Package org.orekit.propagation


This package provides tools to propagate orbital states with different methods.

Propagation is the prediction of the evolution of an initial state. The initial state and the propagated states are represented in OREKIT by a SpacecraftState, which is a simple container for all needed information at a specific date : mass, kinematics, attitude, date, frame. The state provides basic interpolation features allowing to shift it slightly to close dates. For more accurate and farthest dates, several full-featured propagators are available to propagate the state.

Keplerian propagation

The KeplerianPropagator implements the Propagator interface, which ensures that we can obtain a propagated SpacecraftState at any time once the instance is initialized with an initial state. This extrapolation is not a problem with a simple EquinoctialOrbit representation: only the mean anomaly value changes.

Eckstein-Hechler propagation

This analytical model is suited for near circular orbits and inclination neither equatorial nor critical. It considers J2 to J6 potential coefficients correctors, and uses mean parameters to compute the new position. As the Keplerian propagator, it implements the Propagator interface.

TLE propagation

This analytical model allows propagating {org.orekit.propagation.analytical.tle.TLE} data using SGP4 or SDP4 models. It is very easy to initialize, only the initial TLE is needed. As the other analytical propagators, it implements the Propagator interface.

GNSS propagation

These analytical models allow propagating navigation messages such as in GNSS almanacs available thanks to SEM or YUMA files. Each GNSS constellation has its own propagation model availables in org.orekit.propagation.analytical.gnss package.

Numerical propagation

It is the most important part of the OREKIT project. Based on Hipparchus integrators, the NumericalPropagator class realizes the interface between space mechanics and mathematical resolutions. If its utilization seems difficult on first sight, it is in fact quite clear and intuitive.

The mathematical problem to integrate is a 6 dimension time derivative equations system. The six first equations are given by the Gauss equations (expressed in EquinoctialOrbit). This first order system is computed by the TimeDerivativesEquations class. It will be instanced by the propagator and then be modified at each step (a fixed t value) by all the needed force models which will add their contribution, the perturbing acceleration.

The integrators provided by Hipparchus need the state vector at t0, the state vector first time derivate at t0, and then calculates the next step state vector, and ask for the next first time derivative, etc. until it reaches the final asked date.

Semi-analytical propagation

Semi-analytical propagation in Orekit is based on Draper Semi-analytical Satellite Theory (DSST), which is applicable to all orbit types. DSST divides the computation of the osculating orbital elements into two contributions: the mean orbital elements and the short-periodic terms. Both models are developed in the equinoctial orbital elements via the Method of Averaging. Mean orbital elements are computed numerically while short period motion is computed using a combination of analytical and numerical techniques.

Luc Maisonobe, Fabien Maussion, Pascal Parraud