Buid your own DIY RTK Guidance

I use the following setup for my AgOpenGPS autosteer system with the free Iowa CORS correction service. The accuracy here in central Iowa seems to be in the 1 - 2 cm range! The correction takes place outside the autosteer system so the autosteer system is setup as a generic WAAS type antenna (but the GPS shows as RTK Fix or Float accuracy). See documentation under pictures.

I have also built a DIY base/rover setup for use on an AgLeader monitor and can tell you how to convert this one.

OutFARMING dome

DIY 3D printed dome

Under the cover

With the antenna removed

With the antenna disk removed

Base / Rover System

For NTRIP correction you will need to supply a cell phone and install an app for the correction service. I use the free Lefebure NTRIP app on an Android phone. I'm guessomg you might be able to use an IPhone as well.

On Sparkfun.com, go to Home/Tutorials/GPS-RTK2 Hookup Guide and find the "Connecting the ZED-F9P to a Correction Source.

You may need to use the free U-Blox U-Center software to configue the F9P to only output NMEA GGA and VTG sentences to the serial port.

NTRIP Parts:

Sparkfun GPS-RTK2-SMA Breakout - ZED-F9P (Qwiic) $220
Sparkfun GPS Antenna Ground Plate $5
Sparkfun Bluetooth Mate Silver$28
Sparkfun GPS/GNSS Magnetic Mount Antenna SMA - 3m $13
OR the much better antenna: Sparkfun GPS-15192 - $64.95
Sparkfun Female Headers $1.50
My Plastice dome with magnetic mount: $35
Serial to RS232 conversion kit for AgLeader type use: $75

You will need to buy a special data cable to go from the antenna to the cell phone (find on Amazon). The antenna end needs to be a USB type C male connector and the other end needs to be a male connector matching your phone's charging/data port. I use a USB C male to USB C data exchange cable with a USB C to USB micro converter.

DIY Base/Rover Parts:

In 2017 I used an older U-Blox C94-M8P-2 kit with UHF radio communications: $399. See https://www.digikey.com/en/products/detail/u-blox/C94-M8P-2/6150728. The kit contains 2 identical boards. One is used for the base and the other for the router. I would find a way to do it with the newer ZEO-F9P chips but the C099-F8P kit only uses bluetooth and WIFI. The WIFI won't communicate far enough for farm use. Good way might be to use Sparkfun ZEO-F9P boards and XBee 3 Pro units for radio communications.
2 Enclosure boxes @ $24. See https://www.digikey.com/en/products/detail/bud-industries/PN-1334-C/429006. I used one of these and a cheap waterproof camera case? from Menard's hunting supplies.
LM7808 chip for power regulation and prototyping board
TTL to RS232 converter if using with AgLeader type monitor: $16 https://www.sparkfun.com/products/449
12V garden tractor battery for base. Tripod. Various wires and connectors. Contact me if interested.

A second option is to look into the Ardusimple RTK system.