GNSS Timing Breakout - ZED-F9T (Qwiic) Hookup Guide
Using the ZED-F9T in Differential Timing Mode
The receiver has two time pulse signals, each with an accuracy of 5ns. These signals can be configured to have a frequency 0.25Hz to 25MHz with configurable pulse width and polarity. These tightly coupled signals make the ZED-F9T an excellent source as a time base.
Additionally, multiple ZED units can be synchronized by transmitting correction data from a control unit to multiple peripheral units. This allows multiple units, up to 20km (12 miles) apart to have synchronized time pulses with 2.5ns relative accuracy. In other words, differential timing mode will improve the phase alignment of the time pulse between units (and you can have 2 or 20, doesn't matter).
If you're familiar with GNSS RTK and base/rover setups the ZED-F9T is very similar. We will be scratching the surface so if you need additional information be sure to check out What is RTCM? and How to Setup a Temporary Base.
The purpose of a GNSS timing reference station is to not move. Once you tell a receiver that it is a reference station (ie, not moving) it will begin to look at the satellites zooming overhead and calculate its position. As the environment changes the signals from the GNSS (Global Navigation Satellite System - the collective term for GPS, GLONASS, Beidou, Galileo satellites) network change and morph. Because the base knows it is motionless, it can determine the disturbances in the ionosphere and troposphere (as well as other error sources) and begin to calculate the values needed to correct the location that the satellites are reporting to the actual location of the base station. These values are unsurprisingly called ‘correction values’ and are encoded into a format called RTCM. (RTCM stands for Radio Technical Commission for Maritime but is just a name for a standard akin to "802.11". Don’t worry about it.). You will often see the term RTCM3 being used; this is simply correction data being transmitted using version 3 of the RTCM format.
For specifics of configuring the ZED-F9T for reference station mode, please see the ZED-F9T Differential Timing Setup app note from u-blox. This will walk you through how to put a ZED-F9T into stationary mode. Once complete this unit will begin producing RTCM packets once per second.
Once packets are being produced, determine how the ~550 bytes per second will be delivered to the peripheral units to be synchronized. Typically, these packets are pushed to a NTRIP caster over an internet connection (you can read here about Setting up an NTRIP Caster). Alternatively these packets can be broadcast over serial modem radios, buried cable, or any other back haul option capable of supporting ~1kB/s. We've found that using an ESP32 along with a Qwiic connection and WiFi makes setting up an NTRIP connection enticingly easy.
Once a reference station has been setup, the peripheral ZED-F9Ts simply need to access these RTCM packets and have them fed into the module. Generally, this is done over UART2 but can be done over USB, I2C, UART1 or SPI. Don't worry, we've got Arduino examples demonstrating both NTRIP Client and Server that can be leveraged for the ZED-F9T. And if Arduino and RTK2Go are not your cup of tea, u-center has a built in Caster and Client that will make the connection between multiple units over USB using their Windows application.