flightgear/Time/fg_time.c

/**************************************************************************
 * fg_time.c -- data structures and routines for managing time related stuff.
 *
 * Written by Curtis Olson, started August 1997.
 *
 * Copyright (C) 1997  Curtis L. Olson  - curt@infoplane.com
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 *
 * $Id$
 * (Log is kept at end of this file)
 **************************************************************************/


#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#include "fg_time.h"
#include "../constants.h"
#include "../Flight/flight.h"
#include "../Time/fg_time.h"


#define DEGHR(x)        ((x)/15.)
#define RADHR(x)        DEGHR(x*RAD_TO_DEG)


struct fgTIME cur_time_params;


/* Initialize the time dependent variables */

void fgTimeInit(struct fgTIME *t) {
    t->lst_diff = -9999.0;
}


/* given a date in months, mn, days, dy, years, yr, return the
 * modified Julian date (number of days elapsed since 1900 jan 0.5),
 * mjd.  Adapted from Xephem.  */

double cal_mjd (int mn, double dy, int yr) {
    static double last_mjd, last_dy;
    double mjd;
    static int last_mn, last_yr;
    int b, d, m, y;
    long c;

    if (mn == last_mn && yr == last_yr && dy == last_dy) {
	mjd = last_mjd;
	return(mjd);
    }

    m = mn;
    y = (yr < 0) ? yr + 1 : yr;
    if (mn < 3) {
	m += 12;
	y -= 1;
    }

    if (yr < 1582 || (yr == 1582 && (mn < 10 || (mn == 10 && dy < 15)))) {
	b = 0;
    } else {
	int a;
	a = y/100;
	b = 2 - a + a/4;
    }

    if (y < 0) {
	c = (long)((365.25*y) - 0.75) - 694025L;
    } else {
	c = (long)(365.25*y) - 694025L;
    }
    
    d = 30.6001*(m+1);

    mjd = b + c + d + dy - 0.5;

    last_mn = mn;
    last_dy = dy;
    last_yr = yr;
    last_mjd = mjd;

    return(mjd);
}


/* given an mjd, return greenwich mean siderial time, gst */

double utc_gst (double mjd) {
    double gst;
    double day = floor(mjd-0.5)+0.5;
    double hr = (mjd-day)*24.0;
    double T, x;

    T = ((int)(mjd - 0.5) + 0.5 - J2000)/36525.0;
    x = 24110.54841 + (8640184.812866 + (0.093104 - 6.2e-6 * T) * T) * T;
    x /= 3600.0;
    gst = (1.0/SIDRATE)*hr + x;

    return(gst);
}


/* given Julian Date and Longitude (decimal degrees West) compute and
 * return Local Sidereal Time, in decimal hours.
 *
 * Provided courtesy of ecdowney@noao.edu (Elwood Downey) 
 */

double sidereal_precise (double mjd, double lng) {
    double gst;
    double lst;

    /* printf ("Current Lst on JD %13.5f at %8.4f degrees West: ", 
       mjd + MJD0, lng); */

    /* convert to required internal units */
    lng *= DEG_TO_RAD;

    /* compute LST and print */
    gst = utc_gst (mjd);
    lst = gst - RADHR (lng);
    lst -= 24.0*floor(lst/24.0);
    printf ("%7.4f\n", lst);

    /* that's all */
    return (lst);
}


/* return a courser but cheaper estimate of sidereal time */
double sidereal_course(struct tm *gmt, time_t now, double lng) {
    time_t start, start_gmt;
    struct tm mt;
    long int offset;
    double diff, part, days, hours, lst;
    int i;

    printf("COURSE: GMT = %d/%d/%2d %d:%02d:%02d\n", 
           gmt->tm_mon, gmt->tm_mday, gmt->tm_year,
           gmt->tm_hour, gmt->tm_min, gmt->tm_sec);

    mt.tm_mon = 2;
    mt.tm_mday = 21;
    mt.tm_year = gmt->tm_year;
    mt.tm_hour = 12;
    mt.tm_min = 0;
    mt.tm_sec = 0;

    start = mktime(&mt);

    offset = -(timezone / 3600 - daylight);

    printf("Raw time zone offset = %ld\n", timezone);
    printf("Daylight Savings = %d\n", daylight);

    printf("Local hours from GMT = %ld\n", offset);

    start_gmt = start - timezone + (daylight * 3600);

    printf("March 21 noon (CST) = %ld\n", start);
    printf("March 21 noon (GMT) = %ld\n", start_gmt);

    diff = (now - start_gmt) / (3600.0 * 24.0);

    printf("Time since 3/21/%2d GMT = %.2f\n", gmt->tm_year, diff);

    part = fmod(diff, 1.0);
    days = diff - part;
    hours = gmt->tm_hour + gmt->tm_min/60.0 + gmt->tm_sec/3600.0;

    lst = (days - lng)/15.0 + hours - 12;

    while ( lst < 0.0 ) {
	lst += 24.0;
    }

    printf("days = %.1f  hours = %.2f  lon = %.2f  lst = %.2f\n", 
	   days, hours, lng, lst);

    return(lst);
}


/* Update the time dependent variables */

void fgTimeUpdate(struct FLIGHT *f, struct fgTIME *t) {
    double lst_precise, lst_course;

    /* get current Unix calendar time (in seconds) */
    t->cur_time = time(NULL);
    printf("Current Unix calendar time = %ld\n", t->cur_time);

    /* get GMT break down for current time */
    t->gmt = gmtime(&t->cur_time);
    printf("Current GMT = %d/%d/%2d %d:%02d:%02d\n", 
           t->gmt->tm_mon+1, t->gmt->tm_mday, t->gmt->tm_year,
           t->gmt->tm_hour, t->gmt->tm_min, t->gmt->tm_sec);

    /* calculate modified Julian date */
    t->mjd = cal_mjd ((int)(t->gmt->tm_mon+1), (double)t->gmt->tm_mday, 
	     (int)(t->gmt->tm_year + 1900));

    /* add in partial day */
    t->mjd += (t->gmt->tm_hour / 24.0) + (t->gmt->tm_min / (24.0 * 60.0)) +
	   (t->gmt->tm_sec / (24.0 * 60.0 * 60.0));

    /* convert "back" to Julian date + partial day (as a fraction of one) */
    t->jd = t->mjd + MJD0;
    printf("Current Julian Date = %.5f\n", t->jd);

    printf("Current Longitude = %.3f\n", FG_Longitude * RAD_TO_DEG);

    /* Calculate local side real time */
    if ( t->lst_diff < -100.0 ) {
	/* first time through do the expensive calculation & cheap
           calculation to get the difference. */
	printf("First time, doing precise lst\n");
	t->lst = lst_precise = 
	    sidereal_precise(t->mjd, -(FG_Longitude * RAD_TO_DEG));
	lst_course = 
	    sidereal_course(t->gmt, t->cur_time, -(FG_Longitude * RAD_TO_DEG));
	t->lst_diff = lst_precise - lst_course;
    } else {
	/* course + difference should drift off very slowly */
	t->lst = 
	    sidereal_course(t->gmt, t->cur_time, -(FG_Longitude * RAD_TO_DEG))
	    + t->lst_diff;
    }
    printf("Current Sidereal Time = %.3f (%.3f) (diff = %.3f)\n", t->lst,
	   sidereal_precise(t->mjd, -(FG_Longitude * RAD_TO_DEG)),
	   t->lst_diff);
}


/* $Log$
/* Revision 1.4  1997/09/16 15:50:31  curt
/* Working on star alignment and time issues.
/*
 * Revision 1.3  1997/09/13 02:00:08  curt
 * Mostly working on stars and generating sidereal time for accurate star
 * placement.
 *
 * Revision 1.2  1997/08/27 03:30:35  curt
 * Changed naming scheme of basic shared structures.
 *
 * Revision 1.1  1997/08/13 21:55:59  curt
 * Initial revision.
 *
 */