OrbitCore

A Kotlin Multiplatform orbital mechanics library — SGP4/SDP4 propagation, pass prediction and GeoJSON.

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_{Licensed under the Apache License 2.0}

OrbitCore is a satellite orbit propagation library written entirely in Kotlin. Give it a satellite's orbital elements (from a TLE / Celestrak's JSON GP format) and an instant in time, and it tells you where the satellite is — and whether, when, and how it can be seen from anywhere on Earth.

It's built as a Kotlin Multiplatform library, so the same code can be compiled for every platform Kotlin supports and integrated into whatever system you need — a JVM backend, an Android app, an iOS app, a native Linux service, and more, all sharing the exact same propagation logic and test suite.

• SGP4 + SDP4 propagator — a faithful port of the Vallado/Crawford/Hujsak/Kelso (2006) revision of Spacetrack Report #3 (WGS-72), supporting both near-earth orbits (SGP4) and deep-space orbits (SDP4: geostationary, GPS-like semi-synchronous, and highly-eccentric Molniya-type orbits, all of which need lunisolar perturbations and 12h/24h geopotential resonance to propagate correctly).
• High-level Satellite API — position and velocity (TEME), sub-satellite point (lat/lon/altitude), ground speed, orbital period, look angles (azimuth/elevation/range) from any observer on the ground, geometric and naked-eye visibility (accounting for sunlight and the observer's twilight), and next-pass prediction (AOS/TCA/LOS with maximum elevation).
• Ground tracks and coverage footprints, exposed both as plain coordinate lists and as ready-to-use GeoJSON (Feature / FeatureCollection, RFC 7946) — MapLibre, Mapbox, Leaflet, Google Maps and virtually every other map library speak GeoJSON natively, so there's no glue code needed between this library and a map.
• Tested against reference vectors generated from a well-established, independent SGP4/SDP4 implementation, covering near-earth, geostationary, semi-synchronous and Molniya-type regimes — not just "it compiles," the actual propagated state vectors are checked.

OrbitCore publishes artifacts for JVM, Android, iOS (arm64 and simulator arm64), Linux (x64), JS and Wasm/JS, so any of them can be consumed directly as a Kotlin dependency.

JS and Wasm/JS bindings are published, but their JS/TS-idiomatic ergonomics (clean TypeScript types instead of raw Kotlin types like Instant in exported signatures) aren't finished yet — the artifacts exist, direct JS/TS interop is still a work in progress.

OrbitCore is published on Maven Central as com.parodison:orbit-core.

Version catalog (libs.versions.toml):

[versions]
orbit-core = "0.1.0"

[libraries]
orbit-core = { module = "com.parodison:orbit-core", version.ref = "orbit-core" }

Then, in your module's build.gradle.kts:

dependencies {
    implementation(libs.orbit.core)
}

Or without a version catalog:

dependencies {
    implementation("com.parodison:orbit-core:0.1.0")
}

Apache License 2.0 — see LICENSE.