Flyby Error Analysis Based on Contour Plots for Cassini Tour

Abstract

The maneuver cancellation analysis consists of cost contour plots employed by the Cassini maneuver team. The plots are two-dimensional linear representations of a larger six-dimensional solution to a multimaneuver, multiencounter mission at Saturn. This realization and the use of the tool itself is just one of the many significant engineering achievements that have come from the Cassini project. After inserting the cost contour capability with an enhancement (taking account of asymptote changes), a tool that was originally only used for analysis could be used for operations once the accuracy of the plots was determined to be acceptable for operations. The plots have been used extensively since the enhancement. By using contours plotted in the B plane with B R and B T components, it is possible to view the effects on V for various encounter positions in the B plane. The plot is used in operations to help determine if the approach maneuver (ensuing encounter minus three days) and/or the cleanup maneuver (ensuing encounter plus three days) can be cancelled and is a linear check of an integrated solution. The plots have also been used to bias the targets of encounters to save V. Nomenclature B = B vector or B plane B R = vertical axis of the B plane B T = horizontal axis of the B plane C3 = excess velocity or characteristic energy G x;y = grid point value on the contour plot for an approach maneuver, m=s G x;y;c = grid point value on the contour plot for a cleanup maneuver, m=s i = leg index J x;y = V calculation of a B plane grid point for an approach maneuver, m=s J x;y;c = V calculation of a B plane grid point for a cleanup maneuver, m=s K = K matrix, a maneuver capability matrix M = the B plane mapping R = R vector of the B plane r o = nominal trajectory apoapsis maneuver location S = the incoming asymptote T = T vector of the B plane T n = nth encounter of Titan V1 = the magnitude of the velocity for a cleanup maneuver, m=s V2 = the velocity magnitude for an apoapsis maneuver, m=s B A = asymptote change B plane state vector B RT = grid variation vector B xy = the variations in the B plane with a perturbed trajectory B R y = y component of grid points in the B plane B T x = x component of grid points in the B plane r = difference in the apoapsis maneuver location for a perturbed trajectory V = change in velocity, m=s V REF = change in velocity given in the reference trajectory, m=s

Cassini In-Flight Navigation Adaptations

“…If, during the maneuver design process, the contour plots show an advantage by biasing the B-plane target, a second analysis is undertaken to find the true optimal target. Contour plots were originally developed to determine the likelihood of canceling a maneuver [17], which is another way of introducing a target bias. The cost of canceling a maneuver was deduced by determining which contours are crossed by the orbit determination solution.…”