Computing passes…
🔔 Push notifications
The forecast is re-checked whenever you open the app, whenever an alert arrives and, if installed, in the background. A shorter window means fresher weather in your alerts.
What counts as a notification-worthy pass?
Notification timing
ISS Pass Tracker
Know when to look up. Version
What it does
- Shows where the International Space Station is right now, and where to look from your location.
- Predicts the passes you can actually see over the next 10 days, and scores each one for brightness, sky darkness, duration and the weather forecast.
- Sends a push notification before a good pass, so you're outside at the right moment.
How it works
- Orbit: the ISS position is computed on your device with the standard SGP4 model, from orbital elements published by NORAD via CelesTrak and refreshed several times a day. Predictions are typically accurate to a few seconds.
- Visibility: a pass counts only when the ISS is sunlit, your sky is dark and it's at least 10° up. Each visible pass gets a score; see below.
- Weather: hourly cloud forecasts from Open-Meteo for your location.
- Notifications: your phone works out which passes are worth it, then encrypts and signs each alert itself. A small relay server only holds these sealed messages and sends each one at the right time, even when the app is closed. It can't read them, and it never learns where you are. When the forecast changes, the bookings are replaced, so a pass that clouds over is silently dropped. Each arriving alert also re-checks the forecast: a heads-up re-books the later alerts, and a "look up now" alert tells you if clouds moved in since it was booked.
How visibility and the score work
When can you see the ISS?
The ISS has no lights you could see from the ground. It shines by reflecting sunlight. A pass is listed only when, at some point, all three are true:
- The ISS is in sunlight, not in Earth's shadow.
- Your sky is dark enough: the sun is at least 6° below your horizon (after dusk or before dawn).
- The ISS is at least 10° above your horizon, clear of trees and buildings.
That's why visible passes cluster in the hour or two after sunset and before sunrise, and why some weeks have none.
The score (0–100)
Four factors, each from 0 to 100%, multiplied together. Open any pass in Visible passes to see its own numbers.
| Brightness | Estimated magnitude at its brightest moment, as seen from the ground (lower is brighter). It depends on:
| |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Dark sky | Sun's depth below your horizon. −6° gives 65%; −12° or deeper gives 100%. | |||||||||||||||
| Duration | How long it's visible. A blink gives 60%; 5 minutes or more gives 100%. | |||||||||||||||
| Clear sky | Forecast cloud cover at your location during the pass. Low and mid-level clouds count fully; thin high clouds count less than half, because the ISS usually shines through them. 0% cloud gives 100%, 50% gives about 40%, overcast gives 0. Rain likelihood lowers it further. Passes beyond the forecast range are scored without it. |
The Minimum height setting under Alerts is separate from the score: use it if trees or buildings block your view low down.
Privacy
Your location stays on your device. The weather service receives it rounded to about 1 km. Turning on GPS, or long-pressing the map, also looks up a place name. There are no accounts, no tracking and no ads.
Author
Made by Petr Gregor.
App status
Background check log
Nothing yet. Each time the alert schedule is checked (when you open the app, when an alert arrives, or in the background) a line appears here. This needs push notifications turned on under Alerts.
Data & credits
Orbital elements: CelesTrak / NORAD · Weather: Open-Meteo (CC BY 4.0) · Imagery: NASA GIBS Blue Marble & Black Marble · Place names: BigDataCloud, Open-Meteo geocoding · Libraries: satellite.js, Leaflet.
Predictions are estimates. Weather can change faster than any forecast.