Determination of the Density of Air at an Altitude of 3,500 km

Abstract

IN 1963 an investigation was begun of densities of air at heights above 3,000 km. Observations were started in January 1964 on the high Midas satellites (Agena satellites in near-polar orbits at heights mainly between 3,000 and 4,000 km) with a special camera at the prime focus of the 24-in., f/5 reflector of the University of London Observatory. A tilting mirror gives timed breaks in both satellite and star trails and the precision has been found to be about 1″ in position and 5 msec in time. The Midas objects are well recorded to magnitude 9, at angular velocities at 300″/sec, using special film (SO 118) obtained from Eastman Kodak and processed to give a speed of 2,000 A.S.A. A typical frame is shown in Fig. 1. In work recorded here observations were made at selected positions on high passes to maximize the accuracy in determining the satellite latitude, and hence mean anomaly. To derive the mean anomaly from each observation approximate orbit elements were obtained from various sources, including Norad (via the Goddard Space Flight Center) and the Smithsonian Astrophysical Observatory. Near-zenith observations were also used to derive accurate positions for the ascending node and its motion.