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Astronomy

Keplerian Orbital Element / TLE / SGP4 Satellite Position Calculator

· Last updated：2026-08-08

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

Details

What this tool helps you do

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.

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Written by the Elysia Tools team

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category
Astronomy
Input types
textarea, text, number
Output type
html
Sample coverage
4
API ready
Yes

Overview

This local TLE and SGP4 satellite position calculator parses two-line element data, validates checksums, displays classical Keplerian elements, and calculates satellite position, sub-satellite point, ground track, observer visibility, and Doppler shift.

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

  1. 1Paste the satellite's two TLE lines into the required input fields.
  2. 2Optionally enter a UTC propagation time; leaving it empty uses the TLE epoch.
  3. 3Add observer latitude and longitude to calculate azimuth, elevation, and visibility.
  4. 4Enter 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.

Examples

1. ISS position and one-orbit ground track

Satellite observer

Background

An observer wants to inspect the ISS orbit at the epoch included in its TLE and see where the satellite passes over Earth.

Problem

Display the parsed orbital elements, propagated position, sub-satellite point, altitude, and ground track.

How to use

Paste the supplied ISS TLE into the two required fields, leave Propagation time empty, and keep Ground-track samples at 60.

Propagation time: empty; Observer latitude: 0; Observer longitude: 0; Transmit frequency: 0; Ground-track samples: 60.

Outcome

The calculator uses the TLE epoch and returns the satellite's orbital elements, propagated position, sub-satellite coordinates, altitude, and one-orbit ground track.

2. Doppler estimate for a 437.8 MHz downlink

Amateur-radio operator

Background

A radio operator is evaluating an ISS pass from New York City on a 437.8 MHz downlink.

Problem

Calculate the satellite's observer-relative pointing data, visibility, and Doppler shift at a selected UTC time.

How to use

FAQ

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?

Enter a transmit frequency in MHz along with an observer location and propagation time to calculate the Doppler shift.

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Enter the ISS TLE, set Propagation time to 2025-08-08T12:00:00Z, and provide the observer coordinates and transmit frequency.

Observer latitude: 40.7128; Observer longitude: -74.006; Transmit frequency: 437.8 MHz; Ground-track samples: 60.

Outcome

The calculator reports azimuth, elevation, visibility, and the Doppler shift associated with the selected satellite position and frequency.