81–90 of 95 posts
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In mid 90s I worked for a company that developed combined GPS/GLONASS receiver and aimed specifically at high-precision measurement market. With clever math modeling and by letting a receiver rest on one spot for longer periods of time, e.g. 30 min, they could get the precision down to 3 mm (millimeters). And that's without A-GPS, purely through oversampling of satellite data.
That company was Javad, no?
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Ctrl+F -> GLONASS. It's there.
Assisted GPS Digital compass Wi-Fi Cellular I did a CTRL-F on GLONASS and no match is found. I said I did this in my edit... I'd take a screenshot, but I'm at work.
That's what I'm seeing from Europe.
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It's called rtk correction: http://en.wikipedia.org/wiki/Real_Time_Kinematic There's an opensource rtklib which supports a handful of inexpensive receivers: http://gpspp.sakura.ne.jp/rtklib/rtklib.htm
Why don't cell towers do this?
You also need either a lot of storage locally or a fast data link between the base station and the rover - phones actually have both of these but they cost money
Precisions: * GPS: 2.00—8.76 m * GLONASS: 4.46—7.38 m * GPS+GLONASS: 2.37—4.65 m [citation needed] (source: http://en.wikipedia.org/wiki/GLONASS ).
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It's funny you mention that, as I swear the professor said it could be done to accuracy of mm, but after reviewing the wikipedia articles I had concluded I misremembered. Perhaps not...
You can, but you need to need to analyze the phase of the GPS signals at the analog level (not the the code phase)
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Why don't cell towers do this?
You need a receiver in the phone that can phase track, which costs 50x as much as the simple GPS chip they fit. You also need either a lot of storage locally or a fast data link between the base station and the rover - phones actually have both of these but they cost money
So, $0.50 instead of $0.01? :)
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I don't understand from your table how the combined use of GPS+GLONASS would have a worse 'best case', 2.00 for GPS alone vs 2.37 for GPS+GLONASS.
There probably is some general statistical explanation of why you can get worse accuracy from combining multiple sources than each source by itself, but it seems irrelevant. In actual receiver implementation you really get worse accuracy for simpler reason: most of the hardware resources (ie. receiver channels) are same between GPS and GLONASS, so using GLONASS ties up resources for GPS and vice versa.
Precisions: * GPS: 2.00—8.76 m * GLONASS: 4.46—7.38 m * GPS+GLONASS: 2.37—4.65 m [citation needed] (source: http://en.wikipedia.org/wiki/GLONASS ).
though off-topic and a bit of an aside, at university (I did civ eng) I learnt that it was possible to get cm-levels of accuracy out of GPS through clever arrangement of multiple sensors (for e.g. measuring the movement of buildings due to subsidence, etc.) In fact I just found a wikipedia article all about it - http://en.wikipedia.org/wiki/GPS_augmentation :)