For a terrestrial observer, prepare star-independent astrometry parameters for transformations between ICRS and geocentric CIRS coordinates. The Earth ephemeris and CIP/CIO are supplied by the caller.
The parameters produced by this routine are required in the parallax, light deflection, aberration, and bias-precession-nutation parts of the astrometric transformation chain.
Status: support routine.
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
2450123.7D0 0D0 (JD method)
2451545D0 -1421.3D0 (J2000 method)
2400000.5D0 50123.2D0 (MJD method)
2450123.5D0 0.2D0 (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 routine the choice will not be at all critical.
TT can be used instead of TDB without any significant impact on accuracy.
All the vectors are with respect to BCRS axes.
In cases where the caller does not wish to provide the Earth ephemeris and CIP/CIO, the routine APCI13 can be used instead of the present routine. This computes the required quantities using other SOFA routines.
This is one of several routines that inserts into the ASTROM array star-independent parameters needed for the chain of astrometric transformations ICRS <-> GCRS <-> CIRS <-> observed.
The various routines support different classes of observer and portions of the transformation chain:
routines observer transformation
APCG APCG13 geocentric ICRS <-> GCRS
APCI APCI13 terrestrial ICRS <-> CIRS
APCO APCO13 terrestrial ICRS <-> observed
APCS APCS13 space ICRS <-> GCRS
APER APER13 terrestrial update Earth rotation
APIO APIO13 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 array ASTROM produced by this routine is used by ATCIQ and ATICQ.
| Type | Intent | Optional | Attributes | Name | ||
|---|---|---|---|---|---|---|
| real(kind=wp), | intent(in) | :: | date1 | TDB as a 2-part... |
||
| real(kind=wp), | intent(in) | :: | date2 | |||
| real(kind=wp), | intent(in), | dimension(3,2) | :: | ebpv | Earth barycentric position/velocity (au, au/day) |
|
| real(kind=wp), | intent(in), | dimension(3) | :: | ehp | Earth heliocentric position (au) |
|
| real(kind=wp), | intent(in) | :: | x | CIP X (component of unit vector) |
||
| real(kind=wp), | intent(in) | :: | y | CIP Y (component of unit vector) |
||
| real(kind=wp), | intent(in) | :: | s | the CIO locator s (radians) |
||
| real(kind=wp), | intent(inout), | dimension(30) | :: | astrom | star-independent astrometry parameters: (1) PM time interval (SSB, Julian years) (2-4) SSB to observer (vector, au) (5-7) Sun to observer (unit vector) (8) distance from Sun to observer (au) (9-11) v: barycentric observer velocity (vector, c) (12) sqrt(1-|v|^2): reciprocal of Lorenz factor (13-21) bias-precession-nutation matrix (22) unchanged (23) unchanged (24) unchanged (25) unchanged (26) unchanged (27) unchanged (28) unchanged (29) unchanged (30) unchanged |
subroutine APCI ( date1, date2, ebpv, ehp, x, y, s, astrom )
implicit none
real(wp),intent(in) :: date1 !! TDB as a 2-part...
real(wp),intent(in) :: date2 !! ...Julian Date (Note 1)
real(wp),dimension(3,2),intent(in) :: ebpv !! Earth barycentric position/velocity (au, au/day)
real(wp),dimension(3),intent(in) :: ehp !! Earth heliocentric position (au)
real(wp),intent(in) :: x !! CIP X (component of unit vector)
real(wp),intent(in) :: y !! CIP Y (component of unit vector)
real(wp),intent(in) :: s !! the CIO locator s (radians)
real(wp),dimension(30),intent(inout) :: astrom !! star-independent astrometry parameters:
!!
!! (1) PM time interval (SSB, Julian years)
!! (2-4) SSB to observer (vector, au)
!! (5-7) Sun to observer (unit vector)
!! (8) distance from Sun to observer (au)
!! (9-11) v: barycentric observer velocity (vector, c)
!! (12) sqrt(1-|v|^2): reciprocal of Lorenz factor
!! (13-21) bias-precession-nutation matrix
!! (22) unchanged
!! (23) unchanged
!! (24) unchanged
!! (25) unchanged
!! (26) unchanged
!! (27) unchanged
!! (28) unchanged
!! (29) unchanged
!! (30) unchanged
! Star-independent astrometry parameters for geocenter.
call APCG ( date1, date2, ebpv, ehp, astrom )
! CIO based BPN matrix.
call C2IXYS ( x, y, s, astrom(13) )
end subroutine APCI