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1 | /* |
2 | * (C)2012 Michael Duane Rice All rights reserved. | |
3 | * | |
4 | * Redistribution and use in source and binary forms, with or without | |
5 | * modification, are permitted provided that the following conditions are | |
6 | * met: | |
7 | * | |
8 | * Redistributions of source code must retain the above copyright notice, this | |
9 | * list of conditions and the following disclaimer. Redistributions in binary | |
10 | * form must reproduce the above copyright notice, this list of conditions | |
11 | * and the following disclaimer in the documentation and/or other materials | |
12 | * provided with the distribution. Neither the name of the copyright holders | |
13 | * nor the names of contributors may be used to endorse or promote products | |
14 | * derived from this software without specific prior written permission. | |
15 | * | |
16 | * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" | |
17 | * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE | |
18 | * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE | |
19 | * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE | |
20 | * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR | |
21 | * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF | |
22 | * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS | |
23 | * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN | |
24 | * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) | |
25 | * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE | |
26 | * POSSIBILITY OF SUCH DAMAGE. | |
27 | */ | |
28 | ||
29 | /* $Id: gm_sidereal.c 2369 2013-04-28 14:19:35Z swfltek $ */ | |
30 | ||
31 | /* | |
32 | Greenwich Mean Sidereal Time. A sidereal second is somewhat shorter than a standard second, | |
33 | about 1.002737909350795 sidereal seconds per standard second. | |
34 | ||
35 | We resort to fixed point math due to the insufficient resolution of a 'double', using... | |
36 | ||
37 | timestamp * ( 1.002737909350795 << 31 ) | |
38 | --------------------------------------- + Te | |
39 | 1 << 31 | |
40 | ||
41 | Where Te is the sidereal time at the epoch. | |
42 | ||
43 | */ | |
44 | ||
45 | #include <time.h> | |
46 | #include <stdint.h> | |
47 | ||
48 | unsigned long | |
49 | gm_sidereal(const time_t * timer) | |
50 | { | |
51 | uint64_t tmp; | |
52 | ||
53 | tmp = *timer; | |
54 | tmp *= 0x8059B740; | |
55 | tmp /= 0x80000000; | |
56 | tmp += (uint64_t) 23991; | |
57 | ||
58 | tmp %= ONE_DAY; | |
59 | return tmp; | |
60 | } |