Tradeoffs in Functional Allocation between Spacecraft Autonomy and Ground Operations: The NEAR (Near Earth Asteroid Rendezvous) Experience

Today's modem spacecraft often fly a computing power equivalent, or nearly equivalent, to the computing power available to the ground operations team. This enables the spacecraft to perform many functions autonomously that previously could only be planned and carried out from the ground. In some cases, this increased computing power is required to perform functions that must be carried out on the spacecraft. For example, fault detection and correction must be carried out on the spacecraft when the time scale of critical faults is shorter than the time between ground contacts. In many given function. In implementing these functions, tradeoffs between ground operations and spacecraft autonomy must be considered. With a maximum time of 12 days out of ground contact and a round-trip light time as high as 56 minutes, NEAR requires a moderate degree of onboard autonomy to react to faults and safe the spacecraft. Beyond the basic safing requirements, many additional functions can be carried out onboard. For example, momentum management, center-of-mass management during velocity change maneuvers, and optical navigation are all functions considered for onboard autonomy on NEAR. The allocation of these functions to onboard software or to ground operations involves tradeoffs such as development time for onboard software versus ground software, uplink/ downlink bandwidth, resource management, life cycle costs, and spacecraft safety.