Satellite Pass Tracker

Introduction:

A satellite pass is the short part of an orbit when a spacecraft, rocket body, or other cataloged object is above an observer's local horizon. The same object may cross high overhead for one person, skim the horizon for another, and remain below the horizon for someone a few thousand kilometers away. Pass planning is therefore a local-sky problem, not just an orbit problem.

Three facts drive every useful forecast: the object's orbit, the observer's position on Earth, and the time range being searched. The orbit is commonly described by a Two-Line Element set, or TLE. A TLE is a compact set of orbital numbers tied to a specific epoch, which is the moment when those numbers were valid. The observer position supplies latitude, longitude, and height above sea level so the calculation can turn an Earth-centered orbit into what a person or antenna would see from the ground.

Good pass lists answer more than "when does it rise?" A visual observer wants enough elevation to clear rooftops and trees, a radio operator wants a bearing sweep that the antenna can follow, and a scheduler wants a window long enough to prepare before the object sets. The pass path also matters. A north-to-south crossing may be clear from a backyard with an open eastern horizon and useless from a balcony blocked in that direction.

Peak elevation is the quickest pass-quality clue. Zero degrees is the mathematical horizon and 90 degrees is straight overhead. A pass that peaks at 65 degrees usually gives a longer, cleaner view than a pass that peaks at 8 degrees, but the number is not a promise. Terrain, buildings, trees, haze, antenna gain, radio activity, and lighting can still decide whether the event is useful.

TLE forecasts are predictions from an orbital model. They are good enough for many observing and planning tasks, especially with recent element sets, but accuracy degrades as the TLE ages or after maneuvers, drag changes, reboosts, deployments, and catalog updates. For repeatable comparisons, keep the same TLE, observer coordinates, elevation filter, and start time.

How to Use This Tool:

Start by choosing the satellite, then confirm the observer location before widening or narrowing the forecast window.

1. Use Satellite search for a name or NORAD catalog number, or choose a Satellite preset for common targets such as ISS, Hubble, Tiangong, NOAA 15, NOAA 18, or NOAA 19. A search returns selectable matches with names, NORAD IDs, and TLE epochs.
2. Turn on Manual TLE when you want to paste a saved element set. Manual mode accepts the two numbered TLE lines, with an optional satellite name line above them, and skips live satellite lookup.
3. Choose the Observer source. IP Geolocation is approximate and follows the public network location. City Search uses submitted mapped-place results and built-in city shortcuts. Browser GPS asks the browser for a location fix. Manual Coordinates suit cases where you know the exact observing site.
   Switch to Manual Coordinates after a City Search or GPS lookup when you need to refine the site to an antenna, balcony, field, or roof position.
4. Check latitude, longitude, and altitude. Latitude must be from -90 to 90, longitude from -180 to 180, and observer altitude is bounded from -500 m to 9000 m.
5. Set Lookahead from 1 to 168 hours. A short window is useful for tonight's plan, while a longer window helps sparse targets or strict elevation filters.
6. Open Advanced when you need a fixed Start time, a higher Min elevation, a different Step size, or a smaller Max passes list. Leaving Start time blank begins from the current local clock.
7. Click Compute passes. Review the summary, choose a row in Pass Forecast Ledger, inspect the elevation chart when needed, and export CSV, DOCX, JSON, chart images, chart CSV, or Calendar ICS after the result looks plausible.
   If the summary says no passes were found, lengthen Lookahead, lower Min elevation, confirm the observer coordinates, or select the intended NORAD ID before computing again.

Interpreting Results:

The Next Pass summary reports the first qualifying pass after your selected start time. It shows the rise time, maximum elevation, duration, rough direction sweep, selected satellite, observer coordinates, location source, lookahead window, and minimum elevation filter.

Rise, Max, and Set form the pass timeline. Rise is when the object crosses upward through the mathematical horizon, Max is when it reaches its highest elevation in that pass, and Set is when it crosses downward through the horizon. Duration is the rounded time between rise and set.

Max elev is the strongest sorting clue. Treat 10 to 20 degrees as a common practical clearance filter, not as a universal rule. A high pass is easier to see or track from many sites, but a low pass can still be useful when the horizon is open and the satellite or radio link is strong enough.

Rise az and Set az are compass bearings at the start and end of the pass. The labels use degrees plus nearest cardinal or intercardinal direction, such as 315 deg NW. They do not describe every curve in the pass path. Use the sky-arc visual and the Pass Elevation Profile chart for a better sense of the selected pass shape.

A listed pass is not the same as a confirmed sighting or communication opportunity. Visual brightness depends on sunlight, darkness at the observer, and the satellite's reflective properties. Radio usefulness depends on transmitter status, frequency plan, antenna pattern, polarization, Doppler handling, noise, and local rules. Keep one backup pass when the leading row is near the horizon or in a blocked direction.

Technical Details:

The calculation is topocentric: it answers where the satellite appears from the observer, not where the satellite is in an Earth-centered catalog view. After a TLE is available, the orbit is propagated through the selected time window with an SGP4-compatible model. Each sampled satellite position is converted into local look angles for the observer: elevation above the horizon, azimuth clockwise from north, and range used internally by the look-angle conversion.

The search walks forward in time using the selected step size. A pass begins when elevation changes from below 0 degrees to at least 0 degrees and ends when it crosses back below 0 degrees. Rise and set are refined inside their crossing brackets, and the maximum elevation is refined by a denser scan over the pass interval. Passes shorter than 30 seconds are discarded before the minimum-elevation filter is applied.

Forecast Inputs and Effects

Rule Core

1. Parse the selected TLE or the pasted manual TLE.
2. Clamp observer coordinates, altitude, lookahead, minimum elevation, step size, and pass count to supported bounds.
3. Choose the scan start from Start time, or from the current local time when Start time is blank.
4. Propagate the satellite at each coarse step and convert each sample to observer-relative elevation and azimuth.
5. Detect upward and downward horizon crossings at 0 degrees elevation.
6. Refine the rise and set crossing times, scan the pass interval for the highest elevation, discard very short passes, filter by minimum elevation, sort by rise time, and keep the requested number of rows.

Formula Core

Duration and compass labels are deterministic display calculations built from the refined pass times and look angles.

The cardinal index maps clockwise through N, NE, E, SE, S, SW, W, and NW. A raw azimuth of 315 degrees therefore becomes NW, while 359 degrees rounds back to N.

The elevation chart resamples the selected pass from rise to set and plots elevation against local time. It also marks the mathematical horizon and the selected minimum-elevation threshold, so a pass that barely clears the filter stands apart from a pass that climbs high above it.

Precision should be read in context. A newer TLE can shift the result, and an older TLE can drift, especially for low Earth orbit objects affected by drag or maneuvers. Public search data can also contain similar names or multiple catalog entries. For important observing sessions, select by NORAD ID when possible, keep the TLE epoch visible in your notes, and recheck near the actual pass time.

Privacy and Data Notes:

Satellite lookup sends the search text or NORAD ID to the public TLE data service used by the page. Manual TLE mode skips that lookup and uses the pasted lines for prediction.

Location behavior depends on the observer source. IP Geolocation detects the public IP address and looks up an approximate network location. City Search sends the submitted place text to OpenStreetMap Nominatim and applies one returned coordinate. Browser GPS uses the browser permission flow and may include an accuracy estimate. Manual Coordinates keep location entry under the user's control.

The pass calculation, charts, JSON, CSV, DOCX, and ICS text are produced in the browser after the needed TLE and observer coordinates are available. Exports include observer source details so a saved forecast can be audited later.

Advanced Tips:

• Use Manual TLE and a fixed Start time when you need to compare forecasts across repeated runs without mixing in a refreshed element epoch.
• Raise Min elevation for sites with trees, walls, ridgelines, or antenna clearance limits. A 20 degree filter can be more realistic than a 0 degree mathematical horizon.
• Keep Step size at 20 to 60 seconds for routine scans, then reduce it only when a short pass or strict elevation cutoff needs closer sampling.
• Select satellite rows by NORAD ID when names are similar. Catalog names, mission aliases, and debris entries can look alike in broad searches.
• Use the Calendar ICS only after checking the selected row's rise, maximum, and set times. The calendar event records a planning window, not a guarantee of visibility or radio contact.

Worked Examples:

FAQ:

Glossary:

TLE

A Two-Line Element set describing an Earth-orbiting object's mean orbital elements at a specific epoch.

NORAD ID

The catalog number used to identify a tracked artificial Earth-orbiting object.

Epoch

The reference time at which the TLE's orbital values apply.

Elevation

The angle above the local horizon, where 0 degrees is the horizon and 90 degrees is overhead.

Azimuth

The compass bearing to the object, measured clockwise from north.

Rise

The upward horizon crossing that starts a pass.

Set

The downward horizon crossing that ends a pass.

Maximum elevation

The highest elevation reached during a pass.

References:

• A New Way to Obtain GP Data, CelesTrak.
• NORAD Two-Line Element Set Format, CelesTrak.
• Frequently Asked Questions: Two-Line Element Set Format, CelesTrak.
• Orbital Coordinate Systems, Part II, CelesTrak.
• Spot the Station, NASA.
• OpenStreetMap copyright and license, OpenStreetMap contributors.