A. THD Payload Initial Requirements and Guidelines

Based upon the SAPPHIRE microsatellite design constraints, guidelines, and capabilities, a number of initial design requirements for the Tunneling Horizon Detector (THD) payload were loosely specified. Most importantly, the payload was required to be completely designed, constructed, tested, and integrated with the overall spacecraft system within a year in order to meet the SAPPHIRE program schedule. The initial power budget for the THD payload was 1-3 W peak power and less than 0.8 W continuous average power. Input power supply was specified to be approximately 5-15V. Voltage regulation was not initially specified. THD payload volumetric constraints were estimated to be less than 48 cubic in. Payload configuration specifications required the THD to be capable of integration with the other components in the payload tray. Mass requirements limited the horizon detector to less than 3 lb. Due to the experimental nature of the THD payload, it was decided that the component would function solely as a payload and not be integral to mission operations or attitude determination. Consequently, the payload was not required to be active throughout the mission. Furthermore, the output of the payload could be raw data, sampled and stored by the spacecraft computer, and later downlinked to the ground for data interpretation and processing. Initial THD payload data storage was specified as 100 - 200 Kbytes maximum. Desired payload output signal resolution specified a greater than (or equal to)10 bit A/D converter, capable of sampling the output signal at approximately 100 Hz. Additional computer interface control allowed for a few analog and digital signal lines.

Based on these initial requirements and guidelines, it was decided that there was mass, power, and volume budget for 2 completely independent THD payloads on the spacecraft. Two smaller less complex systems were chosen to provide redundancy, minimize risk, provide data correlation, and help ensure success towards "space evaluation" of the TIS. As one component in the overall spacecraft system design, the requirements and design of the THD evolved throughout the spacecraft design process. The next section details the design and development of the TIS followed by the final THD payload design including design tradeoffs and selection rationale.

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