2024-03-26 | By SparkFun Electronics

License: See Original Project Wireless

Courtesy of SparkFun

Guide by NATE

Introduction

The RTK Express from SparkFun is your one stop shop for high precision geolocation and ‎surveying needs. For basic users, it’s incredibly easy to get up and running and for advanced users, ‎the RTK Express is a flexible and powerful tool.‎

SparkFun RTK Express

With just a few minutes of setup, the RTK Express is one of the fastest ways to take centimeter ‎grade measurements.‎

An RTK Fix with 14mm accuracy in SW Maps

By connecting your phone to the RTK Express over Bluetooth, your phone can act as the radio link ‎to provide correction data as well as receive the NMEA output from the device. It’s how $10,000 ‎surveying devices have been operating for the past decade - we just made it easier, smaller, and a ‎lot cheaper.‎

Required Materials

While the RTK Express is nicely enclosed you will need a few cables and antennas to make ‎everything work. We'll go into the specifics of how to hook things together but in general you will ‎need to get a good quality L1/L2 antenna:‎

• GNSS Multi-Band L1/L2 Surveying Antenna - TNC (TOP106)‎
• Interface Cable - SMA Male to TNC Male (300mm)
• ‎Antenna Thread Adapter - 1/4in. to 5/8in.‎

Depending on your setup you may want to use your phone for RTCM correction data. If a source is ‎not available online, you will need a 2nd RTK Facet setup in base mode and a radio link connecting ‎the Base to the Rover. We'll go into details, but we designed RTK Facet to work with these 100mW ‎‎915MHz telemetry radios out of the box.‎

• SiK Telemetry Radio V3 - 915MHz, 100mW‎

To charge the RTK Express you will need a USB C cable and a power supply. SparkFun carries a ‎few options:‎

• USB 2.0 Type-C Cable - 1 Meter
• ‎Reversible USB A to C Cable - 0.8m
• ‎USB-C Wall Adapter - 5.1V, 3A (Black)
• ‎USB Wall Charger - 5V, 2A‎

Suggested Reading

GNSS RTK is an incredible feat of engineering that has been made easy to use by powerful GNSS ‎receivers such as the ZED-F9P by u-blox (the receiver inside RTK Express). The process of ‎setting up an RTK system will be covered in this tutorial but if you want to know more about RTK ‎here are some good tutorials to brush up on:‎

• What is GPS RTK?: Learn about the latest generation of GPS and GNSS receivers to get 14mm ‎positional accuracy!‎
• Getting Started with U-Center for u-blox: Learn the tips and tricks to use the u-blox ‎software tool to configure your GPS receiver.‎
• GPS-RTK2 Hookup Guide: Get precision down to the diameter of a dime with the new ZED-‎F9P from u-blox.‎
• Setting up a Rover Base RTK System: Getting GNSS RTCM correction data from a base ‎to a rover is easy with a serial telemetry radio! We'll show you how to get your high precision RTK ‎GNSS system setup and running.‎
• How to Build a DIY GNSS Reference Station: Learn how to affix a GNSS antenna, use ‎PPP to get its ECEF coordinates and then broadcast your own RTCM data over the internet and ‎cellular using NTRIP to increase rover reception to 10km!‎

Hardware Overview

The RTK Express is a fully enclosed, preprogrammed device. There are very few things to worry ‎about or configure but we will cover the basics.‎

Buttons

The RTK Express uses two buttons, Power and Setup for in-field configuration.‎

Setup

This device can be used in four modes:‎

• GNSS Positioning (~30cm accuracy) - also known as 'Rover'
• GNSS Positioning with RTK (1.4cm accuracy) - also known as 'Rover with RTK Fix'
• GNSS Base Station
• GNSS Base Station NTRIP Server

At power on the device will enter Rover or Base mode; whichever state the device was in at the last ‎power down. When the SETUP button is pressed the RTK Express will toggle ‎between Rover and Base mode. The display will indicate the change with a small car or flag icon.‎

In Rover mode the RTK Express will receive L1 and L2 GNSS signals from the four constellations ‎‎(GPS, GLONASS, Galileo, and BeiDou) and calculate the position based on these signals. Similar to ‎a standard grade GPS receiver, the RTK Express will output industry standard NMEA sentences at ‎‎4Hz and broadcast them over any paired Bluetooth device. The end user will need to parse the ‎NMEA sentences using commonly available mobile apps, GIS products, or embedded devices ‎‎(there are many open-source libraries). Unlike standard grade GPS receivers that have 2500m ‎accuracy, the accuracy in this mode is approximately 300mm horizontal positional accuracy with a ‎good grade L1/L2 antenna.‎

When the device is in Rover mode and RTCM correction data is sent into the radio port or over ‎Bluetooth, the device will automatically enter Positioning with RTK mode. In this mode RTK ‎Express will receive L1/L2 signals from the antenna and correction data from a base station. The ‎receiver will quickly (within a few seconds) obtain RTK float, then fix. The NMEA sentences will ‎have increased accuracy of 14mm horizontal and 10mm vertical accuracy. The RTCM correction ‎data can be obtained from a cellular link to online correction sources or over a radio link to a 2nd ‎RTK Express setup as a base station.‎

In Base mode the device will enter Base Station mode. This is used when the device is mounted to ‎a fixed position (like a tripod or roof). The RTK Express will initiate a survey. After 60 to 120 ‎seconds the survey will complete and the RTK Express will begin transmitting RTCM correction ‎data out the radio port. A base is often used in conjunction with a second RTK Express (or RTK ‎Surveyor) unit set to 'Rover' to obtain the 14mm accuracy. Said differently, the Base sits still and ‎sends correction data to the Rover so that the Rover can output a really accurate position. You’ll ‎create an RTK system without any other setup.‎

Power

RTK Express startup display with firmware version number

The Power button turns on and off the unit. Press and hold the power button until the display ‎illuminates. Press and hold the power button at any time to turn the unit off.‎

RTK Express showing the battery level

The RTK Express has a built-in 1300mAh lithium polymer battery that will enable over 5 hours of ‎field use between charging. If more time is needed a common USB power bank can be attached ‎boosting the field time to 40 hours.‎

Charge LED

The Charge LED is located above the Power button. It will illuminate any time there is an external ‎power source and will turn off when the internal battery is charged. With the unit fully powered ‎down, charging takes approximately 1.5 hours from a 1A wall supply or 3 hours from a standard ‎USB port. The RTK Express can run while being charged but it increases the charge time. Using an ‎external USB battery bank to run the device for extended periods or running the device on a ‎permanent wall power source is supported.‎

Connectors

The SparkFun RTK Express has a variety of connectors

Antenna:‎

This SMA connector is used to connect an L1/L2 type GNSS antenna to the RTK Express. Please ‎realize that a standard GPS antenna does not receive the L2 band signals and will greatly impede ‎the performance of the RTK Express (RTK fixes are nearly impossible). Be sure to use a ‎proper L1/L2 antenna.‎

Configure u-blox:‎

This USB C connector is used for charging the device and/or directly configuring and inspecting ‎the ZED-F9P GNSS receiver using u-center. It’s not necessary in normal operation but is handy for ‎tailoring the receiver to specific applications. As an added perk, the ZED-F9P can be detected ‎automatically by some mobile phones and tablets. If desired, the receiver can be directly connected ‎to a compatible phone or tablet removing the need for a Bluetooth connection.‎

Configure ESP32:‎

This USB C connector is used for charging the device, configuring the device, and reprogramming ‎the ESP32. Various debug messages are printed to this port at 115200bps, and a serial menu can ‎be opened to configure advanced settings.‎

Radio:‎

This 4-pin JST connector is used to allow RTCM correction data to flow into the device when it is ‎acting as a rover or out of the device when it is acting as a base. The connector is a 4-pin locking ‎‎1.25mm JST SMD connector (part#: SM04B-GHS-TB, mating connector part#: GHR-04V-S). The ‎RTK Express comes with a cable to interface to this connector but additional cables can be ‎purchased. You will most likely connect this port to one of our Serial Telemetry Radios if you don’t ‎have access to a correction source on the internet. The pinout is 3.5-5.5V / TX / RX / GND from left ‎to right as pictured (pin labels are shown on the board itself). 3.5V to 5.5V is provided by this ‎connector to power a radio with a voltage that depends on the power source. If USB is connected ‎to the RTK Express, then voltage on this port will be 5V (+/-10%). If running off of the internal ‎battery, then voltage on this port will vary with the battery voltage (3.5V to 4.2V depending on the ‎state of charge). While the port is capable of sourcing up to 2 amps, we do not recommend more ‎than 500mA. This port should not be connected to a power source.

Data:‎

This 4-pin JST connector is used to output and input a variety of data to the RTK Express. The ‎connector is a 4-pin locking 1.25mm JST SMD connector (part#: SM04B-GHS-TB, mating ‎connector part#: GHR-04V-S). The RTK Express comes with a cable to interface to this connector ‎but additional cables can be purchased.‎

Internally the Data connector is connected to a digital mux allowing one of four software selectable ‎setups:‎

• NMEA - The TX pin outputs any enabled messages (NMEA, UBX, and RTCM) at a default of ‎‎460,800bps (configurable 9600 to 921600bps). The RX pin can receive RTCM for RTK and ‎can also receive UBX configuration commands if desired.
• PPS/Trigger - The TX pin outputs the pulse-per-second signal that is accurate to 30ns RMS. ‎The RX pin is connected to the EXTINT pin on the ZED-F9P allowing for events to be ‎measured with incredibly accurate nano-second resolution. Useful for things like audio ‎triangulation. See the Timemark section of the ZED-F9P integration for more information.‎
• I2C - The TX pin operates as SCL, RX pin as SDA on the I2C bus. This allows additional ‎sensors to be connected to the I2C bus.‎
• GPIO - The TX pin operates as a DAC capable GPIO on the ESP32. The RX pin operates as a ‎ADC capable input on the ESP32. This is useful for custom applications.‎

Most applications do not need to utilize this port and will send the NMEA position data over ‎Bluetooth. This port can be useful for sending position data to an embedded microcontroller or ‎single board computer. The pinout is 3.3V / TX / RX / GND. 3.3V from left to right as pictured (pin ‎labels are shown on the board itself), which is provided by this connector to power a remote device ‎if needed. While the port is capable of sourcing up to 600mA, we do not recommend more than ‎‎300mA. This port should not be connected to a power source.‎

microSD:‎

This slot accepts standard microSD cards up to 32GB formatted for FAT16 or FAT32. Logging any ‎of 67 messages at up to 4Hz is supported for all constellations.‎

The following 67 messages are supported for logging:‎

• NMEA-DTM
• NMEA-GBS
• NMEA-GGA
• NMEA-GLL
• NMEA-GNS
• NMEA-GRS
• NMEA-GSA
• NMEA-GST
• NMEA-GSV
• NMEA-RMC
• NMEA-VLW
• NMEA-VTG
• NMEA-ZDA
• NAV-CLOCK
• NAV-DOP
• NAV-EOE
• NAV-GEOFENCE
• NAV-HPPOSECEF
• NAV-HPPOSLLH
• NAV-ODO
• NAV-ORB
• NAV-POSECEF
• NAV-POSLLH
• NAV-PVT
• NAV-RELPOSNED
• NAV-SAT
• NAV-SIG
• NAV-STATUS
• NAV-SVIN
• NAV-TIMEBDS
• NAV-TIMEGAL
• NAV-TIMEGLO
• NAV-TIMEGPS
• NAV-TIMELS
• NAV-TIMEUTC
• NAV-VELECEF
• NAV-VELNED
• RXM-MEASX
• RXM-RAWX
• RXM-RLM
• RXM-RTCM
• RXM-SFRBX
• MON-COMMS
• MON-HW2
• MON-HW3
• MON-HW
• MON-IO
• MON-MSGPP
• MON-RF
• MON-RXBUF
• MON-RXR
• MON-TXBUF
• TIM-TM2
• TIM-TP
• TIM-VRFY
• RTCM3x-1005‎
• RTCM3x-1074
• RTCM3x-1077
• RTCM3x-1084
• RTCM3x-1087
• RTCM3x-1094
• RTCM3x-1097
• RTCM3x-1124
• RTCM3x-1127
• RTCM3x-1230
• RTCM3x-4072-0
• RTCM3x-4072-1‎

Qwiic:‎

This 4-pin Qwiic connector exposes the I2C bus of the ESP32 WROOM module. Currently, there is ‎no firmware support for adding I2C devices to the RTK Express, but support may be added in the ‎future.‎

Power

The RTK Express has a built in 1300mAh battery and consumes approximately 240mA worst case ‎with Bluetooth connection active, GNSS fully tracking, and a 500mW radio broadcasting. This will ‎allow for around 5.5 hours of use in the field. If more time is needed in the field a standard USB ‎power bank can be attached. If a 10,000mAh bank is attached one can estimate 30 hours of run ‎time assuming 25% is lost to efficiencies of the power bank and charge circuit within RTK Express.‎

The RTK Express can be charged from any USB port or adapter. The charge circuit is rated for ‎‎1000mA so USB 2.0 ports will charge at 500mA and USB 3.0+ ports will charge at 1A.‎

To quickly view the state of charge, turn on the unit. The battery icon will indicate the following:‎

• ‎3 bars: >75% capacity remain
• ‎2 bars: >50% capacity remain
• ‎1 bar: >25% capacity remain
• ‎0 bars: <25% capacity remain‎

RTK Express Display showing three battery bars

Hardware Overview - Advanced Features

The RTK Express is a hacker’s delight. Under the hood of the RTK Express is an ESP32 WROOM ‎connected to a ZED-F9P as well as some peripheral hardware (LiPo fuel gauge, microSD, etc.) It is ‎programmed in Arduino and can be tailored by the end user to fit their needs.‎

ZED-F9P GNSS Receiver

The ZED-F9P GNSS receiver is configured over I²C and uses two UARTs to output NMEA (UART1) ‎and input/output RTCM (UART2). In general, the ESP32 harvests the data from the ZED-F9Ps ‎UART1 for Bluetooth transmission and logging to SD.‎

ESP32‎

The ESP32 uses a standard USB to serial conversion IC (CH340) to program the device. You can ‎use the ESP32 core for Arduino or Espressif’s IoT Development Framework (IDF).‎

The CH340 automatically resets and puts the ESP32 into bootload mode as needed. However, the ‎reset pin of the ESP32 is brought out to an external 2-pin 0.1” footprint if an external reset button is ‎needed.‎

Note: If you've never connected a CH340 device to your computer before, you may need to install ‎drivers for the USB-to-serial converter. Check out our section on "How to Install CH340 Drivers" for ‎help with the installation.‎

Measurement Jumpers

To facilitate the measurement of run, charge, and quiescent currents, two measurement jumpers ‎are included. These are normally closed jumpers combined with a 2-pin 0.1” footprint. To take a ‎measurement, cut the jumper and install a 2-pin header and use banana to IC hook cables to a ‎DMM. These can then be closed with a 2-pin jumper.‎

LiPo and Charging

The RTK Express houses a standard 1300mAh 3.7V LiPo. The charge circuit is set to 1A so with an ‎appropriate power source, charging an empty battery should take a little over one hour. USB C on ‎the RTK Express is configured for 2A draw so if the user attaches to a USB 3.0 port, the charge ‎circuit should operate near the 1A max. If a user attaches to a USB 2.0 port, the charge circuit will ‎operate at 500mA. This charge circuit also incorporates a 42C upper temperature cutoff to insure ‎the LiPo cannot be charged in dangerous conditions.‎

Fuel Gauge and Accelerometer

The MAX17048 is a simple to use fuel gauge IC that gives the user a statement of charge (SOC) ‎that is basically a 0 to 100% report. The MAX17048 has a sophisticated algorithm to figure out what ‎the SOC is based on cell voltage that is beyond the scope of this tutorial but for our purposes, ‎allows us to reliably view the battery level when the unit is on.‎

The RTK Express also incorporates a the LIS2DH12 triple-axis accelerometer to aid in leveling in ‎the field.‎

Qwiic

Two Qwiic connectors are included in the unit. The internal Qwiic connector connects to the OLED ‎display attached to the upper lid. The lower Qwiic connector is exposed on the end of the unit. ‎These allow connection to the I²C bus on the ESP32. Currently the stock RTK Express does not ‎support any additional Qwiic sensors or display, but users may add support for their own application.‎

microSD

A microSD socket is situated on the ESP32 SPI bus. Any microSD up to 32GB is supported. RTK ‎Express supports RAWX and NMEA logging to the SD card. Max logging time can also be set ‎‎(default is 10 hours) to avoid multi-gigabyte text files. For more information about RAWX and doing ‎PPP please see this tutorial.‎

Data Port and Digital Mux

The 74HC4052 analog mux controls which digital signals route to the external Data port. This allows ‎a variety of custom end user applications. The most interesting of which is event logging. Because ‎the ZED-F9P has microsecond accuracy of the incoming digital signal, custom firmware can be ‎created to triangulate an event based on the receiver's position and the time delay between multiple ‎captured events. Currently, TM2 event logging is supported.‎

Additionally, this mux can be configured to connect ESP pin 26 (DAC capable) and pin 39 (ADC ‎capable) for end user custom applications.‎

Hardware Assembly

The RTK Express was designed to work with low-cost, off the shelf equipment. Here we’ll describe ‎how to assemble a Rover and Base.‎

Rover

Shown here is the most common RTK Rover setup. A monopole designed for cameras is used. A ‎cell phone holder is clamped to the monopod and the RTK Express is mounted. The ¼” camera ‎thread of the monopole is adapted to ⅝” 11-TPI and a L1/L2 antenna is attached. A Male TNC to ‎Male SMA cable connects the antenna to the RTK Express. No radio is needed because RTCM ‎correction data is provided by a phone over Bluetooth.‎

Basic RTK Express Rover setup with RTCM over Bluetooth

If you’re shopping for a monopole (aka monopod), get one that is 65” in length or greater to ensure ‎that the antenna will be above your head. We’ve had good luck with the Amazon Basics brand.‎

We have done lots of testing with the u-blox L1/L2 antenna and it's very good for the price and size. ‎Mounted to a ground plate you will get good results. It's just a bit ungainly when mounted to the top ‎of a monopole. We recommend the 'ufo' style L1/L2 antennas because they have a larger antenna ‎element and a slightly larger ground plane than the u-blox antenna.‎

u-blox L1/L2 antenna with ground plate

We strongly recommend against using a rigid helical antenna configuration as shown below. The ‎RTK Express is not designed for such configurations and can lead to permanent damage to the ‎antenna connector. The helical antenna becomes a large lever arm. If the unit is dropped this lever ‎is capable of damaging both the SMA connector and where the connector is soldered to the PCB.‎

Dangerous Antenna Configuration

If you’re shopping for a cell phone clamp, be sure to get one that is compatible with the diameter of ‎your monopole and has a knob to increase clamp pressure. Our monopole is 27mm in diameter and ‎we’ve had a good experience with this clamp and this clamp. Your mileage may vary.‎

If you are receiving RTCM correction data over a radio link it’s recommended that you attach a radio ‎to the back of the RTK Express.‎

‎

2nd most common setup with a 915MHz Radio providing RTCM

Picture hanging strips from 3M make a nice semi-permanent mount. Plug the 4-pin to 6-pin JST ‎cable included with the RTK Express from the Radio port to either of the Serial Telemetry ‎Radios (shipped in pairs). We really love these radios because they are paired out of the box, either ‎can send or receive (so it doesn't matter which radio is attached to base or rover) and they have ‎remarkable range. We achieved over a mile range (nearly 1.5 miles or 2.4km) with the 500mW ‎radios and a big 915MHz antenna on the base (see this tutorial for more info.)‎

Temporary Base

A temporary or mobile base setup is needed when you are in the field too far away from a ‎correction source and/or cellular reception. A 2nd RTK Express is mounted to a tripod, and it is ‎configured to complete a survey-in (aka, locate itself), then begin broadcasting RTCM correction ‎data. This data (~1000 bytes a second) is sent to the user's connected radio of choice. For our ‎purposes, the 915MHz 500mW telemetry radios are used because they provide what is basically a ‎serial cable between our base and rover.‎

Temporary RTK Express Base setup

Any tripod with a ¼” camera thread will work. The Amazon Basics tripod works well enough but is a ‎bit light weight and rickety. A cell phone holder is clamped to the tripod and the RTK Express is ‎held in the clamp. The ¼” camera thread is adapted to ⅝” 11-TPI and a L1/L2 antenna is attached. ‎A Male TNC to Male SMA adapter connects the antenna to the RTK Express.‎

Once the base has been setup with a clear view of the sky, turn on the RTK Express. Once on, ‎press the Setup button to put the device in Base mode. The display will show the Survey-In screen ‎for 60-120 seconds. Once the survey is complete the display will show the 'Xmitting' display and ‎begin producing RTCM correction data. You can verify this by viewing the LEDs on the telemetry ‎radio (a small red LED will blink when serial data is received from the RTK Express). The RTK ‎Express is designed to follow the u-blox recommended survey-in of 60s and a mean 3D standard ‎deviation of 5m of all fixes. If a survey fails to achieve these requirements it will auto-restart after 10 ‎minutes.‎

More expensive surveyor bases have a ⅝” 11-TPI thread but the top of the surveyor base will often ‎interfere with the antenna’s TNC connector. If you chose to use a surveyor’s ‘stick’ be sure to ‎obtain a ⅝” extender plate to raise the antenna at least an inch.‎

If you’re shopping for a cell phone clamp, be sure to get one that is compatible with the diameter of ‎your tripod and has a knob to increase clamp pressure. Our tripod is 18mm in diameter and we’ve ‎had a good experience with this clamp. Your mileage may vary.‎

Note: A mobile base station works well for quick trips to the field. However, the survey-in method is ‎not recommended for the highest accuracy measurements because the positional accuracy of the ‎base will directly translate to the accuracy of the rover. Said differently, if your base's calulcated ‎position is off by 100cm, so will every reading your rover makes. If you’re looking for maximum ‎accuracy consider installing a static base with fixed antenna. We were able to pinpoint the antenna ‎on the top of SparkFun with an incredible accuracy +/-2mm of accuracy using PPP!‎

Bluetooth and NTRIP

The RTK Express transmits full NMEA sentences over Bluetooth serial port profile (SPP) at 4Hz ‎and 115200bps. This means that nearly any GIS application that can receive NMEA data over serial ‎port (almost all do) can be used with the RTK Express. As long as your device can open a serial ‎port over Bluetooth (also known as SPP) your device can retrieve industry standard NMEA ‎positional data. The following steps show how to use SW Maps, but the same steps can be followed ‎to connect any serial port-based GIS application.‎

Please see the SparkFun RTK Product Manual for step-by-step instructions.‎

Display

The RTK Express has a 0.96" high-contrast OLED display. While small, it packs various situational ‎data that can be helpful in the field. We will walk you through each display.‎

Please see the SparkFun RTK Product Manual for a description of each display.‎

System Configuration

Out of the box, the SparkFun RTK products are exceptional GNSS receivers out-of-box and can be ‎used with little or no configuration. Additionally, the line of RTK products from SparkFun are ‎immensely configurable. Please see the SparkFun RTK Product Manual for detailed descriptions of ‎all the available features on the RTK products.‎

Firmware Updates and Customization

The RTK Express is open source hardware meaning you have total access to ‎the firmware and hardware.‎

From time-to-time SparkFun will release new firmware for the RTK product line to add and improve ‎functionality. We've made updating the firmware as easy as possible. Please see Updating RTK ‎Firmware for a step-by-step tutorial.‎

Troubleshooting

Not working as expected and need help? ‎

If you need technical assistance and more information on a product that is not working as you ‎expected, we recommend heading on over to the SparkFun Technical Assistance page for some ‎initial troubleshooting.

SPARKFUN TECHNICAL ASSISTANCE PAGE‎ ‎

If you don't find what you need there, the SparkFun Forums are a great place to find and ask for ‎help. If this is your first visit, you'll need to create a Forum Account to search product forums and ‎post questions.

CREATE NEW FORUM ACCOUNT   LOG INTO SPARKFUN FORUMS

Resources and Going Further

We hope you enjoy using the RTK Express as much as we have!‎

Here are the pertinent technical documents for the RTK Express:‎

• ZED-F9P GNSS Receiver Datasheet
• MAX17048 Fuel Gauge IC
• SparkFun RTK Express GitHub Repo (contains the open-source hardware electronics and ‎enclosure)
• SparkFun RTK Firmware GitHub Repo (contains the firmware that runs SparkFun RTK ‎products)