[rv, astrom, pmt, eb, eh, em, v, bm1, bpn, along, xpl, ypl, sphi, cphi, diurab, eral, refa, refb] = ERFA.apio13(utc1, utc2, dut1, elong, phi, hm, xp, yp, phpa, tc, rh, wl)For a terrestrial observer, prepare star-independent astrometry parameters for transformations between CIRS and observed coordinates. The caller supplies UTC, site coordinates, ambient air conditions and observing wavelength.
utc1 double UTC as a 2-part...
utc2 double ...quasi Julian Date (Notes 1,2)
dut1 double UT1-UTC (seconds)
elong double longitude (radians, east +ve, Note 3)
phi double geodetic latitude (radians, Note 3)
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)
astrom ASTROM* star-independent astrometry parameters:
pmt double unchanged
eb double[3] unchanged
eh double[3] unchanged
em double unchanged
v double[3] unchanged
bm1 double unchanged
bpn double[3][3] unchanged
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)
int status: +1 = dubious year (Note 2)
0 = OK
-1 = unacceptable date
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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In cases where the caller wishes to supply his own Earth rotation information and refraction constants, the function Apc can be used instead of the present function.
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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.
eraUtctai UTC to TAI
eraTaitt TAI to TT
eraUtcut1 UTC to UT1
eraSp00 the TIO locator s', IERS 2000
eraEra00 Earth rotation angle, IAU 2000
eraRefco refraction constants for given ambient conditions
eraApio astrometry parameters, CIRS-observed
This revision: 2021 February 24
Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.