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.
This work was done by Stephen F. Peters and Charles A. Vanelli of Caltech for NASA’s Jet Propulsion Laboratory. Please contact Dan Broderick at
This Brief includes a Technical Support Package (TSP).
Heading Versus Tilt Chart for Assessing HGA Occlusion and Flop Risk in MSL Operations
(reference NPO-49662) is currently available for download from the TSP library.
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Overview
The document titled "Avoiding High-Gain Antenna Occlusions and Flops in Mars Science Laboratory Surface Operations" discusses the challenges faced by the Mars Science Laboratory (MSL) rover in maintaining effective communication with Earth through its High-Gain Antenna (HGA). The primary issues addressed are occlusions, which block the HGA's view of Earth, and "flops," which refer to changes in the rover's kinematic solution when the Earth track crosses the HGA's joint limits.
To ensure successful communication during the rover's operations, planners must select end-of-drive headings that are both occlusion-free and flop-free for the upcoming sol's HGA communications pass. The document outlines two key ground tools that assist in this decision-making process: occlusion maps and a tilt-heading chart. These tools help assess the risk of occlusions and flops based on the rover's position and orientation relative to the celestial sphere.
The occlusion maps provide visual representations of areas where the HGA may be obstructed, while the tilt-heading chart allows for quick selection of headings that minimize both occlusion and flop risks. The document emphasizes that occlusion and flop risks can vary seasonally, necessitating careful planning and assessment based on the rover's current and anticipated positions.
Additionally, the document highlights the importance of terrain constraints, which can impact the timing of HGA communications. The kinematic solutions of the rover are influenced by its joint space and azimuth ranges, making it crucial for planners to consider these factors when determining communication strategies.
Overall, the document serves as a technical support package that outlines the methodologies and tools developed by NASA's Jet Propulsion Laboratory (JPL) to enhance the operational efficiency of the MSL rover. By addressing the challenges of HGA occlusions and flops, the document contributes to the broader goal of ensuring reliable communication between the rover and mission control, ultimately supporting the scientific objectives of Mars exploration.
