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A summary is given of the measurements carried out in the framework of OPEX (OLYMPUS Propagation Experimenters Group). In particular, the progress since mid-1990 is presented. In this period two OPEX meetings were held, OPEX 14 (Oct. 1990) and OPEX 15 (Apr. 1991). The First OPEX Workshop was held at ESTEC on 23-24 April 1991.
With the launch of the telecommunications Olympus satellite a new area began for the Olympus Propagation Experiments (OPEX) group. The years of preparations are now paying off - the experiments are underway and the co-operative effort is now turning its attention to the processing and analysis of data and to the interpretation of results. The aim here is to give a short review of the accomplishments made since NAPEX 13 and the work planned for the future. When ESA's Olympus was launched in summer of 1989 it carried a payload producing three unmodulated beacons at 12.5, 19.8, and 29.7 GHz. The main purpose of these beacons is to enable scientists to carry out long term slant path propagation experiments at these frequencies. The OPEX group, which was set up under ESA auspices in 1980, had been preparing for this event very carefully. The specifications for the equipment to be used and the elaboration of standard procedures for data processing and analysis have been worked out jointly. Today the OPEX community includes approximately 30 groups of experimenters. Immediately after achieving platform stability at the orbital location at 341 degrees east, ESA performed the In-Orbit Tests. Most measurements were carried out in Belgium using terminals specially developed for this purpose. A summary of the test results is given.
FORTRAN program for radiation shield optimization using steepest descent method
The Olympus-1 satellite carries four distinct payloads for experimental utilization and research in the field of satellite communications: (1) the Direct Broadcasting Service (DBS) payload; (2) the Specialized Services Payload; (3) the 20/30 GHz Advanced Communications Payload; and (4) the Propagation Payload. Experimental utilization of the first three payloads involves ground transmissions to the satellite and hence sharing of available satellite time among experimenters. This is coordinated through the Olympus Utilization Program.
A group of propagation researchers have joined to prepare for a major campaign of propagation experiments for fixed satellite services using the Olympus satellite. For several years, the receiving hardware and data processing procedures have been planned, now, with the launch of the Olympus satellite imminent, experimenters are eager to start the real work. A short review is given of the accomplishments made since the last NAPEX meeting.
Computer program, OPEX 2, determines minimum weight, volume, or cost for shields. Program incorporates improved coding, simplified data input, spherical geometry, and an expanded output. Method is capable of altering dose-thickness relationship when a shield layer has been removed.
An overview of the organization of the OLYMPUS propagation experimenters group (OPEX) is given. Preparations, participation, and experiments are described. Some examples for first statistical results are also reported. OLYMPUS, a 3-axis stabilized communications satellite was launched in 1989 for providing experimental telecommunications payloads and a propagation beacon payload at 12, 20, and 30 GHz to the European Space Agency. From previous experience (OTS), the Agency undertook to carry out extensive preparations with an eye on obtaining the statistical results needed within the limited available lifetime of the spacecraft. The OLYMPUS propagation experiment was conceived as part of ESA's space telecommunications applications program (ESA/IPC/(79)83) with the emphasis on exploring the possibilities and limitations of Ka-band satellite communications. The objectives of the OLYMPUS propagation campaign were: (1) characterization of the slant-path propagation conditions at 20/30 GHz in the various climatic regions of Europe; (2) improvement of the understanding of the link between atmospheric observable (rain rate, cloud thickness, etc.) to propagation impairments such as attenuation, depolarization, scintillation, etc.; and (3) arrive at improved propagation prediction methods.
A summary of the activities of the OPEX (Olympus Propagation EXperimenters) group is given and some of the recent findings are presented. OLYMPUS, a telecommunication satellite owned by the European Space Agency, was launched on 12 June 1989. After the in-orbit tests were completed (in September 1989) the first propagation experiments started. Throughout 1990 the spacecraft functioned very well and a large number of experimenters received the beacon signals. On 29 May 1991 the spacecraft became inoperational after a major technical problem. With a series of complicated procedures OLYMPUS was recovered on 15 August 1991 - the first time in history that a civilian telecommunications satellite was brought back to service after losing power and telemetry. The propagation experiments were back on track. However, the recovery had used up so much fuel that the North-South station keeping had to be abandoned, which led to a natural increase of inclination at a rate of about 0.8 deg per year. On 10 October 1992 the second 30 GHz beacon tube failed, causing a loss of this beacon signal. The other two beacon frequencies continued to deliver a stable signal for more than two years. On 12 August 1993 the spacecraft experienced another problem with the altitude control, but this time there was not enough fuel left for a recovery maneuver and thus the mission came to an end.
Insulation systems are critical to liquid hydrogen storage tank performance. Tanks in the capacity range of 100 to 1,000 m 3 are typically shop built and designed with high-vacuum (HV) multi-layer insulation (MLI), whereas storage vessels larger than 1,000 m 3 are typically field-erected and supplied with bulk fill insulation working at moderate vacuum (MV) levels (1-100 millitorr). For large, field-erected vessels, two types of bulk fill insulation typically used: perlite and hollow glass microspheres (glass bubbles). Selection of either material is driven by a tradeoff between CAPEX and OPEX, such as the material and construction cost versus operating thermal performance and maintenance. In either case, the vacuum level needed to achieve optimum performance is likely to drive the field testing and commissioning portion of the construction schedule. A primary goal of this paper is to present practical experience and data for warm vacuum pressure (WVP) and cold vacuum pressure (CVP) levels. Recommended WVP levels needed prior to cooldown consider both perlite powder and glass bubbles. Pumping time expected to achieve target vacuum levels, considering a variety of factors, is also discussed. Recommendations for a standard practice in vacuum-insulated tank commissioning are based on historical NASA data, and those collected during recent projects.