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Comment by nodata | Hacker News Reader

They 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.

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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?

Cell towers kind of do by providing an accurate time and other features in AGPS.

Not really. AGPS improves the time it takes to lock on to the GPS signal, but it doesn't improve accuracy.

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

You 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? :)

I think Phase tracking GPS chipsets are still pricey - outside surveying applicatiosn there isn't much demand.

Yes, 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.

Thats very interesting. Do you have any information on how much these farms are automated?

Autosteer 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

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

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.

maybe 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

(Sorry, German language only:)

http://www.sapos.de/ http://sapos.bayern.de/service.php

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