# Skyfold Satellite Pass Predictor

> When the ISS or any satellite flies over you, and whether you can see it, from TLE elements. Predicts rise, culmination and set of satellites over a place with an SGP4/SDP4 propagator verified against the published Vallado test cases, and marks the naked-eye visible part (satellite in sunlight, sky dark). Paste elements or fetch them from CelesTrak.

Part of usefulweb.app (https://usefulweb.app/). Part of [Skyfold](https://skyfold.usefulweb.app/) (ownerApp: skyfold). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://skyfoldsatellitepasses.usefulweb.app/ (interactive tool at https://skyfoldsatellitepasses.usefulweb.app/tool)
- JSON API: POST https://skyfoldsatellitepasses.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://skyfoldsatellitepasses.usefulweb.app/api/v1/tool)
- OpenAPI: https://skyfoldsatellitepasses.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://skyfoldsatellitepasses.usefulweb.app/mcp - tool `skyfold_satellite_passes`

## Inputs
- location (text): Location (latitude, longitude) - Decimal degrees such as 52.09, 5.12 (north and east positive), or DMS, UTM, MGRS, Maidenhead or a plus code.
- elevation (number): Elevation above sea level
- timeZone (text): Time zone - IANA name such as Europe/Berlin or America/New_York, or a fixed offset such as UTC+2.
- date (date): Local date
- time (time): Local time
- source (select): Elements
- tle (textarea): Two-line elements (TLE) - One or more sets: an optional name line, then line 1 and line 2.
- catalogNumbers (text): NORAD catalogue numbers (CelesTrak) - Up to 5 numbers separated by commas, for example 25544 for the ISS. Used when Elements is set to CelesTrak.
- group (select): Or a CelesTrak group
- days (number): Days to search
- minElevation (number): Minimum elevation
- visibleOnly (boolean): Only naked-eye visible passes

## Method
- SGP4/SDP4 after Hoots & Roehrich (Spacetrack Report #3) as revised by Vallado, Crawford, Hujsak and Kelso (AIAA 2006-6753), WGS-72 constants, AFSPC mode; matches the published verification ephemerides to better than 1 m.
- TEME positions are rotated to the Earth-fixed frame with Greenwich mean sidereal time; look angles for a WGS84 observer.
- Passes: elevation sampled every 20–60 s, crossings refined by bisection to 0.5 s, culmination by golden-section search.
- Visible = satellite at least half sunlit (conical Earth shadow) while the Sun is 6° or more below the observer’s horizon.
- CelesTrak elements are cached for 2 hours per request, as CelesTrak asks.

## Limits
- TLE predictions degrade with element age (minutes after a week or two); reboosts of the ISS shift passes. Brightness (magnitude) is not predicted.