# Keplerian Orbital Element / TLE / SGP4 Satellite Position Calculator Parse a TLE, show the six Keplerian elements (with derived semi-major axis & period), propagate with SGP4 for ECI position, sub-satellite point, ground track, elevation visibility and Doppler shift > Canonical page: https://elysiatools.com/en/tools/keplerian-orbital-element-tle-satrec-position - **Category:** Astronomy - **Keywords:** keplerian, tle, sgp4, satrec, orbital elements, satellite position, ground track, doppler, amsat, satnogs, cubesat ## Overview For amateur-satellite operators (AMSAT / SatNOGS), CubeSat teams and space-awareness educators. Paste a two-line TLE: fields are parsed and checksums validated; the six classical Keplerian elements are shown with the semi-major axis and orbital period derived from mean motion; SGP4 propagates the satellite's ECI position at a chosen time, computes the geodetic sub-satellite point, a one-orbit ground track, azimuth/elevation and visibility against a ground observer, and the Doppler shift for a transmit frequency. Based on satellite.js (WGS72 SGP4). Pure local computation, no network. ## Inputs - **TLE line 1** (textarea): 1 25544U 98067A 25220.50000000 .00000000 00000-0 00000-0 0 9996 - **TLE line 2** (textarea): 2 25544 51.6400 200.0000 0001000 90.0000 270.0000 15.50000000123455 - **Propagation time (UTC)** (text): Leave empty = epoch. ISO 8601, e.g. 2026-08-08T12:00:00Z - **Observer latitude** (number): e.g. 40.7128 - **Observer longitude** (number): e.g. -74.006 - **Transmit frequency (MHz)** (number): e.g. 437.800 (ISS packet), 145.800 - **Ground-track samples** (number) ## When to use - Inspect a satellite's six Keplerian orbital elements, derived semi-major axis, and orbital period from a TLE. - Propagate a satellite to a specific UTC time and view its ECI position, sub-satellite point, and one-orbit ground track. - Estimate observer azimuth, elevation, visibility, and Doppler shift for an amateur-radio transmit frequency. ## How it works - Paste the satellite's two TLE lines into the required input fields. - Optionally enter a UTC propagation time; leaving it empty uses the TLE epoch. - Add observer latitude and longitude to calculate azimuth, elevation, and visibility. - Enter a transmit frequency and choose the number of ground-track samples to calculate Doppler shift and generate the track. ## Use cases - Amateur-satellite operators can check pass geometry, elevation, and Doppler shift for radio communication planning. - CubeSat teams can inspect TLE-derived orbital elements, propagated positions, and ground tracks. - Students and educators can explore how orbital elements produce satellite positions and sub-satellite points. ## Frequently asked questions ### What input does the calculator require? It requires a valid two-line element set: TLE line 1 and TLE line 2. ### What happens if I leave the propagation time empty? The calculator propagates the satellite at the TLE epoch. ### Which orbital elements are displayed? It displays the six classical Keplerian elements, plus derived semi-major axis and orbital period. ### Can I calculate visibility from a ground location? Yes. Enter observer latitude and longitude to calculate azimuth, elevation, and visibility. ### How is Doppler shift calculated? 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