|OGO - |
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The purpose of the six Orbiting Geophysical Observatories was to conduct diversified geophysical experiments to obtain a better understanding of the earth as a planet and to develop and operate a standardized observatory-type satellite. OGO consisted of a main body, two solar panels, each with a solar-oriented experiment package (SOEP), two orbital plane experiment packages (OPEP) and six appendages, EP-1 through EP-6, supporting the boom experiment packages. The main body of the spacecraft was attitude controlled by means of horizon scanners and gas jets so that its orientation was maintained constant with respect to the earth and the sun. The solar panels rotated on a horizontal axis extending transversely through the main body of the spacecraft. The rotation of the panels was activated by sun sensors so that the panels received maximum sunlight. Seven experiments were mounted on the solar panels (the SOEP package). An additional axis, oriented vertically across the front of the main body, carried seven experiments (the OPEP package). Nominally, these sensors observed in a forward direction in the orbital plane of the satellite. The sensors could be rotated more than 90 deg relative to the nominal observing position and more than 90 deg between the upper and lower OPEP groups mounted on either end of this axis.
Total Mass: 588 kg.
Two experiment booms failed to properly deploy, with one of the booms obscuring a horizon scanner's view of earth. As a result, the spacecraft attitude could not be earth oriented and OGO 1 remained spin stabilized at 5 rpm. Nevertheless, data from all 20 experiments on board was received, although at a 'less than expected capacity' from some of them. Twelve of the experiemnts were particle studies and two were magnetic field studies. In addition, there was one experiment for each of the following types of studies: interplanetary dust, VLF, Lyman-alpha, Gegenschein, atmospheric mass, and radio astronomy. During September 1964, acceptable data were received over 70% of the orbital path. By June 1969, data acquisition was limited to 10% of the orbital path. Spacecraft operation was restricted to Spring and Fall due to power supply limitations. There were 11 such 3-month periods prior to the spacecraft being put into stand-by mode on 25 November 1969. By April 1970 the spacecraft perigee had increased to 46,000 km and the inclination had increased to 58.8 deg. All support was terminated November 1, 1971.
OGO 2 was a large observatory instrumented with 20 experiments designed to make simultaneous, correlative observations of aurora and airglow emissions, energetic particles, magnetic field variations, ionospheric properties, etc., especially over the polar areas. Soon after achieving orbit, difficulties in maintaining earth lock with horizon scanners caused exhaustion of attitude control gas by October 23, 1965, 10 days after launch. At this time, the spacecraft entered a spin mode (about 0.11 rpm) with a large coning angle about the previously vertical axis. Five experiments became useless when the satellite went into this spin mode. Six additional experiments were degraded by this loss of attitude control. By April 1966, both batteries had failed, so subsequent observations were limited to sunlit portions of the orbit. By December 1966, only eight experiments were operational, five of which were not degraded by the spin mode operation. By April 1967, the tape recorders had malfunctioned and only one third of the recorded data could be processed. Spacecraft power and periods of operational scheduling conflicts created six large data gaps so that data were observed on a total of about 306 days of the 2-yr, 18-day total span of observed satellite data to November 1, 1967. The spacecraft was shut down on November 1, 1967, with eight experiments still operational. It was reactivated for 2 weeks in February 1968 to operate the rubidium vapor magnetometer experiment.
Orbiting Geophysical Observatory 3. All 21 experiments returned good data. At the time, this was the largest experimental complement ever put into orbit. There were 4 cosmic ray instruments (1 of which included a gamma-ray spectrometer), 4 plasma, 2 trapped radiation, 2 magnetic fields, 5 ionosphere, 3 radio/optical, and 1 micrometeoroid detectors. OGO 3 maintained 3-axis stabilization for 46 days. At that point, an attitude controller failed and the spacecraft was put into a spin on 23 July 1966. The spin period varied from 90-125 seconds. By June 1969, data acquisition was limited to 50% of the orbital path. Routine spacecraft operation was discontinued on December 1, 1969, after which only data from Heppner's experiment (Rubidium + Fluxgate magnetometer) was acquired. By March 1971 spacecraft perigee had increased to 16,400 km and the inclination had increased to 75.8 deg. All spacecraft support terminated on February 29, 1972.
OGO 4 was a large observatory instrumented with experiments designed to study the interrelationships between the aurora and airglow emissions, energetic particle activity, geomagnetic field variation, ionospheric ionization and recombination, and atmospheric heating which take place during a period of increased solar activity. After the spacecraft achieved orbit and the experiments were deployed into an operating mode, an attitude control problem occurred. This condition was corrected by ground control procedures until complete failure of the tape recording systems in mid-January 1969. At that time, due to the difficulty of maintaining attitude control without the tape recorders, the attitude control system was commanded off, and the spacecraft was placed into a spin-stabilized mode about the axis which was previously maintained vertically. In this mode, seven of the remaining experiments were turned off since no meaningful data could be observed by them. On October 23, 1969, the satellite was turned off. It was reactivated again in January 1970 for 2 months to obtain VLF observations.
OGO 5 carried 25 experiments, 17 of which were particle studies, and two were magnetic field studies. In addition, there was one each of the following types of experiments: radio astronomy, UV spectrum, Lyman-alpha, solar X ray, plasma waves, and electric field. By April 1971, spacecraft perigee had increased to 26,400 km and inclination had increased to 54 deg. The spacecraft attitude control failed on August 6, 1971, after 41 months of normal operation. The spacecraft was placed in a standby status on October 8, 1971. Four experiments (Meyer, Blamont, Thomas, and Simpson) were reactivated for the period from June 1 to July 13, 1972, after which all operational support terminated. Spacecraft orbit parameters changed significantly over the spacecraft life.
OGO 6 was a large observatory instrumented with 26 experiments designed to study the various interrelationships between, and latitudinal distributions of, high-altitude atmospheric parameters during a period of increased solar activity. On June 22, 1969, the spacecraft potential dropped significantly during sunlight operation and remained so during subsequent sunlight operation. This unexplained shift affected seven experiments which made measurements dependent upon knowledge of the spacecraft plasma sheath. During October 1969, a string of solar cells failed, but the only effect of the decreased power was to cause two experiments to change their mode of operation. Also during October 1969, a combination of manual and automatic attitude control was initiated, which extended the control gas lifetime of the attitude control system. In August 1970, tape recorder (TR) no. 1 operation degraded, so all recorded data were subsequently taken with TR no. 2. By September 1970, power and equipment degradation left 14 experiments operating normally, 3 partially, and 9 off. From October 14, 1970, TR no. 2 was used only on Wednesdays (world days) to conserve power and extend TR operation. In June 1971, the number of 'on' experiments decreased from 13 to 7, and on June 28, 1971, the spacecraft was placed in a spin-stabilized mode about the yaw (Z) axis and turned off due to difficulties with spacecraft power. OGO 6 was turned on again from October 10, 1971, through March 1972, for operation of experiment 25 by The Radio Research Laboratory, Japan.