There will be a serial connection between the host computer and the GPS module. SatPulse needs to know the device name on the host computer, and also the speed (baud rate).

SatPulse can work with a 1-way connection where it only receives data from the GPS module. But for configuration it needs a 2-way connection where it can also send data to the GPS module.

Determining the device name

The device name depends on how the GPS module is connected:

  • a connection to the pins on a Raspberry Pi CM4/CM5 will usually be /dev/ttyAMA0
  • a USB-to-RS232 or USB-to-TTL converter will usually show up as /dev/ttyUSB0, but occasionally may show up as /dev/ttyACM0
  • an RS232 connection using a DB9 port on the PC will usually show up as /dev/ttyS0
  • a GPS in M.2 slot will usually be /dev/ttyACM0
  • a GPS with a USB connection will be either /dev/ttyACM0 or /dev/ttyUSB0

You may need to change 0 in the device name to a larger number.

/dev/ttyUSB0 and /dev/ttyACM0 are USB devices and they exist in /dev only when they are connected, so you can use ls to find what you have available. When you plug such a device in, there will be a kernel log message, which you can see using dmesg | tail.

Determining the speed

You also need to know the speed the GPS module is using. This will often be included in the product description when you buy the module. The default speed for every GPS module I have seen (and I have dozens) is 9600, 38400 or 115200. Older modules use 9600. Some newer u-blox modules use 38400. Modern Chinese modules that are not using u-blox chips, especially higher-end models, often use 115200.

If you are not sure, then just try each of 9600, 38400 and 115200 until you find one that works.

SatPulse assumes serial connection uses 8 data bits, no parity and 1 stop bit (called 8N1); many modules support only this and it is the default on all modules.

My understanding is that if the device is /dev/ttyACM0 any speed should work. But you should still set it to match the speed of the module.

Serial device permissions

Typically on Linux, serial devices are configured to allow access by anybody in the dialout group.

It’s convenient if you are in the dialout group so you can access the serial devices

sudo usermod -G dialout -a jjc

Here jjc is your username. You’ll need to logout and then login again for this to take effect.

Verifying with satpulsetool

Do, for example:

satpulsetool gps -s 9600 -d /dev/ttyAMA0

where 9600 is the speed and /dev/ttyAMA0 is the device name.

With just those arguments, satpulsetool gps will read packets from the GPS and also send a probe packet to determine whether it supports the UBX protocol.

If it detects a GPS, it will tell you some information about what packets it received, with more details if the probe succeeded.

Verifying without satpulsetool

Do, for example:

(stty 9600 -echo -icrnl; cat) </dev/ttyAMA0

where 9600 is the speed and /dev/ttyAMA0 is the device name.

You should see lines starting with $. In particular look for a line starting with $GPRMC or $GNRMC. The number following that should be the current UTC time; for example, 025713.00 means 02:57:13.00 UTC. After another 8 commas, there will be a field that should have the current UTC date; for example, 140923 means 14th September 2023.