How SpaceTracker.live Works

SpaceTracker.live combines public orbital data, established propagation methods, observer geometry and NASA/JPL ephemerides to create educational tracking tools. The important point is that the website does not receive a continuous direct telemetry stream from the spacecraft. Most numbers you see are calculated from source data for a specific time.

1. Earth-orbit data: where the orbit starts

For Earth-orbiting objects, the server retrieves public general-perturbations orbital records from CelesTrak and caches them. These records contain the parameters needed by an SGP4/SDP4 propagator, together with an epoch describing when the fitted orbital state applies. The catalog also supplies identifiers such as the NORAD catalog number and international designator.

A NORAD ID is especially useful because spacecraft names can change or be similar. The Satellite Directory and catalog search therefore let you find objects by name or identifier before opening an individual tracker.

2. SGP4 turns orbital elements into a position at a chosen time

The browser uses the satellite.js implementation of SGP4/SDP4 to propagate the orbital record. Propagation means calculating where the model predicts the object should be at another time. The resulting Earth-centred position and velocity are converted into quantities that are easier to understand on a map, including latitude, longitude, altitude and speed.

The moving marker is therefore a continuously recalculated estimate. It is not a GPS signal from the satellite and not a live camera feed. If a fresher orbital record becomes available, the propagated position can shift because the calculation now starts from updated source data.

3. What the map actually represents

Latitude and longitude on a tracker represent the sub-satellite point: the point on Earth directly beneath the propagated spacecraft position. Altitude is the calculated height above Earth’s surface. The orbit line is a predicted ground track generated from the same model across a range of times.

Earth rotates beneath the orbital plane, so successive ground tracks normally move across longitude. Low-Earth-orbit spacecraft such as the ISS cross the map quickly, while geostationary spacecraft such as those in the GOES directory can appear almost fixed near one longitude.

4. Observer geometry: converting an orbit into your sky

A global satellite position does not tell you where to look from your own location. Once you set observer coordinates, the browser calculates the topocentric geometry between you and the propagated satellite. The key values are azimuth, elevation and range.

• Azimuth is the compass direction around the horizon.
• Elevation is the angular height above the geometric horizon; 0° is on the horizon and 90° is overhead.
• Range is the line-of-sight distance between the observer and the spacecraft.

The Satellites Above Me page uses the same idea to identify supported objects that are currently above a chosen minimum elevation.

5. How pass predictions are calculated

A geometric pass is a period during which the propagated spacecraft rises above a selected elevation threshold for your location. The pass routine searches forward in time, finds the rise, maximum-elevation and set portions of the event, and reports the corresponding local times and directions. The Satellite Pass Predictions page applies this method across supported catalog objects.

Your location matters: a pass visible geometrically from one city can be below the horizon from another. The age of the orbital record also matters because prediction error generally grows as the source elements become stale, especially for low-altitude or maneuvering spacecraft.

6. Geometric pass does not mean visible pass

Being above the mathematical horizon is only one requirement for actually seeing a satellite. Naked-eye visibility also depends on whether the spacecraft is illuminated by the Sun, whether your sky is dark enough, the object’s size and orientation, atmospheric transparency, cloud, light pollution and local obstructions.

This is why SpaceTracker.live labels ordinary pass calculations as geometric. The ISS viewing guide and azimuth/elevation guide explain how to turn the numbers into a practical observing plan.

7. Why deep-space missions use a different system

Voyager, JWST and interplanetary missions cannot be represented usefully as ordinary Earth satellites propagated from a near-Earth GP element set. SpaceTracker.live therefore keeps deep-space tracking separate and requests ephemeris information from NASA/JPL Horizons.

Horizons can provide time-dependent vectors and distances for supported spacecraft and Solar System bodies. The site caches these values server-side and presents quantities such as distance from Earth, distance from the Sun, one-way light time and speed where available. The three-dimensional visual scenes use compressed or illustrative scales because true interplanetary distances cannot be displayed usefully alongside planets and spacecraft at one simple linear scale.

8. Moon tools are calculations too

The Moon tools calculate phase, illumination and observer-relative geometry from astronomical models and user location where required. Moonrise and moonset are location-dependent horizon events, while the exact instant of full moon is a global astronomical event that can fall on different civil dates in different time zones.

9. Caching and source availability

Caching reduces repeated requests to public data providers and helps the site continue responding efficiently. It also means the newest provider record may not appear at the exact instant it is published. If an upstream service is unavailable, a page can temporarily show cached data, a waiting state or an unavailable message depending on the feature.

For current mission status, launches, spacecraft health, warnings or operational decisions, use the responsible agency or operator. SpaceTracker.live is designed for education, public awareness and general observing—not mission operations.

10. Location and browser storage

Device location is requested only after you choose Use My Location. You can also search for a city, town, village or place; the required coordinates are calculated automatically in the background. Observer settings and favorites may be stored in the browser so they can be reused across compatible tools. Read the Privacy Policy for information about browser storage, analytics, advertising and third-party services.

11. How to judge tracker accuracy

There is no single universal accuracy number for every object and every future time. Accuracy depends on the source-data quality, age of the element set or ephemeris, propagation model, orbital regime, maneuvers and how far the calculation is from the source epoch. A tracker is most useful when it clearly states what data it uses and what the displayed value represents.

For a deeper comparison of the two tracking systems, read TLE/SGP4 vs JPL Horizons and How Accurate Are Live Satellite Trackers?.

Primary technical references

• CelesTrak: General Perturbations data formats
• CelesTrak: NORAD orbital documentation
• NASA/JPL Horizons
• satellite.js project documentation

Educational-use limitation: Do not use SpaceTracker.live for navigation, collision avoidance, aviation, maritime operations, military decisions, emergency response or spacecraft operations.

Modern satellite catalogs also include six-digit NORAD catalog numbers. Traditional TLE files cannot represent these newer identifiers, so data providers such as CelesTrak offer newer GP data formats, including OMM-compatible JSON and CSV. Availability for individual satellites depends on the data formats supported by each tracking tool.