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jahn |
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C $Header$ |
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C $Name$ |
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#include "RADTRANS_OPTIONS.h" |
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CBOP |
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C !ROUTINE: RADTRANS_GHA2000 |
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C !INTERFACE: ====================================================== |
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subroutine radtrans_gha2000 (radeg, iyr, imon, day, gha) |
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C !DESCRIPTION: |
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c This subroutine computes the Greenwich hour angle in degrees for the |
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c input time. It uses the model referenced in The Astronomical Almanac |
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c for 1984, Section S (Supplement) and documented in Exact |
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c closed-form geolocation algorithm for Earth survey sensors, by |
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c F.S. Patt and W.W. Gregg, Int. Journal of Remote Sensing, 1993. |
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c It includes the correction to mean sideral time for nutation |
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c as well as precession. |
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c Calling Arguments |
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c Name Type I/O Description |
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c |
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c iyr I*4 I Year (four digits) |
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c day R*8 I Day (time of day as fraction) |
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c gha R*8 O Greenwich hour angle (degrees) |
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c Subprograms referenced: |
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c |
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c JD Computes Julian day from calendar date |
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c EPHPARMS Computes mean solar longitude and anomaly and |
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c mean lunar lontitude and ascending node |
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c NUTATE Compute nutation corrections to lontitude and |
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c obliquity |
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c |
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c |
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c Program written by: Frederick S. Patt |
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c General Sciences Corporation |
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c November 2, 1992 |
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c |
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c Modification History: |
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c |
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C !USES: =========================================================== |
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IMPLICIT NONE |
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#include "RADTRANS_VARS.h" |
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C !INPUT PARAMETERS: =============================================== |
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INTEGER myThid |
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_RL radeg |
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INTEGER iyr, imon, day |
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C !OUTPUT PARAMETERS: ============================================== |
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_RL gha |
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C !FUNCTIONS: ====================================================== |
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INTEGER radtrans_jd |
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EXTERNAL radtrans_jd |
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C !LOCAL VARIABLES: ================================================ |
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integer iday,jday,nt |
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_RL fday, t, gmst, xls, gs, xlm, omega |
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data nutime /-99999/ |
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CEOP |
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c Compute days since J2000 |
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iday = int(day) |
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fday = day - iday |
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jday = radtrans_jd(iyr,imon,iday) |
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t = jday - 2451545.5D0 + fday |
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c Compute Greenwich Mean Sidereal Time (degrees) |
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gmst = 100.4606184D0 + 0.9856473663D0*t + 2.908D-13*t*t |
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c Check if need to compute nutation correction for this day |
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nt = int(t) |
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if (nt.ne.nutime) then |
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nutime = nt |
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call radtrans_ephparms (t, xls, gs, xlm, omega) |
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call radtrans_nutate (radeg, t, xls, gs, xlm, omega, dpsi, eps) |
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end if |
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c Include apparent time correction and time-of-day |
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gha = gmst + dpsi*cos(eps/radeg) + fday*360.0D0 |
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gha = mod(gha,360.0) |
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if (gha.lt.0.0D0) gha = gha + 360.0D0 |
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return |
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end |
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