1 /**************************************************************************
2 * fg_time.c -- data structures and routines for managing time related stuff.
4 * Written by Curtis Olson, started August 1997.
6 * Copyright (C) 1997 Curtis L. Olson - curt@infoplane.com
8 * This program is free software; you can redistribute it and/or
9 * modify it under the terms of the GNU General Public License as
10 * published by the Free Software Foundation; either version 2 of the
11 * License, or (at your option) any later version.
13 * This program is distributed in the hope that it will be useful, but
14 * WITHOUT ANY WARRANTY; without even the implied warranty of
15 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
16 * General Public License for more details.
18 * You should have received a copy of the GNU General Public License
19 * along with this program; if not, write to the Free Software
20 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
23 * (Log is kept at end of this file)
24 **************************************************************************/
33 #include "../constants.h"
34 #include "../Flight/flight.h"
37 #define DEGHR(x) ((x)/15.)
38 #define RADHR(x) DEGHR(x*RAD_TO_DEG)
41 struct fgTIME cur_time_params;
44 /* given a date in months, mn, days, dy, years, yr, return the
45 * modified Julian date (number of days elapsed since 1900 jan 0.5),
46 * mjd. Adapted from Xephem. */
48 double cal_mjd (int mn, double dy, int yr) {
49 static double last_mjd, last_dy;
51 static int last_mn, last_yr;
55 if (mn == last_mn && yr == last_yr && dy == last_dy) {
61 y = (yr < 0) ? yr + 1 : yr;
67 if (yr < 1582 || (yr == 1582 && (mn < 10 || (mn == 10 && dy < 15)))) {
76 c = (long)((365.25*y) - 0.75) - 694025L;
78 c = (long)(365.25*y) - 694025L;
83 mjd = b + c + d + dy - 0.5;
94 /* given an mjd, return greenwich mean siderial time, gst */
96 double utc_gst (double mjd) {
98 double day = floor(mjd-0.5)+0.5;
99 double hr = (mjd-day)*24.0;
102 T = ((int)(mjd - 0.5) + 0.5 - J2000)/36525.0;
103 x = 24110.54841 + (8640184.812866 + (0.093104 - 6.2e-6 * T) * T) * T;
105 gst = (1.0/SIDRATE)*hr + x;
111 /* given Julian Date and Longitude (decimal degrees West) compute and
112 * return Local Sidereal Time, in decimal hours.
114 * Provided courtesy of ecdowney@noao.edu (Elwood Downey)
117 double sidereal (double mjd, double lng) {
121 printf ("Current Lst on JD %13.5f at %8.4f degrees West: ",
124 /* convert to required internal units */
127 /* compute LST and print */
129 lst = gst - RADHR (lng);
130 lst -= 24.0*floor(lst/24.0);
131 printf ("%7.4f\n", lst);
138 /* Update the time dependent variables */
140 void fgTimeUpdate(struct FLIGHT *f, struct fgTIME *t) {
141 /* get current Unix calendar time (in seconds) */
142 t->cur_time = time(NULL);
143 printf("Current Unix calendar time = %ld\n", t->cur_time);
145 /* get GMT break down for current time */
146 t->gmt = gmtime(&t->cur_time);
147 printf("Current GMT = %d/%d/%2d %d:%02d:%02d\n",
148 t->gmt->tm_mon+1, t->gmt->tm_mday, t->gmt->tm_year,
149 t->gmt->tm_hour, t->gmt->tm_min, t->gmt->tm_sec);
151 /* calculate modified Julian date */
152 t->mjd = cal_mjd ((int)(t->gmt->tm_mon+1), (double)t->gmt->tm_mday,
153 (int)(t->gmt->tm_year + 1900));
155 /* add in partial day */
156 t->mjd += (t->gmt->tm_hour / 24.0) + (t->gmt->tm_min / (24.0 * 60.0)) +
157 (t->gmt->tm_sec / (24.0 * 60.0 * 60.0));
159 /* convert "back" to Julian date + partial day (as a fraction of one) */
160 t->jd = t->mjd + MJD0;
161 printf("Current Julian Date = %.5f\n", t->jd);
163 /* Calculate local side real time */
164 t->lst = sidereal(t->mjd, -(FG_Longitude * RAD_TO_DEG));
165 printf("Current Sidereal Time = %.3f\n", t->lst);
170 /* Revision 1.3 1997/09/13 02:00:08 curt
171 /* Mostly working on stars and generating sidereal time for accurate star
174 * Revision 1.2 1997/08/27 03:30:35 curt
175 * Changed naming scheme of basic shared structures.
177 * Revision 1.1 1997/08/13 21:55:59 curt