Explorer 54, also known as AE-D (Atmosphere Explorer-D), was a NASA scientific satellite belonging to the Atmosphere Explorer series launched on 6 October 1975 from Vandenberg Air Force Base aboard a Thor-Delta 2910 launch vehicle.
Mission The purpose of 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 the 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 seconds. The measurements allowed the 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 allowed 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.
Cold Cathode Ion Gauge The cold cathode-ion gauge flown on AE-D was primarily an engineering experiment to provide data on spacecraft operation. However, data from this experiment were correlated with accelerometer and capacitance manometer data to evaluate satellite drag performance. The ion gauge, also referred to as Pressure Sensor A (PSA), measured atmospheric pressure in the region between and above the Earth's surface for values of atmospheric pressure between 1.3E-3 to 1.3E-7 mb. The estimated accuracy of the PSA was ± 20%. The cylindrically shaped sensor package consisted of a wedge-shaped orifice, a cathode near ground potential, an anode operating at about 1300 VDC, and a permanent magnetic field of about 1600 Gauss. The gauge contained no primary source of ionizing electrons. The discharge was initiated by field emission and was self-sustaining at a pressure above 1.3E-7 mb. The ion current was collected at the cathode. The sensor was mounted on the spacecraft with the orifice perpendicular to the spacecraft spin axis, which was normal to the orbital plane. The instrument was operated in two modes, spinning or despun. When the spacecraft was in a spinning mode, the PSA alternately sampled the ram and wake pressure. When the spacecraft was in the despun mode, the PSA faced 30° from the direction of motion. Data from this experiment were not tape-recorded but were observed in real-time.
Cylindrical Electrostatic Probe (CEP) The CEP consisted of two identical instruments designed to measure electron temperatures, electron and ion concentrations, ion mass, and spacecraft potential. One probe was oriented along the spin axis of the spacecraft (normally perpendicular to the orbit plane), and the other radially so that it could observe in the direction of the velocity vector once each 15-second spin period. Each instrument was a retarding potential Langmuir probe device that produced a current-voltage (I-V) curve for a known voltage pattern placed on the collector. Electrometers were used to measure the current. There were two systems of operation (one with two modes and another with three modes) using collector voltage patterns between ± 5 volts. Most modes involved an automatic or fixed adjustment of collector voltage limits (and/or electrometer output) such that the region of interest on the I-V profile provided high resolution. Each system was designed for use with only one of the probes, but they could be interswitched to provide backup redundancy. The best measurements in the most favorable modes provided one-second time resolution; electron temperature between 300K and 10,000K (10% accuracy); ion density between 10,000 and 10,000,000 ions/cc (10-20% accuracy); electron density between 50 and 1,000,000 electrons/cc; and ion mass at ion densities above 10,000 ions/cc. Each probe had a collector electrode extending from the central axis of a cylindrical guard ring. The -long guard ring was at the end of a boom, and the collector extended another beyond the guard ring. The boom, guard, and collector were in diameter.
Low-Energy Electrons (LEE) This experiment furnished direct measurements of the energy input into the upper atmosphere due to electrons and protons (ions) in the energy range 0.2 to 25 keV. The fluxes of electrons and ions were measured with cylindrical electrostatic analyzers and Spiraltron electron multipliers. There were 19 detectors, one ion-stepped-energy analyzer, and two electron-stepped analyzers mounted at different angles. In addition, there were 16 fixed energy detectors that obtained high-time-resolution angular distributions, in the spacecraft one revolution-per-orbit mode, at five energies between 0.72 and 18 keV.
Magnetic Ion-Mass Spectrometer (MIMS) A magnetic ion-mass spectrometer was flown to measure in situ the concentrations of the ambient positive ion species in the mass range from 1 to 90 atomic mass units (u). Mounted on the satellite equator normal to the spin axis, the entrance aperture faced forward when the spacecraft was in the despun mode. The electric and magnetic fields were arranged to produce a mass spectrum along the focal plane following the magnetic analyzer. Three slits were placed along this plane in appropriate places to simultaneously collect ions in the mass ratio 1-4-16. Ionospheric ions were accelerated into the analyzer system by a negative voltage that varied from -1060 to -225 Volts. The three mass ranges measured simultaneously were 1 to 4, 4 to 16, and 14 to 72 units. Following each slit was an electron multiplier and logarithmic electrometer-amplifier detector. The detector output was either measured directly for an analog output or was supplied to a "peak" circuit that determined the amplitude of each peak in the spectrum. Only the amplitude of each peak was telemetered in the "peak" mode, and in this mode the time required to simultaneously sweep all three mass ranges was 1 second. Other modes of operation were possible. In the analog short mode, the three mass ranges were swept in 2-seconds, alternating with 1-second "peak" mode scans. An 8-second sweep time was required in the analog long mode, again alternating with 1-second "peak" mode scans. An option existed in the locked mode to continuously measure any set of mass numbers in the ratio 1-4-16 to give high spatial resolution.
Neutral Atmosphere Composition (NACE) This experiment measured in situ the spatial distribution and temporal changes of the concentrations of the neutral atmospheric species. In addition, new insight into in situ measurement techniques was obtained from comparisons of these measurements with other onboard experiments: namely, open-source spectrometer (1975-096A-07), solar EUV spectrophotometer (1975-096A-06), and density accelerometer (1975-096A-02). The mass-spectrometer sensor included a gold-plated stainless steel thermalizing chamber and ion source, a hyperbolic-rod quadrupole analyzer, and an off-axis electron multiplier. Five different sequences of mass selection were available and, expressed in atomic mass units (u), were as follows: (1) geophysical: -1, 2, 4, total, 16, 28, 32, selected, 40, (2) analytical: -12, 14, 18, 20, 22, 30, 44, calibrate, zero, (3) individual: -selected, selected, selected, ... (any mass 1 to 44), (4) sweep digital: -1, 2, 3, 4, 5, ... 45 (in 3/16-units steps), (5) sweep analog: 2, 3, 4, 5, ... 45 (continuous). Five operational formats were available and selected by ground command. When operating in the "normal" format, the analyzer measured all masses in the range 1 to 44 with emphasis on hydrogen, helium, oxygen, nitrogen, and argon. Another format was optimized for minor constituent studies of any individual gas species in the measured range. Spatial resolution was determined primarily by the mode of spacecraft operation. In orbit, the pre-sealed spectrometer was opened, and the atmospheric constituents passed through a knife-edged orifice into the thermalization chamber and ion source. Selected ions left the quadrupole analyzer through a weak focusing lens and were accelerated into a 14-stage electron multiplier, where they were turned 90° to strike the first dynode. For each impacting ion, the multiplier output was a pulse of 2.E6 electrons. These output pulses constituted the measurement and the count rate was proportional to the chamber density of the selected species. These density values were converted to ambient concentrations. The analyzer normally operated at a resolution of 1 unit over the mass range, so that a mass peak one-thousandth the amplitude of an adjacent peak could be measured. For the dynamic range required, pulses occurring during 0.015-second integration intervals were accumulated in a 16-bit counter. Multiple integration periods (up to 16) were assigned to each measurement for less dense atmospheric species. Automatically selected ranges of ionizing electron currents were used. The overall range of the measurements was planned to be greater than 10,000,000. There was a provision for the instrument orifice to be covered during spacecraft thruster operations.
Neutral Atmosphere Temperature (NATE)