#include "erfa.h" void eraRefco(double phpa, double tc, double rh, double wl, double *refa, double *refb) /* ** - - - - - - - - - ** e r a R e f c o ** - - - - - - - - - ** ** Determine the constants A and B in the atmospheric refraction model ** dZ = A tan Z + B tan^3 Z. ** ** Z is the "observed" zenith distance (i.e. affected by refraction) ** and dZ is what to add to Z to give the "topocentric" (i.e. in vacuo) ** zenith distance. ** ** Given: ** phpa double pressure at the observer (hPa = millibar) ** tc double ambient temperature at the observer (deg C) ** rh double relative humidity at the observer (range 0-1) ** wl double wavelength (micrometers) ** ** Returned: ** refa double* tan Z coefficient (radians) ** refb double* tan^3 Z coefficient (radians) ** ** Notes: ** ** 1) The model balances speed and accuracy to give good results in ** applications where performance at low altitudes is not paramount. ** Performance is maintained across a range of conditions, and ** applies to both optical/IR and radio. ** ** 2) The model omits the effects of (i) height above sea level (apart ** from the reduced pressure itself), (ii) latitude (i.e. the ** flattening of the Earth), (iii) variations in tropospheric lapse ** rate and (iv) dispersive effects in the radio. ** ** The model was tested using the following range of conditions: ** ** lapse rates 0.0055, 0.0065, 0.0075 deg/meter ** latitudes 0, 25, 50, 75 degrees ** heights 0, 2500, 5000 meters ASL ** pressures mean for height -10% to +5% in steps of 5% ** temperatures -10 deg to +20 deg with respect to 280 deg at SL ** relative humidity 0, 0.5, 1 ** wavelengths 0.4, 0.6, ... 2 micron, + radio ** zenith distances 15, 45, 75 degrees ** ** The accuracy with respect to raytracing through a model ** atmosphere was as follows: ** ** worst RMS ** ** optical/IR 62 mas 8 mas ** radio 319 mas 49 mas ** ** For this particular set of conditions: ** ** lapse rate 0.0065 K/meter ** latitude 50 degrees ** sea level ** pressure 1005 mb ** temperature 280.15 K ** humidity 80% ** wavelength 5740 Angstroms ** ** the results were as follows: ** ** ZD raytrace eraRefco Saastamoinen ** ** 10 10.27 10.27 10.27 ** 20 21.19 21.20 21.19 ** 30 33.61 33.61 33.60 ** 40 48.82 48.83 48.81 ** 45 58.16 58.18 58.16 ** 50 69.28 69.30 69.27 ** 55 82.97 82.99 82.95 ** 60 100.51 100.54 100.50 ** 65 124.23 124.26 124.20 ** 70 158.63 158.68 158.61 ** 72 177.32 177.37 177.31 ** 74 200.35 200.38 200.32 ** 76 229.45 229.43 229.42 ** 78 267.44 267.29 267.41 ** 80 319.13 318.55 319.10 ** ** deg arcsec arcsec arcsec ** ** The values for Saastamoinen's formula (which includes terms ** up to tan^5) are taken from Hohenkerk and Sinclair (1985). ** ** 3) A wl value in the range 0-100 selects the optical/IR case and is ** wavelength in micrometers. Any value outside this range selects ** the radio case. ** ** 4) Outlandish input parameters are silently limited to ** mathematically safe values. Zero pressure is permissible, and ** causes zeroes to be returned. ** ** 5) The algorithm draws on several sources, as follows: ** ** a) The formula for the saturation vapour pressure of water as ** a function of temperature and temperature is taken from ** Equations (A4.5-A4.7) of Gill (1982). ** ** b) The formula for the water vapour pressure, given the ** saturation pressure and the relative humidity, is from ** Crane (1976), Equation (2.5.5). ** ** c) The refractivity of air is a function of temperature, ** total pressure, water-vapour pressure and, in the case ** of optical/IR, wavelength. The formulae for the two cases are ** developed from Hohenkerk & Sinclair (1985) and Rueger (2002). ** ** d) The formula for beta, the ratio of the scale height of the ** atmosphere to the geocentric distance of the observer, is ** an adaption of Equation (9) from Stone (1996). The ** adaptations, arrived at empirically, consist of (i) a small ** adjustment to the coefficient and (ii) a humidity term for the ** radio case only. ** ** e) The formulae for the refraction constants as a function of ** n-1 and beta are from Green (1987), Equation (4.31). ** ** References: ** ** Crane, R.K., Meeks, M.L. (ed), "Refraction Effects in the Neutral ** Atmosphere", Methods of Experimental Physics: Astrophysics 12B, ** Academic Press, 1976. ** ** Gill, Adrian E., "Atmosphere-Ocean Dynamics", Academic Press, ** 1982. ** ** Green, R.M., "Spherical Astronomy", Cambridge University Press, ** 1987. ** ** Hohenkerk, C.Y., & Sinclair, A.T., NAO Technical Note No. 63, ** 1985. ** ** Rueger, J.M., "Refractive Index Formulae for Electronic Distance ** Measurement with Radio and Millimetre Waves", in Unisurv Report ** S-68, School of Surveying and Spatial Information Systems, ** University of New South Wales, Sydney, Australia, 2002. ** ** Stone, Ronald C., P.A.S.P. 108, 1051-1058, 1996. ** ** Copyright (C) 2013-2016, NumFOCUS Foundation. ** Derived, with permission, from the SOFA library. See notes at end of file. */ { int optic; double p, t, r, w, ps, pw, tk, wlsq, gamma, beta; /* Decide whether optical/IR or radio case: switch at 100 microns. */ optic = ( wl <= 100.0 ); /* Restrict parameters to safe values. */ t = ERFA_GMAX ( tc, -150.0 ); t = ERFA_GMIN ( t, 200.0 ); p = ERFA_GMAX ( phpa, 0.0 ); p = ERFA_GMIN ( p, 10000.0 ); r = ERFA_GMAX ( rh, 0.0 ); r = ERFA_GMIN ( r, 1.0 ); w = ERFA_GMAX ( wl, 0.1 ); w = ERFA_GMIN ( w, 1e6 ); /* Water vapour pressure at the observer. */ if ( p > 0.0 ) { ps = pow ( 10.0, ( 0.7859 + 0.03477*t ) / ( 1.0 + 0.00412*t ) ) * ( 1.0 + p * ( 4.5e-6 + 6e-10*t*t ) ); pw = r * ps / ( 1.0 - (1.0-r)*ps/p ); } else { pw = 0.0; } /* Refractive index minus 1 at the observer. */ tk = t + 273.15; if ( optic ) { wlsq = w * w; gamma = ( ( 77.53484e-6 + ( 4.39108e-7 + 3.666e-9/wlsq ) / wlsq ) * p - 11.2684e-6*pw ) / tk; } else { gamma = ( 77.6890e-6*p - ( 6.3938e-6 - 0.375463/tk ) * pw ) / tk; } /* Formula for beta from Stone, with empirical adjustments. */ beta = 4.4474e-6 * tk; if ( ! optic ) beta -= 0.0074 * pw * beta; /* Refraction constants from Green. */ *refa = gamma * ( 1.0 - beta ); *refb = - gamma * ( beta - gamma / 2.0 ); /* Finished. */ } /*---------------------------------------------------------------------- ** ** ** Copyright (C) 2013-2016, NumFOCUS Foundation. ** All rights reserved. ** ** This library is derived, with permission, from the International ** Astronomical Union's "Standards of Fundamental Astronomy" library, ** available from http://www.iausofa.org. ** ** The ERFA version is intended to retain identical functionality to ** the SOFA library, but made distinct through different function and ** file names, as set out in the SOFA license conditions. The SOFA ** original has a role as a reference standard for the IAU and IERS, ** and consequently redistribution is permitted only in its unaltered ** state. The ERFA version is not subject to this restriction and ** therefore can be included in distributions which do not support the ** concept of "read only" software. ** ** Although the intent is to replicate the SOFA API (other than ** replacement of prefix names) and results (with the exception of ** bugs; any that are discovered will be fixed), SOFA is not ** responsible for any errors found in this version of the library. ** ** If you wish to acknowledge the SOFA heritage, please acknowledge ** that you are using a library derived from SOFA, rather than SOFA ** itself. ** ** ** TERMS AND CONDITIONS ** ** Redistribution and use in source and binary forms, with or without ** modification, are permitted provided that the following conditions ** are met: ** ** 1 Redistributions of source code must retain the above copyright ** notice, this list of conditions and the following disclaimer. ** ** 2 Redistributions in binary form must reproduce the above copyright ** notice, this list of conditions and the following disclaimer in ** the documentation and/or other materials provided with the ** distribution. ** ** 3 Neither the name of the Standards Of Fundamental Astronomy Board, ** the International Astronomical Union nor the names of its ** contributors may be used to endorse or promote products derived ** from this software without specific prior written permission. ** ** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT ** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS ** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE ** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, ** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, ** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; ** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER ** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT ** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE ** POSSIBILITY OF SUCH DAMAGE. ** */