Attitudes

The org.orekit.attitudes package provides classes to represent simple attitudes.

Attitudes Presentation

Some force models, such as the atmospheric drag for maneuvers, need to
know the spacecraft orientation in an inertial frame. Orekit uses a simple
container for Attitude which includes both the geometric part (i.e. rotation)
and the kinematic part (i.e. the instant spin axis). The components
held by this container allow to convert vectors from inertial frame to spacecraft
frame along with their derivatives. This container is similar in spirit to the various
extensions of the abstract Orbit class: it represents a state at a specific instant.

In order to represent attitude evolution in time, the AttitudeProvider interface
is available.
At a higher level, attitude laws defined by a ground pointing law are also available.
This corresponds to a real situation where satellite attitude law is defined in order
to perform a mission, i.e. pointing a specified point/area. All these laws are collected
under an abstract class called "GroundPointing".
Finally, there exist attitude laws that wrap a "base" attitude law, and add to this
base attitude law a complementary rotation in order to fulfill specific mission constraints.

Description of attitudes providers

Basic attitude laws

  • InertialProvider, which represents an inertial attitude law, perfectly
    aligned with the EME2000 frame.

  • FixedRate, which represents a rotation around a fixed axis at
    constant rate.

  • CelestialBodyPointed, i.e satellite pointing axis directed towards given
    celestial body.

  • SpinStabilized, which is handled as a wrapper for an underlying
    non-rotating law. This underlying law is typically an instance
    of CelestialBodyPointed with the pointing axis equal to
    the rotation axis, but can in fact be anything...

  • LofOffset, defined as a given angular offset around three axes from local orbital
    frame at given date.

  • TabulatedLofOffset, defined by interpolating within a user-provided ephemerides
    with resepct to a local orbital frame, using any number of interpolation points and
    either using or ignoring the tabulated rotation rates.

  • TabulatedProvider, defined by interpolating within a user-provided ephemerides
    with respect to an inertial frame, using any number of interpolation points and
    either using or ignoring the tabulated rotation rates.

Ground pointing laws

These classes are designed to represent attitude laws used to fulfill pointing missions.
Several pointing laws are modelized :

  • BodyCenterPointing, where satellite pointing axis is directed towards
    reference body frame center.

  • LofOffsetPointing, defined by a lof offset simple attitude law
    and completed with ground pointing corresponding functions.

  • NadirPointing, where satellite pointing axis is aligned on subtrack
    point vertical direction.

  • TargetPointing, where satellite pointing axis is directed towards given
    point on reference body shape.

  • GroundPointingWrapper, which is an abstract class used for complex pointing
    laws described herebelow.

    All these ground pointing laws are relative to corresponding body frame,
    which is used for their construction. Depending on their nature, each ground pointing
    law also have its own specific construction arguments.
    For each of these laws, satellite attitude state at any time in any given frame
    can be computed, as well as the observed ground point, or a target in the body frame.

Complex pointing laws

Several classes have been implemented in order to represent attitude laws in which a
base attitude law is used, and a complementary rotation is added in order to fulfill specific
mission constraints. They are gathered under abstract class GroundPointingWrapper.
At this point, implemented laws of this kind are:

  • YawCompensation: this law is used to fulfill ground observation constraints
    that reduce geometrical distortion. Yaw angle is changed a little from
    the basic ground pointing attitude, so that the apparent motion of ground points is
    along a prescribed axis (orthogonal to the optical sensors rows), taking into account
    all effects. It is the impact of earth proper rotation on ground points that is
    compensated.

  • YawSteering: this law is mainly used for low Earth orbiting satellites
    with no mission-related constraints on yaw angle. It sets the yaw angle in
    such a way that the solar arrays have maximal lighting without changing the
    roll and pitch.

Attitude sequence

The AttitudeSequence class manages a sequence of different attitude laws activated
in rows according to switching events. Only one attitude law in the sequence is in
an active state. When one of the switch events associated with the active law occurs,
the active law becomes the one specified with the event.

It is possible to have perpetually alternating laws, for example when eclipse entry
triggers a switch from a day light attitude to a night attitude and eclipse exit
triggers the reverse (possibly with intermediate transition modes).

Package organization

attitude class diagram

attitude-class-diagram.png - attitude class diagram (149 KB) Luc Maisonobe, 2014-07-01 17:24