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Mars Science LaboratoryNavfilter
Trajectory Reconstruction
Fred Serricchio
Miguel San Martin, Edward C. Wong Jet Propulsion Laboratory, California Institute of Technology
IPPWSan Jose , CA
June 17 - 21 , 2013
(c) 2011 California Institute of Technology. Government sponsorship acknowledged.
Outline
•GNC Challenges•Navfilter Architecture (IMU, TDS)•Performance•Velocity Analysis•Ground Estimates•Attitude Initialization•TDS Performance•Conclusions/Open Items
9/13/07 2
MSL EDL/GN&C 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)
NavFilter Architecture
•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)
9/13/07 4
IMU
• 2 IMUs (primary and backup)• Honeywell MIMU
• It provides 3 axis position and attitude knowledge in the form of:
• Accumulated angle for each DIMU axis.• Accumulated sensed acceleration for each
DIMU axis.
• Operation:• One IMU operational at a time.• IMU selection done before EDL.
DIMU (MIMU)
http://www51.honeywell.com/aero/common/documents/myaerospacecatalog-documents/MIMU.pdf
Terminal Descent Sensor
• The Radar Terminal Descent Sensor produces line of sight range and velocity measurements on 6 independent narrow (~3.5 deg) beams
– Single antenna per beam, with 3 beams canted 20o off nadir, 2 beams canted 50o off nadir, and 1 nadir beam
– Ka-band (35.75 GHz) center frequency
– Pulse doppler radar design
• The Radar Terminal Descent Sensor produces line of sight range and velocity measurements on 6 independent narrow (~3.5 deg) beams
– Single antenna per beam, with 3 beams canted 20o off nadir, 2 beams canted 50o off nadir, and 1 nadir beam
– Ka-band (35.75 GHz) center frequency
– Pulse doppler radar design
Descent Stage Configuration
TerminalDescentSensor
(TDS) 1x
Descent IMU(DIMU) 2x
MarsLandingEngines
(MLEs) 8x
Entry RCS 8x
Descent StageEntry Configuration Descent Stage
Skycrane Configuration
Terminal Descent Sensor (TDS)
NavFilter Architecture (cont.)
• 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
9/13/07 8
NavFilter ArchitectureInertial Propagation (64 Hz)
9/13/07 9
AttitudePropagator
AttitudePropagator
DIMU200 Hz
DIMU200 Hz
VelocityPropagator
VelocityPropagator
PositionPropagator
PositionPropagator
CoordinatesFrames
&Center of
NavigationTransforms
CoordinatesFrames
&Center of
NavigationTransforms
PositionCorrectionPropagator
PositionCorrectionPropagator
Vcorr(ti) + Acorr(ti) x ( t – ti )
∆h0
Vcorr(ti)
dimuAtt_q_Dimu_J
+ +
+
-
- - dimuPos_J
dimuVel_J
deltaV_Dimu
deltaTheta_Dimu
From Cruise: dimuAtt_q_Dimu_J(0)
From Ground Nav: Pos_J(t0), Vel_J(t0)
To TDS Batch Filter (Foreground)
To PD Controller
q_Msf_MPosMsf_M
MarsRotation
MarsRotation
q_M_J , wMars_J
wMars_JRotRate
&Acc
Filter
RotRate&
AccFilter
deltaTheta_Dimu, deltaV_Dimu
w_Ds, wDot_Ds, acc_Ds
uncorr_dimuPos_Juncorr_dimuVel_J
dsPos_MsfdsVel_Msfq_Ds_Jq_Msf_J
NavFilter ArchitectureMeasurement Update
10
TDS20 Hz
TDS20 Hz
SanityChecksSanity
Checks
ComputeVelocityResidual
ComputeVelocityResidual
ComputeRaw
GroundAltitude
ComputeRaw
GroundAltitude
ForegroundBuffer(1 sec)
ForegroundBuffer(1 sec)
Data EditingData
EditingFinal
FitFinal
FitValidityChecksValidityChecks
GroundAltitude
Rectification&
FormSolutionBuffers
GroundAltitude
Rectification&
FormSolutionBuffers
Data EditingData
Editing
DataEditingData
Editing
Final Fit
Final Fit
Final Fit
Final Fit
ValidityChecksValidityChecks
ValidityChecksValidityChecks
Primary Solution
Secondary Solution
FinalSolutionSelection
FinalSolutionSelection
BackgroundBuffer
(20 sec)
BackgroundBuffer
(20 sec)
timeTag, beam_ID, vel_residual, beam_direction_J
timeTag, beam_ID, raw_ground_alt_inMsf
tepoch, Vcorr(tepoch) , Acorr(tepoch)
ground_alt
To Inertial Propagator
To PD Controller
Forward (64 Hz)
Background (1 Hz)
FormSolutionBuffer
FormSolutionBuffer
Flight Reported Performance
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 converged113.4 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 photosUsing OD179 770 meters ( Flight , 5 day old at time of EDL)
Using OD230 162 meters ( After landing rerun standalone Navfilter using od230 state)
• The Navfilter converged at an altitude of 8500 meters – Needed to converge by 2500 meters to accomplish EDL activites before
safe touchdown
Flight Reported Velocity Solutions
• The first thing we noticed was the acceleration term in the velocity solution for Z axis
Acceleration of 450 micro G’s
Local Gravity At Gale Crater
We were able to match the slope in the velocity solutions we saw in flight
That resulted in a larger velocity Eastward at touchdown than reported landing night
Reconstructed Velocity
Touchdown Velocity Analysis
•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
Descent Stage Velocity at Touchdown [.1694 -.0482 -.6312 ] m/s in MSF frame
Versus Desired [ 0 0 -.75 ] m/s, +/- 0.1 m/s 3σ
Msf: Mars Surface Frame
Mars Surface Frame (Msf)– Origin: Intersection of s/c position vector at first Nav. Filter TDS solution with Mars
Spherical model with radius equivalent to radius at landing target (or radius at edl_update_param?)
– Orientation: ENU (East, North, Up)> East:> North:> Up: along radial vector from center of Mars.
AGL altitude
Ground altitude
Mars Sphere(Radius = radius at landing target)
SC position vector
Msf
Power approach target altitude =
+ dh_const_decel
+ dh_const_vel_accordion
+ h_sky_crane_start
Msf frame defined at first Nav Filter Solution using
TDS data
Ground Estimate
We see some vehicle dynamics and ground effect but once we converged the estimates of the ground were not changing that much
10 m
~70 m
Attitude Initialization Error
• 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
Best Estimate of Attitude Init error is 30 millidegrees
TDS Performance
– 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
Conclusions/Open Items
• 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