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)

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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

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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