Fixed-point simulation of onboard orbit determination.

The application of optimum estimation techniques to the problem of onboard orbit determination has received considerable attention. However, most studies and simulations have not considered the constraints imposed by typical onboard computers, such as fixed-point arithmetic with a limited word length. This paper discusses such computational limitations and presents means for their solution. These techniques include selecting a nonstandard set of observables so as to minimize computational problems, normalizing the units so as to eliminate scaling problems, and judicious selection of the filter equations. Simulation results are presented of fixed-point operation of onboard orbit determination with a horizon scanner sensor using word lengths varying from 22 to 32 bits. For comparison, the results of floating-poin t, 54-bit arithmetic simulation are also presented. These results demonstrate that a fixed-point formulation of the onboard orbit determination equations is feasible with only a limited number of scale factors and shiftings.