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The fixed 20/30 GHz ground terminal is described in detail; it is being used for path diversity measurements in the ATS-6 millimeter wavelength propagation experiment. The current status and summary of operations are reviewed.
The real time digital data acquisition system used to acquire ATS-6 propagation data at the fixed, transportable, and unmanned terminals is described. This system performs all digital conversion, formating, merging, and recording. All data from the remote sites are transferred to the fixed terminal for recording on a common digital tape. The current status and summary of operations are also reviewed.
At the present time 'Applications Technology Satellite (ATS)-6' is the world's largest communication satellite. It handles telecommunications in the frequency range of 40 - 30,000 MHz. Power levels on board the spacecraft range from -110 dBm to 52.5 dBw. Consequently considerable care was required in the design and test of this spacecraft, in order to provide assurances that the spacecraft would perform properly in its own RF environments. The testing was performed first by placing the earth viewing module (EVM) in a specially constructed 'small' anechoic chamber with an overhead parabolic reflector section, of 8' in diameter, instead of the 30' reflector of the full scale design. The near field analysis of this paper proves that this test configuration leads to a desirable overtest for the spacecraft. The test requirements, procedure and results are also explained.
Two one-millipound-thrust cesium-bombardment ion thrusters have been developed and integrated on the ATS-F spacecraft for the purpose of demonstrating compatible north-south stationkeeping of a synchronous communication satellite. In addition to the two flight units, an identical system is undergoing extended testing on the ground to demonstrate operating lifetime. On July 17, 1974, approximately 50 days after launch, preliminary operation of one of the thrusters was begun. Completely successful operation was achieved on the first run. In addition to verifying operation, the principal accomplishments were the demonstration of a total absence of interference with the complete array of spacecraft-to-ground communication links and the verification of the predicted spacecraft operating potential of approximately -10 volts. Subsequent attempts to operate the same thruster have not been successful and an investigation of possible explanations is underway.
On May 30, 1974, NASA launched the ATS-6 experimental communications satellite into a geosynchronous orbit at a station centered over the United States. The 1400 kg satellite was designed to be body-stabilized with a 3-axis control system capable of precision offset pointing. It deployed a 9.1 meter (30 foot) parabolic reflector antenna with a transponder that covered a frequency range from VHF through C-band. The high RF gains obtained with the antenna were to be used for many dramatic communications experiments, one of which was the Health/Education Telecommunications Experiment (HET), a demonstration of direct broadcast of color television to low cost terminals in remote regions of the United States. More than 120 terminals with 3-meter antennas were deployed in Alaska, Washington, the Rocky Mountains, and Appalachia to provide educational and health services to selected community centers. After 11 months of nearly continuous service, the performance of both the satellite and the experiment have exceeded all expectations.
The modified power system design adopted for the ATS-6 satellite is described. This design featured a shunt-boost configuration to provide direct power from the solar array to the loads for maximum efficiency. In addition, shunt power dissipators were used, not only as a means of power system regulation, but also for thermal control. A brief description of the spacecraft and of the initial series regulator power system design (which was unable to meet mission requirements) precede the discussion. The shunt boost system provides excellent utilization of array power with an 8 to 10% increase in efficiency over the original configuration; good array-battery static and dynamic load sharing; excellent flexibility in accommodating mission requirements; and low electromagnetic interference.
The results of data obtained at The University of Texas at Austin in conjunction with the ATS-6 millimeter wave experiment are presented. Attenuation measurements at 30 GHz and sky noise data at 20 GHz were obtained simultaneously at each of two sites separated by 11 km. Space diversity reduces outage time for a system in Austin, Texas with a 10 dB fade margin at 30 GHz from 15 hours to 16 minutes per year. The maximum cloud height shows a good correlation to the maximum attenuations measured.
The mathematical formulation is presented of CNTRLF, the maneuver control program for the Applications Technology Satellite-F (ATS-F). The purpose is to specify the mathematical models that are included in the design of CNTRLF.
Predictions of satellite downlink attenuation through the use of ground based measurements form a substantial part of the ATS-6 millimeter wave experiment (MWE). At the downlink frequencies (20 and 30 GHz), the major causes of attenuation are the density and the size distribution of rain drops along the propagation path. Ground station data, which include radar and rain gauge records, measure quantities related to the meteorological parameters of interest and thereby provide a prediction of downlink attenuation with which the measured attenuation can be compared. The calibration and data analysis procedures used in the MWE are reviewed with the object of improving the accuracy of such ground based predictions.
Behavior of the plasma sheet around its earthward edge during substorms was studied by using high resolution (every 2.6 sec) measurements of proton and electron fluxes by ATS-5. In the injection region near midnight the flux increase at the expansion-phase onset is shown to lag behind the onset of the low-latitude positive bay by several minutes. Depending upon the case, before the above increase (1) the flux stays at a constant level, (2) it gradually increases for some tens of minutes, or (3) it briefly drops to a low level. Difference in the position of the satellite relative to the earthward edge and to the high-latitude boundary of the plasma sheet is suggested as a cause of the above difference in flux variations during the growth phase of substorms. Magnetograms and tables (data) are shown.
Antenna patterns, principally associated with the 9.1 meter parabolic antenna of the ATS-6 spacecraft, were measured while in orbit at quasi-stationary synchronous altitude. Controlling the spacecraft attitude permitted a scanning of the spacecraft antenna pattern over the Rosman ground station, thus achieving the measurement of the antenna pattern contour. Patterns were determined in terms of relative gain referenced in position to the spacecraft body coordinates by means of signal power measurements made using a linear detector. These data were subsequently correlated with the attitude data to define the antenna patterns. Antenna patterns measured are presented and compared with available preflight patterns.
The development of an L-band trilateration network capable of locating the ATS-5 satellite, determining the satellite's orbital elements, and predicting the satellite position was investigated. An automatic tone-code ranging transponder was used to compare ranging measurements and communications reliability for the VHF and L-band. The L-band transponder network, analytical techniques, and the determination of the Kepler orbit parameters are described along with the calibration procedures, operation procedures, and verification of trilateration position.
The Ohio State University participation in the ATS-6 Millimeter Wavelength Propagation Experiment is summarized. Attenuation was measured simultaneously at 20 and 30 GHz on earth space propagation paths to two ground terminals located at Columbus, Ohio. In addition, 20 and 30 GHz radiometric temperatures were measured along the same propagation paths; and the 20 GHz radiometric temperature was also measured at a third ground terminal. The results of these measurements are presented, and diversity gains for the four pairs of propagation paths are discussed. The scintillation characteristics of the received signals are also presented.
The Aerospace Corporation energetic electron-proton spectrometer operating on ATS-6 is described. This experiment detects energetic electrons in four channels between 140 keV and greater than 3.9 MeV, measures energetic protons in five energy channels between 2.3 and 80 MeV and energetic alpha particles in three channels between 9.4 and 94 MeV. After more than a year of operation in orbit, the experiment continues to return excellent data on the behavior of energetic magnetospheric electrons as well as information regarding the fluxes of solar protons and alpha particles.
Although the ATS 5 satellite failed to de-spin, a rather intricate analysis procedure allows the extraction of scintillation information at spectral rates from 20 to 60 Hz, as well as below 0.5 Hz. The procedure has been applied to 15.3 GHz signals. Distributions and spectra were obtained for a limited amount of data, representing a variety of meteorological conditions. A definite correlation of scintillation strength and variability with rainfall is apparent. The data analysis is continuing.
Low frequency oscillations of the magnetic field at ATS-1 were analyzed for the 25 month data interval, Dec., 1966 through 1968. Irregular oscillations and those associated with magnetic storms were excluded from the analysis. Of the 222 events identified, 170 were found to be oscillating predominantly transverse to the background magnetic field. The oscillations were observed to occur most frequently in the early afternoon hours. They also seemed to occur more frequently during Dec., Jan., and Feb. than at any other time of the year. During a given event, the frequency was fairly constant. The event duration varied between a minimum of 10 min. and a maximum of 14 hrs and 26 min. During a given event the amplitude varied.
All the meteorological data for the 32-hour period before the ATS-F launch from Kennedy Space Center at 0900 EDT on May 30, 1974, are archived at the Marshall Space Flight Center. These data were collected in support of the NASA rocket exhaust effluent prediction and monitoring program. This data set is unique in that soundings were made hourly from T-12 to T-0 hours, providing high temporal resolution. All supporting data, such as synoptic charts and surface data, are also included. This is the third in a series of seven data reports.
The ATS-6 has been subject to potential charges as high as 11,000 volts based on onboard measurements. Thus, special measures were required to protect the spacecraft surface areas, - primarily the Earth Viewing Module, the reflector support truss, and the solar array boom that were specially processed for thermal control - from electrical discharges whose accompanying fields could destroy sensitive receiving circuits. The paper describes thermal blanket grounding criteria and techniques applied to prevent undesired discharge effects, and discusses the disturbance rate from static discharge in orbit.