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Communications System Utilizing Passive Satellites The Space Congress® Proceedings 1966 (3rd) The Challenge of Space Mar 7th, 8:00 AM Communications System Utilizing Passive Satellites Robert T. Hart Collins Radio Company Follow this and additional works at: https://commons.erau.edu/space-congress-proceedings Scholarly Commons Citation Hart, Robert T., "Communications System Utilizing Passive Satellites" (1966). The Space Congress® Proceedings. 5. https://commons.erau.edu/space-congress-proceedings/proceedings-1966-3rd/session-1/5 This Event is brought to you for free and open access by the Conferences at Scholarly Commons. It has been accepted for inclusion in The Space Congress® Proceedings by an authorized administrator of Scholarly Commons. For more information, please contact [email protected]. COMMUNICATIONS SYSTEMS UTILIZING PASSIVE SATELLITES Robert T. Hart Collins Radio Company Dallas, Texas Summary Dallas, Texas. The geographical locations of the sta­ tions permitted mutual visibility on many passes The era of communications systems utilizing with the passive satellites was inaugurated by the launch of satellite at an orbital height of approximately 700 miles. Operational Echo I in 1960. This was followed by Echo II in 1964. frequencies in the S-band region These preliminary experiments have proven system were chosen for maximum radio reflectivity of the sat­ feasibility and indicated deficiencies of the satellite ellite consistent with available equipment in the ground station materials and configurations. Echo III will be launched complex. in the spring of 1966 to test the latest available A set of disappointing circumstances caused the materials. Echo II satellite to be less than a perfect radio reflec­ This paper outlines the history of the passive tor, primarily due to ejection of the balloon from the satellite communications experiments to date and canister. One of the two clam shells failed to open, offers solutions to the deficiencies that have been in­ imparting a sideways velocity to the satellite. The ma­ dicated. The solutions include a preview of studies terial used was similar to that in Echo I mylar plastic of new materials and satellite configurations. Com­ and aluminum foil. This material has the property of munications system capability utilizing the proposed becoming rigid in a vacuum after the pressurization satellites is detailed for conventional and specialized system stresses the material to the yield point. Before networks. the material had rigidized completely, the satellite lost pressurization prematurely. As a result, the sat­ The Echo I Satellite ellited shape is somewhat like that of a M prune n . The tumbling motion of the sphere due to improper ejection The Echo I satellite, launched in August, 1960, from the canister and the rough surface causes severe is a 90-foot-diameter sphere composed of aluminum scintillation of the reflected radio signals. The sphere foil and mylar plastic. It has been in orbit for al­ is made in a segmented configuration using many gores most six years without any significant changes in its with two polar caps. Several times during the test pro­ surface characteristics after the first few months. gram the satellite was oriented in such a manner that The inflation system produced a good sphere; however, the axis of rotation coincided with the reflecting sur­ after the first few months the surface of the satellite face and scintillation was reduced to almost zero. The became wrinkled, causing severe scintillation of the characteristics of the Echo satellites are illustrated reflected radio signals. in figures 1, 2, and 3. Tests on the reflective characteristics of Echo I Description of have 'been conducted throughout its lifetime to deter­ Experimental Stations mine the effects of the space environment. Immedi­ Before discussing the tests and results of the ex­ ately following orbital insertion and inflation, the perimental program on the Echo I and Ecfco II satellites, Collins Radio Company successfully transmitted a the ground station equipment should be described in facsimile picture between Dallas, Texas, and Cedar brief. Stepids, Iowa, via the Echo I satellite. The U. S. Naval Research Laboratory station em­ The Echo II Satellite ploys a 6 0-foot-diameter antenna, low-noise TWT re­ ceiving preamplifier, and a pfease-lock receiver utiliz­ The success of Echo I initiated a program for im­ ing special provisions to yield "analog data. Data was proved materials and, inflation techniques ; and the re­ taken at 2380 MHz, 2260 MHz, and 2190 MHz. Also, sultant launch of Echo II in June, 1964. In preparation a 10-kilowatt transmitter at the station was utilized in lor the launch, the Goddard Space Flight Center of the communications experiments. The NRL antenna NASA fead organized a network of ground stations to was pointed by means of a programmed track using study, during its first year in orbit, the communica­ predicted look angle tapes, corrected by optical sight­ tions capability of the Echo II satellite. The network ing when possible and by peaking the signal return consisted of the U. S. Naval Research Laboratory sta­ when the satellite was not optically visible. tion near Washington, D. C. ; the Ohio State University It should be noted that Research Foundation station at Columbus, Ohio; and all experiments by this net­ work were communications the Collins Radio Company experimental station near oriented and NASA provided 1 all satellite ephemeris data. Each station received a that expected for a perfect sphere at the predicted ianga set of weekly predictions. While the predictions were The data indicated that the satellite characteristics did adequate for the communications program, they were not change significantly with time, and that the Echo II poor in comparison to ephemeris data on other satel­ exhibits a cross section average of 1060 square meters lites; the limited use of Echo I and Echo II not justify­ as compared to the physical cross section of 1320 sq. ing expenditures for high accuracy satellite orbit pre­ meters. The Echo I exhibits a cross section of about diction programs. 3 db less than Echo II, which is approximately equal to the ratio of their physical cross sections. Analog re­ University Research Foundation The Ohio State cordings of received signal level are shown in figures dishes and parametric (OSU) station has four 30-foot 4 and 5. amplifiers feeding phase-lock receivers with phase- locked demodulators for deriving analog signal strength Time and Amplitude Distribution measurements. The station employs monopulse track­ of Fades of the four antennas. ing for the pointing Another item of extreme interest concerning the The Collins Radio Company (CRC) experimental reflected signal is its variability. The signal returns station employs a 60-foot and a 28-foot antenna. This are characterized by strong lobes persisting for a sec­ station was used primarily for transmitting to the ond or so, and short deep nulls between lobes. The other two stations on each of the three frequencies in­ depth of the nulls is normally about 20 db. to receive the 2380 MHz volved, but was also used As was previously indicated, the scintillations vir­ from NRL at each of the two antennas, for a signal tually disappeared on several occasions. We believe In the normal mode of space diversity experiment. this was due to the line-of-sight coinciding with the axis 10-kilowatt transmitter operating on a 50- operation a of rotation of the satellite. percent duty cycle was used in the 28-foot dish to il­ luminate the satellites. The 60-foot dish and receiver Essentially all of the power fluctuations occur at tracked the reflected signals and completed the radar frequencies below 3 or 4 cycles. By removing the var­ loop. Simultaneously, on one of the other two fre­ iations due to change in range between the satellite and quencies, a 10-kilowatt transmitter in the 60-foot dish the ground stations, it was found that the amplitude illuminated the satellites for the other stations. fades could be approximated by the Rayleigh distribu­ tion curve. The characteristics of the reflected signals from the Collins station and from the NRL Data are indicated in figures 6 through 10. station was reduced and processed by Collins, and the data from the OSU station was reduced and processed Bandwidth and Diversity by OSU. Measurements The data obtained from the program is unclassi­ An item of importance in the evaluation of a com­ fied and may be made available by Collins Radio Com­ munication medium is the coherent bandwidth of the pany with the permission of NASA to interested parties. medium. It is also of interest, if the medium produces While the complete program cannot be detailed here, scintillations, to discover what sort of spacings will some of the more interesting aspects are included. provide for satisfactory diversity operation. The co­ herent bandwidth in this case is defined as that band­ Typical Communication Experiments width over which the fading is highly correlated. Fre­ quency diversity measurements were first attempted by interest centered on the effective In general, the modulating the carrier with 6 MHz to produce sidebands section of the satellites (Echo I radar reflection cross with 12-MHz spacing. This was also tried at 70 and the characteristics of the scintillation of and Echo II), 190 MHz. Due to equipment limitations, the wider the effects of these two par­ the reflected signals, and spacings combined space and frequency diversity. From link. ameters on the communications the data it can be concluded that both Echo I and Echo II Average Cross Section have a coherent bandwidth in excess of 12 MHz; fre­ quency diversity operation could be successful at 190 single characteristic Perhaps the most important MHz or greater; and space diversity would not normally to communications of the Echo II satellite as it relates improve the system. The effects of diversity operation This param­ is its effective reflection cross section.
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