MARS EXPLORATION ROVERS -A NEW STANDARD FOR INTERPLANETARY NAVIGATION

摘要

The twin Mars Exploration Rovers (MER), Spirit and Opportunity, arrived at Mars for landings respectively at Gusev Crater (on January 4, 2004) and Meridiani Planum (on January 25, 2004). During the development of the mission, the capability of the navigation system to deliver the landers within a particular accuracy played a major role in landing site selection. This process ultimately resulted in commitments to deliver each lander within a specified landing ellipse (about 70 km x 5 km) determined to be safe for landing and also judged to be scientifically interesting. Achieving atmospheric entry delivery accuracies consistent with this landing requirement necessitated significant improvements to the interplanetary navigation system used for MER. These improvements included new processes and software for orbit determination, propulsive maneuver design, and entry, descent, and landing (EDL) trajectory simulation; and aggressive, mission-critical use of interferometric delta differenced one-way ranging (ADOR) tracking data. Because these advances pressed the state of the art, innovative methods to verify the assumptions in the pre-launch covariance analyses were also developed. The actual achieved atmospheric entry accuracies for Spirit and Opportunity significantly bettered the requirements. At the navigation data cutoff for the fifth trajectory correction maneuver (TCM-5) final design, the orbit determination entry flight path angle (FPA) knowledge errors were ±0.028° (3σ) for Spirit and ±0.035° (3σ) for Opportunity. These FPA accuracies correspond to B-plane errors of less than 1 km, setting a new standard for interplanetary delivery accuracy. Because of the exceptionally accurate navigation performance, TCM-5 (entry minus 2 [E-2] days) and TCM-6 (E-4 hours) were canceled for both Spirit and Opportunity. The actual landing locations (determined from in situ Doppler tracking between the MER rovers and the Mars Odyssey orbiter) differed from the target landing points by 10.1 km (downtrack) for Spirit and 24.6 km (downtrack) for Opportunity. The majority of the landing position offsets for both landers was primarily caused by variations in atmosphere and spacecraft aerodynamic modeling from what was predicted. The amount of the landing position offset caused by navigation-only errors was only 3.3 km (uptrack) for Spirit and 9.7 km (downtrack) for Opportunity.