Satellite Tracking Basics: How Real-Time Orbital Tracking Works

Every satellite circling Earth follows a predictable path governed by gravity and orbital mechanics. By knowing a satellite's current position and velocity, we can compute where it will be at any point in the near future. This is the foundation of real-time satellite tracking, and it is how Track The Sky places thousands of objects on an interactive 3D globe.

TLE and OMM: The Data Behind Tracking

The United States Space Command tracks objects in orbit and publishes their orbital parameters through NORAD. These parameters are distributed in two standard formats: Two-Line Element sets (TLEs) and the newer Orbital Mean-elements Message (OMM) in JSON. Both formats encode six key values known as Keplerian elements that define the shape, orientation, and timing of an orbit. CelesTrak, maintained by Dr. T.S. Kelso, is the primary public source for this data, and Track The Sky refreshes its catalog from CelesTrak every two hours.

SGP4: Predicting Satellite Positions

Raw orbital elements describe an orbit at a specific moment in time called the epoch. To find out where a satellite is right now, a mathematical model called SGP4 (Simplified General Perturbations 4) propagates those elements forward in time. SGP4 accounts for Earth's oblateness, atmospheric drag on low-orbit objects, and gravitational effects from the sun and moon. For most satellites below geostationary altitude, SGP4 provides positional accuracy within a few kilometers over prediction windows of a day or two.

From Math to Map: How Track The Sky Works

Track The Sky uses the open-source satellite.js library to run SGP4 propagation in your browser. For each satellite in the catalog, the app computes its Earth-Centered Inertial (ECI) position, converts that to Earth-Centered Earth-Fixed (ECEF) coordinates, and maps the result to latitude, longitude, and altitude on a CesiumJS 3D globe. This calculation runs every second, producing smooth real-time movement for over 9,000 objects simultaneously.

Key Orbital Parameters Explained

• Inclination: The tilt of the orbit relative to the equator. The ISS has an inclination of about 51.6 degrees.
• Eccentricity: How circular (0) or elliptical (closer to 1) the orbit is. Most LEO satellites have near-circular orbits.
• Period: The time to complete one orbit. LEO satellites take roughly 90 minutes; geostationary satellites take 24 hours.
• Apogee and Perigee: The highest and lowest points of the orbit above Earth's surface.

Accuracy and Limitations

SGP4 is reliable for short-term predictions, but accuracy degrades over time because TLE data is a snapshot. Atmospheric drag, solar radiation pressure, and maneuvers can shift a satellite from its predicted path. This is why Track The Sky updates its orbital data every two hours, ensuring predictions stay current. For precise conjunction analysis or reentry forecasting, specialized agencies use higher-fidelity models, but SGP4 remains the standard for publicly available tracking.

Explore these concepts in action by opening Track The Sky and clicking on any satellite to view its orbital elements, ground track, and real-time coordinates.