Operators’ Requirements for SSA Services

Abstract

The space data association (SDA), an association of global satellite operators working to ensure a controlled, reliable, and efficient space environment, has run a survey among its members to gather data on their Conjunction Assessment concept of operations. These include collision avoidance Go/No-Go metrics, collision avoidance targets, and operational constraints. This paper assesses the various positional accuracy requirements of space situational awareness (SSA) data associated with each of these diverse "Go/No-Go" metrics as employed in the conjunction mitigation processes used for space traffic coordination and spaced traffic management. These metrics include miss distance at the time of closest approach (TCA), componentized miss distance (e.g., TCA radial separation to preclude collision even when in-track or cross-track separations or uncertainties are unknown), and maximum collision probability and estimated actual probability. A common practice is to approximate a spacecraft’s hardbody with an encapsulating sphere. This one-shape-fits-all approach eliminates the need to determine orientation, but results in an overestimated object volume and an overinflated probability unless both satellites are actually spheres. The dependence of collision probability on orientation and configuration/shape of the satellites at TCA is examined in contrast to the use of an encapsulating spheres to produce more representative probabilities. To overcome the lack of knowledge of the enveloping box's orientation a spectrum of collision probability values corresponding to a range of box orientations, from which the interrelationship between attitudinal knowledge and position accuracy required for a given collision probability threshold can be determined. It was found that such an approach can typically reduce probability by a factor of 3 or more. The interrelationships between SSA positional accuracy, the operator-selected Go/No-Go metric and its threshold, timeliness, and resulting maneuver frequency is also explored. For example, the necessity to perform a collision avoidance maneuver adhere to a squared relationship on the adopted miss-distance threshold. The miss distance threshold adopted by the operator should, if done properly, be a function of the estimated accuracy of the primary and secondary objects as a function of time. This paper concludes by comparing the accuracy requirements derived for each metric above with estimates of positional accuracy observed in actual SSA data fusion experiments conducted this past year. In many situations, the accuracy of legacy and commercial SSA systems is insufficient to support the adopted Go/No-Go metric without using comprehensive data fusion techniques. Recommendations for operators are provided.