How to Feed NMEA Data to gpsfake for Testing with gpsd

If you’re working with GPS data, testing your software with real-world data can be challenging. Thankfully, tools like gpsfake and gpsd make it possible to simulate GPS data. gpsfake allows you to replay NMEA data streams, while gpsd acts as the GPS daemon to manage and process the GPS data. This guide walks you through the steps to feed NMEA data to gpsfake and use it effectively with gpsd.

Prerequisites

Before proceeding, ensure the following tools are installed on your system:

1. gpsd: The GPS daemon that handles GPS data.
2. gpsfake: A tool to simulate GPS data for testing purposes.
3. NMEA Data File: You’ll need a file containing GPS data in NMEA format. If you don’t have one, you can find sample files online or record your own using GPS hardware and gpspipe.

You can install these tools on Linux with:

sudo apt update
sudo apt install gpsd gpsd-clients python3-gps

Step 1: Prepare Your NMEA Data

Ensure your NMEA file is formatted correctly. It should look something like this:

$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47
$GPRMC,123519,A,4807.038,N,01131.000,E,022.4,084.4,230394,003.1,W*6A

You can name the file something like testdata.nmea.

Step 2: Start the gpsd Daemon

First, ensure that gpsd is not already running:

sudo systemctl stop gpsd.socket
gpsd.service

Now, start gpsd manually to avoid conflicts:

sudo gpsd -N -D5 /dev/null

• The -N flag keeps gpsd running in the foreground.
• The -D5 flag sets the debug level to 5 for detailed logs.
• /dev/null is a placeholder since we’ll be feeding data from gpsfake.

Step 3: Feed NMEA Data to gpsfake

Run gpsfake with your NMEA file:

gpsfake -D 5 -c testdata.nmea

• -D 5: Sets the debug level.
• -c: Feeds the data to gpsd in real-time.

gpsfake will replay the NMEA sentences, simulating a live GPS device.

Step 4: Verify the Data with gpsd

To ensure that gpsd is receiving the simulated data, use tools like cgps or gpsmon:

• Run cgps to view the data:

cgps

• You should see the coordinates, satellite information, and other data.
• Alternatively, use gpsmon for detailed NMEA sentence analysis:

gpsmon

Both tools should display the data as if it were coming from a real GPS device.

Troubleshooting

• Ensure Correct File Permissions: Make sure your NMEA file is readable:

chmod +r testdata.nmea

• Check for Conflicting Services: Ensure no other instances of gpsd are running.

ps aux | grep gpsd
sudo kill <PID>

• Verify NMEA Data Integrity: Ensure your NMEA file contains valid sentences. Use tools like gpsbabel to validate:

gpsbabel -i nmea -f testdata.nmea -o nmea -F /dev/null

By combining gpsfake and gpsd, you can simulate GPS data streams for testing and debugging GPS-enabled applications. This setup is particularly useful for software development and testing in environments where real GPS hardware isn’t available or practical.

Now that you have your simulated environment ready, you can focus on building and testing your GPS-based applications with ease.