Py ERFA:docs

- Index
- Modules

### Navigation

- next »
- « previous |
- PyERFA v2.0.1.5 »
- erfa Package »
- apco13

# apco13

erfa.apco13(utc1, utc2, dut1, elong, phi, hm, xp, yp, phpa, tc, rh, wl)

  For a terrestrial observer, prepare star-independent astrometry parameters for transformations between ICRS and observed coordinates.

  Parameters:
    utc1 double array
    utc2 double array
    dut1 double array
    elong double array
    phi double array
    hm double array
    xp double array
    yp double array
    phpa double array
    tc double array
    rh double array
    wl double array
  Returns:
    astrom eraASTROM array
    eo double array

  Notes

  Wraps ERFA function `eraApco13`. The ERFA documentation is:

      - - - - - - - - - -
       e r a A p c o 1 3
      - - - - - - - - - -

      For a terrestrial observer, prepare star-independent astrometry
      parameters for transformations between ICRS and observed
      coordinates.  The caller supplies UTC, site coordinates, ambient air
      conditions and observing wavelength, and ERFA models are used to
      obtain the Earth ephemeris, CIP/CIO and refraction constants.

      The parameters produced by this function are required in the
      parallax, light deflection, aberration, and bias-precession-nutation
      parts of the ICRS/CIRS transformations.

      Given:
         utc1   double     UTC as a 2-part...
         utc2   double     ...quasi Julian Date (Notes 1,2)
         dut1   double     UT1-UTC (seconds, Note 3)
         elong  double     longitude (radians, east +ve, Note 4)
         phi    double     latitude (geodetic, radians, Note 4)
         hm     double     height above ellipsoid (m, geodetic, Notes 4,6)
         xp,yp  double     polar motion coordinates (radians, Note 5)
         phpa   double     pressure at the observer (hPa = mB, Note 6)
         tc     double     ambient temperature at the observer (deg C)
         rh     double     relative humidity at the observer (range 0-1)
         wl     double     wavelength (micrometers, Note 7)

      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       longitude + s' (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)
         eo     double          equation of the origins (ERA-GST, radians)

      Returned (function value):
                int        status: +1 = dubious year (Note 2)
                                    0 = OK
                                   -1 = unacceptable date

      Notes:

      1)  utc1+utc2 is quasi Julian Date (see Note 2), apportioned in any
          convenient way between the two arguments, for example where utc1
          is the Julian Day Number and utc2 is the fraction of a day.

          However, JD cannot unambiguously represent UTC during a leap
          second unless special measures are taken.  The convention in the
          present function is that the JD day represents UTC days whether
          the length is 86399, 86400 or 86401 SI seconds.

          Applications should use the function eraDtf2d to convert from
          calendar date and time of day into 2-part quasi Julian Date, as
          it implements the leap-second-ambiguity convention just
          described.

      2)  The warning status "dubious year" flags UTCs that predate the
          introduction of the time scale or that are too far in the
          future to be trusted.  See eraDat for further details.

      3)  UT1-UTC is tabulated in IERS bulletins.  It increases by exactly
          one second at the end of each positive UTC leap second,
          introduced in order to keep UT1-UTC within +/- 0.9s.  n.b. This
          practice is under review, and in the future UT1-UTC may grow
          essentially without limit.

      4)  The geographical coordinates are with respect to the ERFA_WGS84
          reference ellipsoid.  TAKE CARE WITH THE LONGITUDE SIGN:  the
          longitude required by the present function is east-positive
          (i.e. right-handed), in accordance with geographical convention.

      5)  The polar motion xp,yp can be obtained from IERS bulletins.  The
          values are the coordinates (in radians) of the Celestial
          Intermediate Pole with respect to the International Terrestrial
          Reference System (see IERS Conventions 2003), measured along the
          meridians 0 and 90 deg west respectively.  For many
          applications, xp and yp can be set to zero.

          Internally, the polar motion is stored in a form rotated onto
          the local meridian.

      6)  If hm, the height above the ellipsoid of the observing station
          in meters, is not known but phpa, the pressure in hPa (=mB), is
          available, an adequate estimate of hm can be obtained from the
          expression

                hm = -29.3 * tsl * log ( phpa / 1013.25 );

          where tsl is the approximate sea-level air temperature in K
          (See Astrophysical Quantities, C.W.Allen, 3rd edition, section
          52).  Similarly, if the pressure phpa is not known, it can be
          estimated from the height of the observing station, hm, as
          follows:

                phpa = 1013.25 * exp ( -hm / ( 29.3 * tsl ) );

          Note, however, that the refraction is nearly proportional to
          the pressure and that an accurate phpa value is important for
          precise work.

      7)  The argument wl specifies the observing wavelength in
          micrometers.  The transition from optical to radio is assumed to
          occur at 100 micrometers (about 3000 GHz).

      8)  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.

      9)  In cases where the caller wishes to supply his own Earth
          ephemeris, Earth rotation information and refraction constants,
          the function eraApco can be used instead of the present function.

      10) 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.

      11) The context structure astrom produced by this function is used
          by eraAtioq, eraAtoiq, eraAtciq* and eraAticq*.

      Called:
         eraUtctai    UTC to TAI
         eraTaitt     TAI to TT
         eraUtcut1    UTC to UT1
         eraEpv00     Earth position and velocity
         eraPnm06a    classical NPB matrix, IAU 2006/2000A
         eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
         eraS06       the CIO locator s, given X,Y, IAU 2006
         eraEra00     Earth rotation angle, IAU 2000
         eraSp00      the TIO locator s', IERS 2000
         eraRefco     refraction constants for given ambient conditions
         eraApco      astrometry parameters, ICRS-observed
         eraEors      equation of the origins, given NPB matrix and s

      This revision:   2022 May 3

      Copyright (C) 2013-2023, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.