How Satellite Tracking Works: TLEs and SGP4

Every day, thousands of satellites orbit Earth, from the International Space Station to Starlink internet satellites. But how do we know where they are at any given moment?

Two-Line Element Sets (TLEs)

TLEs are a standardized format for describing satellite orbits. Despite being developed in the 1960s, they remain the primary way orbital data is shared.

A TLE looks like this:

ISS (ZARYA)
1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9993
2 25544  51.6400 123.4567 0007890  90.1234 270.1234 15.49000000000000

What’s in a TLE?

Line 1 contains:

• NORAD Catalog Number (25544 for ISS)
• Classification (U = Unclassified)
• International Designator (98067A = launched in 1998)
• Epoch time (when the TLE was valid)
• Drag terms (B* coefficient)

Line 2 contains the orbital elements:

• Inclination: Angle from the equator (51.64° for ISS)
• RAAN: Where the orbit crosses the equator going north
• Eccentricity: How elliptical the orbit is (nearly circular for ISS)
• Argument of Perigee: Orientation within the orbital plane
• Mean Anomaly: Position along the orbit at epoch
• Mean Motion: Orbital revolutions per day (15.49 for ISS = ~92 min orbit)

SGP4: Predicting Satellite Positions

TLEs don’t tell you where a satellite is right now—they describe the orbit at a specific moment (the epoch). To find the current position, we use SGP4 (Simplified General Perturbations 4).

SGP4 accounts for:

• Earth’s non-spherical gravity (J2 oblateness)
• Atmospheric drag (for low orbits)
• Solar and lunar gravitational effects
• Other perturbations

The algorithm takes a TLE and a time, then outputs position and velocity in the TEME (True Equator, Mean Equinox) coordinate frame.

Reference Frames Explained

Satellite positions are expressed in different reference frames:

ECI (Earth-Centered Inertial)

• Origin at Earth’s center
• Fixed relative to distant stars
• X-axis points to vernal equinox
• Used for orbital calculations

ECEF (Earth-Centered Earth-Fixed)

• Origin at Earth’s center
• Rotates with Earth
• X-axis points to 0° longitude
• Used for ground-based applications

Geodetic

• Latitude, Longitude, Altitude
• What we typically use for maps
• Referenced to WGS84 ellipsoid

Our Reference Frame Converter lets you transform between these frames.

TLE Accuracy and Limitations

TLEs are predictions, and they degrade over time:

Orbit Type	Accuracy After 1 Day	Accuracy After 7 Days
LEO (ISS)	~1-2 km	~5-10 km
MEO	~100 m	~1 km
GEO	~1-2 km	~5 km

Factors affecting accuracy:

• Solar activity: Affects atmospheric density
• Space weather: Geomagnetic storms
• Maneuvers: Satellites actively change orbits
• Age of TLE: Older = less accurate

Getting TLE Data

TLEs are published by:

1. Space-Track.org: Official US source (requires account)
2. CelesTrak: Aggregates and redistributes TLEs
3. Our Satellite Tracker: Updated daily from authoritative sources

Practical Applications

• Amateur radio: Tracking communication satellites
• Astronomy: Avoiding satellites in observations
• Photography: Capturing ISS passes
• Aviation: Space situational awareness
• Research: Studying orbital debris

Try It Yourself

1. Use our Orbit Viewer to decode any TLE or OMM
2. Track satellites in real-time with our Satellite Ground Track
3. Convert between reference frames with the Frame Converter

Further Reading

• NASA Orbital Mechanics
• CelesTrak: TLE Formats
• SGP4 Algorithm Paper