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RTK, cell phones, and other questions.......
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DG N. AL
Posted 3/18/2007 15:54 (#121772)
Subject: RTK, cell phones, and other questions.......



Hillsboro, AL
Sorry guys.  I have been planting corn all week except for the few hours (not nearly enough) that I have been sleeping.  There are several questions scattered down the page below.  I will try to answer most of them here.
  1. RTK means there is a base station somewhere.  You do not get RTK without a base.  Someone owns and maintains that base.  It may be you or some one else.
  2. What does the cell phone do/replace?  With RTK, you must transmit a correction signal from the base station to the rover unit/tractor.  The traditional way of doing this has been with 900 MHz radios.  900 MHz is the same frequency that many cordless telephones use.  It is favored because it does not require and FFC license.  But in reality it is a poor choice for this application.  The 900 MHz frequency does not penetrate trees or foliage very well at all.  It does not bend around buildings.  The 900 MHz radios being sold with RTK systems are nothing more than (very expensive) souped up cordless phones.  The range of these radios can be extended with (again expensive) repeater radios.  The cell phone replaces the radios in the system.  The kicker is that you must have good cell phone coverage.  Also, you must get the correction signal to the rover unit fast.  In the past cell phones have had latency issues, but newer cell phone technology is fixing the latency problems.  I am getting a consistent 1 second correction age.  That means that the signal is getting from the base station to the rover in one second or less.
  3. How close to the base station do you need to be?  This is a very debatable question.  At first the industry told us you had to be within 6-7 miles, because that was the maximum range of the above radios.  Then they told us that you could stretch that to 12 miles with repeaters.  We are finding that you can get 2” accuracy at 20 miles with a high quality system.  All systems are not created equal.  Some use more advanced algorithms than others.  I am using a free publicly broadcast signal from a CORS station.  I am running at a maximum of 29 miles from the station.  I am finding the accuracy to be acceptable at that range.  As the algorithms get better we may find that we can increase that range.
  4. What is the advantage of using the cell phone over the radios?  Range is obviously the first advantage.  Cost is the second.  If your are setting up an RTK base station network, you have to have a station every 12-14 miles with radios.  With cell phones, you can extend that to at least a station every 18-20 miles and maybe farther than that.  Base stations are not cheap.  The fewer you need the better.
  5. Another advantage to the cell phone is that it is in actuality just an Internet connection.  As long as you have enough bandwidth (and bandwidth is increasing every day) you can piggy back other Internet needs on the signal.  You could check the radar in the tractor.  You could email the broker etc.
  6. The question was asked what happens if the call is dropped?  I happened to be blessed to be in an area with very good cell coverage.  I am using a dedicated cell modem with an external antenna on the tractor cab.  I rarely have a dropped call, but if that does happen, my modem is set to automatically reconnect.  It will reconnect and be running again usually in less than 2 minutes (probably 90 seconds on average). 

I know this is getting long, but I will add just a few more comments.  A lot is said about using mobile base stations.  That may work for some, but I don’t consider it to be an option.  First of all if you want repeatability, the base has to be returned to the EXACT same position every time.  Some say use a fence post or concrete marker.  That fence post can not start to lean.  You can not hit that concrete marker with a bush hog.  You must have power to run the base, either AC or battery power.  If battery power is used, the battery has to have a good charge.  You don’t just set the base down and turn it own.  You have to program the location into it so that it knows where it is and it need to “warm up” for lack of a better term.  You have to move the base around with you.  They don’t fit in the tractor cab very well.

There are getting to be more and more of these free RTK signals out there and we will see this trend continue for a variety of reasons that I will not go into now.

 

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