Function apcg 

pub fn apcg(
    date1: f64,
    date2: f64,
    ebpv: &[[f64; 3]; 2],
    ehp: &[f64; 3],
    astrom: &mut IauAstrom,
)

Prepare for ICRS <−> GCRS, geocentric, special

For a geocentric observer, prepare star-independent astrometry parameters for transformations between ICRS and GCRS coordinates. The Earth ephemeris is supplied by the caller.

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

This function is part of the International Astronomical Union’s SOFA (Standards of Fundamental Astronomy) software collection.

Status: support function.

Given:

   date1  double       TDB as a 2-part...
   date2  double       ...Julian Date (Note 1)
   ebpv   double[2][3] Earth barycentric pos/vel (au, au/day)
   ehp    double[3]    Earth heliocentric position (au)

Returned:

   astrom iauASTROM*   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. 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

     iauApcg iauApcg13    geocentric      ICRS <-> GCRS
     iauApci iauApci13    terrestrial     ICRS <-> CIRS
     iauApco iauApco13    terrestrial     ICRS <-> observed
     iauApcs iauApcs13    space           ICRS <-> GCRS
     iauAper iauAper13    terrestrial     update Earth rotation
     iauApio iauApio13    terrestrial     CIRS <-> observed

Those with names ending in “13” use contemporary SOFA 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.

4. The context structure astrom produced by this function is used by iauAtciq* and iauAticq*.

Called:

   iauApcs      astrometry parameters, ICRS-GCRS, space observer