Spacecraft safing designs generally have minimal goals with loose pointing requirements.Safe pointing orientations for three-axis stabilized spacecraft areusually chosen to put the spacecraft into a thermally safe and power-positiveorientation. In addition, safe mode designs are required to be simple and reliable.This simplicity lends itself to the usage of analog sun sensors, becausedigital sun sensors will add unwanted complexity to the safe hold mode.The Global Precipitation Measurement (GPM) Mission Core Observatory willlaunch into lower earth orbit (LEO) at an inclination of 65 degrees. The GPMinstrument suite consists of an active radar system and a passive microwaveimager to provide the next-generation global observations of rain and snow.The complexity and precision of these instruments along with the operationalconstraints of the mission result in tight pointing requirements during allphases of the mission. To ensure the instruments are not damaged duringspacecraft safing, thermal constraints dictate that the solar pointing orientationmust be maintained to better than 6.5 degrees. This requirement is outside thecapabilities of a typical analog sun sensor suite, primarily due to the effects ofEarth's albedo. To ensure mission success, a new analog sensor, along with theappropriate algorithms, is needed.This paper discusses the design issues involving albedo effects on spacecraftpointing and the development of a simple, low-cost analog sensor and algorithmthat will address the needs of the GPM mission. In addition, the algorithmsare designed to be easily integrated into the existing attitude determinationsoftware by using common interfaces. The sensor design is based on aheritage, commercial off-the-shelf analog sun sensors with a limitedfield-of-view to reduce the effects of Earth's albedo. High fidelity simulationresults are presented that demonstrate the efficacy of the design.
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