This paper documents the design, analysis, construction, test, and predicted on- orbit performance and operation of the Tunneling Horizon Detector (THD) payload, an experimental infrared Earth horizon detector for microsatellites. In addition, descriptions of payload error sources, mission operations, data validation, and performance verification are included.

The primary goal of the THD is to "evaluate" the space based performance and sensitivity characteristics of the Tunneling Infrared Sensor (TIS) as a possible first step towards the process of space qualification of this sensor and its sensing technique. The secondary goal of this work is to design a 1st generation, generic, inexpensive, low power ( < 1 W), light (< 3 lb. ), small (< 48 cubic in.) Earth horizon detector that can be utilized by SQUIRT (microsatellite) space vehicles. To accomplish both goals, the THD payload was constructed, tested, and integrated into the SAPPHIRE spacecraft for subsequent launch, and payload operation.

As part of the Stanford Spacecraft Design Program, and under the direction of Professor Robert Twiggs, SAPPHIRE is being designed, analyzed, constructed, tested, and operated by a integrated product development team of 10 - 15 Stanford graduate students. Started in March 1994, SAPPHIRE is the first SQUIRT project. It contains three primary payloads: an experimental infrared Earth horizon detector (THD), a digital camera, and a voice synthesizer. SAPPHIRE is expected to be launched in the winter quarter of 1996.

The THD is fixed to the payload tray of the spinning SAPPHIRE spacecraft, and utilizes a slit aperture in the side of the spacecraft to define its field of view. The basis for the THD sensing mechanism is a TIS. The TIS is an experimental silicon micromachined sensor designed and constructed by Stanford Professor Tom Kenny and colleagues of his at the Center for Space Microelectronics Technology (CSMT) at the Jet Propulsion Laboratory (JPL). It is a thermopneumatic infrared sensor that utilizes an electron tunneling displacement transducer to detect infrared radiation induced volume changes of a small amount of gas trapped between flexible membranes. THD operation is controlled by the SAPPHIRE computer and its outputs include 2 analog signals proportional to the variation in incident infrared radiation. Preliminary Earth horizon simulation testing indicates that the THD system has a sensitivity of approximately 1,500 V/W, which meets the performance requirements of Earth horizon detection providing a 30-40 mV output signal variation at horizon crossings. Due to the THD payload's low power (< 1 W), size (< 16 cubic in.), and mass (< 6 oz.), it is believed that a horizon detector based upon the results and "lessons learned" from the SAPPHIRE mission should be valuable to microsatellites. In addition, the expected operational requirements, performance, and sensitivity characteristics of the TIS make it a viable candidate for many future spacecraft missions.

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