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WAAS.....how accurate ?
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torn
Posted 8/18/2017 07:10 (#6194470 - in reply to #6194273)
Subject: RE: WAAS.....how accurate ?


roaming
iknow - 8/17/2017 23:06

was at a sprayer dealership today, was asking questions about guidance units, since we might need to do our own spraying, we lost our local
sprayer this past month.
I was wanting something to spray with, and to plant with. we are dryland, so having things exactly spaced is not that important.
but want it fairly close in spacing.......I was going to get an inexpensive unit this spring, but was told WAAS it was plus or minus 20 inches........
today, I was told it is plus or minus 6 inches, with a hocky puck antenna......this was a ravin cruiser II unit.

which is correct ?

thank you


Both are basically correct statements, and illustrate the importance of details when it comes to understanding GPS accuracy.

WAAS is generally understood to have a static or absolute accuracy of approximately 1 meter. This means that if you were to plant a WAAS receiver in a static location and let it collect points for 24 hours, most of the points would fall into a circle with a 1-meter radius. "Most" will generally mean either 68% or 95% (1 sigma or 2 sigma), depending on the type of antenna, quality of GPS chipset, and manufacturer's decision. Static/absolute accuracy is also referred to as "repeatable" accuracy, but I avoided that term here as WAAS is not considered a repeatable signal. Static/absolute/repeatable accuracy is the number you want to pay attention to if you want to be able to come back later (next day, next week, next month, or in future years) and drive the same path (i.e. for strip-till, controlled traffic, drip-tape, row-crop cultivation, side-dressing, etc.) Most manufacturers will quote WAAS at "sub-meter" absolute accuracy.

The other number you were quoted, plus or minus 6 inches, is referring to pass-to-pass accuracy. This gives you a good indication of the accuracy you can expect with in 15 minutes, or approximately the average time it takes to make one full pass (back and forth) in the field. This is the number you want to pay attention to if you're concerned about skips and overlap during a single continuous field operation. Most manufacturers will quote WAAS at between 6 and 12 inches, pass-to-pass.

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