The Mars Science Laboratory (MSL) high-gain antenna (HGA) sits low on the deck, leaving the sky occluded in many directions by other parts of the rover. Each drive must end with the rover at a heading where the Earth will be unoccluded during the next HGA communications pass. This is a multidimensional problem that can take considerable time to assess in detail. For a portion of heading/tilt space, the Earth track starts outside joint limits for one kinematic pointing solution, and ends outside joint limits for the other. Tracking would either stop at the joint limit, or go off Earth point (“flop”) in the middle of the pass in order to change kinematic solutions to complete the pass. Special attention to uplink must be made when a drive ends at a heading where there is risk of a flop.
A chart of heading versus magnitude of tilt for a given HGA pass time divides the heading/tilt space into regions: certainly unoccluded and flop-free, occluded when tilt is in an unhelpful direction, and risk of flop. Detailed further analysis is not needed when rover planners are able to choose a certainly unoccluded flopfree end-of-drive heading.
Everyday surface operations include developing rover plans meeting a large number of constraints in a limited time. In this environment, quick “good enough” solutions are helpful. Rover attitude can be equally represented as heading, pitch, and roll, or as heading, magnitude of tilt, and direction of tilt. The three-dimensional space of rover attitude is collapsed into a two-dimensional chart of heading versus magnitude of tilt, quantized into 5° × 5° cells.
For a given HGA pass time and given attitude, the path of the Earth relative to occlusions on the celestial sphere is determined, and the time to encountering the first occlusion can be computed for each kinematic solution (“branch”) of the HGA. If the longer of the two unoccluded durations is longer than the pass, the Earth is unoccluded for this rover attitude.
Holding heading and magnitude of tilt constant, when this check is repeated for all directions of tilt (at 5° intervals), and all directions are unoccluded, the heading at that magnitude of tilt is guaranteed to be clear of occlusions and the cell is colored tan. If any direction of tilt is occluded, the direction of tilt determines whether or not that heading is occluded, and the cell is colored gray, indicating a need for deeper analysis. A conservative computational simplification is employed to test directions of tilt at 90° intervals relative to the direction of Earth.
If the extended Earth track passes outside joint limits on only one branch, the heading is guaranteed to be free of flop risk. If the extended Earth track passes outside joint limits on both branches and the magnitude of tilt matches the closest distance of the Earth track to zenith, there is a possibility that a flop may be needed to complete the pass, and the cell is marked blue, indicating flop risk.