  April 1998 Issue 9  by Charles Stelzried and Michael Klein In this issue's lead article, "TMOD Reorganization: Changes for the Better" , Leslie Deutsch, the manager of the Telecommunications and Mission Operations Directorate (TMOD) Technology (TMOT) Program, describes the logic behind the recent TMOD reorganization. TMOD will become more responsible for spacecraft functions. It will provide a single organization that is responsible for the end-to-end system and will result in a higher infusion of new technology into the deep space communications and operations system. The number of simultaneous, deep space missions is expected to grow from about 10 today to over 25 by 2003. It is especially important that new technology provide increased efficiencies with resultant lower costs, since TMOD must provide all of the current services to these additional missions with a budget that is not expected to grow. Roger Linfield's article, "Improvement in the Water Vapor Emission Model from GPS/WVR Comparisons" , describes an advanced calibration system for determining tropospheric parameters being developed for the Cassini Radio Science experiments. The success of this effort will determine the ultimate sensitivity of the Gravitational Wave Experiment. A factor of two improvement, relative to current water vapor radiometers (WVR) emission coefficient of water vapor, is required for this new system. It is expected that this can be accomplished using comparisons between Global Positioning System (GPS) and WVR calibrations of microwave delay from the earth's troposphere. Comparisons at a warm, humid site, such as Florida or Hawaii, will be made for this purpose. Paul Richter's and David Rochblatt's article, "Raster-Scan for Calibration of DSN Antennas" , provides details for a proposed raster scan technique of radio sources for calibration of Deep Space Network (DSN) antennas. This is expected to improve the accuracy relative to the current radio source boresight method. The boresight method is particularly susceptible to errors using radio sources that are not point sources and are not 'uniform' in brightness. Improved station instrumentation for taking data 'on the fly' is needed and currently under development at the TMOD Goldstone DSS13 research 34 m Beam Waveguide (BWG) antenna. It is expected that this will reduce the uncertainty of a typical X-band (8.4 GHz) radio source noise temperature measurement from about 0.14 K to about 0.03 K. L. Cangahuala, et al article, "ARTSN: An Automated Real-Time Spacecraft Navigation System" , describes a new class of navigation software initiated in 1994 now allowing for automated real-time spacecraft navigation system (ARTSN). This system will greatly improve the efficiency of the navigation analysis process and thus, enable simultaneous support of additional spacecraft by a single analyst. Additionally, faster orbit solution generation, relative to manual systems, reduces the turn-around time between critical events and the required response, allowing new modes of operation with increased reliability and reduced costs. This is especially important during critical events such as the launch phase, aerobraking, maneuver monitoring and approach navigation.
Finally, Valery Altunin's and Tom Kuiper's article, "Space VLBI Co-Observing at the DSN" , explains the contribution of the DSN 70-m antennas as elements of a space very-long-baseline interferometer (SVLBI) mission that combines space-based and ground-based radio telescopes to perform high resolution radio astronomy observations. The authors describe equipment upgrades that enable DSN participation in current and future SVLBI missions. The advantage of space-ground VLBI, allowing baselines greater than two earth diameters, is demonstrated with an image of a distant quasar. The unprecedented resolution was produced from interferometer fringes obtained between the VSOP space radio telescope and the 70-m Tidbinbilla radio telescope.
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