Overview (ORbit Extrapolation KIT 7.1 API)

Packages
Package	Description
org.orekit.attitudes	This package provides classes to represent simple attitudes.
org.orekit.bodies	This package provides interface to represent the position and geometry of space objects such as stars, planets or asteroids.
org.orekit.data	This package provide base classes for exploring the configured data directory tree and read external data that can be used by the library.
org.orekit.errors	This package provides classes to generate and handle exceptions.
org.orekit.files	Top level package for various parsers.
org.orekit.files.ccsds	This package provides a parser for orbit data stored in CCSDS Orbit Data Message format.
org.orekit.files.general	This package provides interfaces for orbit file representations and corresponding parsers.
org.orekit.files.sp3	This package provides a parser for orbit data stored in SP3 format.
org.orekit.forces	This package provides the interface for force models that will be used by the `NumericalPropagator`, as well as some classical spacecraft models for surface forces (spherical, box and solar array ...).
org.orekit.forces.drag	This package provides all drag-related forces.
org.orekit.forces.gravity	This package provides all gravity-related forces.
org.orekit.forces.gravity.potential	This package provides classes to read gravity field files and supports several different formats.
org.orekit.forces.maneuvers	This package provides models of simple maneuvers.
org.orekit.forces.radiation	This package provides all radiation pressure related forces.
org.orekit.frames	This package provides classes to handle frames and transforms between them.
org.orekit.models	Top level package for physical models.
org.orekit.models.earth	This package provides models that simulate certain physical phenomena of Earth and the near-Earth environment.
org.orekit.models.earth.tessellation	This package provides ways to do tessellation and sampling of zones of interest over an ellipsoid surface.
org.orekit.orbits	This package provides classes to represent orbits.
org.orekit.propagation	This package provides tools to propagate orbital states with different methods.
org.orekit.propagation.analytical	Top level package for analytical propagators.
org.orekit.propagation.analytical.tle	This package provides classes to read and extrapolate tle's.
org.orekit.propagation.conversion	This package provides tools to convert a given propagator or a set of `SpacecraftState` into another propagator.
org.orekit.propagation.events	This package provides interfaces and classes dealing with events occurring during propagation.
org.orekit.propagation.events.handlers	This package provides an interface and classes dealing with events occurrence only.
org.orekit.propagation.integration	Utilities for integration-based propagators (both numerical and semi-analytical).
org.orekit.propagation.numerical	Top level package for numerical propagators.
org.orekit.propagation.sampling	This package provides interfaces and classes dealing with step handling during propagation.
org.orekit.propagation.semianalytical	Top level package for semi-analytical propagators.
org.orekit.propagation.semianalytical.dsst	This package provides an implementation of the Draper Semi-analytical Satellite Theory (DSST).
org.orekit.propagation.semianalytical.dsst.forces	This package provides force models for Draper Semi-analytical Satellite Theory (DSST).
org.orekit.propagation.semianalytical.dsst.utilities	This package provides utilities for Draper Semi-analytical Satellite Theory (DSST).
org.orekit.propagation.semianalytical.dsst.utilities.hansen	This package provides Hansen coefficient related utilities for Draper Semi-analytical Satellite Theory (DSST).
org.orekit.time	This independent package provides classes to handle epochs, time scales, and to compare instants together.
org.orekit.utils	This package provides useful objects.

1. Purpose

OREKIT library is a low-level library for space mechanics projects. It is implemented in the JAVA language.

OREKIT project was initiated by CS with the following goals in mind:

provide a common base layer for space projects
support both quick developments with loose requirements and complex projects with rich physical modeling, fine customization, and high accuracy
be user-extensible through clear and as simple as possible architecture
use unified interfaces to allow models and algorithms switching with almost no effort
provide both basic and rich models that can be plugged interchangeably to ease applications validation
separate models configuration from models usage to hide away complexity and keep caller code simple
support both development of standalone and embedded applications (web services, plugins, scripting frameworks ...)
use up-to-date techniques for low cost development and maintenance
remove platforms and system dependencies and leverage environment-hell (environment variables, dynamic libraries versions, operating system, filesystem layout ...)

2. Features

Orekit can be used for many purposes, from simple geometrical or orbital tools up to complex orbits propagators with several perturbing forces, maneuvers and discrete events. Here is a short list of the features offered by the library:

Time
- high accuracy absolute dates
- time scales (TAI, UTC, UT1, GPS, TT, TCG, TDB, TCB, GMST, GST ...)
- transparent handling of leap seconds
- support for CCSDS time code standards
Geometry
- frames hierarchy supporting fixed and time-dependent (or telemetry-dependent) frames
- predefined frames (EME2000/J2000, ICRF, GCRF, ITRF93, ITRF97, ITRF2000, ITRF2005, ITRF2008 and intermediate frames, TOD, MOD, GTOD and TEME frames, Veis, topocentric, tnw and qsw local orbital frames, Moon, Sun, planets, solar system barycenter, Earth-Moon barycenter, ecliptic)
- user extensible (used operationally in real time with a set of about 60 frames on several spacecraft)
- transparent handling of IERS Earth Orientation Parameters (for both new CIO-based frames following IERS 2010 conventions and old equinox-based frames)
- transparent handling of JPL DE 4xx (405, 406 and more recent) and INPOP ephemerides
- transforms including kinematic combination effects
- composite transforms reduction and caching for efficiency
- extensible central body shapes models (with predefined spherical and ellipsoidic shapes)
- Cartesian and geodesic coordinates, kinematics
Spacecraft state
- Cartesian, elliptical Keplerian, circular and equinoctial parameters
- Two-Line Elements
- transparent conversion between all parameters
- automatic binding with frames
- attitude state and derivative
- Jacobians
- mass management
- user-defined associated state (for example battery status, or higher order derivatives, or anything else)
Maneuvers
- analytical models for small maneuvers without propagation
- impulse maneuvers for any propagator type
- continuous maneuvers for numerical propagator type
Propagation
- analytical propagation models (Kepler, Eckstein-Heschler, SDP4/SGP4 with 2006 corrections)
- numerical propagators
  - customizable force models
    - central attraction
    - gravity models including time-dependent like trends and pulsations (automatic reading of ICGEM (new Eigen models), SHM (old Eigen models), EGM and GRGS gravity field files formats, even compressed)
    - atmospheric drag (DTM2000, Jacchia-Bowman 2006, Harris-Priester and simple exponential models), and Marshall solar Activity Future Estimation
    - third body attraction (with data for Sun, Moon and all solar systems planets)
    - radiation pressure with eclipses
    - solid tides, with or without solid pole tide
    - ocean tides, with or without ocean pole tide
    - general relativity
    - multiple maneuvers
  - state of the art ODE integrators (adaptive stepsize with error control, continuous output, switching functions, G-stop, step normalization ...)
  - computation of Jacobians with respect to orbital parameters and selected force models parameters
  - serialization mechanism to store complete results on persistent storage for later use
- semi-analytical propagation model (DSST) with customizable force models
- tabulated ephemerides
  - file based
  - memory based
  - integration based
- unified interface above analytical/numerical/tabulated propagators for easy switch from coarse analysis to fine simulation with one line change
- all propagators can be used in several different modes
  - slave mode: propagator is driven by calling application
  - master mode: propagator drives application callback functions
  - ephemeris generation mode: all intermediate results are stored during propagation and provided back to the application which can navigate at will through them, effectively using the propagated orbit as if it was an analytical model, even if it really is a numerically propagated one, which is ideal for search and iterative algorithms
- handling of discrete events during integration (models changes, G-stop, simple notifications ...)
- predefined discrete events
  - eclipse (both umbra and penumbra)
  - ascending and descending node crossing
  - anomaly, latitude argument or longitude argument crossings, with either true, eccentric or mean angles
  - apogee and perigee crossing
  - alignment with some body in the orbital plane (with customizable threshold angle)
  - raising/setting with respect to a ground location (with customizable triggering elevation and ground mask)
  - date
  - latitude, longitude, altitude crossing
  - latitude, longitude extremum
  - elevation extremum
  - anomaly, latitude argument, or longitude argument crossings, either true, mean or eccentric
  - moving target detection in spacecraft sensor Field Of View (any shape, with special case for circular)
  - spacecraft detection in ground based Field Of View (any shape)
  - sensor Field Of View (any shape) overlapping complex geographic zone
  - complex geographic zone traversal
  - impulse maneuvers occurrence
- possibility of slightly shifting events in time (for example to switch from solar pointing mode to something else a few minutes before eclipse entry and reverting to solar pointing mode a few minutes after eclipse exit)
- possibility of filtering events based on their direction (for example to detect only eclipse entries and not eclipse exits)
- possibility of filtering events based on an external enabling function (for example to detect events only during selected orbits and not others)
Attitude
- extensible attitude evolution models
- predefined laws
  - central body related attitude (nadir pointing, center pointing, target pointing, yaw compensation, yaw-steering)
  - orbit referenced attitudes (LOF aligned, offset on all axes)
  - space referenced attitudes (inertial, celestial body-pointed, spin-stabilized)
  - tabulated attitudes, either respective to inertial frame or respective to Local Orbital Frames
Orbit file handling
- loading of SP3-a and SP3-c orbit files
- loading of CCSDS Orbit Data Messages (both OPM, OEM, and OMM types are supported)
Earth models
- tropospheric delay (modified Saastamoinen)
- tropospheric refraction correction angle (Recommendation ITU-R P.834-7 and Saemundssen's formula quoted by Meeus)
- geomagnetic field (WMM, IGRF)
- geoid model from any gravity field
- tessellation of zones of interest as tiles
- sampling of zones of interest as grids of points
Customizable data loading
- loading from local disk
- loading from classpath
- loading from network (even through internet proxies)
- support for zip archives
- support from gzip compressed files
- plugin mechanism to delegate loading to user defined database or data access library

3. Dependency

OREKIT has only one dependency: the Apache Commons Math open-source library. This library provides mathematical tools for many purposes: geometry, polynomial equations, differential equations, and more... It is distributed under the business-friendly Apache Software License version 2.0.

OREKIT is based on several Apache Commons Math packages:

geometry, which provides very useful objects as vectors and rotations (internally represented by quaternions)
ode, which provides numerical solutions to solve Ordinary Differential Equations (ODE)
optim, which provides multi-dimensional optimization
solvers, which provides root-finding algorithms
differentiation, which provides analytical computation of partial derivatives to arbitrary order and arbitrary number of free parameters

The use of Apache Commons Math is hidden in OREKIT as much as possible so users don't need to fully master this library. If they wish, they can limit themselves to understanding the simple classes Vector3D and Rotation from the geometry package, and the interface FirstOrderIntegrator from the ode package.