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Propagators

Introduction

An orbit propagator (or propagator in short) takes in an initial condition (e.g. satellite position and velocity at a given time) and outputs where the satellite is going to be at a target time, usually with intermediate points depicting the trajectory while proceeding towards this target time.

The basic properties of the orbit propagators are:

  • Initial condition type: This could be cartesian position and velocity vector or orbital elements. Both of them are defined at a reference time.
  • Integration type: This could be analytical or numerical. Analytical propagators can compute the coordinates at a target time directly, without computing intermediate points. Numerical integrators must compute intermediate points numerically until target time is reached.
  • Force model: Each propagator has a force model that defines which forces (or accelerations) are taken into account during the propagation.

The accuracy of an orbit propagator is a function of both the force model and the analytical or numerical model. As a general rule, a numerical propagator is more accurate than an analytical one with a similar force model. Furthermore, among the numerical propagators, a higher order propagation scheme generally yields better accuracy than a lower order scheme. However, there are a lot of other parameters that determine the short and long term accuracy of a propagation scheme.

Propagators in SatMAD

A propagator in SatMAD accepts an initial orbital state (cartesian coordinates or orbital elements at a reference time) and outputs a {py:class}.Trajectory object through a gen_trajectory() method. This trajectory starts at the initial time and ends around the target time, with intermediate points at each stepsize (given by the common {py:meth}.AbstractPropagator.stepsize property). Note that, the trajectory may exceed the target time to ensure enough output points exist for a proper interpolation.

Currently, the following propagators are implemented in SatMAD:

---
maxdepth: 1
---
sgp4_propagator
numerical_propagation

Reference/API

.. automodule:: satmad.propagation.base_propagator
    :members:
    :undoc-members: