source: branches/FACT++_lidctrl_usb/erfa/src/tdbtcb.c@ 19875

Last change on this file since 19875 was 18711, checked in by tbretz, 10 years ago
Updated to ERFA 1.3.0 (no relevant code change except the leap second at the beginning of 2017)
File size: 5.5 KB
Line 
1#include "erfa.h"
2
3int eraTdbtcb(double tdb1, double tdb2, double *tcb1, double *tcb2)
4/*
5** - - - - - - - - - -
6** e r a T d b t c b
7** - - - - - - - - - -
8**
9** Time scale transformation: Barycentric Dynamical Time, TDB, to
10** Barycentric Coordinate Time, TCB.
11**
12** Given:
13** tdb1,tdb2 double TDB as a 2-part Julian Date
14**
15** Returned:
16** tcb1,tcb2 double TCB as a 2-part Julian Date
17**
18** Returned (function value):
19** int status: 0 = OK
20**
21** Notes:
22**
23** 1) tdb1+tdb2 is Julian Date, apportioned in any convenient way
24** between the two arguments, for example where tdb1 is the Julian
25** Day Number and tdb2 is the fraction of a day. The returned
26** tcb1,tcb2 follow suit.
27**
28** 2) The 2006 IAU General Assembly introduced a conventional linear
29** transformation between TDB and TCB. This transformation
30** compensates for the drift between TCB and terrestrial time TT,
31** and keeps TDB approximately centered on TT. Because the
32** relationship between TT and TCB depends on the adopted solar
33** system ephemeris, the degree of alignment between TDB and TT over
34** long intervals will vary according to which ephemeris is used.
35** Former definitions of TDB attempted to avoid this problem by
36** stipulating that TDB and TT should differ only by periodic
37** effects. This is a good description of the nature of the
38** relationship but eluded precise mathematical formulation. The
39** conventional linear relationship adopted in 2006 sidestepped
40** these difficulties whilst delivering a TDB that in practice was
41** consistent with values before that date.
42**
43** 3) TDB is essentially the same as Teph, the time argument for the
44** JPL solar system ephemerides.
45**
46** Reference:
47**
48** IAU 2006 Resolution B3
49**
50** Copyright (C) 2013-2016, NumFOCUS Foundation.
51** Derived, with permission, from the SOFA library. See notes at end of file.
52*/
53{
54
55/* 1977 Jan 1 00:00:32.184 TT, as two-part JD */
56 static const double t77td = ERFA_DJM0 + ERFA_DJM77;
57 static const double t77tf = ERFA_TTMTAI/ERFA_DAYSEC;
58
59/* TDB (days) at TAI 1977 Jan 1.0 */
60 static const double tdb0 = ERFA_TDB0/ERFA_DAYSEC;
61
62/* TDB to TCB rate */
63 static const double elbb = ERFA_ELB/(1.0-ERFA_ELB);
64
65 double d, f;
66
67
68/* Result, preserving date format but safeguarding precision. */
69 if ( tdb1 > tdb2 ) {
70 d = t77td - tdb1;
71 f = tdb2 - tdb0;
72 *tcb1 = tdb1;
73 *tcb2 = f - ( d - ( f - t77tf ) ) * elbb;
74 } else {
75 d = t77td - tdb2;
76 f = tdb1 - tdb0;
77 *tcb1 = f + ( d - ( f - t77tf ) ) * elbb;
78 *tcb2 = tdb2;
79 }
80
81/* Status (always OK). */
82 return 0;
83
84}
85/*----------------------------------------------------------------------
86**
87**
88** Copyright (C) 2013-2016, NumFOCUS Foundation.
89** All rights reserved.
90**
91** This library is derived, with permission, from the International
92** Astronomical Union's "Standards of Fundamental Astronomy" library,
93** available from http://www.iausofa.org.
94**
95** The ERFA version is intended to retain identical functionality to
96** the SOFA library, but made distinct through different function and
97** file names, as set out in the SOFA license conditions. The SOFA
98** original has a role as a reference standard for the IAU and IERS,
99** and consequently redistribution is permitted only in its unaltered
100** state. The ERFA version is not subject to this restriction and
101** therefore can be included in distributions which do not support the
102** concept of "read only" software.
103**
104** Although the intent is to replicate the SOFA API (other than
105** replacement of prefix names) and results (with the exception of
106** bugs; any that are discovered will be fixed), SOFA is not
107** responsible for any errors found in this version of the library.
108**
109** If you wish to acknowledge the SOFA heritage, please acknowledge
110** that you are using a library derived from SOFA, rather than SOFA
111** itself.
112**
113**
114** TERMS AND CONDITIONS
115**
116** Redistribution and use in source and binary forms, with or without
117** modification, are permitted provided that the following conditions
118** are met:
119**
120** 1 Redistributions of source code must retain the above copyright
121** notice, this list of conditions and the following disclaimer.
122**
123** 2 Redistributions in binary form must reproduce the above copyright
124** notice, this list of conditions and the following disclaimer in
125** the documentation and/or other materials provided with the
126** distribution.
127**
128** 3 Neither the name of the Standards Of Fundamental Astronomy Board,
129** the International Astronomical Union nor the names of its
130** contributors may be used to endorse or promote products derived
131** from this software without specific prior written permission.
132**
133** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
134** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
135** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
136** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
137** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
138** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
139** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
140** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
141** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
142** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
143** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
144** POSSIBILITY OF SUCH DAMAGE.
145**
146*/
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