.
What we should be doing is replacing $1 GPS receive chipsets in phones with procession positions chipsets, and taking advantage of CORS reference stations (http://geodesy.noaa.gov/CORS/) for sub-centimeter positioning.
you can google the spec sheet for the receiver described. it can measure carrier phase to 0.2mm rms. that translates to an accuracy after post-processing of 3mm horizontal (with a fancy antenna).
the trick is to use the carrier itself, which is higher frequency than the signal modulated on it. also, relative offsets are easier that absolutes (removes many systematic errors).
i wrote (the software for part of) one of these (not for leica, for some geophysical survey company) back in the day (although it was not mm resolution!)
Current GPS receivers have a couple sources of error, including processing slowness and ionospheric delays. If you don't know what the ionosphere looks like, you can't be accurate to 0.2mm. There are ground stations that measure this (based on the fact that they aren't moving so any change in the GPS-calculated position means ionospheric changes) and transmit the correction data to other GPSes, but this doesn't get you to 0.2mm, at least not for a moving object.
TO be fair, Andrew wasn't claiming 0.2mm accuracy in position measurement - he was quoting the devices ability to resolve phase difference in the carrier wave (and noted that is an order of magnitude or so better than the resulting position accuracy).
Having said that, I wonder what the magnitude of ionospheric changes have on the phase difference of the carrier signals from satellites in different directions?
(Even though I know how it works, the idea of getting millimeter precision in measuring distances to something that's at least 20,000km away and traveling at almost 4km/sec seems like very black magic to me… Surely that can't actually _work_ in practice…)
i don't know what current state of the art is and even 3mm sounds crazy good when i think about it (i was just repeating the spec sheet). i wonder if it that also requires separate / multiple receivers to fully model the ionosphere?
As I understand it - once you can get the time-based position fix accurate enough, you add in the phase information from the 1.2GHz carrier wave - with a wavelength of ~200mm, resolving that to 0.2mm seems reasonable.
The problem of working out which of the peak/troughs in the carrier wave you're in almost certainly requires terrestrial DGPS assistance (http://en.wikipedia.org/wiki/Differential_GPS). If you can use that to get ~100mm precision - that allows you to use the phase difference in the 200mm
wave length to get sub mm measurements.
Like I said, I understand how it works – I just find it hard to believe it's actually practically possible… Deep magic…
There is a network of hundreds of (static) GPS receivers to monitor seismic activity. They are post-processed to identify and remove the ionospheric effects you mention.
They used to (ca. 2000) get ~1mm accuracy in the plane of the Earth, and ~1cm accuracy in the radial direction. The accuracy seems to have increased in the meantime, and it appears to be ~0.1mm in the plane of the Earth.
GPS is combined with other sensors like strain meters, etc., into something called Plate Boundary Observatory: http://pbo.unavco.org/instruments/gps
this is actually easier in some ways because it's so slow you can integrate forever and so reduce noise (both "normal" noise and the added stuff, if they are still doing that).
A new GPS made for commercial, certified use (eg. for use in aviation) will cost way more that $5K. Just a software update to it might cost as much. Technology is getting cheaper, certifying it isn't. In don't know about ships, but I assume there is some form of certification for nav equipment there as well.
I don't know about presently, but in the past the GPS transmitters had an introduced error known as "Selective Availability," the purpose of which was to deny pin-point accuracy to unauthorized devices.
For a price, one could acquire the necessary hardware/software to mitigate Selective Availability in order to increase the accuracy of GPS readings, although I forget what those prices were/are.
I remember reading a fascinating article (which seems to elude me on Google right now) about the competitors in the 1990 BOC single handed Around the World yacht race noticing the CEP (circular error probable) on their GPS devices drop from ~100m down to ~10m or better in late July – a week or so before the Gulf War officially started.
They didn't permanently switch off Selective Availability til mid 2000, but they could and did temporarily switch it off when it suited them (and, no doubt, could easily switch it back on today if they thought it worthwhile… They claim any satellites launched since 2008 haven't had SA capability, but I suspect that's only a small-ish percentage of the GPS constellation today…)
GPS used to be that way to give the military access to accurate positioning and provide lower accuracy to others. They can still do this selectively to whole areas of the world should they want to.
The US claim that since 2008 they've been launching GPS satellites without selective availability capabilities.
That means 27 of the current working GPS satellites are older ones which do have SA which could be switched back on at any time, and 12 are newer allegedly non-SA equipped satellites.
Errr, as I read it, the cost is for new eLoran receivers, intended to be an alternative backup to the Mark I eyeball if GPS flakes out for whatever reason. Not going to be widely deployed like GPS receivers, and big ships need reliability ... in fact, the only customers are those who really need this sort of thing, unless it becomes mandated.
There are different grades of GPS receivers, all licensed differently. Consumer-grade receivers have a maximum accuracy of something like 3 meters and a slow response time. There are also aviation grade and military grade receivers, which cost more (build & calibration) but also require a special license.
I interned at Trimble in the radio group. Industrial GPS receivers could be purchased by more / less anyone, however they were built and priced for heavy-duty industrial consumption (mines, farms, ships, aircraft, etc.). The unassisted accuracy without kinematic corrections was around +- 1 m horizonal and ~10 m vertical, constantly changing as the constellation moves. This would be for a top-of-the-line 24 channel parallel receiver. WITH kinematic GPS, +- 10 mm (not a typo) horizonal and ~3 m vertical. It was so good that receivers could be installed on the left and right sides of a grader's blade that the angle and position could be known with great accuracy and precision. Kinematic means a nearby stationary reference ground base station sending updates over radio frequencies to another GPS receiver that could be in motion. This was also used to subtract SA pre-Clinton because the injected error would be the same in nearby locations.
Hmm, yes kind of like how a new GPS costs $5K, because the first .mil model did cost that much. Or all computers currently cost millions of dollars because the first ones did.