RTK Corrections for tractor auto steer project


First Post here so go easy on me.

I currently run Trimble CFX-750 display with EZ-Pilot assisted steering system on my tractors.
I plan to keep this setup in tack.^

The CFX-750 is currently being fed RTK correction data from a Raven Slingshot RTK module.
The base station portion of this slingshot system is owned by a equipment dealership who charges me a hefty subscription fee.

I would like to setup my own base station and replace the Raven RTK Slingshot in my tractors.

Any input would be greatly appreciated.

CFX-750 Display

Slingshot RTK

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Base Station:

  • Permanently setup in a static Location.
  • Send out corrections via internet.
  • send corrections to more than one rover. (Not mission critical) (base station server?)

-Receive corrections via LTE

  • interface with the CFX-750


  • finding proper communications settings for the CFX-750
  • rover being rugged enough to ride around in a tractor.
  • accuracy of 3cm or better, preferably 1cm or better.
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If your 750 display will accept standard NMEA as input for GNSS position through one of the connectors, then a couple of Reach devices and one or two cellular modems will get you started. I guess the most important thing is to find out if anyone has been able to interface their 750 display with an RS232 NMEA GNSS position source. It appears that Trimble doesn’t like to publish that kind of info, and I didn’t get any definitive answer in my few minutes of searching.

I guess one could look in the 750’s software settings to see if you can specify one of the rear ports to be an input; to decode NMEA protocol; and to be able to specify the baud rate. That would be a good start.

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I think right now we are using RTCM3.
I will sort through my owners manual and get back to you.

In the meantime here are some screenshots from the 750’s rtk setup menus.
Right now I am talking to the 750 through the slingshot rtk modem via rs232.

I have a good feeling about this.
Appreciate the input.

Yes, please let us know how it works. I get this kind of question frequently.

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I noticed one option hidden off-screen here:

Do you know what it is? The upper ones in that list look so proprietary.

If you had a laptop with with a serial port (RS232), or even a USB to serial adapter, then you could take someone’s position log (NMEA .pos file from another Reach device) and send the file to the serial port and see if the 750 display can read the data. Of course it would be better to try with an actual Reach, but if you don’t have one, there’s nothing stopping you from simulating that you do. At least for testing purposes. :slight_smile:

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so this screenshot you have to select which source is gonna provide the RTK corrections.
Basically all the ones above “Generic CMR Radio” are proprietary Trimble RTK bridges.

Is feed by a Raven slingshot modem.

I just started to do some more digging on it and actually found some information.


I probably could get my hands on a serial port laptop.
I wonder if there is a program I could use to connect to the Slingshots RS-232 port and see what format the data is currently being spit out in? Hyperterminal?

Your totally right tho, Trimble is tight lipped with any tech data.

I am 99% sure I am going to be picking up a couple of the new Reach M+ and attempting this.

Yes, that should work just fine.

I may have misspoke about getting a position log to try. That would be for the case where you Reach as a rover and the 750 doesn’t do any RTK processing. It would just take in the position data.

The other case is where you just use Reach as a base and send the corrections directly to your 750 for processing. I assume it already has it’s own antenna.

so actually started up my tractor and took it for a rip in the snow today to see if I could see anything else about the data format.

This is all I could see off the trimble.

Took a couple pictures of the back of the Slingshot.
Can anyone fill me in on what diversity does?

That’s correct.
All correction data is calculated between the base station server and the slingshot rover.
and the fed to the cfx750 to do the auto steer portion.
The cfx750 has a waas antenna for backup, but it is a nightmare, tractor sways back and forth 2-8ft at best.
RTK I am within 3CM.
There is a host of differnt unlock codes you can get for the CFX750, lots of them are derivatives of RTK.
But anything Trimble is mega subscription $$$$

Interesting read from Trimble^

So I am using the RTCM3 format currently with Slingshot.

“The RTCM 10402.3 standards defined the messages for differential correction information. There are 64 types of messages. The message format is a sequence of 30 bits. The messages 1 to 17 are available in older RTCM versions, while messages 18-21 have been added in version 2.3 to made the standard applicable to RTK corrections.”


And it looks like the Reach supports RTCM3 so the trick will be to figure out which messages are actually required to optimize bandwidth.


Anyone have any suggestions on a good cellular modem with highgain anttennas?

Ok sorry I missed an important part of your question here, I went back and grabbed another picture.

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Diversity is a scheme where you usually have two antennas, and the device with antenna diversity will pick the best antenna of the two. That may change depending on which way the tractor is facing, etc.

…just came across a quote in their FAQ as well:

The way I read it, you have a base station somewhere (at your equipment dealer, or at your house) and that is connected to the Internet, so it can serve out RTK corrections (probably RTCM3). Then in your roving vehicle (tractor), you have a Slingshot field hub, which is basically a cellular modem with some extras. It has a GPS antenna, but that is not for RTK, only for reporting your location back to someone. The field hub must have some internal computer which can be set to log in to a NTRIP caster to get RTCM3 messages and then repeat them out the RS232 port. Your 750 must be taking those corrections in over RS232, and processing them against it’s own rover data.

Does that sound correct?

If so, then in the tractor, you would have:
Internet --> Cell modem --USB-cable–> Reach RS+ --RS-cable–> --Trimble-cable–> CFX-750

At the base station, you would have:
Reach RS+ --Wi-Fi–> Router --> Internet

(of course there could be a few variations on that)

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How are the Reach M+ coming along?

Estimated ship date is on the product page:

I see it has been changed recently. That’s all I know.

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Hi, i am having the same issue, hope someone will find a solution for connecting the 750 on reach.