Timothy P. McElrath

2001 Mars Odyssey Orbit Determination During Interplanetary Cruise

On 24 October 2001 coordinated universal time, following a seven-month journey to Mars, Odyssey executed a nominal orbit insertion burn to be captured successfully into orbit around Mars. The excellent navigation performance during the interplanetary cruise resulted in arrival conditions over the north pole of Mars well within 1‐σ of the designed values. The achieved altitude above the north pole was less than 1 km away from the 300-km target altitude. Several sources of error made the orbit determination process for Odyssey challenging. The largest of these errors was caused by the periodic autonomous angular momentum desaturation events. Several navigational aids were brought forth to mitigate the error sources and improve the accuracy of Odyssey’s interplanetary cruise navigation. The most significant of these included the incorporation of very long baseline interferometry, delta-differential one-way range tracking data into the orbit determination filtering process and the placement of the spacecraft into a low-torque attitude during the final two months of interplanetary cruise. Orbit determination solution consistency was routinely evaluated through a battery of filter strategies and data combinations. The orbit determination processes and results of Mars Odyssey from launch to orbit insertion at Mars are discussed.

Mars Exploration Rovers orbit determination filter strategy.

§†† ‡‡ §§ §§ , The successful delivery of the Mars Exploration Rover (MER) landers to well within the boundaries of their surface target areas in January of 2004 was the culmination of years of orbit determination analysis. The process began with a careful consideration of the filter parameters used for pre-launch covariance studies, and continued with the refinement of the filter after launch based on operational experience. At the same time, tools were developed to run a plethora of variations around the nominal filter and analyze the results in ways that had never been previously attempted for an interplanetary mission. In addition to achieving sub-kilometer Mars-relative orbit determination knowledge, the filter strategy and process detected unexpected error sources, while at the same time proving robust by indicating the correct solution. Consequently, MER orbit determination set a new standard for interplanetary navigation. Nomenclature

Orbit Determination for the 2007 Mars Phoenix Lander

The Phoenix mission is designed to study the arctic region of Mars. To achieve this goal, the spacecraft must be delivered to a narrow corridor at the top of the Martian atmosphere, which is approximately 20 km wide. This paper will discuss the details of the Phoenix orbit determination process and the effort to reduce errors below the level necessary to achieve successful atmospheric entry at Mars. Emphasis will be placed on properly modeling forces that perturb the spacecraft trajectory and the errors and uncertainties associated with those forces. Orbit determination covariance analysis strongly influenced mission operations scenarios, which were chosen to minimize errors and associated uncertainties.