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Trajectory Reconstruction for Beagle 2

Trajectory Reconstruction for Beagle 2. Paul Withers, 5 th year PhD student at the University of Arizona/LPL Worked on MGS accelerometer data from its aerobraking in upper atmosphere Advisor to MER (NASA 2004 Mars Landers) for trajectory (and atmospheric structure) reconstruction

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Trajectory Reconstruction for Beagle 2

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  1. Trajectory Reconstruction for Beagle 2 • Paul Withers, 5th year PhD student at the University of Arizona/LPL • Worked on MGS accelerometer data from its aerobraking in upper atmosphere • Advisor to MER (NASA 2004 Mars Landers) for trajectory (and atmospheric structure) reconstruction • John Zarnecki, Martin Towner, Brijen Hathi of the Open University (Great Britain) • Huygens DTWG#6, JPL, 2002.12.02

  2. Background • Beagle 2 = UK/ESA Mars Lander, delivered by ESA’s Mars Express in Dec 2003. • Accelerometer is a small part of John Zarnecki’s Environmental Sensor package • I spent summer 2001 at Open University writing trajectory (and atmospheric structure) reconstruction programs • At DTWG to learn about Huygens’s techniques for doing this and share plans from Beagle 2

  3. Aims of Summer Work • Develop general techniques for trajectory reconstruction, then specialize for Beagle 2’s instruments and capabilities • Current status: Simple, general techniques without error analysis • Apply to entry phase accelerometer data using specified entry position and velocity only. • I will highlight two areas I found complicated

  4. (1) Constraining Spacecraft Attitude • Head-on: Attitude adjusts to keep main accelerometer axis parallel to relative velocity between spacecraft and atmosphere • Drag-only: Direction of measured 3-component acceleration is parallel to relative velocity • Acceleration Ratios: Ratio of axial and normal accelerations gives spacecraft attitude • Gyroscopes: Nice if you have them • Current status: Head-on or Drag-only

  5. (2) Coordinate Frames • Aerodynamic accelerations measured in spacecraft frame • Equations of motion simplest in an inertial frame • Final trajectory most useful in planet-fixed rotating frame • My choice: Work in a different inertial frame at each timestep, which is coincident with planet-fixed rotating frame at that timestep. There are many other choices.

  6. Testing on Mars Pathfinder • Use poor aerodynamic database without additional constraints of near-landing radar altimeter and landed position • Difference between our traj and PDS in latitude and longitude is 0.05o or less, same scale as errors in entry position. • Discovered inconsistency within PDS archive at high altitude • (Atmospheric results are good)

  7. Digression on Quick T(z) • Trajectory + aerodynamic database gives density profile • Density + gravity gives pressure profile • Density + pressure gives temperature profile • Can get reasonable T(z) without any aerodynamic database at all since CD changes slowly

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