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Satellite tracking software predicts where a satellite is at any moment and when it will pass within range of your location, so you can point an antenna, schedule a contact, or just watch it cross the sky. The core inputs are simple: your observer coordinates and a current set of orbital data (TLEs). Gpredict is a free, real-time satellite tracking and orbit prediction program for Linux, Mac OS X, and Windows, and it is a useful concrete example of how these tools work. This explainer covers the mechanism, the setup steps, and the problems that most often break a tracking session.

What satellite tracking actually means

Tracking has two related meanings, and it helps to keep them apart:

• Position tracking — knowing where a satellite is right now, usually shown on a world map with its ground track and footprint (the area of Earth from which it is visible).
• Pass prediction — knowing when that satellite will rise above your local horizon, how high it will get (maximum elevation), and when it will set.

For radio work, a third layer matters: Doppler shift. As a satellite moves toward you and then away, the frequency you hear drifts. Tracking software calculates this offset so you can retune during a pass.

Why orbit data (TLEs) drives everything

Tracking software does not measure the satellite directly. It propagates an orbital model forward in time. The standard input is a TLE — a Two-Line Element set, a compact text description of a satellite's orbit at a given epoch.

Key consequences:

• TLEs go stale. Atmospheric drag and other forces make the real orbit drift from the prediction. A TLE a few days old may be fine for a rough look; for precise antenna pointing or Doppler tuning, fresher data is better.
• You need a current source. Most trackers can download TLEs automatically from a catalog such as Celestrak or Space-Track. The update interval is a setting worth checking.
• The epoch matters. If your software is using an old element set, your pass times will be off even if your clock and coordinates are perfect.

Core features to expect

Setting up a tracker like Gpredict

The general workflow is the same across most tracking programs:

1. Set your observer location. Enter latitude, longitude, and altitude. This is the single most common source of wrong results — a sign error or a swapped lat/long puts you on the other side of the planet.
2. Check your clock and time zone. Predictions are time-sensitive. Make sure the system clock is accurate and the time zone is correct, especially if you compare pass times with other people.
3. Load or download TLEs. Add the satellites you want and point the software at a current TLE source, or import a file you downloaded.
4. Refresh the elements. Set an update interval, or manually re-download before an important session.
5. Read the pass list. Look at start time, maximum elevation, and duration. Higher maximum elevation generally means a better, longer contact.
6. Optionally connect hardware. If you have a rotator or radio, configure the interface so the software can point the antenna and apply Doppler correction.

Expected result: a list of upcoming passes for your location, and a map showing the satellite's current position and footprint.

Common problems and how to spot them

• Stale TLEs — pass times drift by minutes. Symptom: the satellite is not where the software says. Fix: refresh the elements.
• Wrong observer coordinates — every pass looks wrong or the satellite never rises. Symptom: predictions that make no sense for your area. Fix: re-check latitude, longitude, and sign conventions.
• Time-zone or clock errors — pass times are offset by whole hours. Symptom: consistent shift between your software and a reference. Fix: verify system time and zone.
• Confusing UTC with local time — most tracking tools display UTC. Symptom: you show up an hour or more early or late. Fix: convert deliberately.
• Elevation mask too high or low — you either miss low passes or get unusable ones. Fix: adjust the minimum elevation setting to match your horizon and antenna.

A note on the source

The site at oz9aec.dk, which describes Gpredict as free, real-time satellite tracking and orbit prediction software for Linux, Mac OS X, and Windows, currently shows only an "under construction" page. Treat the description as the project's stated purpose rather than a live download or documentation source; for current files and instructions, look for the project's own distribution channel.

A satellite orbit is the curved path a satellite follows around a central body—usually Earth—under the influence of gravity. That path is predictable because it is governed by well-understood physics: once you know a satellite's current position and velocity (or an equivalent set of orbital elements), you can calculate where it will be at any future time. Tracking and pass prediction are simply the practical application of that predictability—converting an orbit description into "when and where will this satellite appear in my sky?" This article explains the mechanics behind that conversion so you can understand what tracking software is actually doing, and what limits its accuracy.

Orbit vs. trajectory: what's the difference?

An orbit is a repeating, gravitationally bound path. A trajectory is the more general term for any path through space, including non-repeating ones (a rocket launch, an escape trajectory, a re-entry).

For most satellites, the orbit is close enough to periodic that you can describe it with a small set of numbers and predict passes days or weeks ahead. That periodicity is what makes tracking software possible.

The orbital elements that determine everything

A satellite's orbit is typically described by a set of parameters called orbital elements. The most important for tracking purposes:

You don't need to memorize these to use tracking software, but understanding them explains why two satellites behave so differently. A satellite at 400 km altitude with 51° inclination (like the ISS) passes overhead frequently but briefly. A satellite at 35,786 km altitude with 0° inclination (geostationary) never appears to move relative to the ground.

Common orbit types and what they mean for visibility

• LEO (Low Earth Orbit, roughly 160–2,000 km): Fast-moving, frequent passes, short visibility windows (typically 5–15 minutes). Most tracking hobbyists observe LEO satellites.
• MEO (Medium Earth Orbit, roughly 2,000–35,786 km): Slower apparent motion, longer passes, used by navigation constellations like GPS.
• GEO (Geostationary Orbit, ~35,786 km): Appears fixed over one point on the equator. No "passes" in the usual sense—it's either visible from your location or it isn't.
• Polar orbit: High inclination (~90°), covers the entire globe as Earth rotates underneath.
• Sun-synchronous orbit: A special polar orbit where the satellite crosses each latitude at roughly the same local solar time—useful for imaging because lighting is consistent.

The orbit type determines whether "tracking" means watching for a brief overhead pass or simply pointing at a fixed spot.

How orbital data becomes a pass prediction

The workflow from orbit to usable prediction has four steps:

1. Get the orbital elements. For most satellites, this comes as a TLE (Two-Line Element set)—a standardized text format published by organizations like NORAD/Celestrak. A TLE encodes the orbital elements at a specific epoch (timestamp).

2. Propagate the orbit. Software takes the TLE and mathematically advances the satellite's position forward in time. The simplest method uses Keplerian mechanics (treating the orbit as a clean ellipse), but real-world propagation models like SGP4 also account for perturbations such as atmospheric drag and Earth's non-spherical shape.

3. Compute look angles. For your specific location (latitude, longitude, altitude), the software calculates azimuth (compass direction) and elevation (angle above the horizon) for the satellite at each moment. A pass begins when elevation rises above your minimum threshold (often 0° or 10°) and ends when it drops back below.

4. Plan the observation. The output is a list of passes with start time, peak elevation, duration, and direction. You use this to point an antenna, aim a camera, or schedule a radio contact.

Why predictions drift: common sources of error

• Stale TLEs. A TLE is only accurate near its epoch. For LEO satellites, atmospheric drag changes the orbit continuously; a TLE more than a few days old can produce pass-time errors of minutes. GEO satellites are far more stable.
• Atmospheric drag. Stronger during high solar activity, it causes LEO satellites to gradually lose altitude and eventually re-enter.
• Propagation model limits. Simplified models ignore some perturbations; higher-fidelity models reduce but don't eliminate error.
• Your location accuracy. Wrong coordinates or altitude shift predicted pass times and peak elevations.

The practical takeaway: refresh your TLE data regularly, and treat predictions for LEO satellites as approximate beyond a few days.

What this means for choosing tracking software

Any tracking tool—whether it's Gpredict (a free, open-source tracker for Linux, macOS, and Windows) or another package—is doing the same four steps above. When evaluating one, the meaningful questions are:

• How does it obtain and update TLE data?
• Which propagation model does it use?
• How does it handle your observer location and minimum elevation?
• Does it present passes in a form you can act on (timing, azimuth/elevation, visual map)?

The orbit mechanics don't change between tools; the differences are in data freshness, model fidelity, and usability.

Satellite orbit prediction is the process of calculating where a satellite will be at a future time, using a mathematical model of its orbit plus a set of orbital parameters measured at a known reference time. It is what lets tracking software tell you, in advance, when a satellite will rise above your horizon, how high it will get, and which direction to point an antenna. Live tracking, by contrast, only tells you where the satellite is right now. Prediction is what turns a current position into a usable schedule.

Prediction vs. live tracking

These two are often confused because they appear in the same software window.

Most tracking programs do both: they predict a pass, then use the same model in real time to keep the antenna pointed as the pass unfolds.

The inputs a prediction needs

Prediction is only as good as the orbit description you feed it.

Two-line elements (TLEs)

A TLE is a compact, standardized text format that encodes a satellite's orbital parameters at a specific moment. It contains the mean motion, inclination, eccentricity, argument of perigee, right ascension of the ascending node, mean anomaly, and a drag term. The moment the data refers to is the epoch, printed in the first line.

Keplerian elements

These are the classical six parameters that describe an ellipse in space: semi-major axis, eccentricity, inclination, right ascension of the ascending node, argument of perigee, and true (or mean) anomaly. A TLE is essentially a packaged, epoch-stamped version of these plus drag and mean-motion derivatives.

Epoch and its age

The epoch is the timestamp the elements are valid for. As time passes beyond the epoch, the prediction degrades, because the real orbit drifts from the model. This is why TLE age matters more than almost anything else for accuracy.

How predictions become pass forecasts

Once the software has the elements and a propagation model, it computes the satellite's position in an Earth-centered frame at each future time step. It then converts that to a topocentric frame relative to your station's latitude, longitude, and altitude.

From there it derives:

• Azimuth — the compass bearing to look toward
• Elevation — the angle above the horizon
• Range — distance to the satellite
• Doppler shift — the frequency offset caused by relative motion

A pass is simply the interval during which elevation stays above your minimum threshold (often 0° or a few degrees to avoid terrain). Within that interval the software reports acquisition of signal (AOS), the time of maximum elevation (TCA), and loss of signal (LOS). Those three numbers are the core of any pass forecast.

Why predictions go wrong

Errors come from a few well-understood sources:

• TLE age. The older the epoch relative to your prediction time, the larger the position error. Low-Earth-orbit satellites decay fastest in accuracy.
• Atmospheric drag. The drag term in a TLE is a fit, not a physical constant. It changes with solar activity, so predictions drift during high solar flux.
• Simplified models. SGP4/SDP4 are the standard propagators for TLEs and are accurate enough for most amateur and operational tracking, but they are not full numerical integrations.
• Station coordinates. A wrong latitude, longitude, or altitude shifts every predicted pass time.
• Maneuvers. If the satellite fires thrusters, the TLE no longer describes its orbit until a fresh one is published.

Practical rule: refresh TLEs before any critical pass, and treat predictions more than a few days out as approximate.

How prediction drives tracking software

In a typical workflow, prediction and tracking are two stages of one loop:

1. Load a current TLE for the target satellite.
2. Enter your station coordinates.
3. Let the software compute upcoming passes and pick one.
4. At AOS, the same propagator runs in real time, converting the predicted position into live azimuth/elevation commands.
5. Those commands drive a rotator, or guide you manually, while Doppler correction tunes the radio.

This is the mechanism behind tools like Gpredict, which is described as free, real-time satellite tracking and orbit prediction software for Linux, Mac OS X, and Windows. Note that the site at oz9aec.dk currently shows only an "under construction" placeholder, so treat that description as the project's stated purpose rather than something you can verify from the page itself.

What to check before trusting a forecast

• Is the TLE epoch recent relative to your pass?
• Are your station coordinates correct to within a few kilometers?
• Is your minimum elevation threshold realistic for your horizon and obstructions?
• Has the satellite performed a maneuver since the last TLE?

If all four are satisfied, a pass forecast is usually good enough to schedule a contact and pre-point an antenna. If any are not, expect the satellite to arrive early, late, or lower than predicted.

Gpredict is a free, real-time satellite tracking and orbit prediction application for Linux, Mac OS X, and Windows. It is designed to show where satellites are now and when they will pass over your location, so it suits anyone who wants to follow amateur radio satellites, weather satellites, or other tracked objects without paying for software. The official site at oz9aec.dk is currently under construction, so the software itself is best obtained from its established distribution channels rather than from that page.

What Gpredict does

Gpredict combines two related jobs:

• Real-time tracking — it continuously calculates and displays satellite positions as they move.
• Orbit prediction — it projects future positions so you can see upcoming passes over your ground station.

This makes it useful for planning observations, radio contacts, or antenna pointing, because you can look ahead instead of only watching the current position.

Supported platforms

Gpredict runs on the three major desktop platforms named by the project:

Because the project website is under construction, use your operating system's package manager or the project's established download locations to get the current version.

How to track satellites with Gpredict

1. Install the application

Install Gpredict using the method appropriate to your platform. On Linux this is typically a package manager; on Windows and Mac OS X it is a downloaded build. The expected result is that Gpredict launches and shows a default ground station and module layout.

2. Set your ground station

Your location determines every pass prediction. Enter your latitude, longitude, and altitude so Gpredict can calculate when satellites rise above your horizon. If your position is wrong, every pass time will be wrong.

3. Add satellites and update orbit data

Satellites are tracked using TLE (Two-Line Element) data, which describes their orbits. You need to:

• Add the satellites you care about to a module.
• Update the TLE data regularly, because orbits change over time.

Stale TLE data is the most common reason predictions drift or look wrong. Refresh it before relying on any pass.

4. Read the tracking and prediction views

Once configured, Gpredict provides:

• A map view showing satellite positions and ground tracks.
• Pass predictions listing when a satellite rises, reaches maximum elevation, and sets for your location.
• Tracking displays for following a selected satellite in real time.

Use the pass list to plan ahead, and the map and tracking views to follow the satellite as the pass happens.

Common setup issues

• Wrong pass times — almost always a ground station location error or outdated TLE data.
• Satellite missing from predictions — the satellite may not be added to the active module, or its TLE may not have been updated.
• No passes shown — check that your location is set and that the satellite's orbit actually passes over your latitude.

Where to get help

Since the official site is under construction, look for the project's documentation, user community, and distribution channels for installation help and troubleshooting. The application's own help resources and the amateur satellite community are the practical places to resolve setup problems.