Next Passes: How to Predict Satellite Overheads

Knowing a satellite is up there is one thing. Knowing exactly when it will cross your sky is another. The Next Passes feature on SatFleet Live takes your GPS location, combines it with real-time TLE orbital data for over 14,000 satellites, and calculates precise predictions for every visible overhead pass — up to 7 days ahead.

This guide explains how satellite pass prediction works, how to read each data field in the results, and how to use SatFleet Live to plan your next ISS sighting, Starlink train observation, or military satellite spotting session.

What is a Satellite Pass?

A satellite pass is the window of time during which a satellite rises above your local horizon, travels across the sky, and sets again on the other side. For a LEO satellite like the ISS at 408 km altitude, a typical pass lasts between 4 and 10 minutes. Higher MEO satellites move more slowly and their passes last much longer, though they appear far dimmer.

Not every pass is visible to the naked eye. Three conditions must be met simultaneously for a satellite to be visually observable:

1. The satellite must be above your horizon SatFleet Live uses a minimum elevation of 10° to filter out passes too low to see clearly above buildings, trees, or haze.
2. It must be dark at your location The observer needs to be in twilight or full darkness — the Sun should be at least 6° below your horizon. Passes during daylight are invisible to the naked eye.
3. The satellite must be in sunlight Even when it is dark where you are, the satellite needs to still be catching sunlight at its higher altitude — reflecting it down toward you. This is why the best passes happen just after sunset or before sunrise.

SatFleet Live's Next Passes calculator checks all three conditions simultaneously for every satellite in its database, returning only the passes that are genuinely worth watching.

How Pass Prediction Works

Every satellite pass prediction on SatFleet Live is computed in real time in your browser using the SGP4 orbital propagation model — the same mathematical framework used by space agencies worldwide. Here is how it flows:

1. Your location is detected SatFleet Live requests your GPS coordinates. If you decline, it falls back to your last known position stored locally, or defaults to Madrid. Your coordinates never leave your device — all computation is local.
2. TLE data is loaded The latest Two-Line Element sets are fetched from the local TLE file (updated from CelesTrak every 48 hours). Each TLE encodes a satellite's orbital parameters at a reference time. Want to understand TLEs? Read our TLE guide.
3. SGP4 projects each satellite forward in time For each satellite, the algorithm steps through the next 1–7 days in 2-minute increments, computing position at each step and checking all three visibility conditions.
4. Visible windows are extracted When a satellite enters a visible window, the pass is recorded with its start time, peak elevation, direction, duration, and estimated brightness magnitude.
5. Results are sorted and displayed Passes are ranked by your chosen sort order — earliest first, brightest first, highest elevation, or longest duration — and shown as interactive cards.

How to Read the Pass Data

Each pass card shows four key fields. Here is a sample of what you will see — and exactly what each value means:

Start Time

The moment the satellite rises above 10° elevation and becomes potentially visible. Go outside a minute or two early to let your eyes adjust to the darkness and orient yourself in the right direction.

Max Elevation

The highest point the satellite reaches in your sky, measured in degrees from the horizon (0° = horizon, 90° = directly overhead). A pass above 40° is generally excellent — the satellite is high, bright, and clear of ground haze. Passes below 20° are harder to spot and often blocked by buildings or trees.

Direction

The compass path the satellite travels across your sky, shown as start → end (e.g. SW → NE). Face the starting direction a couple of minutes before the pass begins and scan slowly along the predicted arc.

Brightness (Magnitude)

Estimated apparent magnitude — the lower (more negative) the number, the brighter the satellite. This is the same scale used for stars:

Practical Example: Predicting an ISS Pass

The International Space Station is the most spectacular satellite you can observe. At magnitude −2 to −4 on a good pass, it outshines everything in the night sky except the Moon and Venus — a brilliant, steady point of white light moving silently but quickly overhead.

1. Open Next Passes and allow location access Go to SatFleet Live Next Passes. Allow the location prompt so predictions are accurate for your exact position — ISS passes can differ by several minutes just 50 km apart.
2. Set the filter to "Space Stations" This narrows results to the ISS, Tiangong CSS, and visiting crewed vehicles — the most rewarding passes to observe.
3. Calculate for 7 days ISS visible passes cluster in active windows of several days, then go quiet as the geometry shifts. 7 days gives you the best selection of passes to pick from.
4. Sort by "Brightest first" or "Highest elevation" These filters surface the most spectacular passes at the top. An ISS pass above 60° with magnitude below −2 is an unforgettable experience.
5. Hit "Show on Map" just before the pass The button opens the live map with the ISS highlighted. Watch its real-time position approach your horizon — then step outside and look up.

Practical Example: Spotting Starlink Trains

Shortly after a Starlink launch, the newly deployed satellites travel in a tight formation called a train — a chain of bright dots crossing the sky in single file, one every few seconds, for up to 15 minutes of continuous spectacle. It is one of the most striking things you can observe with the naked eye and consistently goes viral on social media when people see it for the first time.

Fresh Starlink satellites are significantly brighter than operational ones. The window to see a train is typically the first 1–2 weeks after launch, before the satellites raise their orbits and orient their solar panels edge-on to Earth.

1. Select "Starlink / Internet" from the filter This category contains thousands of satellites so the calculation takes a couple of minutes. The ‼️ warning in the dropdown is there for exactly this reason — be patient.
2. Sort by "Earliest first" and look for clusters A Starlink train appears as many consecutive pass cards from STARLINK-XXXX satellites all starting within a few minutes of each other and travelling in the same direction. That cluster is your train.
3. Go outside early and scan the predicted direction Once you spot the first dot, the rest of the train will follow along exactly the same track, roughly evenly spaced. A fresh train can have 20–60 visible satellites in a single pass.

Tips for a Successful Observation

Frequently Asked Questions