required is significant. Also, mission operations grow more complex and costly as the number of satellites needed to perform a mission increases.

Multisensor Platforms

Requirements for spatial and temporal simultaneity among measurements have traditionally led to the use of multisensor platforms. Placing multiple sensors on a common platform ensures contemporaneous measurements and permits coalignment such that they view the same scene simultaneously. This is always desirable when the measurements are complementary and is in some cases essential if full value of the data is to be realized.

While single-sensor platforms provide the greatest mission flexibility, multisensor platforms have a higher probability of delivering an equivalent number of sensors to orbit because they require fewer launches and use more reliable launch vehicles (e.g., Delta, Atlas).2 Multisensor platforms also offer simpler ground system operations with fewer spacecraft to control and fewer data downlinks.3 On the other hand, design compromises may be needed because all sensors must have a common orbit, and payload accommodations must account for possible optical (fields-of-view), mechanical (jitter), or electromagnetic sensor interference as well as the possible multiplexing of resources (data handling, power, etc.). Further, with a multisensor platform, a launch or satellite bus failure results in the loss of all sensors.

Clusters

When spatial or temporal simultaneity of measurements is required, flying several single-(or few) sensor spacecraft in formation as a cluster is an alternative to a larger multisensor platform. This addresses the requirements for simultaneity of measurements while still providing the programmatic flexibility associated with small spacecraft: For example, the cluster can be built up incrementally, and, in the case of failure or obsolescence, sensors can be replaced one (or a few) at a time.

Formation flying is feasible but has not been proven in operational use.4 The difficulty and cost associated with this approach depend on how stringent the requirements are for coalignment and simultaneity of measurements. In all cases, on-board propulsion systems are needed to maintain relative position within the cluster. Coalignment of sensors on separate platforms to better than a few tenths of a degree requires sophisticated spacecraft attitude control and/or sensor pointing systems. In addition, active control links between spacecraft are

2  

 A spate of launch vehicle failures occurred in the months before this report was completed, including failures on larger and historically more reliable launch vehicles (Delta, Titan) as well as newer launch vehicles (Athena). On May 4, 1999, the second stage of a Delta III rocket failed, leaving a $150 million satellite in a useless orbit. On April 30, 1999, a Titan IV rocket failed to place an $800 million Milstar communications satellite into its proper orbit. On April 27, 1999 a Lockheed Martin Athena 2 rocket failed to place a commercial space imaging satellite into polar orbit. On April 9, 1999, a $250 million missile warning satellite was left stranded in the wrong orbit after its upper stage booster failed on a Titan IV rocket. On August 26, 1998, a Delta III exploded on its maiden flight, destroying a communications satellite. On August 12, 1998, a Titan IV rocket carrying a $1 billion classified payload exploded shortly after launch. See the extended launch and explore the archives pages at FLORIDA TODAY Space Online (1999).

3  

 The size, complexity, and cost of a satellite ground system are important issues when there are potentially many satellites in the same orbit and ground contacts overlap. The National Oceanic and Atmospheric Administration and the Air Force typically retire (essentially through lack of attention) older polar-orbiting meteorological satellites because of the load on their tracking and processing facilities. In addition, generation of command lists, ephemerides, and the other data required to operate a satellite requires personnel, time, and money. To better manage these problems, operation of multisatellite systems requires approaches to operations that are more automated and less taxing of human resources.

4  

 Earth Orbiter-1 (EO-1) is the first of a series of Earth orbiting missions for the National Aeronautics and Space Administration's New Millennium Program. The mission, which is scheduled for launch on April 13, 2000, is designed to validate a number of technologies that would provide Land Remote Sensing Satellite (Landsat) follow-on instruments with increased performance at lower cost. The mission's centerpiece is an Advanced Land Imager (ALI) instrument. Once on orbit, EO-I will provide 100 to 200 paired scene comparisons between ALI and the Landsat 7 imager, ETM+ (Enhanced Thematic Mapper Plus), to validate ALI's novel multispectral technology. The mission will also demonstrate formation flying, since the EO-orbit will be associated with that of Landsat 7. The EO-1 spacecraft will fly in a Sun-synchronous orbit at the same altitude as, but slightly offset from, that of Landsat 7. Thus, EO-1 will fly over the same ground track as Landsat 7, but several minutes behind it. For more information on the mission, see EO-1 (1999).



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