GB691310A - Logarithmic calculators for astronavigational purposes - Google Patents

Abstract

691,310. Calculating-apparatus. BARTELSKI, J. M. June 22, 1950 [Dec. 31, 1949], No. 33449/49. Class 106 (i). In astronavigation from aircraft, a short interval, generally about four minutes, always elapses between the actual observation and the time of the fix. The invention comprises a logarithmic computer for calculating the change in altitude of a star during this interval so that the observed altitude can be corrected to the true altitude at the time of the fix. Calculations are based on the formulµ: (1) rate of change in altitude due to star movement= cosine latitude of the observer X sine azimuth of the star X constant, and (2) rate of change in altitude due to aircraft movement = cosine angle between flight track and star azimuth x aircraft speed X constant. Separate sections of the computer evaluate altitude change due to star and aircraft movement, and the results are added to provide the correction. The computer comprises a base disc a with a fixed reference arm f, and four superposed rotatable discs b-e. One section comprises the discs band c, disc c having a log. sine star azimuth scale and a reading aperture for a log. cosine aircraft latitude scale on the disc b which also has a log altitude change scale on its exposed periphery. The other section, comprising the discs d and e, is similar, disc e having a log. cosine track angle scale and a reading aperture for a log. speed scale on the disc d which again has a log. altitude change scale on its exposed periphery. In use, the scales on discs c and e are set against the reference arm f according to azimuth and track angle, the inner scales on discs band d are set in the reading apertures according to latitude and speed, and the two corrections are then read off the outer scales on the discs b and d against the reference arm. The reference arm f is fixed in the 4-minute position on a log time scale marked on the base disc a. A second reference arm g, adjustable over the time scale, is used for other time periods.