SGP4SDP4

SGP4SDP4 refers to the suite of orbital propagation models used to forecast positions of Earth-orbiting satellites from Two-Line Element sets. The SGP4 (Simplified General Perturbations Model 4) and SDP4 (Simplified Deep-space Perturbations Model 4) algorithms were developed in the 1980s for NORAD and remain the standard method for propagating most TLEs in near-Earth and deep-space regimes. In practice, an implementation labeled SGP4SDP4 provides both propagators and chooses the appropriate one based on the satellite's orbit and epoch.

A TLE encodes a satellite’s state as a set of mean orbital elements at a given epoch and a drag term B*. The propagator uses perturbation theory to advance the state forward in time, applying short- and long-period terms. SGP4 is intended for near-Earth objects with lower altitudes, while SDP4 extends the modelling to deep-space regimes, including highly elliptical orbits and higher altitudes, where third-body effects become significant. The models incorporate perturbations from Earth's oblateness (J2), atmospheric drag, solar radiation pressure, and third-body gravitational effects from the Moon and Sun.

The output of SGP4SDP4 is a position and velocity vector, typically in an Earth-centered inertial frame, at the requested epoch, from which orbital elements or ground-track predictions can be derived. The model is widely implemented in software libraries across languages such as C, C++, Python, and JavaScript, and remains a standard tool for satellite tracking, mission planning, and ground-station pass computations. While accurate for many purposes and short to medium forecast horizons, SGP4/SDP4 are approximations and rely on up-to-date TLEs for reliable long-term predictions.