Explorer 54, also called as AE-D (Atmospheric Explorer-D), was a NASA scientific satellite belonging to series Atmosphere Explorer, being launched on 6 October 1975 from Vandenberg Air Force Base board a Thor-Delta 2910 launch vehicle.
Mission
The purpose of the Explorer 54 was to continue the investigation begun by Explorer 51 (AE-C) of the chemical processes and energy transfer mechanisms that control the structure and behavior of the Earth's atmosphere and ionosphere in the region of high absorption of solar energy. This mission was planned to sample the high latitude regions at the same time that the Explorer 55 (AE-E) mission was sampling the equatorial and low latitude regions. The same type of spacecraft as Explorer 54 was used, and the payload consisted of the same types of instruments except for deletion of the extreme solar ultraviolet monitor and the Bennett ion mass spectrometer, which were part of the Explorer 55 (AE-E) payload.
Experiments
Miniature Electrostatic Accelerometer (MESA) The Miniature Electrostatic Analyzer (MESA) obtained data on the neutral density of the atmosphere in the altitude range of to by the measurements of satellite deceleration due to aerodynamic drag. The instrument consisted of three single-axis accelerometers, mounted mutually at right angles, two in the spacecraft X-Y plane and the other in the Z-axis. The instrument determined the applied acceleration from the electrostatic force required to recenter a proof mass. The output of the device was a digital pulse rate proportional to the applied acceleration. The sample time of each measurement was 0.25-second. The measurements allowed determination of the density of the neutral atmosphere, monitored the thrust of the "Orbit-Adjust Propulsion System" (OAPS), determined the satellite minimum altitude, measured spacecraft roll, and provided some attitude-sensing information. Spacecraft nutations of less than 0.01° were monitored. The instrument had three sensitivity ranges: 8.E-3 Earth's gravity (G) in OAPS monitor mode; 4.E-4 G between (± 2%) and (± 10%); and 2.E-5 G between (± 2%) and (± 10%). Numbers in parentheses represent errors. A systematic error of up to ± 5% due to drag coefficient uncertainty was also possible. The highest measurement altitude was determined assuming the instrument could sense to 0.2% of full scale.
Capacitance Manometer
The capacitance manometer was primarily an engineering experiment to provide data on spacecraft operations. However, data from this experiment were also correlated with accelerometer and ion gauge data in evaluating satellite drag. The manometer, also referred to as Pressure Sensor B (PSB), provided a direct measure of atmospheric pressure in the region below 200 km. The accuracy of the PSB gauge varied from about 10% at to about 40% at . The PSB consisted of two spherical, thermally controlled chambers, separated by a thin membrane stretched flat and under radial tension. Any deflection of the diaphragm caused by a pressure differential between the two sides caused a change in capacitance between the diaphragm and an adjacent electrode which biased an AC bridge circuit. Air was allowee into one of the chambers through two ports 180° apart and perpendicular to the spacecraft spin axis. Thus the wake-ram pressure differential was sampled twice each spacecraft revolution.