/* Copyright 2002-2017 CS Systèmes d'Information
* Licensed to CS Systèmes d'Information (CS) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* CS licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.orekit.time;
import org.hipparchus.RealFieldElement;
import org.hipparchus.util.FastMath;
import org.orekit.utils.Constants;
/** Greenwich Mean Sidereal Time.
* <p>The Greenwich Mean Sidereal Time is the hour angle between the meridian of Greenwich
* and mean equinox of date at 0h UT1.</p>
* <p>This is intended to be accessed thanks to the {@link TimeScalesFactory} class,
* so there is no public constructor.</p>
* @author Luc Maisonobe
* @see AbsoluteDate
* @since 5.1
*/
public class GMSTScale implements TimeScale {
/** Serializable UID. */
private static final long serialVersionUID = 20131209L;
/** Duration of one julian day. */
private static final double FULL_DAY = Constants.JULIAN_DAY;
/** Duration of an half julian day. */
private static final double HALF_DAY = Constants.JULIAN_DAY / 2.0;
/** Coefficient for degree 0. */
private static final double C0 = 24110.54841;
/** Coefficient for degree 1. */
private static final double C1 = 8640184.812866;
/** Coefficient for degree 2. */
private static final double C2 = 0.093104;
/** Coefficient for degree 3. */
private static final double C3 = -0.0000062;
/** Universal Time 1 time scale. */
private final UT1Scale ut1;
/** Reference date for GMST. */
private final AbsoluteDate referenceDate;
// GST 1982: 24110.54841 + 8640184.812866 t + 0.093104 t2 - 6.2e-6 t3
// GST 2000: 24110.5493771 + 8639877.3173760 tu + 307.4771600 te + 0.0931118 te2 - 0.0000062 te3 + 0.0000013 te4
/** Package private constructor for the factory.
* @param ut1 Universal Time 1 scale
*/
GMSTScale(final UT1Scale ut1) {
this.ut1 = ut1;
this.referenceDate = new AbsoluteDate(2000, 1, 1, 12, 0, 0.0, ut1);
}
/** {@inheritDoc} */
@Override
public double offsetFromTAI(final AbsoluteDate date) {
// julian seconds since reference date
final double ts = date.durationFrom(referenceDate);
// julian centuries since reference date
final double tc = ts / Constants.JULIAN_CENTURY;
// GMST at 0h00 UT1 in seconds = offset with respect to UT1
final double gmst0h = C0 + tc * (C1 + tc * (C2 + tc * C3));
// offset with respect to TAI
final double offset = gmst0h + ut1.offsetFromTAI(date);
// normalize offset between -43200 and +43200 seconds
return offset - FULL_DAY * FastMath.floor((offset + HALF_DAY) / FULL_DAY);
}
/** {@inheritDoc} */
@Override
public <T extends RealFieldElement<T>> T offsetFromTAI(final FieldAbsoluteDate<T> date) {
// julian seconds since reference date
final T ts = date.durationFrom(referenceDate);
// julian centuries since reference date
final T tc = ts.divide(Constants.JULIAN_CENTURY);
// GMST at 0h00 UT1 in seconds = offset with respect to UT1
final T gmst0h = tc.multiply(C3).add(C2).multiply(tc).add(C1).multiply(tc).add(C0);
// offset with respect to TAI
final T offset = gmst0h.add(ut1.offsetFromTAI(date));
// normalize offset between -43200 and +43200 seconds
return offset.subtract(FULL_DAY * FastMath.floor((offset.getReal() + HALF_DAY) / FULL_DAY));
}
/** {@inheritDoc} */
public String getName() {
return "GMST";
}
/** {@inheritDoc} */
public String toString() {
return getName();
}
}