What Is a Satellite Orbit and How Does It Enable Tracking and Pass Prediction?

Updated

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:

Element What it controls Why it matters for tracking
Inclination Tilt of the orbit relative to the equator Determines the maximum latitude the satellite reaches; a satellite at 51.6° inclination never passes over the poles
Altitude (semi-major axis) Size of the orbit Sets the orbital period and how fast the satellite appears to move
Eccentricity How circular vs. elliptical the orbit is Affects whether the satellite spends more time over certain regions
Period Time for one full revolution Directly determines how often passes occur
Right ascension of the ascending node Where the orbit crosses the equator Shifts over time due to Earth's oblateness, changing pass timing

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.