What Is Gpredict and Why Use It?

Gpredict is a free, open-source real-time satellite tracking application that runs on Linux, macOS, and Windows. It uses TLE (Two-Line Element) data to calculate satellite positions and predict when a satellite will pass over your location. Whether you're trying to catch an ISS pass or listen in on a weather satellite, Gpredict is one of the most accessible tools available to amateur observers.

Installing Gpredict

Gpredict is available from its official repository and most Linux package managers. On Ubuntu or Debian:

sudo apt-get install gpredict

On Windows and macOS, download the installer from the official Gpredict website. Installation is straightforward — no special dependencies are required for basic use.

Setting Your Ground Station Location

Before you can generate accurate pass predictions, you need to tell Gpredict where you are:

  1. Open Edit → Preferences → Ground Stations.
  2. Click Add New and enter your location name.
  3. Input your latitude and longitude (use a GPS app or map service to find these).
  4. Set your altitude above sea level in meters.
  5. Click OK and make this your default station.

Accuracy matters here — even a few kilometers of error can shift pass windows by a minute or more.

Updating TLE Data

TLE (Two-Line Element) sets are the mathematical descriptions of a satellite's orbit. They decay in accuracy over time, so you should refresh them regularly — at least weekly for active satellites.

  • Go to Edit → Update TLE Data from Network.
  • Gpredict will fetch the latest sets from Celestrak or Space-Track automatically.
  • For the ISS, use the stations or visual TLE catalog.

Predicting a Satellite Pass

Once your location and TLEs are set:

  1. Select a satellite module (e.g., Amateur or ISS).
  2. Right-click a satellite in the list and choose Predict future passes.
  3. Gpredict will display a table of upcoming pass windows including AOS (Acquisition of Signal), TCA (Time of Closest Approach), and LOS (Loss of Signal) times.
  4. Note the maximum elevation — passes above 40° are considered excellent; anything above 10° is generally workable.

Understanding the Polar Map View

The polar map shows the satellite's path across your sky dome. The center is your zenith (directly overhead), and the outer ring is the horizon. The arc traced across the map shows exactly where in the sky to point your antenna or eyes. Color coding indicates signal strength windows — useful when planning antenna rotator commands.

Tips for Better Predictions

  • Update TLEs before any important session — old data means shifted pass times.
  • Filter by minimum elevation to only show useful passes (avoid sub-10° passes when starting out).
  • Use UTC times consistently to avoid timezone confusion, especially when coordinating with others.
  • Cross-check with Heavens-Above for a second opinion on high-value passes like the ISS.

Next Steps

Once you're comfortable generating predictions in Gpredict, consider connecting it to a software-defined radio (SDR) to automatically tune your receiver as the satellite's Doppler shift changes. Gpredict supports rig control via Hamlib, enabling fully automated tracking sessions. That's where the real fun begins.