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Friday, 28 December 2018

Stationary Mode - Really?


I've been fiddling quite a bit this week with frequency standards (man with two watches never know correct time et cetera). All of the GPS based frequency standards rely on the 1 PPS signal from the GPS receiver, and the accuracy of this can be improved by putting the GPS receiver into "Stationary" mode i.e. not moving. This mode is intended for timing applications and is clearly the correct mode for any fixed GPS being used as part of a frequency reference.

I have recently bought a couple of GT-U7 GPS modules from Amazon. They are as cheap as chips and the documentation for the chipset can be found online. This documents the process of squirting a bunch of data down the serial comms link to the GPS module to put the receiver in "Stationary" mode.

So, in order to do this I have stolen some C code and modified it a bit (the Syncronisation command being looked for wasn't correct) and stuck it on my webspace.

I've connected the GPS module to a Raspberry Pi:

Its just the TXD and RXD lines of the Pi Serial0 connected to the GPS TX and RX serial lines.

You need a bit of jiggery pokery on the Pi before you can use the Serial Port in the way we want:

  1. Edit the config.txt file in the /boot directory and make sure you have the line enable_uart=1
  2. Edit the cmdline.txt file in the /boot directory (make sure you keep only one line in this file) and remove the text "console=serial0,115200" - this removes the need for a login when you connect to the serial port
Once you have done that, reboot your Pi and then from the comand line:

>sudo gcc Stationary.c -o StationaryMode
>sudo chmod +x StationaryMode

And the software should respond with a message to say the configuration has been updated and saved correctly.

What could be simpler?

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