sgp4/

SGP4 is the standard model for predicting where a satellite will be from its two-line element set (TLE), and SDP4 extends it to deep-space orbits. The widely used reference implementation is David Vallado's, published by CelesTrak.

I wanted a version that drops into small systems: plain C99, no dynamic allocation, fixed-size buffers, and error codes instead of exceptions. The whole library is one source file and one header, so it can be copied into a project or pulled in with clib.

sgp4_tle_t tle;
sgp4_parse_tle_2line(line1, line2, &tle);

sgp4_elements_t elements;
sgp4_tle_to_elements(&tle, &elements);

sgp4_state_t state;
sgp4_init(&state, &elements);

sgp4_result_t result;
sgp4_propagate(&state, tsince_min, &result);  /* km and km/s, TEME frame */

• No heap. Every structure (sgp4_tle_t, sgp4_state_t, sgp4_result_t, and the rest) is a fixed-size value the caller owns, so the library works where malloc is unavailable or unwelcome.
• Near-earth and deep-space. Initialization picks SGP4 or SDP4 from the orbital period, so the same calls handle low-earth and deep-space orbits.
• More than propagation. TLE parsing, checksum validation, and formatting, plus the coordinate transforms needed to use the result: ECI to ECEF, ECEF to geodetic, and look angles for pointing an antenna.
• Runs in the browser. The satellite tracker on this site's home page is this library compiled to WebAssembly, fed live elements from CelesTrak.

I had Claude Code port CelesTrak's Vallado reference implementation to C99. The test suite checks the math helpers, TLE parsing against real ISS and Vanguard elements, propagation at epoch and over a full orbit, and round trips through each coordinate transform. It builds warning-free with -std=c99 -Wall -Wextra -pedantic.

My C++ satellite tracker, SatTrack, has its own SGP4 port; this library is the standalone C version.