NASA Ocean Color

ocssw V2022

 /*
  *----------------------------------------------------------------------
  * @(#) gmha.c      1.1 20 Mar 93   <shc>
  * Copyright (c) 1993, CSIRO Division of Oceanography
  *----------------------------------------------------------------------
  *
  * gmha --
  *
  *  Greenwich Mean Hour angle.
  *
  * Results:
  *
  *  gmha() returns the Greenwich mean hour angle at a given time,
  *  which must be in modified Julian date format (number of days since
  *  noon on the 31st Dec 4714 minus 2,400,000).  It does not exhibit
  *  the slight day-to-day discontinuity of algorithms based on day and
  *  fraction.  Lifted and adapted from Chris Rizos's code which was
  *  derived from a U.S. Naval Observatory circular.  dgmha() returns
  *  the derivative of mean hour angle in radians/sec.
  *
  * Side effects:
  *  None.
  *
  * History:
  *  1.0 28 Nov 90   <shc>
  *     Initial FORTRAN version.
  *
  *  1.1 14 May 92   <shc>
  *     Changed gha() to gmha().
  *
  *  1.1 20 Mar 93   <shc>
  *     Converted from FORTRAN to C.
  *
  *  1.2 27 Jun 95   <shc>
  *     Added dgmha().
  *
  *----------------------------------------------------------------------
  */
  
 #include <math.h>
 #include "orbit.h"
  
  
 /*
  * Polynomial coefficients for GMST from USNO circular #163, P.A3,
  * converted to radians and divided by 36525.0**N (N=0..3)
  */
  
 static double gmstc[] = {
     0.4894961212823058751375704430e+01,
     0.6300388098984893552276513720e+01,
     0.5075209994113591478053805523e-14,
     -0.9253097568194335640067190688e-23
 };
  
 /*
  * Greenwich mean hour angle (radians)
  */
  
 double
 gmha(tjd)
 double tjd; /* Time in Julian days mod 2400000 */
 {
     double t, sid, gmha;
  
     t = tjd - J2000;
     sid = gmstc[0] + t * (gmstc[1] + t * (gmstc[2] + t * gmstc[3]));
     gmha = fmod(fmod(sid, D2PI) + D2PI, D2PI);
  
     return (gmha);
 }
  
 /*
  * Greenwich mean hour angle derivative (radians/day)
  */
  
 double
 gmhadot(tjd)
 double tjd; /* Time in Julian days mod 2400000 */
 {
     double t, dsid;
  
     t = tjd - J2000;
     dsid = gmstc[1] + t * (2.0 * gmstc[2] + t * 3.0 * gmstc[3]);
  
     return (dsid);
 }

data_t t[NROOTS+1]

Definition: decode_rs.h:77

gmhadot

double gmhadot(double tjd)

Definition: gmha.c:78

gmha

double gmha(double tjd)

Definition: gmha.c:61

orbit.h

D2PI

#define D2PI

Definition: orbit.h:45

J2000

#define J2000

Definition: orbit.h:72