Precompute apparent to observed place parameters required by slaAopqk and slaOapqk.
date d UTC date/time (Modified Julian Date, JD-2400000.5) dut d delta UT: UT1-UTC (UTC seconds) elongm d mean longitude of the observer (radians, east +ve) phim d mean geodetic latitude of the observer (radians) hm d observer's height above sea level (metres) xp d polar motion x-coordinate (radians) yp d polar motion y-coordinate (radians) tdk d local ambient temperature (DegK; std=273.155) pmb d local atmospheric pressure (mB; std=1013.25) rh d local relative humidity (in the range 0.0-1.0) wl d effective wavelength (micron, e.g. 0.55) tlr d tropospheric lapse rate (DegK/metre, e.g. 0.0065)
(0) geodetic latitude (radians) (1,2) sine and cosine of geodetic latitude (3) magnitude of diurnal aberration vector (4) height (hm) (5) ambient temperature (tdk) (6) pressure (pmb) (7) relative humidity (rh) (8) wavelength (wl) (9) lapse rate (tlr) (10,11) refraction constants A and B (radians) (12) longitude + eqn of equinoxes + sidereal DUT (radians) (13) local apparent sidereal time (radians)
1) 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. 2) The date argument is UTC expressed as an MJD. This is, strictly speaking, wrong, because of leap seconds. However, as long as the delta UT and the UTC are consistent there are no difficulties, except during a leap second. In this case, the start of the 61st second of the final minute should begin a new MJD day and the old pre-leap delta UT should continue to be used. As the 61st second completes, the MJD should revert to the start of the day as, simultaneously, the delta UTC changes by one second to its post-leap new value. 3) The delta UT (UT1-UTC) is tabulated in IERS circulars and elsewhere. It increases by exactly one second at the end of each UTC leap second, introduced in order to keep delta UT within +/- 0.9 seconds. 4) IMPORTANT -- TAKE CARE WITH THE LONGITUDE SIGN CONVENTION. The longitude required by the present routine is east-positive, in accordance with geographical convention (and right-handed). In particular, note that the longitudes returned by the slaObs routine are west-positive, following astronomical usage, and must be reversed in sign before use in the present routine. 5) The polar coordinates xp,yp can be obtained from IERS circulars and equivalent publications. The maximum amplitude is about 0.3 arcseconds. If xp,yp values are unavailable, use xp=yp=0.0. See page B60 of the 1988 Astronomical Almanac for a definition of the two angles. 6) The height above sea level of the observing station, HM, can be obtained from the Astronomical Almanac (Section J in the 1988 edition), or via the routine slaObs. If p, the pressure in millibars, is available, an adequate estimate of hm can be obtained from the expression hm = -8149.9415 * log(p/1013.25); (See Astrophysical Quantities, C.W.Allen, 3rd edition, section 52.) Similarly, if the pressure p is not known, it can be estimated from the height of the observing
p = 1013.25 * exp(-hm/8149.9415); Note, however, that the refraction is proportional to the pressure and that an accurate p value is important for precise work. Defined in slamac.h: D2PI, DS2R
slaGeoc, slaRefco, slaEqeqx, slaAoppat P.T.Wallace Starlink 31 October 1993