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| Millhouse
I am going to start with your comments and work my way through all that have contributed so far to the discussion.
I do not want to start a color war, but some GPS receivers perform (accuracy) better than others.
As far as you not seeing much "difference" between SF2 and JD RTK points to some of JD's problems with RTK. I have seen SF2 here in California for about 4 years and yes it is USUALLY in the 1-4" range but not always and it is rated at +/-4" witch means it can be 4" to the left or 4" to the right of a proposed line. We are seeing similar accuracy with WAAS.
Initially JD would not quote an accuracy for their RTK system, and just because it is RTK does not mean it is sub-inch accurate. Also they will only talk about accuracy at the antenna which is 10' in the air, not what the tractor will do in the dirt. Their receiver is only accurate 66% of the time vs the industry standard of 95%. What does this mean? 34% of the time their RTK unit (accuracy measured 10' above the ground) is outside its rated accuracy. So if their receiver is rated at 1" accuracy (remember that is +/-1"), then for 20 minutes out of every hour it outside its accuracy rating, greater than 1" from the line. How much is it off by, depends on many factors.
So what I am saying is look at other GPS mfg and look at their RTK. You will see a difference in accuracy between JD RTK and say Trimble or Autofarm or the newest entry to AG RTK....Topcon.
Next SF1, SF2, Omnistar VBS, HP, XP, and WAAS are all DGPS ( differential/delayed GPS) signals. That means your receiver is getting a signal from all the GPS sats in view along with a separate "correction" signal from either a landbased tower or a privately funded(Omnistar, Greenstar, Navcom) Satellite. These signals are delayed and can take a few seconds to receive, hence their accuracy.
RTK (Real Time Kinematic) means that the receiver on the tractor and the receiver at the base station communicate via their radio network multiple (50-100) times per second as to the location of the "rover" or in this case, a tractor. Couple the accuracy of the receiver and how often the "base" and "rover" talk, and you have why it is called real time. All the systems use radios for the ag market. Some have stronger radios and some actually have a better GPS correction algorithm that allows them to be farther than 6 miles and still maintain their target accuracy. (have been out at 10 miles and within 2")
In the construction and survey market, they have been using RTK GPS for a number of years and can use it with cellular services in the place of a range limited radio. There is a limitation, their situations do not require that the rover and base communicate 50-100 times per second, cellular technology has latency or time delay issues. (Think about how long it takes to a VM in some locations)
So what you have to ask yourself is what does it cost for accuracy(+/- inches) and what does it cost for precision(+/- sub-inch)?
They are different!
Accuracy is hitting the target, precision is hitting the same spot on the target over and over and over.
Sorry for being so long winded, but want eliminate some of the misunderstandings of GPS.
Scott | |
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Is RTK really necessary?
