User Manual 3.3 Attitude ephemeris

User Manual 3.3 Attitude ephemeris

Introduction

Scope

The purpose is to extend the current Orekit attitude package with classes and methods to compute and process attitude ephemeris.

Javadoc

The ephemeris objects are available in the package org.orekit.attitudes in the Orekit addons library.

Library	Javadoc
Orekit addons	Package org.orekit.attitudes

Links

Orekit Attitudes architecture description, Modèle:SiteLink label="Orekit site" jd="OR" suffix="architecture/attitudes.html"/.

Useful Documents

None as of now.

Package Overview

The attitude ephemeris conception is described hereafter :

Legend :

• green : new objects
• grey : modified existing objects
• blue : existing objects

Features Description

Generation of attitude ephemeris

Attitude ephemeris are generated from a sequence of attitude laws using a fixed time step; one or more options can be added when computing ephemeris:

• the time interval of generation can be smaller than the attitude laws sequence time interval (but it must be contained in it);

• attitude can be computed when there is a transition between attitude laws in the sequence: the user can choose to compute one value (the attitude of the law starting at the transition point), or two values (the attitude of the law starting and that of the law ending at the transition point);

• the time step can be variable: in this case, the user must choose a minimum and maximum time step, and a threshold value for the angular distance. The time step is then computed for each ephemeris; the angular distance between two consecutive ephemeris is calculated: when is bigger than the threshold value, the time step is reduced using an iterative algorithm.

Fixed step ephemeris generation

final AbsoluteDate date0 = AbsoluteDate.J2000_EPOCH;
final AbsoluteDate dateF = date0.shiftedBy(86400);
final AbsoluteDateInterval interval = new AbsoluteDateInterval(IntervalEndpointType.OPEN, date0,
dateF, IntervalEndpointType.OPEN);
 
AttitudeLegsSequence sequence = new AttitudeLegsSequence(orbit);
double fixedStep = 10;
FixedStepAttitudeEphemerisGenerator generator = new FixedStepAttitudeEphemerisGenerator(sequence, fixedStep);
final Set<Attitude> ephemeris = generator.generateEphemeris(interval, FramesFactory.getGCRF());

Variable step ephemeris generation

final AbsoluteDate date0 = AbsoluteDate.J2000_EPOCH;
final AbsoluteDate dateF = date0.shiftedBy(86400);
final AbsoluteDateInterval interval = new AbsoluteDateInterval(IntervalEndpointType.OPEN, date0,
                dateF, IntervalEndpointType.OPEN);
 
AttitudeLegsSequence sequence = new AttitudeLegsSequence(orbit);
final double stepMin = 1;
final double stepMax = 10;
final double angMax = 0.15;
VariableStepAttitudeEphemerisGenerator generator = new VariableStepAttitudeEphemerisGenerator(sequence, 
                stepMin, stepMax, angMax, AbstractAttitudeEphemerisGenerator.START_TRANSITIONS);
final Set<Attitude> ephemeris = generator.generateEphemeris(interval, frame);

Contents

Classes

Class	Summary	Javadoc
AbstractAttitudeEphemerisGenerator	Abstract class containing the methods to generate attitude ephemeris from an attitude laws sequence.	...
FixedStepAttitudeEphemerisGenerator	Class computing attitude ephemeris from an attitude laws sequence, using a fixed time step.	...
VariableStepAttitudeEphemerisGenerator	Class computing attitude ephemeris from an attitude laws sequence, using a variable time step.	...