GPS Fence - RTK GPS Modules


To achieve more accurate positioning data, we can use an RTK system. RTK systems can achieve GPS coordinate accuracy of a few cm and originally cost over a thousand Euros/Dollars, but these systems have now come down in price to be affordable for hobby projects.

What is RTK GPS?

There are many websites explaining the function of RTK, but I'll try and put in context of the Lawn Mower project which is the ultimate goal of this module in my projects.

Standard GPS modules rely on triangulation of many satellites to calculate the position of the rover on the earth.  The satellites send lines of data/code which are received by the GPS module.  The code transmitted from the satellites is the same, therefore the time when the section of code arrives at the GPS module will determine the distance the GPS module is from that satellite. 

With many satellites an accurate location can be calculated as the GPS module knows where the satellite is and what delay (t) the GPS module is seeing when the code arrives.  This would be great in a perfect world but there are additional factors which can mess up this "simpler" equation.

Weather, atmospheric conditions etc can add additional delay to the signal sent , which then ruins the internal calculation of the GPS module.

RTK attempts to solve this issue by having a fixed ground station with fixed coordinates.  The ground station can then look at the satellite signals as they arrive and compare them to what it would expect to see (as the ground station hasn't moved).  These corrections in the signal are then sent by radio transmission to the rover GPS module.  The corrections can then be incorporated into the equation to give a much better calculation of the rovers position.

With this correction data, GPS coordinates with cm accuracy can be acheived instead of meter accuracy with standard GPS modules.

The closer the Rover is to the Base Station the more accurate the correction data is, as the same atmospheric and weather conditions are present.  The further away the rover gets from the ground station, the more likely it is that the base station has different conditions and therefore different errors than the rover.


Which RTK GPS Module to use?

To make RTK work we need to invest in 2 RTK GPS modules.  One module is setup as a base station and one module is used for the rover.   Once a base station is setup, this can be used to "correct" multiple rover modules.

There are various RTK systems available on the market.  For this project we need a compact system that outputs the messages via a UART (Serial communication) and can be read by the ESP32 board which we will use to calculate the GPS position of the mower relative to the fence.

I decided to use a C94-M8P-2-10 Application set from UBLOX.  This package aleady comes with all the necessary antennas ground planes, GPS modules and boards.  This ensures compatibility and simplify the setup





Which RTK GPS Module is best?

I am using a UBLOX C94-M8P-2-10 application board.  This set comes complete with 2 RTK GPS boards, GPS antenna and UHF radios.  1 board is used as a base station and the other identical board is used in the Mower/Rover...Its basically ready to go. 

Is this the best board? ..... I dont know.....  Here is an article which compares various RTK GPS boards to the price level.

For this project we need an RTK GPS board which can output NMEA signals via UART (Serial communication).  The UBLOX board is relatively easy to setup for this.  If you decide to purchase another board then you can substitute the board in the wiring diagrams I show.

When buying the board ensure that the UHF antenna is specified for your region with the correct allowed radio frequency.