Changeset 3371 for trunk/MagicSoft/Mars/mbase

Timestamp:

03/01/04 15:33:06 (21 years ago)

Author:

tbretz

Message:

*** empty log message ***

Location:

trunk/MagicSoft/Mars/mbase

Files:

• MTime.cc (modified) (1 diff)
• MTime.h (modified) (2 diffs)

trunk/MagicSoft/Mars/mbase/MTime.cc

@@ -317 +317 @@
     return kTRUE;
 
+}
+
+// --------------------------------------------------------------------------
+//
+//  Conversion from Universal Time to Greenwich mean sidereal time,
+//  with rounding errors minimized.
+//
+//  The result is the Greenwich Mean Sidereal Time (radians)
+//
+//  There is no restriction on how the UT is apportioned between the
+//  date and ut1 arguments.  Either of the two arguments could, for
+//  example, be zero and the entire date+time supplied in the other.
+//  However, the routine is designed to deliver maximum accuracy when
+//  the date argument is a whole number and the ut argument lies in
+//  the range 0 to 1, or vice versa.
+//
+//  The algorithm is based on the IAU 1982 expression (see page S15 of
+//  the 1984 Astronomical Almanac).  This is always described as giving
+//  the GMST at 0 hours UT1.  In fact, it gives the difference between
+//  the GMST and the UT, the steady 4-minutes-per-day drawing-ahead of
+//  ST with respect to UT.  When whole days are ignored, the expression
+//  happens to equal the GMST at 0 hours UT1 each day.
+//
+//  In this routine, the entire UT1 (the sum of the two arguments date
+//  and ut) is used directly as the argument for the standard formula.
+//  The UT1 is then added, but omitting whole days to conserve accuracy.
+//
+//  The extra numerical precision delivered by the present routine is
+//  unlikely to be important in an absolute sense, but may be useful
+//  when critically comparing algorithms and in applications where two
+//  sidereal times close together are differenced.
+//
+Double_t MTime::GetGmst() const
+{
+    const Double_t ut = (Double_t)(fNanoSec/1e6+(Long_t)fTime)/kDay;
+
+    // Julian centuries since J2000.
+    const Double_t t = (ut -(51544.5-fMjd)) / 36525.0;
+
+    // GMST at this UT1
+    const Double_t r1 = 24110.54841+(8640184.812866+(0.093104-6.2e-6*t)*t)*t;
+    const Double_t r2 = 86400.0*ut;
+
+    const Double_t sum = (r1+r2)/(24*3600);
+
+    return fmod(sum, 1)*TMath::TwoPi()+TMath::TwoPi();
 }
 

trunk/MagicSoft/Mars/mbase/MTime.h

@@ -56 +56 @@
         Set(tm);
     }
-    MTime(Double_t d);
+    MTime(Double_t mjd);
     MTime(const MTime& t) : fMjd(t.fMjd), fTime(t.fTime), fNanoSec(t.fNanoSec)
     {
@@ -85 +85 @@
     void     SetMjd(Double_t m);
     Double_t GetMjd() const;
+    Double_t GetGmst() const;
     TString  GetString() const;
     TString  GetStringFmt(const char *fmt=0) const;