Box 2.6 Special Orbits

The infrared performance of the ATD/NTOT is likely to be maximized if the telescope is operated at as low a temperature as possible. The largest obvious contributors to the heating of the telescope are sunshine (the same for all geocentric orbits) and earthshine (varying drastically with geocentric distance and direction). Studies of alternative orbits, from the point of view of both cooling and efficient operations, should be carried out during the final design phase. The orbits to be considered might include IRAS-like polar orbits orthogonal to the solar direction, IUE-like geosynchronous or even geostationary orbits, and SIRTF-like heliocentric orbits. The current communications capability might be inadequate for some of these (it would certainly be inadequate for the heliocentric orbit), and some orbits may be incompatible with DOD’s need for evaluating its technology. These considerations deserve further exploration.

capability for lossless data compression, by at least a factor of two for essentially all data and by a factor of five for many types of data. It also assumes that some on-board processing will also be available that can, for example, take the median of several images to remove cosmic-ray hits (in cases for which that is desired) so that only median images need be transmitted to Earth. These assumptions provide a wide margin for the anticipated load of communications during the astronomical phases of the mission.

The power supplied by the F-SAT bus in the baseline mission is estimated at more than 3 kW at the end of a 6-year lifetime. Of this, approximately 1200 W would be used for spacecraft operations and communications, leaving some 2 kW for payload components. It is assumed that the payload provided for the DOD’s technology demonstrations will take only a small fraction of this capability. Thus at least 1 kW of power should be available at the end of the ATD/NTOT’s anticipated lifetime for any additions to the payload required by the astronomical tasks.

REFERENCE

1. Proton Launch Vehicle and Launch Services: User’s Guide, Lockheed-Khrunichev-Energia International, Inc., San Jose, Calif., 1993.



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