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Comment by jonah | Hacker News Reader

Hmm, so are they using laser transits as the input to adjust implement height when leveling fields? I always assumed it was dGPS, but 1m in the vertical isn't nearly enough.

Robots have been (helping) grow our food for quite a while now and we just don't think of it that way.

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RTK GPS is quite common for levelling applications. It's closer to 25mm in the vertical when a local (~10km) base station is used. Lasers are used but have line-of-sight and atmospheric distortion issues.

This [1] is a good overview of how RTK works.

[1] http://www.novatel.com/an-introduction-to-gnss/chapter-4-adv...

Yes, lasers. Even with the higher accuracy and more expensive real time dGPS systems the vertical accuracy isn't as good as horizontal. When you're looking for a very shallow gradient in a field for irrigation GPS just isn't as good as a laser level.

Much of the problem is geometry, while you're likely to have a good spread of satellites around your receiver horizontally they're all on one side of the vertical solution your receiver is calculating.

3 sats are needed for a two point fix, 4 for a one point fix, and 5+ help with accuracy if they're not clumped near each other (the constellation's always moving). At higher altitudes such as in a plane at FL 350 with sats below the receiver, vertical accuracy increases some compared to ground, which makes these obvious: http://www.gps.gov/applications/space/

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