C-band FET power amplifier for TWTA replacement

The technology of gallium arsenide field-effect transistors has advanced to the point where these devices can serve to replace traveling wave tubes in spaceborne communication transponders.This paper describes interim results of a C-band FET amplifier which is currently being developed by RCA for INTELSAT. The FETA is designed to provide a 6-W output with an efficiency greater than 25%, and a backed-off 1.5-W output level with low distortion and efficiency of 13%.The third order intermodulation ($\text{C}\text{I}$) of a FETA at saturation is typically 15 dB as compared to 10 dB for a TWTA. At 10 dB input power back-off the $\text{C}\text{I}$ improves to 24 dB while that of a TWTA is only 16 dB.We fabricated a breadboard 5-W FETA which demonstrates that a linear operation can be achieved at an output level of 1.5 W with 10% efficiency. By contrast the efficiency of a typical 5 W TWTA in the same linear region is no greater than 3–5%.     

New Techniques in Undersea Technology

The successful development of expendable instruments in oceanography is certainly one of the most exciting advances in the rapidly evolving technology of oceanography. It appears that microelectronics may be especially well qualified in this field. The economics of ship operations and expediency tend to dictate this new development.     

Operational life improvement of SSME high-pressure turbopumps

The current Space Shuttle Main Engine Phase I engine has demonstrated excellent flight performance at power levels up to 104% of rated power level. Early certification testing at 109% power level showed the operational life of the high pressure turbopumps to be limited. A development program, Phase II, has been under way since 1983 to increase the operational life and provide additional margin. The history of the SSME Phase I engine and the design and results of the SSME Phase II Program are presented. Operational life limits and high-maintenance areas of the high-pressure turbopumps are identified. The Phase II design improvements, supporting analyses, and test results are described. The formal certification test program is discussed. Modifications have been made to the high-pressure fuel turbopump to reduce turbine operating temperature, to extend the life of the first- and second-stage turbine blades, and to reduce sheet metal maintenance. Modifications have been made to the high-pressure oxidizer turbopump to improve bearing life and to eliminate subsynchronous whirl. These are discussed in detail, as is the design and use of special instrumentation to provide for rapid evaluation of merit.     

Implementation of an ESA delta-DOR capability

This paper describes the implementation of delta-DOR (delta-differential one-way ranging) receivers within the ESA Deep Space ground station network. Delta-DOR provides very accurate plane-of-sky measurements of spacecraft position which complement existing line-of-sight ranging and Doppler measurements. We discuss how this technique has been adapted and implemented at the two ESA deep-space ground stations using existing equipment and infrastructure. These new capabilities were added by writing new software modules for the standard ESA digital receiver (the intermediate frequency modem system—IFMS). With these upgrades the receiver has the ability to record accurately timetagged signals from up to eight IF sub-channels. These sub-channels can have bandwidths of 50 kHz–2 MHz with a sample quantization of 1–16 bits per component. The IF samples are stored locally for subsequent retrieval over a WAN by the correlator facility at ESOC.     

A reappraisal of the habitability of planets around M dwarf stars

Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.     

Modeling international cooperation for space exploration

Space exploration into the twenty-first century is contingent upon the ability of states to forge an appropriate vehicle for international cooperation. A theoretical framework that explains international cooperation in space exploration is proposed. This framework encompasses scientific, technological, political, and economic initial conditions, state and nonstate political actors, and models of cooperation that explain how initial conditions and actors interact to realize cooperative outcomes. It is hypothesized that the prevailing initial conditions favor certain political actors over others which, in turn, promote a specific model of cooperation. Cooperative policy outcomes are examined and assessed vis-à-vis case studies of cooperation in space exploration. On this basis, policy recommendations that engender effective cooperative outcomes in space exploration are suggested.     

Non-Gaussian clutter characterization applied to OTHR using a mixture of two Rayleigh probability density functions

A practical technique for characterizing non-Gaussian radar clutter is specified and demonstrated using Over The Horizon Radar (OTHR) data, as an example. The technique employs maximum likelihood to fit the probability density of the clutter amplitude returns to a mixture of two Rayleigh probability densities instead of the single Rayleigh density typically used for Gaussian clutter. This model for non-Gaussian clutter is fully specified for any set of clutter amplitudes by a log likelihood, two Rayleigh parameters, and a mixing coefficient. A 3D plot of these values yields an easily-visualized clutter characterization, as is illustrated using OTHR data. This technique is a demonstration of clutter characterization using OTHR data, but the method can be applied to characterize other types of clutter data.     

A review of global satellite-derived snow products

Snow cover over the Northern Hemisphere plays a crucial role in the Earth’s hydrology and surface energy balance, and modulates feedbacks that control variations of global climate. While many of these variations are associated with exchanges of energy and mass between the land surface and the atmosphere, other expected changes are likely to propagate downstream and affect oceanic processes in coastal zones. For example, a large component of the freshwater flux into the Arctic Ocean comes from snow melt. The timing and magnitude of this flux affects biological and thermodynamic processes in the Arctic Ocean, and potentially across the globe through their impact on North Atlantic Deep Water formation.