NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction -- A general formula for all heights and altitudes
From: Frank Reed CT
Date: 2005 Aug 25, 16:21 EDT
From: Frank Reed CT
Date: 2005 Aug 25, 16:21 EDT
I neglected to mention that Alt is input in degrees and Ht is in meters. Alt should be between -3 and 90. Ht should be between 0 and 8000m -- outside this range is ok but with lower accuracy. >>>>> kk=180/pi 'Alt is the angular height (corrected for dip). Ht is the observer's height above sea level. IF Alt < 0 THEN 'use new below zero degrees altitude formula: refx = EXP(3.537 - .369 * Alt + .051 * Alt * Alt) ELSEIF Alt < 15 THEN 'use standard low altitude refraction formula (Bennett): refx = .998 / TAN((Alt + 7.31 / (Alt + 4.4)) / kk) ELSE 'use standard high altitude refraction: refx = .972 / TAN(Alt / kk) END IF 'refx as calculated so far is the sea level refraction at standard temperature and 'pressure of 10 deg C and 1010 millibars. The result is in minutes of arc. 'Now adjust for non-standard tempreature and pressure. 'Temp and Press are sea level temperature in deg C and pressure in mbar: refx = refx * (Press / 1010) * (283.15 / (273.15 + Temp)) 'Now scale for height above sea level: 'Primary scaling with height above sea level: refx = refx * EXP(-ht / (11278 - ht / 13)) 'A correction in scaling for very low angular altitudes, proportional to observer altitude: refx = refx - (ht / 10000) * EXP(-Alt / 14) 'refx is in minutes of arc. <<<<< -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars
