Development of Honeywell's Earth Reference Attitude Determination System (erads)

In 1992 Honeywell began development of Earth Reference Attitude Determination System (ERADS), a very small low cost/power/weight attitude reference system designed for small satellite applications. We undertook this development because it appeared to us that small satellites require significantly smaller, lighter, and lower cost attitude reference systems than are currently available. ERADS was conceived as a single, strapdown, three axis sensor that would image the entire Earth's limb in the ultraviolet. The spectral band was selected because it provided feature stability comparable to what is seen in the infrared along with sensor cost and weight characteristic of visible sensors. Although an Earth reference sensor was originally envisioned, it became evident that the ultraviolet was an excellent spectral band to observe stars as well, providing a providing a combined star/sun/Earth sensing capability in a single package. As a result, the current system can provide both threeaxis attitude and autonomous navigation data from a single solid-state sensor. A prototype sensor was fabricated in late 1992. In order to meet the weight and field of view requirements a highly innovative optical and detector assembly was developed. The optical assembly through the eeD has a volume smaller than a coke can and weighs less than a pound. The associated electronics, including an embedded R3000 processor, occupies two 8x 10 inch boards. The system was originally designed to provide three axis accuracy of .05°. Subsequent evaluations indicated that a .02° accuracy can be obtained. In 1993 the optical system has been modified to be more compatible with typical satellite real estate priorities. The field of view has been extended to provide a clear 30° area in the center in addition to the original annular field. These modifications will make it easier to integrate ERADS with satellites and will also improve performance. The resulting sensor package now has a configuration more closely approximated by a tuna can. A processor design incorporating high density interconnect technology is being developed which will greatly reduce the weight and dimensions. The resulting package should fit within the tuna can envelope. As the system has evolved, it has become clear that better accuracy can be obtained by relying more heavily on stars for attitude determination, and using the earth limb data primarily for navigation purposes. The combination of earth and star sensing in a single small package should serve to further reduce the burdens of attitude determination for smallsats. The processor section of ERADS is scheduled for a flight test in 1994. The entire: system is under consideration for an experiment on another 1994 flight.