Developed at the University of Texas at Dallas, the Retarding Potential Analyzer and Ion Drift Meter were flown aboard the three Atmosphere Explorer spacecraft.
For this purpose, each spacecraft carried more than twelve scientific instruments. These instruments simultaneously measured incoming solar radiation and the atmosphere to provide information about the physical processes that govern the composition of the lower thermosphere and the ionosphere. The measurements made possible a study of the closely interlocked cause-and-effect relationships that control the near-space environment of Earth. The instruments included spectrophotometers, photo meters, spectrometers, electrostatic probes, accelerometers, retarding potential analyzers, and ultraviolet radiation monitors.
Following the success of two earlier Atmosphere Explorers (17 and 32), three missions were planned for spacecraft equipped with onboard propulsion systems to compensate for atmospheric drag. The first, whose orbit (i = 68 degrees) was selected so that the perigee would move very slowly, concentrated on gathering data over a wide range of altitudes. The second and third spacecraft orbits (90 and 19 degrees) were designed to study the thermosphere over a wide range of latitudes and local time variations, respectively.
Variations were expected over the ranges of latitude, local time, and season. Atmosphere Explorer C provided a stable perigee location and a constancy of altitude at various locations. It acted as a prototype to cover measurements over a range of altitude. Atmosphere Explorer D permitted rapid latitude surveys, and Atmosphere Explorer E made measurements over local time variations without large changes of latitude.
At the beginning of the first operational phase, the first satellite (Atmosphere Explorer C) was to be placed in an orbit with an intermediate inclination (68 degrees) to the Earth's Equator. The perigee was to be near 150 km and the apogee near 4000 km, and the orbit was to be maintained near these levels for approximately 8 months. During these months, the orbit would itself move around the Earth with the perigee moving very slowly, allowing measurements at particular altitudes at a variety of local times. Occasionally, the propulsion system would be used to assist the apsidal rotation.
Also during this period, the perigee would be lowered to 130 to 135 km for brief excursions into the lower region of the thermosphere. This would provide, for the first time, information on the behavior of the lower thermosphere and E-region of the ionosphere with suitable spatial and temporal resolution.
During the second operational phase, circular orbits of altitudes between 600 and 250 km were needed. At the end of the first phase in elliptical orbit, therefore, the altitude of the perigee was to be reduced to the lowest useful value-about 129 km. The resultant increase in drag would lower the apogee of the orbit. When it had decayed to 600 km, the propul sion system would be fired at apogee to raise the perigee, lower the apogee, and make the orbit circular. The satellite would then be kept in the circular orbit indefinitely, gathering data at all points along the orbit.
The propulsion system carried by each Atmosphere Explorer spacecraft permitted it to achieve orbits quite close to a true circle, so that only irregularities in the shape of the Earth produced significant changes in the altitude of the satellite as it orbited the Earth. The resulting increased accuracy of measurement of global pressure gradients could lead to more realistic calculations of the system of global winds in the upper atmosphere. Before all the propellant was exhausted, the spacecraft would be placed in a suitably positioned circular orbit so that it would decay slowly.
The primary difference between the high inclination orbit for D and the intermediate inclination orbit for C was important to aeronomy because the motion of the perigee of D would translate into latitude coverage of the thermosphere, whereas that of C would give good local time coverage because of the slow motion of perigee.
The Atmosphere Explorer D orbit would be circularized similar to the Atmosphere Explorer C orbit and would be stepped through a similar altitude range. A pair of satellite orbits for studying global and local time behavior of the thermosphere was of great value. Similarly, Atmosphere Explorer E made possible a complementary low-inclination orbit mission for extensive equatorial investigations. Its inclination corresponded to the latitude of the Arecibo, Puerto Rico, incoherent backscatter station, permitting extensive correlative studies.
In summary, the Atmosphere Explorer C space craft was launched in 1973 by the improved Thor/Delta 1900 launch vehicle. The orbit of the satellite was inclined at 68 degrees. Atmosphere Explorer D was launched in 1975 by a Thor/Delta 2910 launch vehicle. Its orbit was inclined at 90 degrees. Both of these satellites were launched from the Western Test Range. Atmosphere Explorer E was launched in 1975 by a Thor/Delta 2910 launch vehicle from the Eastern Test Range. Its orbit was inclined at 19.7 degrees. All three satellites were intended to be placed in initial orbits with perigees near 150 km and apogees of 3000 to 4000 km. Mission profiles were determined by the aeronomy teams to meet the science objectives.
The instrument consisted of four sensor heads, their ground planes, and a main electronics box. Each of the sensor heads contained its own amplifiers. Sensor heads I and 4 were located on the forward-facing surface of the spacecraft when it was in the despin mode. Heads 2 and 3 were located 110 and 130 degrees from heads 1 and 4, respectively. Each sensor head consisted of a small cylinder with an aperture at one end through which charged particles passed before striking a solid collector. The path between the aperture and the collector was electrically segmented by a series of grids. By controlling the potentials on the grids, the investigators could screen particles of different energies (e.g., low-energy electrons could be screened out when ions were to be measured).
All the grids through which the ions and electrons entered the chamber of the instrument were square meshes of 25-micrometer wires having either 40 or 20 wires per cm. Heads 2 and 3 were like head 1, except that they had an extra grid separating the entrance grids from the retarding grids. The extra grid could be biased positively to protect the inner grids from ion bombardment when measuring electrons. The other significant difference was that heads 2 and 3 could be biased slightly positive (less than 1.5 volts) on command so that thermal electrons could be encouraged to enter the collector. The collector of head 4, which was called the ion drift meter (IDM), was divided symmetrically into four equal pie shaped segments, and it had a square aperture with sides parallel to the pie cuts. Therefore, any off-axis flow of ions would result in different currents in the four segments. This per mitted the transverse components of ion drift velocity to be measured. When the other sen sors faced along the spacecraft's velocity vector, the measured ion energy spectra could be used to deduce the component of ion drift in that direction. Together with the IDM data, therefore, the complete ion drift vector was measured.
A ground rule for the program was to operate the spacecraft 4 to 5 hours per day, process all the data that the spacecraft gathered, and put this amount of data in the unified abstract file. The overall system worked well, and data were quickly transmitted. On special occasions, an investigator with a terminal actually had access in his own office to data that had been acquired in space only 6 hours earlier. Before Atmosphere Explorer, it often took 8 to 9 months to gain access to such data.