Gnc challenges Navfilter Architecture (imu, tds)

• 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

• Sandy radar effects and the near ground effects of radar