SparkFun RTK Facet Hookup Guide

Contributors: Nate

Hardware Overview

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

Power/Setup Button

The RTK Facet has one button used for both Power and Setup for in-field configuration changes. Pressing and holding the Power button will cause it to power on or off. Short pressing the button will cause the RTK Facet to change modes.

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 POWER/SETUP button is pressed momentarily, a menu is presented to change the RTK Facet to Rover or Base mode. The display will indicate the change with a small car or flag icon.

In Rover mode the RTK Facet 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 Facet 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 over Bluetooth or into the radio port, the device will automatically enter Positioning with RTK mode. In this mode RTK Facet will receive L1/L2 signals from the antenna and correction data from a base station. The receiver will quickly (within a second) obtain RTK float, then fix. The NMEA sentences will have increased accuracy of 14mm horizontal and 10mm vertical accuracy. The RTCM correction data is most easily obtained over the internet using a free app on your phone (see SW Maps or Lefebure NTRIP) and sent over Bluetooth to the RTK Facet but RTCM can also be delivered over an external cellular or radio link to a 2nd RTK Facet 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 Facet will initiate a survey. After 60 to 120 seconds the survey will complete and the RTK Facet will begin transmitting RTCM correction data out the radio port. A base is often used in conjunction with a second RTK Facet (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

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.

The RTK Facet has a large, built-in 6000mAh lithium polymer battery that will enable over 25 hours of field use between charging. If more time is needed a common USB power bank can be attached boosting the field time to any amount needed.

Charge LED

The Charge LED is located on the front face. 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 6 hours from a 1A wall supply or 12 hours from a standard USB port. The RTK Facet 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

There are a variety of connectors protected by a dust flap.

USB:

This USB C connector is used for three purposes:

• Charging the device
• Configuring the RTK Facet, and reprogramming the ESP32
• Directly configuring and inspecting the ZED-F9P GNSS receiver

There is a USB hub built into the RTK Facet. When you attach the device to your computer it will enumerate as two COM ports.

In the image above, the USB Serial Device is the ZED-F9P and the USB-SERIAL CH340 is the ESP32.

Configuring the RTK Facet can be done over the USB-Serial CH340 COM port via serial text menu. Various debug messages are printed to this port at 115200bps and a serial menu can be opened to configure advanced settings.

Configuring the ZED-F9P can be configured over the USB Serial Device port 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.

Radio:

This port is used when an external cellular or radio link is needed. This port is not used if you transfer RTCM from your phone to the RTK Facet over Bluetooth.

This 4-pin JST connector can be 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 Facet 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. 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 Facet 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). This port is capable of sourcing up to 600mA and is protected by a PTC (resettable fuse). This port should not be connected to a power source.

Data:

This port is used when an external system is connected such as a rover, car, timing equipment, camera triggers, etc. This port is not used if you transfer NMEA positional data to your phone from the RTK Facet over Bluetooth.

This 4-pin JST connector is used to output and input a variety of data to the RTK Facet. The connector is a 4-pin locking 1.25mm JST SMD connector (part#: SM04B-GHS-TB, mating connector part#: GHR-04V-S). The RTK Facet 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, 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:

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 Facet but support may be added in the future.

Antenna:

It's built in! Housed under the dome of the RTK Facet is a surveyor grade L1/L2 antenna. It is the same element found within our GNSS Multi-Band L1/L2 Surveying Antenna. Its datasheet is available here.

The built in antenna has an antenna phase center (APC) of 61.4mm from the base if the device to the measuring point of the L1 antenna and an APC of 57.4mm to the measuring point of the L2 antenna.

Power

The RTK Facet has a built in 6000mAh battery and consumes approximately 240mA worst case with Bluetooth connection active and GNSS fully tracking. This will allow for around 25 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 56 hours of run time assuming 25% is lost to efficiencies of the power bank and charge circuit within RTK Facet.

The RTK Facet 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