sparkfun RTK Express Documento tecnico
SparkFun RTK Express Hookup Guide
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.
With just a few minutes of setup, the RTK Express is one of the fastest ways to take centimeter grade
measurements.
SparkFun RTK Express
GPS-18442
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:
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 Express setup in base mode and a radio link connecting the Base to the Rover.
Again, we'll go into details but we designed RTK Express to work with these 500mW 915MHz telemetry radios out
of the box:
To charge the RTK Express you will need a USB C cable and a power supply. SparkFun carries a few options:
GNSS Multi-Band L1/L2 Surveying Antenna
(TNC) - TOP106
GPS-17751
Interface Cable - SMA Male to TNC Male
(300mm)
CAB-17833
1/4" to 5/8" Antenna Thread Adapter
PRT-17546
Serial Telemetry Radio Kit - 915MHz, 500mW
WRL-17255
USB 2.0 Type-C Cable - 1 Meter
CAB-16905
Reversible USB A to C Cable - 0.8m
CAB-15425
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:
USB Wall Charger - 5V, 2A
TOL-16893
USB-C Wall Adapter - 5.1V, 3A (Black)
TOL-16272
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.
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.
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!
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
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