apcs¶
- erfa.apcs(date1, date2, pv, ebpv, ehp)[source]¶
For an observer whose geocentric position and velocity are known, prepare star-independent astrometry parameters for transformations between ICRS and GCRS.
- Parameters:
- date1double array
- date2double array
- pvdouble array
- ebpvdouble array
- ehpdouble array
- Returns:
- astromeraASTROM 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) 2013-2023, NumFOCUS Foundation. Derived, with permission, from the SOFA library. See notes at end of file.