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.
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.
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 routine is right-handed, i.e. east-positive, in accordance with geographical convention.
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 routine APCO13 can be used instead of the present routine. This starts from UTC and weather readings etc. and computes suitable values 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 ATIOQ, ATOIQ, 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 pos/vel (au, au/day, Note 2) |
|
| real(kind=wp), | intent(in), | dimension(3) | :: | ehp | Earth heliocentric position (au, Note 2) |
|
| 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(in) | :: | theta | Earth rotation angle (radians) |
||
| real(kind=wp), | intent(in) | :: | elong | longitude (radians, east +ve, Note 3) |
||
| real(kind=wp), | intent(in) | :: | phi | latitude (geodetic, radians, Note 3) |
||
| real(kind=wp), | intent(in) | :: | hm | height above ellipsoid (m, geodetic, Note 3) |
||
| real(kind=wp), | intent(in) | :: | xp | polar motion coordinate (radians, Note 4) |
||
| real(kind=wp), | intent(in) | :: | yp | polar motion coordinate (radians, Note 4) |
||
| real(kind=wp), | intent(in) | :: | sp | the TIO locator s' (radians, Note 4) |
||
| real(kind=wp), | intent(in) | :: | refa | refraction constant A (radians, Note 5) |
||
| real(kind=wp), | intent(in) | :: | refb | refraction constant B (radians, Note 5) |
||
| real(kind=wp), | intent(out), | 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) longitude + s' (radians) (23) polar motion xp wrt local meridian (radians) (24) polar motion yp wrt local meridian (radians) (25) sine of geodetic latitude (26) cosine of geodetic latitude (27) magnitude of diurnal aberration vector (28) "local" Earth rotation angle (radians) (29) refraction constant A (radians) (30) refraction constant B (radians) |
subroutine APCO ( date1, date2, ebpv, ehp, x, y, s, &
theta, elong, phi, hm, xp, yp, sp, &
refa, refb, 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 pos/vel (au, au/day, Note 2)
real(wp),dimension(3),intent(in) :: ehp !! Earth heliocentric position (au, Note 2)
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),intent(in) :: theta !! Earth rotation angle (radians)
real(wp),intent(in) :: elong !! longitude (radians, east +ve, Note 3)
real(wp),intent(in) :: phi !! latitude (geodetic, radians, Note 3)
real(wp),intent(in) :: hm !! height above ellipsoid (m, geodetic, Note 3)
real(wp),intent(in) :: xp !! polar motion coordinate (radians, Note 4)
real(wp),intent(in) :: yp !! polar motion coordinate (radians, Note 4)
real(wp),intent(in) :: sp !! the TIO locator s' (radians, Note 4)
real(wp),intent(in) :: refa !! refraction constant A (radians, Note 5)
real(wp),intent(in) :: refb !! refraction constant B (radians, Note 5)
real(wp),dimension(30),intent(out) :: 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) longitude + s' (radians)
!! (23) polar motion xp wrt local meridian (radians)
!! (24) polar motion yp wrt local meridian (radians)
!! (25) sine of geodetic latitude
!! (26) cosine of geodetic latitude
!! (27) magnitude of diurnal aberration vector
!! (28) "local" Earth rotation angle (radians)
!! (29) refraction constant A (radians)
!! (30) refraction constant B (radians)
real(wp) :: sl, cl, r(3,3), pvc(3,2), pv(3,2)
! Longitude with adjustment for TIO locator s'.
astrom(22) = elong + sp
! Polar motion, rotated onto the local meridian.
sl = sin(astrom(22))
cl = cos(astrom(22))
astrom(23) = xp*cl - yp*sl
astrom(24) = xp*sl + yp*cl
! Functions of latitude.
astrom(25) = sin(phi)
astrom(26) = cos(phi)
! Refraction constants.
astrom(29) = refa
astrom(30) = refb
! Local Earth rotation angle.
call APER ( theta, astrom )
! Disable the (redundant) diurnal aberration step.
astrom(27) = 0.0_wp
! CIO based BPN matrix.
call C2IXYS ( x, y, s, r )
! Observer's geocentric position and velocity (m, m/s, CIRS).
call PVTOB ( elong, phi, hm, xp, yp, sp, theta, pvc )
! Rotate into GCRS.
call TRXPV ( r, pvc, pv )
! ICRS <-> GCRS parameters.
call APCS ( date1, date2, pv, ebpv, ehp, astrom )
! Store the CIO based BPN matrix.
call CR ( r, astrom(13) )
end subroutine APCO