Gnc challenges Navfilter Architecture (imu, tds)


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  • GNC Challenges

  • Navfilter Architecture (IMU, TDS)

  • Performance

  • Velocity Analysis

  • Ground Estimates

  • Attitude Initialization

  • TDS Performance

  • Conclusions/Open Items



EDL Challenge

  • EDL Challenge

  • Land a 950Kg Rover over terrain with:

    • < 30 degree slopes, and
    • < 1.2 meter rocks
  • Response

  • 6-Axis Powered-Descent + SkyCrane

    • Parachute terminal velocity
      • Vertical = 65 to 100 m/sec
      • Horizontal = 0 to 40 m/sec
    • Powered-Descent Start Altitude
      • 1.5 to 2 km
    • Rover touchdown velocity
      • Vertical = 0.75 m/sec, +/- 0.1m/sec (3-sigma)
      • Horizontal = 0 m/sec, +/- 0.5 m/sec (3-sigma)
    • Descent Stage touchdown velocity
      • Vertical = 0.75 m/sec, +/- 0.1 m/sec (3-sigma)
      • Horizontal = 0 m/sec, +/- 0.1 m/sec (3-sigma)


  • Requirements

    • Attitude Initialization
      • Original .25 Deg 3σ, Reduced to 0.1 Deg 3σ before landing
    • Touchdown Velocity Knowledge Error
      • 0.1 m/s 3σ all axes
  • Provide state information to all modules throughout EDL

  • Inertial Propagation

    • 200 Hz (DIMU sample rate) integration of Attitude, Position, and Velocity
    • DIMU internal Coning and Sculling compensation (Internal DIMU sampling rate is 1600Hz)
    • J2 Gravity Model
  • Attitude Initialization from Cruise ACS

  • Position and Velocity state Initialization from Ground Navigation in EdlParms (Epoch at E-9minutes)









TDS Description

  • TDS Description

    • Ka-band (35.75 GHz) center frequency
    • Pulse doppler radar design, LOS Velocity and LOS Slant Range
    • 6 beams: 3 beams canted 20° off nadir, 2 beams canted 50° off nadir,1 nadir beam
  • TDS data is used to correct errors in Inertially propagated Velocity, Acceleration and Altitude

  • Velocity Filter is 6 state Least Squares Filter

    • 3 Axis Velocity Error
    • 3 Axis Acceleration Error (attitude, misalignment, etc)
    • Recursive (but finite) data editing
  • Altitude Filter

    • Estimates ground over MSF frame
    • Mean of ground altitudes in measurement buffer






Flight Reported Values

  • Flight Reported Values

  • Velocity Correction when Navfilter converged

    • [ .5884 -.4567 .1942 ] m/s (MSF Frame, East-North-Up)
    • Velocity correction .77 m/s
  • Altitude Correction when Navfilter converged

    • 113.4 meters
  • Touchdown Velocity

    • [ -.02203 -.003505 -.76501 Z ] m/s ( MSF Frame, East-North-Up)
  • Position of Descent Stage Reported at Touchdown

    • [ 1597.74 246.3 7.677 ] meters ( MSF Frame, East-North-Up )
  • Knowledge Error at Touchdown Based on MRO photos

    • Using OD179 770 meters ( Flight , 5 day old at time of EDL)
    • Using OD230 162 meters ( After landing rerun standalone Navfilter using od230 state)


The first thing we noticed was the acceleration term in the velocity solution for Z axis

  • The first thing we noticed was the acceleration term in the velocity solution for Z axis





  • Re-Run the Navfilter without any TDS data after 935 seconds past T0 (~450 m above terrain)

    • This was done to remove any potential sandy radar effects on TDS measurements close to the ground
    • This matched the touchdown velocity from the RIMU (Rover IMU) very well






  • Analysis

    • Using best estimate of reconstructed trajectory to determine the best estimate of gravity using both RIMU and DIMU data
    • Obtain the best estimate from gravity at touchdown location from the Navigators using MGM2011 knowledge


    • The TDS produced no ambiguities during all of EDL
    • The Navfilter Data Editing was never invoked
    • TDS passed 1 measurement of the Heat shield and the Navfilter rejected it in the pre-filter


  • Performance

    • Attitude Init. 30 millidegrees
    • Position Knowledge error 162 meters
    • Velocity at touchdown [.1694 -.0482 -.6312 ] m/s
    • Velocity Knowledge error [ -.19 .045 -.134 ] m/s (almost 5σ)
  • Open Items




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