STRAPDOWN OR GIMBALLED IMU'S FOR THE SPACE SHUTTLE

T. S. Bettwy, E. A. Goldberg, and H. A. Irwin

Peer Reviewed

Abstract: Traditionally, inertial navigation has been performed with gimballed IMU's. The navigation system on the Apollo CSM and primary navigation system on the LM utilized such a system. Bolstered by technical advances over the past decade that have made strapdown systems feasible, advocates of such systems have been pressing for theri use. The abort guidance system on the LM is a recently developed strapdown system. Strapdown systems for other space vehicles, such as the Delta and Agena boosters, are currently being developed. With the advent of the Space Shuttle Program, the question of gimballed or strapdown IMU again arises. In this paper, a rational attempt is made to answer the question. The space shuttle requirements (such as accuracy, redundancy, maintainability, operatiosn) are analyzed and candidate system configurations developed. Analytical and historical data are employed to justify reliability conclusions. Redundancy aspects are considered. Cost-of-ownership is considered from the point of view of both minimum total program cost and minimum cost to initial operational capability (minimum development cost). Operational considerations (calibration, alignment) that impact the choice of system are explored. The conclusion reached in this paper is that little cost difference exists between an approach using four gimballed systems or four three-pack strapdown systems. A system employing two strapdown six-packs is shown to be a less cost effective approach. A system employing a single six-pack is shown to be competitive from a cost-of-ownership point of view but more vulnerable to localized vehicle damage. Gimballed systems are currently recommendd as the shuttle baseline because of the simplified ground operations that result with their use. However, strapdown systems should not be eliminated from consideration at this early date. Specificatioins and requirements should be written so that they can be satisfied with either type of system. Final system selection should be based upon the capabilities of each candidate and a firm cost quotation from the manufacturer.
Published in: NAVIGATION, Journal of the Institute of Navigation, Volume 18, Number 3
Pages: 331 - 343
Cite this article: Bettwy, T. S., Goldberg, E. A., Irwin, H. A., "STRAPDOWN OR GIMBALLED IMU'S FOR THE SPACE SHUTTLE", NAVIGATION, Journal of The Institute of Navigation, Vol. 18, No. 3, Fall 1971, pp. 331-343.
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