Adding USB Attached GPS to your Raspberry Pi Projects

This quick learning guide will show you everything you need to do to add position tracking to your Pi project using the open source GPS daemon ‘gpsd’ and an inexpensive USB to TTL adapter cable or via direct-wiring to the built-in Pi UART pins

Please note this guide installs a system service called gpsd which you can then query for data. You may be better off just using pure python to read data from the GPS, its less complex in many cases

What you’ll need:

The easiest way to get started is to use an inexpensive USB to TTL adapter cable with your GPS module.

You can of course use the Hardware UART directly on the Pi, but as you can see in this tutorial (Raspberry PI: Connecting a Neo-6M  GPS Module) it’s a bit more complicated, and there are no secondary consequences with the USB adapter.

This tutorial will assume that we are using the USB to TTL cable mentioned above, and that we are running on Raspbian. Raspbian already has the drivers for PL2303-based cables pre-installed, so you just need to plug it in and it should show up as /dev/ttyUSB0).

Hooking The Breakout Up

The first thing you’ll need to do is to hook your GPS Breakout up to the Pi with the adapter cable. The following diagram shows you what you need to know, essentially just connecting the cables of the same color together.

  • GPS Vin  to USB 5V or 3V (red wire on USB console cable)
  • GPS Ground to USB Ground (black wire)
  • GPS RX to USB TX (green wire)
  • GPS TX to USB RX (white wire)

While the module on the GPS Breakout has an exceptionally sensitive antenna and may work indoors as is, you may want to pick up an external GPS Antenna and an SMA to u.FL adapter cable if this is for indoor use. This will allow you to keep the Pi and GPS breakout indoors, but run the antenna out a window or at least near one for improved reliability ans signal integrity.

Setting up the USB Adapter

Once you plug the USB cable into the Pi, the adapter should show up as /dev/ttyUSB0 (though the ‘0’ may be different if you have other ttyUSB adapters present).

You can see a list of all ttyUSB devices by entering the following into the console (I’m using the ‘terminal’ feature in Adafruit’s browser-based WebIDE here for convenience sake!):

ls /dev/ttyUSB*

If you have any problems, you can enter the following command to see the USB devices on your Pi:

sudo lsusb

Which should show you the USB adapter (Prolific PL2303), as follows:

If you just want to do a quick check to see what data is coming out of the GPS, you can enter the following command, followed by Ctrl-C to quit:

sudo cat /dev/ttyUSB

Installing a GPS Daemon (gpsd)

The next step is installing some software on your Raspberry Pi that understands the serial data that your GPS module is providing via /dev/ttyUSB0.

Thankfully other people have already done all the hard work for you of properly parsing the raw GPS data, and we can use (amongst other options) a nice little package named ‘gpsd’, which essentially acts as a layer between your applications and the actual GPS hardware, gracefully handling parsing errors, and providing a common, well-defined interfaces to any GPS module.

To install gpsd, simply run the following commands from the console:

sudo apt-get install gpsd gpsd-clients

… which will install the required packages (an internet connection will be required for this step!)

>Raspbian Jessie systemd service fix

Note if you’re using the Raspbian Jessie or later release you’ll need to disable a systemd service that gpsd installs.  This service has systemd listen on a local socket and run gpsd when clients connect to it, however it will also interfere with other gpsd instances that are manually run (like in this guide).  You will need to disable the gpsd systemd service by running the following commands:

sudo systemctl stop gpsd.socket
sudo systemctl disable gpsd.socket

Should you ever want to enable the default gpsd systemd service you can run these commands to restore it (but remember the rest of the steps in this guide won’t work!):

sudo systemctl enable gpsd.socket
sudo systemctl start gpsd.socket

After disabling the gpsd systemd service above you’re ready to try running gpsd manually.  Now run the following command to manually start gpsd and point it at the GPS breakout on the USB serial adapter port:

sudo gpsd /dev/ttyUSB0 -F /var/run/gpsd.sock

… which will point the gps daemon to our GPS device on the USB to TTY adapter cable (simply substitute ‘/dev/ttyUSB0’ for another destination if required).

Testing it Out

After a few seconds, gpsd should open up the proper socket and if the GPS is locked we should be able to get some data from the GPS module.

To test this, we can use the following command:

cgps -s

If you have a fix, you’ll see something like the following information in the terminal window:

If you have any problems and cgps always displays ‘NO FIX’ under status and then aborts after a few seconds, you may need to restart the gpsd service. You can do that via the following commands:

sudo killall gpsd
sudo gpsd /ttyUSB0 -F/var/run/gpsd.sock

Using your GPS

Now that your GPS is up and running, and gpsd is playing nice with it, it’s time to do something with the data!

The easiest way to get started is using a bit of Python code to access gpsd.  First, install the gpsmodule from PyPI:

pip3 install gps

Next, use nano or the text editor of your choice to save the following code in a file called gpstest.py:

import gps
 
# Listen on port 2947 (gpsd) of localhost
session = gps.gps("localhost", "2947")
session.stream(gps.WATCH_ENABLE | gps.WATCH_NEWSTYLE)
 
while True:
try:
report = session.next()
# Wait for a 'TPV' report and display the current time
# To see all report data, uncomment the line below
# print(report)
if report['class'] == 'TPV':
if hasattr(report, 'time'):
print(report.time)
except KeyError:
pass
except KeyboardInterrupt:
quit()
except StopIteration:
session = None
print("GPSD has terminated")

This should give you something similar to the following (with an update every second or so):

Looking for position data rather than just the timestamp? Essentially, all you have to do is parse the ‘report’ data following the example above.

To see what data is available, you can uncomment the print(report) line, and then just look at the different values and class names and pick and choose whatever you want.

For example, you could use the following code to get the current speed using the TPV class:

if report['class'] == 'TPV':
if hasattr(report, 'speed'):
print(report.speed * gps.MPS_TO_KPH)

That’s it! It’s pretty painless, and now it’s up to you to figure out what you want to do with you latitude, longitude, date and time, speed, altitude, etc.!

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