Function apco

Source

pub fn apco(
    date1: f64,
    date2: f64,
    ebpv: &[[f64; 3]; 2],
    ehp: &[f64; 3],
    x: f64,
    y: f64,
    s: f64,
    theta: f64,
    elong: f64,
    phi: f64,
    hm: f64,
    xp: f64,
    yp: f64,
    sp: f64,
    refa: f64,
    refb: f64,
    astrom: &mut IauAstrom,
)

Expand description

Prepare for ICRS <−> observed, terrestrial, special

For a terrestrial observer, prepare star-independent astrometry parameters for transformations between ICRS and observed coordinates. The caller supplies the Earth ephemeris, the Earth rotation information and the refraction constants as well as the site coordinates.

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 PV (au, au/day, Note 2)
   ehp    double[3]    Earth heliocentric P (au, Note 2)
   x,y    double       CIP X,Y (components of unit vector)
   s      double       the CIO locator s (radians)
   theta  double       Earth rotation angle (radians)
   elong  double       longitude (radians, east +ve, Note 3)
   phi    double       latitude (geodetic, radians, Note 3)
   hm     double       height above ellipsoid (m, geodetic, Note 3)
   xp,yp  double       polar motion coordinates (radians, Note 4)
   sp     double       the TIO locator s' (radians, Note 4)
   refa   double       refraction constant A (radians, Note 5)
   refb   double       refraction constant B (radians, Note 5)

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       adjusted longitude (radians)
    xpl    double       polar motion xp wrt local meridian (radians)
    ypl    double       polar motion yp wrt local meridian (radians)
    sphi   double       sine of geodetic latitude
    cphi   double       cosine of geodetic latitude
    diurab double       magnitude of diurnal aberration vector
    eral   double       "local" Earth rotation angle (radians)
    refa   double       refraction constant A (radians)
    refb   double       refraction constant B (radians)

Notes:

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.

The vectors eb, eh, and all the astrom vectors, are with respect to BCRS axes.
The geographical coordinates are with respect to the WGS84 reference ellipsoid. TAKE CARE WITH THE LONGITUDE SIGN CONVENTION: the longitude required by the present function is right-handed, i.e. east-positive, in accordance with geographical convention.

The adjusted longitude stored in the astrom array takes into account the TIO locator and polar motion.
xp and yp are the coordinates (in radians) of the Celestial Intermediate Pole with respect to the International Terrestrial Reference System (see IERS Conventions), measured along the meridians 0 and 90 deg west respectively. sp is the TIO locator s’, in radians, which positions the Terrestrial Intermediate Origin on the equator. For many applications, xp, yp and (especially) sp can be set to zero.

Internally, the polar motion is stored in a form rotated onto the local meridian.
The refraction constants refa and refb are for use in a dZ = Atan(Z)+Btan^3(Z) model, where Z is the observed (i.e. refracted) zenith distance and dZ is the amount of refraction.
It is advisable to take great care with units, as even unlikely values of the input parameters are accepted and processed in accordance with the models used.
In cases where the caller does not wish to provide the Earth Ephemeris, the Earth rotation information and refraction constants, the function iauApco13 can be used instead of the present function. This starts from UTC and weather readings etc. and computes suitable values using other SOFA functions.
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.
The context structure astrom produced by this function is used by iauAtioq, iauAtoiq, iauAtciq* and iauAticq*.

Called:

   iauIr        initialize r-matrix to identity
   iauRz        rotate around Z-axis
   iauRy        rotate around Y-axis
   iauRx        rotate around X-axis
   iauAnpm      normalize angle into range +/- pi
   iauC2ixys    celestial-to-intermediate matrix, given X,Y and s
   iauPvtob     position/velocity of terrestrial station
   iauTrxpv     product of transpose of r-matrix and pv-vector
   iauApcs      astrometry parameters, ICRS-GCRS, space observer
   iauCr        copy r-matrix

apco

Function apco Copy item path

Function apco