SUBROUTINE sla_REFCOQ (TDK, PMB, RH, WL, REFA, REFB) *+ * - - - - - - - * R E F C O Q * - - - - - - - * * Determine the constants A and B in the atmospheric refraction * model dZ = A tan Z + B tan**3 Z. This is a fast alternative * to the sla_REFCO routine - see notes. * * 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: * TDK d ambient temperature at the observer (deg K) * PMB d pressure at the observer (millibar) * RH d relative humidity at the observer (range 0-1) * WL d effective wavelength of the source (micrometre) * * Returned: * REFA d tan Z coefficient (radian) * REFB d tan**3 Z coefficient (radian) * * The radio refraction is chosen by specifying WL > 100 micrometres. * * Notes: * * 1 The model is an approximation, for moderate zenith distances, * to the predictions of the sla_REFRO routine. The approximation * is maintained across a range of conditions, and applies to * both optical/IR and radio. * * 2 The algorithm is a fast alternative to the sla_REFCO routine. * The latter calls the sla_REFRO routine itself: this involves * integrations through a model atmosphere, and is costly in * processor time. However, the model which is produced is precisely * correct for two zenith distance (45 degrees and about 76 degrees) * and at other zenith distances is limited in accuracy only by the * A tan Z + B tan**3 Z formulation itself. The present routine * is not as accurate, though it satisfies most practical * requirements. * * 3 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) and (iii) variations in tropospheric * lapse rate. * * The model was tested using the following range of conditions: * * lapse rates 0.0055, 0.0065, 0.0075 deg/metre * latitudes 0, 25, 50, 75 degrees * heights 0, 2500, 5000 metres 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 direct use of the sla_REFRO routine * 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 degK/metre * latitude 50 degrees * sea level * pressure 1005 mB * temperature 280.15 degK * humidity 80% * wavelength 5740 Angstroms * * the results were as follows: * * ZD sla_REFRO sla_REFCOQ 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). * * The results from the much slower but more accurate sla_REFCO * routine have not been included in the tabulation as they are * identical to those in the sla_REFRO column to the 0.01 arcsec * resolution used. * * 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 * expressions 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), expression 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 but not radio, wavelength. The formulae * for the two cases are developed from the Essen and Froome * expressions adopted in Resolution 1 of the 12th International * Geodesy Association General Assembly (1963). * * The above three items are as used in the sla_REFRO routine. * * 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 expression 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), expression 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. * * Hohenkerk, C.Y., & Sinclair, A.T., NAO Technical Note No. 63, 1985. * * International Geodesy Association General Assembly, Bulletin * Geodesique 70 p390, 1963. * * Stone, Ronald C., P.A.S.P. 108 1051-1058, 1996. * * Green, R.M., "Spherical Astronomy", Cambridge University Press, 1987. * * P.T.Wallace Starlink 4 June 1997 * * Copyright (C) 1997 Rutherford Appleton Laboratory *- IMPLICIT NONE DOUBLE PRECISION TDK,PMB,RH,WL,REFA,REFB LOGICAL OPTIC DOUBLE PRECISION T,P,R,W,TDC,PS,PW,WLSQ,GAMMA,BETA * Decide whether optical/IR or radio case: switch at 100 microns. OPTIC = WL.LE.100D0 * Restrict parameters to safe values. T = MIN(MAX(TDK,100D0),500D0) P = MIN(MAX(PMB,0D0),10000D0) R = MIN(MAX(RH,0D0),1D0) W = MIN(MAX(WL,0.1D0),1D6) * Water vapour pressure at the observer. IF (P.GT.0D0) THEN TDC = T-273.15D0 PS = 10D0**((0.7859D0+0.03477D0*TDC)/(1D0+0.00412D0*TDC))* : (1D0+P*(4.5D-6+6D-10*TDC*TDC)) PW = R*PS/(1D0-(1D0-R)*PS/P) ELSE PW = 0D0 END IF * Refractive index minus 1 at the observer. IF (OPTIC) THEN WLSQ = WL*WL GAMMA = ((77.532D-6+(4.391D-7+3.57D-9/WLSQ)/WLSQ)*P : -11.2684D-6*PW)/T ELSE GAMMA = (77.624D-6*P-(12.92D-6-0.371897D0/T)*PW)/T END IF * Formula for beta adapted from Stone, with empirical adjustments. BETA=4.4474D-6*T IF (.NOT.OPTIC) BETA=BETA-0.0074D0*PW*BETA * Refraction constants from Green. REFA = GAMMA*(1D0-BETA) REFB = -GAMMA*(BETA-GAMMA/2D0) END