GPS-RTK2 Hookup Guide

2026-06-19 | By SparkFun Electronics

Wired I2C / TWI USB Wireless GPS Adapters Arduino Qwiic

Courtesy of SparkFun Electronics

Guide by Nate

Introduction

The SparkFun GPS-RTK2 and GPS-RTK-SMA raise the bar for high-precision GPS. Utilizing the latest ZED-F9P module from u-blox the RTK2 and RTK-SMA is capable of 10mm 3 dimensional accuracy. Yes, you read that right, these boards can output your X, Y, and Z location that is roughly the width of your fingernail. With great power comes a few requirements: high precision GPS requires a clear view of the sky (sorry, no indoor location) and a stream of correction data from an RTCM source. We’ll get into this more in a later section but as long as you have two ZED-F9P breakout boards, or access to an online correction source, your ZED-F9P can output lat, long, and altitude with centimeter grade accuracy.

Note: For those looking for the bare minimum without a microcontroller, check out the GPS-RTK-SMA Kit. This includes the GNSS multi-band antenna, USB cable, ZED-F9P breakout board with SMA connector, and GPS antenna ground plate.

SparkFun GPS-RTK-SMA Kit

Hardware Overview

One of the key differentiators between the ZED-F9P and almost all other low-cost RTK solutions is the ZED-F9P is capable of receiving both L1 and L2 bands.

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Communication Ports

The ZED-F9P is unique in that it has five communication ports which are all active simultaneously. You can read NMEA data over I²C while you send configuration commands over the UART and vice/versa. The only limit is that the SPI pins are mapped onto the I²C and UART pins so it’s either SPI or I²C+UART. The USB port is available at all times.

Image of GPS-RTK2 Hookup Guide SparkFun GPS-RTK2 Board

Image of GPS-RTK2 Hookup Guide SparkFun GPS-RTK-SMA Board

USB

The USB C connector makes it easy to connect the ZED-F9P to u-center for configuration and quick viewing of NMEA sentences. It is also possible to connect a Raspberry Pi or other single board computer over USB. The ZED-F9P enumerates as a serial COM port and it is a separate serial port from the UART interface. See Getting Started with U-Center for more information about getting the USB port to be a serial COM port.

Image of GPS-RTK2 Hookup Guide The USB port and power GPS-RTK2

Image of GPS-RTK2 Hookup Guide The USB port and power GPS RTK-SMA

A 3.3V regulator is provided to regulate the 5V USB down to 3.3V the module requires. External 5V can be applied or a direct feed of 3.3V can be provided. Note that if you’re provide the board with 3.3V directly it should be a clean supply with minimal noise (less than 50mV VPP ripple is ideal for precision locating).

The 3.3V regulator is capable of sourcing 600mA from a 5V input and the USB C connection is capable of sourcing 2A.

I²C (a.k.a DDC)

The u-blox ZED-F9P has a “DDC” port which is really just an I²C port (without all the fuss of trademark issues). These pins are shared with the SPI pins. By default, the I²C pins are enabled. Be sure the DSEL jumper on the rear of the board is open. The GPS-RTK2 and GPS-RTK-SMA from SparkFun also includes two Qwiic connectors to make daisy chaining this GPS receiver with a large variety of I²C devices. Checkout Qwiic for your next project.

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Image of GPS-RTK2 Hookup Guide Highlighted I2C port and Qwiic connectors on the RTK2 and RTK-SMA

All features are accessible over the I²C ports including reading NMEA sentences, sending UBX configuration strings, piping RTCM data into the module, etc. We’ve written an extensive Arduino library showing how to configure most aspects of the ZED-F9P making I²C our preferred communication method on the ZED. You can get the library through the Arduino library manager by searching ‘SparkFun u-blox GNSS’. Checkout the SparkFun u-blox Library section for more information.

UART/Serial

The classic serial pins are available on the ZED-F9P but are shared with the SPI pins. By default, the UART pins are enabled. Be sure the DSEL jumper on the rear of the board is open.

TX/MISO = TX out from ZED-F9P
RX/MOSI = RX into ZED-F9P

Image of GPS-RTK2 Hookup Guide

Image of GPS-RTK2 Hookup Guide Serial pins on SparkFun ZED-F9P breakout boards highlighted

There is a second serial port (UART2) available on the ZED-F9P that is primarily used for RTCM3 correction data. By default, this port will automatically receive and parse incoming RTCM3 strings enabling RTK mode on the board. In addition to the TX2/RX2 pins we have added an additional ‘RTCM Correction’ port where we arranged the pins to match the industry standard serial connection (aka the 'FTDI' pinout). This pinout is compatible with our BlueSMiRF v2 and Serial Basic so you can send RTCM correction data from a cell phone or computer. Note that RTCM3 data can also be sent over I²C, UART1, SPI, or USB if desired.

Image of GPS-RTK2 Hookup Guide UART2 and RTCM Correction Port on the GPS-RTK2

Image of GPS-RTK2 Hookup Guide UART2 and RTCM Correction Port on the GPS-RTK-SMA

The RTCM correction port (UART2) defaults to 38400bps serial but can be configured via software commands (checkout our Arduino library) or over USB using u-center. Keep in mind our BlueSMiRF defaults to 115200bps. If you plan to use Bluetooth for correction data (we found it to be easiest), we recommend you increase this port speed to 115200bps using u-center. Additionally, but less often needed, the UART2 can be configured for NMEA output. In general, we don’t use UART2 for anything but RTCM correction data, so we recommend leaving the in/out protocols as RTCM.

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If you’ve got the ZED-F9P setup for base station mode (also called survey-in mode) the UART2 will output RTCM3 correction data. This means you can connect a radio or wired link to UART2 and the board will automatically send just RTCM bytes over the link (no NMEA data taking up bandwidth). Here is an example using the RTK2 but you can also use the RTK-SMA using the same setup.

Image of GPS-RTK2 Hookup Guide Base station setup to send RTCM bytes out over Bluetooth

SPI

The ZED-F9P can also be configured for SPI communication. By default, the SPI port is disabled. To enable SPI close the DSEL jumper on the rear of the board. Closing this jumper will disable the UART1 and I²C interfaces (UART2 will continue to operate as normal).

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Image of GPS-RTK2 Hookup Guide The SPI pins highlighted on the SparkFun RTK2 and RTK-SMA

Control Pins

The control pins are highlighted below.

Image of GPS-RTK2 Hookup Guide Highlighted control pins of the SparkFun GPS-RTK2

Image of GPS-RTK2 Hookup Guide Highlighted control pins of the SparkFun GPS-RTK-SMA

These pins are used for various extra control of the ZED-F9P:

FENCE: Geofence output pin. Configured with U-Center. Will go high or low when a geofence is setup. Useful for triggering alarms and actions when the module exits a programmed perimeter.
RTK: Real Time Kinematic output pin. Remains high when module is in normal GPS mode. Begins blinking when RTCM corrections are received and module enters RTK float mode. Goes low when module enters RTK fixed mode and begins outputting cm-level accurate locations.
PPS: Pulse-per-second output pin. Begins blinking at 1Hz when module gets basic GPS/GNSS position lock.
RST: Reset input pin. Pull this line low to reset the module.
SAFE: Safeboot input pin. This is required for firmware updates to the module and generally should not be used or connected.
INT: Interrupt input/output pin. Can be configured using U-Center to bring the module out of deep sleep or to output an interrupt for various module states.

Note: For those that need to connect a SMA connector to the PPS pin, the RTK-SMA includes a footprint so that you can manually solder the connector to the board for your application. The PPS output is helpful as a clock source correction when synchronizing equipment (not 1 Hz but in many MHz). The PPS output can be configured to output a very accurate clock which scientists use to correct less accurate, but much faster clocks. To configure, you can use the u-center to adjust The NEO-F9P's setting under View > Configuration View > TP (TimePulse).

SMA Connector

Antenna

The ZED-F9P requires a good quality GPS or GNSS (preferred) antenna. A U.FL connector is provided. Note: U.FL connectors are rated for only a few mating cycles (about 30) so we recommend you set it and forget it. You may need to secure the u.FL to SMA cable depending on your application. Otherwise, you could use the RTK-SMA version.

Image of GPS-RTK2 Hookup Guide U.FL antenna connector and SMA cut-out on the GPS RTK2

Image of GPS-RTK2 Hookup Guide SMA antenna connector on the GPS-SMA

A cutout for the SMA bulkhead is available for those who want an extra sturdy connection. We recommended installing the SMA into the board only when the board is mounted in an enclosure. Otherwise, the cable runs the risk of being damaged when compressed (for example, students carrying the board loose in a backpack).

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A U.FL to SMA cable threaded through the mounting hole provides a robust connection that is also easy to disconnect at the SMA connection if needed. Check below in Connecting an Antenna for more information.

Low-cost magnetic GPS/GNSS antennas can be used (checkout the u-blox white paper) but a 4” / 10cm metal disc is required to be placed under the antenna as a metal ground plane.

Image of GPS-RTK2 Hookup Guide

LEDs

The board includes four status LEDs as indicated in the image below.

Image of GPS-RTK2 Hookup Guide Status LEDs on the GPS-RTK2

Image of GPS-RTK2 Hookup Guide Status LEDs on the GPS-SMA

PWR: The power LED will illuminate when 3.3V is activated either over USB or via the Qwiic bus.
PPS: The pulse per second LED will illuminate each second once a position lock has been achieved.
RTK: The RTK LED will be illuminated constantly upon power up. Once RTCM data has been successfully received it will begin to blink. This is a good way to see if the ZED-F9P is getting RTCM from various sources. Once an RTK fix is obtained, the LED will turn off.
FENCE: The FENCE LED can be configured to turn on/off for geofencing applications.

Jumpers

There are five jumpers used to configure the GPS-RTK2.

Image of GPS-RTK2 Hookup Guide

Image of GPS-RTK2 Hookup Guide User jumpers on the SparkFun RTK2 and RTK-SMA

Closing DSEL with solder enables the SPI interface and disables the UART and I²C interfaces. USB will still function.

Cutting the I²C jumper will remove the 2.2k Ohm resistors from the I²C bus. If you have many devices on your I²C bus you may want to remove these jumpers. Not sure how to cut a jumper? Read here!

Cutting the JP1, JP2, JP3 jumpers on the RTK2 will disconnect of the various status LEDs from their associated pins. These have been labeled on the RTK-SMA version. We have included a jumper for the PPS on the top and bottom of the RTK-SMA.

Backup Battery

The MS621FE rechargeable battery maintains the battery backed RAM (BBR) on the GNSS module. This allows for much faster position locks (a.k.a. hot start). The BBR is also used for module configuration retention. The battery is automatically trickle charged when power is applied and should maintain settings and GNSS orbit data for up to two weeks without power.

Image of GPS-RTK2 Hookup Guide The backup battery on the SparkFun RTK2

Image of GPS-RTK2 Hookup Guide The backup battery on the SparkFun RTK-SMA

Connecting an Antenna

U.FL connectors are very good but they are a designed to be implemented inside a small embedded application like a laptop. Exposing a U.FL connector to the wild risks it getting damaged. To prevent damaging the U.FL connection on the GPS-RTK2 we recommend threading the U.FL cable through the stand-off hole, then attach the U.FL connectors. This will provide a great stress relief for the antenna connection. Now attach your SMA antenna of choice.

Image of GPS-RTK2 Hookup Guide

Be Careful! U.FL connectors are easily damaged. Make sure the connectors are aligned, flush face to face (not at an angle), then press down using a rigid blunt edge such as the edge of a PCB or point of a small flat head screwdriver. For more information checkout our tutorial Three Quick Tips About Using U.FL.

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Three Quick Tips About Using U.FL

Quick tips regarding how to connect, protect, and disconnect U.FL connectors.

Additionally, a bulkhead cut-out is provided to screw the SMA onto the PCB if desired.

Image of GPS-RTK2 Hookup Guide

While this method decreases stress from the U.FL connector it is only recommended when the board has been permanently mounted. If the board is not mounted, the cable on the U.FL cable is susceptible to being kinked causing impedance changes that may decrease reception quality.

If you are using the RTK-SMA, there is no need to worry about this since there is a SMA connector soldered on the board providing a sturdy connection.

Image of GPS-RTK2 Hookup Guide

If you’re indoors you must run a SMA extension cable long enough to locate the antenna where it has a clear view of the sky. That means no trees, buildings, walls, vehicles, or concrete metally things between the antenna and the sky. Be sure to mount the antenna on a 4”/10cm metal ground plate to increase reception.

Image of GPS-RTK2 Hookup Guide

Board Dimensions

The overall board dimensions for each board is ~1.70" x ~1.70" (~43.18mm x ~43.18mm). Keep in mind the connectors are not flush with the board, which adds additional length to the board. Make sure to take that into account when mounting and placing the board in an enclosure.

Image of GPS-RTK2 Hookup Guide Board Dimensions for RTK2

Image of GPS-RTK2 Hookup Guide Board Dimensions for RTK-SMA

Connecting the ZED-F9P to a Correction Source

Before you go out into the field it’s good to understand how to get RTCM data and how to pipe it to the GPS-RTK2 and GPS-RTK-SMA. We recommend you read Connecting a Correction Source section of the original GPS-RTK tutorial. This will give you the basics of how to get a UNAVCO account and how to identify a Mount Point within 10km of where your ZED-F9P rover will be used. This section builds upon these concepts.

Image of GPS-RTK2 Hookup Guide GPS RTK Hookup Guide: Connecting a Correction Source

For this example, we will show how to get correction data from the UNAVCO network and pull that data in using the Android app called NTRIP Client. The correction data will then be transmitted from the app over Bluetooth to the ZED-F9P using the SparkFun BlueSMiRF v2.

Required Materials

1x GPS-RTK2 or GPS-RTK-SMA
1x GPS or GNSS Antenna
1x Metal Plate of 4” or larger
1x SMA extension cable (if needed to get a clear view of the sky)
1x USB C 2.0 Cable
1x BlueSMiRF v2
1x male and female header pair

Image of GPS-RTK2 Hookup Guide GNSS antenna sitting on a metal ground plate elevated with clear view of the sky

Now setup your GPS receiver such that you can work from your desk but have the antenna outdoors with a clear view of the sky.

Required Software

Credentials with a free RTCM provider such as UNAVCO
U-Center
Get the NTRIP By Lefebure app from Google Play. There seem to be NTRIP apps for iOS but we have not been able to verify any one app in particular. If you have a favorite, please let us know.

First, we need to attach the Bluetooth Module to the ZED-F9P's breakout board. Solder a female header to the BlueSMiRF (Bluetooth Mate is shown, but the assembly is the same) so that it hangs off the end.

Image of GPS-RTK2 Hookup Guide

On the GPS-RTK2 or GPS_RTK-SMA board we recommend soldering the right-angle male header underneath the board. This will allow the Bluetooth module to be succinctly tucked under the board.

When attaching the BlueSMiRF to board be sure to align the pins so that the GND indicator align on both boards. Once Bluetooth has been installed attach your GNSS antenna and connect the board over USB. This will power the board and the BlueSMiRF.

Image of GPS-RTK2 Hookup Guide

Where the male and female headers go is personal preference. For example, here are two Bluetooth Mates; one with male headers, one with female.

Image of GPS-RTK2 Hookup Guide

Soldering a female header to a Bluetooth Mate makes it easier to add Bluetooth to boards that have a 'FTDI' style connection like our OpenScale, Arduino Pro, or Simultaneous RFID Reader. Whereas, soldering a male header to the Bluetooth Mate makes it much easier to use in a breadboard. It's really up to you!

The Bluetooth Mate and BlueSMiRF v2 both default to 115200bps whereas the ZED-F9P is expecting serial over UART2 to be 38400bps. To fix this we need to open u-center and change the port settings for UART2. If you haven’t already, be sure to checkout the tutorial Getting Started with U-Center to get your bearings.

Open the Configure window and navigate to the PRT (Ports) section. Drop down the target to UART2 and set the baud rate to 115200. Finally, click on the ‘Send’ button.

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By this time you should have a valid 3D GPS lock with ~1.5m accuracy. It’s about to get a lot better.

We are going to assume you’ve read the original RTK tutorial and obtained your UNAVCO credentials including the following:

Username
Password
IP Address for UNAVCO (69.44.86.36 at time of writing)
Caster Port (2101 at time of writing)
Data Stream a.k.a. Mount Point (‘P041_RTCM3’ if you want the one near Boulder, CO - but you should really find one nearest your rover location)

The BlueSMiRF should be powered up. From your phone, discover the BlueSMiRF and pair with it. The module used in this tutorial was discovered as RNBT-E0DC where E0DC is the last four characters of the MAC address of the module and should be unique to your module.

Once you have your UNAVCO credentials and you’ve paired with the Bluetooth module open the NTRIP client.

Image of GPS-RTK2 Hookup Guide

From the home screen, click on the gear in upper right corner then Receiver Settings.

Image of GPS-RTK2 Hookup Guide

Verify that the Receiver Connection is set to Bluetooth then select Bluetooth Device and select the Bluetooth module you just paired with. Next, open NTRIP settings and enter your credentials including mounting point (a.k.a. Data Stream).

Image of GPS-RTK2 Hookup Guide

This example demonstrates how to obtain correction data from UNAVCO’s servers but you could similarly setup your own base station using another ZED-F9P and RTKLIB to broadcast the correction data. This NTRIP app would connect to your RTKLIB based server giving you some amazing flexibility (the base station could be anywhere there’s a laptop and WiFi within 10km of your rover).

Ok. You ready? This is the fun part. Return to the main NTRIP window and click Connect. The app will connect to the Bluetooth module. Once connected, it will then connect to your NTRIP source. Once data is flowing you will see the number of bytes increase every second.

Image of GPS-RTK2 Hookup Guide

Within a few seconds you should see the RTK LED on the ZED-F9P's breakout board turn off. This indicates you have an RTK fix. To verify this, open u-center on your computer. The first thing to notice is that Fix Mode in the left hand black window has changed from 3D to 3D/DGNSS/FIXED.

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Navigate to the UBX-NAV-HPPOSECEF message. This will show you a high-precision 3D accuracy estimate. We were able to achieve 17mm accuracy using a low-cost GNSS antenna with a metal plate ground plane and we were over 10km from the correction station.

Congrats! You now know where you are within the diameter of a dime!

SparkFun u-blox Arduino Library

Note: This example assumes you are using the latest version of the Arduino IDE on your desktop. If this is your first time using Arduino, please review our tutorial on installing the Arduino IDE. If you have not previously installed an Arduino library, please check out our installation guide.

The SparkFun u-blox Arduino library enables the reading of all positional datums as well as sending binary UBX configuration commands over I²C. This is helpful for configuring advanced modules like the ZED-F9P but also the NEO-M8P-2, SAM-M8Q and any other u-blox module that use the u-blox binary protocol.

Note: We support two versions of the SparkFun u-blox GNSS library. Version 2 and Version 3. Version 3 uses the u-blox Configuration Interface (VALSET and VALGET) to configure the module, instead of the deprecated UBX-CFG messages. For modules like the F9 and M10, we recommend upgrading to Version 3. However, older modules like the M8 do not support the Configuration Interface. For those you will need to keep using Version 2 of the library. We will continue to support both.

The SparkFun u-blox Arduino library can be downloaded with the Arduino library manager by searching 'SparkFun u-blox GNSS v3' or you can grab the zip here from the GitHub repository to manually install.

SparkFun u-blox Arduino Library v3 (ZIP)

Once you have the library installed checkout the various examples.

Basics

Example1: Read NMEA sentences over I²C using u-blox module SAM-M8Q, NEO-M8P, etc.
Example2: Parse NMEA sentences using MicroNMEA library. This example also demonstrates how to overwrite the processNMEA function so that you can direct the incoming NMEA characters from the u-blox module to any library, display, radio, etc. that you prefer.
Example3: Get latitude, longitude, altitude, and satellites in view (SIV). This example also demonstrates how to turn off NMEA messages being sent out of the I²C port. You’ll still see NMEA on UART1 and USB, but not on I²C. Using only UBX binary messages helps reduce I²C traffic and is a much lighter weight protocol.
Example4: Displays what type of a fix you have the two most common being none and a full 3D fix. This sketch also shows how to find out if you have an RTK fix and what type (floating vs. fixed).
Example5: Shows how to get the current speed, heading, and dilution of precision.
Example7: Demonstrates how to increase the output rate from the default 1 per second to many per second; up to 30Hz on some modules!
Example8: Older modules like the SAM-M8Q utilize an older protocol (version 18) whereas the newer modules like the ZED-F9P deprecate some commands using the latest protocol (version 27). This sketch shows how to query the module to get the protocol version.
Example9: u-blox modules use I²C address 0x42 but this is configurable via software. This sketch will allow you to change the module’s I²C address.
Example10: Altitude is not a simple measurement. This sketch shows how to get both the ellipsoid based altitude and the MSL (mean sea level) based altitude readings.
Example11: Sometimes you just need to do a hard reset of the hardware. This sketch shows how to set your u-blox module back to factory default settings.

ZED-F9P

ZED-F9P Example1: This module is capable of high precision solutions. This sketch shows how to inspect the accuracy of the solution. It’s fun to watch our location accuracy drop into the millimeter scale.
ZED-F9P Example2: The ZED-F9P uses a new u-blox configuration system of VALGET/VALSET/VALDEL. This sketch demonstrates the basics of these methods.
ZED-F9P Example3: Setting up the ZED-F9P as a base station and outputting RTCM data.
ZED-F9P Example4: This is the same example as ZED-F9P's Example3. However, the data is sent to a serial LCD via I²C.

This SparkFun u-blox library really focuses on I²C because it's faster than serial and supports daisy-chaining. The library also uses the UBX protocol because it requires far less overhead than NMEA parsing and does not have the precision limitations that NMEA has.

Setting the ZED-F9P as a Correction Source

Heads up! This section is a bit out of date. We've got a shiny new How to Build a GNSS Reference Station tutorial that provides up to date information. We plan to keep this section for reference.

If you’re located further than 20km from a correction station you can create your own station using the ZED-F9P. u-blox provides a setup guide within the ZED-F9P Integration Manual showing the various settings needed via u-Center. We’ll be covering how to setup the ZED-F9P using I²C commands only. This will enable a headless (computerless) configuration of a base station that outputs RTCM correction data.

Before getting started we recommend you configure the module using u-center. Checkout our tutorial on using U-Center then read section 3.5.8 Base Station Configuration of the u-blox Integration Manual for getting the ZED-F9P configured for RTK using u-center. Once you’ve been successful controlling the module in the comfort of your lab using u-center, then consider heading outdoors.

For this exercise we’ll be using the following parts:

1x SparkFun GPS-RTK2 Board or RTK-SMA Board
1x U.FL to SMA Cable for the RTK2
1x SparkFun BlackBoard makes I²C easy
1x USB C 2.0 Cable if you need one
1x Two Qwiic Cables
1x 20x4 SerLCD with Qwiic Adapter soldered on
1x Antenna L1/L2 GNSS 3-5V Magnetic Mount
1x GPS Antenna Ground Plate
1x 20+ft SMA extension can be handy when first experimenting with base stations so you can sit indoors with a laptop and analyze the output of the GPS-RTK
1x A standard camera tripod

The ZED-F9P can be configured using Serial, SPI, or I²C. We’re fans of the daisy chain-ability of I²C so we’ll be focusing on the Qwiic system. For this exercise we’ll be connecting the an LCD and GPS-RTK2 to a BlackBoard using two Qwiic cables. You can also use the RTK-SMA as an alternative.

Image of GPS-RTK2 Hookup Guide

For the antenna, you’ll need a clear view of the sky. The better your antenna position the better your accuracy and performance of the system. We designed the GPS Antenna Ground Plate to make this setup easy. The plate has a ¼” threaded hole that threads directly onto a camera tripod. The plate thickness was chosen to be thick enough so that the threaded screw is flush with the plate so it won’t interfere with the antenna. Not sure why we’re using a ground plate? Read the u-blox white paper on using low-cost GNSS antennas with RTK. Mount your magnetic mount antenna and run the SMA cable to the U.FL to SMA cable to the GPS-RTK2 board. If you have the RTK-SMA, you just need to run the SMA cable to the board's connector.

Image of GPS-RTK2 Hookup Guide

There are only three steps to initiating a base station:

Enable Survey-In mode for 1 minute (60 seconds)
Enable RTCM output messages
Being Transmitting the RTCM packets over the backhaul of choice

Be sure to grab the SparkFun Arduino Library for u-blox. You can easily install this via the library manager by searching ‘SparkFun u-blox GNSS’. Once installed click on File -> Examples -> SparkFun_u-blox_GNSS_Arduino_Library.

The ZED-F9P subfolder houses a handful of sketches specific to its setup. Example3 of the library demonstrates how to send the various commands to the ZED-F9P to enable Survey-In mode. Let’s discuss the important bits of code.

Copy Code

response = myGNSS.enableSurveyMode(60, 5.000); //Enable Survey in, 60 seconds, 5.0m

The library is capable of sending UBX binary commands with all necessary headers, packet length, and CRC bytes over I²C. The enableSurveyMode(minimumTime, minimumRadius) command does all the hard work to tell the module to go into survey mode. The module will begin to record lock data and calculate a 3D standard deviation. The survey-in process ends when both the minimum time and minimum radius are achieved. u-blox recommends 60 seconds and a radius of 5m. With a clear view of the sky, with a low cost L1/L2 GNSS antenna mounted to a ground plate we’ve seen the survey complete at 61 seconds with a radius of around 1.5m.

Copy Code

response &= myGNSS.enableRTCMmessage(UBX_RTCM_1005, COM_PORT_I2C, 1); //Enable message 1005 to output through I2C port, message every second
response &= myGNSS.enableRTCMmessage(UBX_RTCM_1074, COM_PORT_I2C, 1);
response &= myGNSS.enableRTCMmessage(UBX_RTCM_1084, COM_PORT_I2C, 1);
response &= myGNSS.enableRTCMmessage(UBX_RTCM_1094, COM_PORT_I2C, 1);
response &= myGNSS.enableRTCMmessage(UBX_RTCM_1124, COM_PORT_I2C, 1);
response &= myGNSS.enableRTCMmessage(UBX_RTCM_1230, COM_PORT_I2C, 10); //Enable message every 10 seconds

These six lines enable the six RTCM output messages needed for a second ZED-F9P to receive correction data. Once these sentences have been enabled (and assuming a survey process is complete) the ZED-F9P base module will begin outputting RTCM data every second after the NMEA sentences (the RTCM_1230 sentence will be output once every 10 seconds). You can view an example of what this output looks like here.

The size of the RTCM correction data varies but in general it is approximately 2000 bytes every second (~2500 bytes every 10th second when 1230 is transmitted).

Copy Code

//This function gets called from the SparkFun u-blox GNSS Arduino Library.
//As each RTCM byte comes in you can specify what to do with it
//Useful for passing the RTCM correction data to a radio, Ntrip broadcaster, etc.
void SFE_UBLOX_GNSS::processRTCM(uint8_t incoming)
{
  //Let's just pretty-print the HEX values for now
  if (myGNSS.rtcmFrameCounter % 16 == 0) Serial.println();
  Serial.print(" ");
  if (incoming < 0x10) Serial.print("0");
  Serial.print(incoming, HEX);
}

If you have a ‘rover’ in the field in need of correction data you’ll need to get the RTCM bytes to the rover. The SparkFun u-blox library automatically detects the difference between NMEA sentences and RTCM data. The processRTCM() function allows you to ‘pipe’ just the RTCM correction data to the channel of your choice. Once the base station has completed the survey and has the RTCM messages enabled, your custom processRTCM() function can pass each byte to any number of channels:

A wireless system such as LoRa or Cellular
Posting the bytes over the internet using WiFi or wired ethernet over an Ntrip caster
Over a wired solution such as RS485

The power of the processRTCM() function is that it doesn’t care; it presents the user with the incoming byte and is agnostic about the back channel.

Heads up! We’ve been experimenting with various LoRa solutions and the bandwidth needed for the best RTCM (~500 bytes per second) is right at the usable byte limit for many LoRa setups. It’s possible but you may need to adjust your LoRa settings to reach the throughput necessary for RTK.

What about configuring the rover? u-blox designed the ZED-F9P to automatically go into RTK mode once RTCM data is detected on any of the ports. Simply push the RTCM bytes from your back channel into one of the ports (UART, SPI, I²C) on the rover's ZED-F9P and the location accuracy will go from meters to centimeters. The rover's NMEA messages will contain the improved Lat/Long data and you'll know where you are with mind-bending accuracy. It’s a lot of fun to watch!

Can I Really Use NMEA with a High Precision GPS Receiver?

Yes! Except that NMEA sentences are right on the edge of enough precision. NMEA sentences look something like this:

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$GNGGA,012911.00,4003.19080,N,10416.95542,W,1,12,0.75,1647.1,M,-21.3,M,,*4F

NMEA outputs coordinates in the ddmm.mmmmm format. So what is the weight of the least significant digit? Said differently, what is the impact of one digit change?

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104 16.95542

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104 16.95543

If we know 1 degree of latitude is 111.3km at the equator, we can glean the change of a fraction of a minute:

1 degree = 60 minutes
1 minute = 1 degree/60 = 111.32km / 60 = 1.855km
1 minute = 1855m
0.1min = 185.5m
0.01min = 18.55m
0.001min = 1.855m
0.0001min = .1855m = 185.5mm
0.00001min = 0.0185m = 18.55mm = 1.855cm

Thankfully u-blox has thought about this and offers a setting to increase the NMEA precision from 5 decimal places to 7.

Image of GPS-RTK2 Hookup Guide

High precision NMEA increases the number of decimal places from 5 to 7. If 0.1855mm is not enough precision you're on your own!

Image of GPS-RTK2 Hookup Guide

Correctly parsing and loading a float variable with 7 digits of significance can be tricky. Consider using the UBX protocol which can output up to 8 digits of precision in dd.dddddddd format. Be sure to checkout the examples in the SparkFun u-blox Arduino Library. We have various examples outputting the full 8 digits of precision over I²C without the burden of parsing NMEA sentences.