Py ERFA:docs

- Index
- Modules

### Navigation

- next »
- « previous |
- PyERFA v2.0.0.1 »
- erfa Package »
- apcs

# apcs

erfa.apcs(date1, date2, pv, ebpv, ehp)

  For an observer whose geocentric position and velocity are known, prepare star-independent astrometry parameters for transformations between ICRS and GCRS.

  Parameters
    date1 double array
    date2 double array
    pv double array
    ebpv double array
    ehp double array
  Returns
    astrom eraASTROM array

  Notes

  Wraps ERFA function `eraApcs`. The ERFA documentation is:

      - - - - - - - -
       e r a A p c s
      - - - - - - - -

      For an observer whose geocentric position and velocity are known,
      prepare star-independent astrometry parameters for transformations
      between ICRS and GCRS.  The Earth ephemeris is supplied by the
      caller.

      The parameters produced by this function are required in the space
      motion, parallax, light deflection and aberration parts of the
      astrometric transformation chain.

      Given:
         date1  double       TDB as a 2-part...
         date2  double       ...Julian Date (Note 1)
         pv     double[2][3] observer's geocentric pos/vel (m, m/s)
         ebpv   double[2][3] Earth barycentric PV (au, au/day)
         ehp    double[3]    Earth heliocentric P (au)

      Returned:
         astrom eraASTROM         star-independent astrometry parameters:
          pmt    double       PM time interval (SSB, Julian years)
          eb     double[3]    SSB to observer (vector, au)
          eh     double[3]    Sun to observer (unit vector)
          em     double       distance from Sun to observer (au)
          v      double[3]    barycentric observer velocity (vector, c)
          bm1    double       sqrt(1-|v|^2): reciprocal of Lorenz factor
          bpn    double[3][3] bias-precession-nutation matrix
          along  double       unchanged
          xpl    double       unchanged
          ypl    double       unchanged
          sphi   double       unchanged
          cphi   double       unchanged
          diurab double       unchanged
          eral   double       unchanged
          refa   double       unchanged
          refb   double       unchanged

      Notes:

      1) The TDB date date1+date2 is a Julian Date, apportioned in any
         convenient way between the two arguments.  For example,
         JD(TDB)=2450123.7 could be expressed in any of these ways, among
         others:

                date1          date2

             2450123.7           0.0       (JD method)
             2451545.0       -1421.3       (J2000 method)
             2400000.5       50123.2       (MJD method)
             2450123.5           0.2       (date & time method)

         The JD method is the most natural and convenient to use in cases
         where the loss of several decimal digits of resolution is
         acceptable.  The J2000 method is best matched to the way the
         argument is handled internally and will deliver the optimum
         resolution.  The MJD method and the date & time methods are both
         good compromises between resolution and convenience.  For most
         applications of this function the choice will not be at all
         critical.

         TT can be used instead of TDB without any significant impact on
         accuracy.

      2) All the vectors are with respect to BCRS axes.

      3) Providing separate arguments for (i) the observer's geocentric
         position and velocity and (ii) the Earth ephemeris is done for
         convenience in the geocentric, terrestrial and Earth orbit cases.
         For deep space applications it maybe more convenient to specify
         zero geocentric position and velocity and to supply the
         observer's position and velocity information directly instead of
         with respect to the Earth.  However, note the different units:
         m and m/s for the geocentric vectors, au and au/day for the
         heliocentric and barycentric vectors.

      4) In cases where the caller does not wish to provide the Earth
         ephemeris, the function eraApcs13 can be used instead of the
         present function.  This computes the Earth ephemeris using the
         ERFA function eraEpv00.

      5) This is one of several functions that inserts into the astrom
         structure star-independent parameters needed for the chain of
         astrometric transformations ICRS <-> GCRS <-> CIRS <-> observed.

         The various functions support different classes of observer and
         portions of the transformation chain:

              functions         observer        transformation

           eraApcg eraApcg13    geocentric      ICRS <-> GCRS
           eraApci eraApci13    terrestrial     ICRS <-> CIRS
           eraApco eraApco13    terrestrial     ICRS <-> observed
           eraApcs eraApcs13    space           ICRS <-> GCRS
           eraAper eraAper13    terrestrial     update Earth rotation
           eraApio eraApio13    terrestrial     CIRS <-> observed

         Those with names ending in "13" use contemporary ERFA models to
         compute the various ephemerides.  The others accept ephemerides
         supplied by the caller.

         The transformation from ICRS to GCRS covers space motion,
         parallax, light deflection, and aberration.  From GCRS to CIRS
         comprises frame bias and precession-nutation.  From CIRS to
         observed takes account of Earth rotation, polar motion, diurnal
         aberration and parallax (unless subsumed into the ICRS <-> GCRS
         transformation), and atmospheric refraction.

      6) The context structure astrom produced by this function is used by
         eraAtciq* and eraAticq*.

      Called:
         eraCp        copy p-vector
         eraPm        modulus of p-vector
         eraPn        decompose p-vector into modulus and direction
         eraIr        initialize r-matrix to identity

      This revision:   2021 February 24

      Copyright (C) 2013-2021, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.