5 ms·
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 arran
by singular 15y ago
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 http://en.wikipedia.org/wiki/GPS_augmentation :)
- nodata 15y agoThey use this on automated farms. At known co-ordinates, they stick a GPS receiver in the ground. The difference between the known co-ordinates and what that GPS receiver sees is then continuously transmitted to the vehicles on the farm.
- mikepurvis 15y agoIt's called rtk correction: http://en.wikipedia.org/wiki/Real_Time_Kinematic 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 http://gpspp.sakura.ne.jp/rtklib/rtklib.htm
- modeless 15y agoWhy don't cell towers do this?
- HeyItsDiogenes 15y agoBecause cellular technology is actually designed to handle long distance with obstructions?
- fleitz 15y agoCell towers kind of do by providing an accurate time and other features in AGPS.
- modeless 15y agoNot really. AGPS improves the time it takes to lock on to the GPS signal, but it doesn't improve accuracy.
- nobody314159 15y agoYou 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
- jrockway 15y agoYou need a receiver in the phone that can phase track, which costs 50x as much as the simple GPS chip they fit. So, $0.50 instead of $0.01? :)
- nobody314159 15y agoI think Phase tracking GPS chipsets are still pricey - outside surveying applicatiosn there isn't much demand.
- wyclif 15y agoYes, until the property market crashed in the US I was a land surveyor. This is the same principle used in RTK/GPS (real-time kinematic) measurement. A "zephyr" GPS antenna is placed on a coordinate "base" usually 2m above it on a tripod, which then continually updates and measures the difference in distance between it and the "rover" antenna carried by the surveyor, as well as the satellite network. I used to have an updating map on the web of all the GLONASS satellites the Russian government had put into orbit, but it was taken down.
- FrojoS 15y agoThats very interesting. Do you have any information on how much these farms are automated?
- randomdata 15y agoAutosteer is quickly gaining in popularity. From looking at the farm equipment in my area, I'd guess that 40% have GPS systems on their tractors/combines based on observing the antennas on the equipment. Precision farming is becoming important to increasing yields and reducing costs. Right now, Kinze are heavily touting their autonomous system for grain carts: http://www.youtube.com/watch?v=MhA5aIw7xNk http://www.youtube.com/watch?v=MhA5aIw7xNk John Deere and CNH have similar systems in actual production, but still require an operator to be present for safety reasons. And safety seems to be the big limiting factor. While the technology is basically there, nobody wants to remove the operator just yet. Though in this video, someone disabled the safety sensors to go fully autonomous: http://www.youtube.com/watch?v=zU4liQvrcm4 http://www.youtube.com/watch?v=zU4liQvrcm4 Most farmers are pulling their RTK data over wireless/mobile internet connections, rather than having their own base station. I don't know of anyone who maintains their own station. A few companies have placed base stations around various places in the countryside and will happily sell you their data.
- rmoriz 15y agomaybe interesting: In Europe (and of course in Germany) the federal governments offer realtime gps correction data via IP or 3G (radio was shut down a few years ago). They offer a precision of 1cm-2cm used together with GPS/GLONASS PDF: http://www.sapos.de/pdf/Flyer/2004Flyer_e.pdf http://www.sapos.de/pdf/Flyer/2004Flyer_e.pdf (Sorry, German language only:) http://www.sapos.de/ http://www.sapos.de/ http://sapos.bayern.de/service.php http://sapos.bayern.de/service.php
- eps 15y agoIn 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.
- omgtehlion 15y agoThat company was Javad, no?
- eps 15y agoIt was Javad's, but before Javad, the company.
- singular 15y agoIt'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...
- juiceandjuice 15y agoYou can, but you need to need to analyze the phase of the GPS signals at the analog level (not the the code phase)
- CamperBob 15y agoSo the carrier phase is derived from the Cs/Rb clocks on the satellites? That's pretty neat, if so.
- Symmetry 15y agoI'd imagine that if you just left a GPS receiver in the same place for long enough, the movement of the GPS satellites would provide some decent dithering for the signal, letting you get fairly decent accuracy just by averaging a lot of measurements. I'm sure there are other methods that work more quickly, though.
- ghshephard 15y agoIn 1995 I worked for a GPS developer, Avcan Avionics, and we built software that would allow tundra devices in the Arctic to travel between a supply depot and remote mines in 100% whiteout conditions, navigating only by a path that had been loaded into GPS. Our variant of this process was called "Differential GPS" - basically leaving a base station in single location for 24+ Hours where it would eliminate the S/A error which was presumably random. We successfully navigated a car down a twisting road, about 10' wide, without looking out the window, so it was a useful mechanism. It became less important, of course, once the US Government removed the S/A error from the GPS signal and the commercial use of GPS took off.
- Shenglong 15y agoI know carrier phase differential GPS is used on satellites to get amazing accuracy. I'm not quite sure what the implications are on mobile phones.
- teyc 15y agoAt mine sites, we used http://en.wikipedia.org/wiki/Differential_GPS http://en.wikipedia.org/wiki/Differential_GPS