Mars Odyssey Navigation

2001 Mars Odyssey

Mars Odyssey Navigation at CU BoulderApril 2, 2002 Page - 1

Mars Odyssey Navigation

Moriba Jah

Jet Propulsion LaboratoryCalifornia Institute of Technology

2001 Mars Odyssey


Spacecraft Mission

• Investigate the Martian environment on a global scale, over a period of 917 Earth days.

• Serve as a relay for information to Earth, following the science phase.

2001 Mars Odyssey


Spacecraft Mission constraintsTo achieve the mission, the spacecraft must:

• Be injected into an orbit with a period of less than 22 hours, while having a 300 km periapse altitude (+/- 25 km) and an inclination of 93.5º (+/- 0.2º), including MOI burn execution errors. This is equivalent to hitting a golf ball from NY to Paris and making it in the hole in only 4 swings. (achieved: 18:36 period 300.75 km and 93.51º)

• Employ aerobraking over a 3-month period (walk-in, main phase, end-game/walk-out) in order to maximize payload mass and minimize propellant expense.

• By the end of aerobraking, stabilize in a 400 km “circular”, frozen, sun-synchronous orbit with a 2PM LMST AEQUAX.

2001 Mars Odyssey


Trajectory Selection: Pork-Chop plot

Courtesy of Rodney Anderson

2001 Mars Odyssey


Interplanetary Trajectory

10 day time ticks

2001 Mars Odyssey


Collecting Navigation Data

2001 Mars Odyssey


Radiometric Data Types • Doppler

– Measurements are comparisons of transmitted frequency (from ground station or spacecraft) with received frequency on ground; typical frequencies are at S-band (2 GHz) and X-band (7-8 GHz)

– Highly reliable; used in all interplanetary missions to date

• Range– Measurements are typically two-way light time for radio signal to

propagate between ground stations and spacecraft with a turn-around time; typical frequencies are also at S- and X-band

– Used in nearly all interplanetary missions since late 1960s

Collecting Navigation Data

2001 Mars Odyssey


Range and Doppler Tracking

2001 Mars Odyssey


Radial vs Angular Measurements

1999 Capability Position VelocityRadial Error 2 m 0.1 mm/sAngular Error (at 1 AU) 3 km* 0.1 m/s

• For most interplanetary missions, S/C position uncertainty is much smaller in Earth-spacecraft (“radial”) direction than in any angular (“plane-of-sky”) direction– Radial components of position and velocity are directly measured by range and

Doppler observations– In absence of other data, angular components are much more difficult to determine --

they require either changes in geometry between observer and spacecraft or additional simultaneous observer, neither of which is logistically simple to accomplish

– Angular errors are more than 1000 x radial errors even under the most favorable conditions (see below) when depending on range and Doppler measurements

*Equivalent to angle subtended by quarter atop Washington Monument as viewed from Chicago

2001 Mars Odyssey


Navigation Data Types

Delta Differential One-Way Range (DDOR)

• DDOR is a measurement technique that utilizes two ground stations to simultaneously view the spacecraft and then a known radio source (quasar or another S/C) to provide an angular position determination

• Two stations viewing the same signal allows for geometric plane-of-sky angular position measurement (Differential)

• By viewing two sources, common errors cancel and the angular separation can be calculated (Delta)

2001 Mars Odyssey


Very Long Baseline Interferometry - ΔDOR

2001 Mars Odyssey


DDOR Campaign• Project requirement to use DDOR as independent data type

– VLBI implementation effort led by Jean Patterson and Jim Border– 9 successful MGS demonstrations (Jan 2001)– 5 more scheduled on MGS (Aug-Sep 2001)

• North-South baseline only geometric opportunity for majority of cruise– Provides critical plane-of-sky information– East-West measurements possible beginning in October

• Campaign began as soon as geometrically possible– Two measurements per week started 04-June-01– All opportunities successful (except for E-W baseline “low elevation”)– Total of 45 measurements scheduled (40 N-S, 5 E-W)

• Traditional S/C-Quasar-S/C Measurements– Measurement Accuracy 0.12 nsec (0.27 km - 1s)

2001 Mars Odyssey


DSN Viewperiods

2001 Mars Odyssey


Navigation Processes

• Trajectory/Mission Design

• Orbit Determination

• Maneuver Design & Analysis

2001 Mars Odyssey


Trajectory Targeting Process• Targets are designed pre-launch, updated as necessary

• Cruise Targets (encounter at Mars) usually defined with Closest Approach metrics– Orbiter: Radius (Altitude) of Periapse, Inclination, Time– Lander: Entry Radius, Entry Latitude, Entry Flight Path Angle, Time– Can be expressed in other coordinates (B-plane)

• Aerobraking trajectory defined by a “corridor”– Corridor defined by spacecraft and trajectory constraints– Dynamic Pressure (structural), Heat Rate (Thermal), Density (Trajectory)– Target Altitude and Time at Periapsis

• Mapping Orbit Targets are usually orbital elements– Semi-Major Axis, Eccentricity, Longitude on Asc/Desc Node– Node often described via True or Local Mean Solar Time– Orbit can be described via orbit Beta angle

2001 Mars Odyssey


Our Targeting Plane: B-plane

2001 Mars Odyssey


Orbit Determination

Orbit Determination is the process of adjusting trajectory models/apriori information to best match the observed tracking data, and quantify the error associated with the trajectory estimate

• The collected tracking data are the actual or Observed measurements • Trajectory models produce predicted or Computed measurements

• Data Residuals = Observed – Computed

• OD method is to minimize residuals by adjusting the trajectory models– Minimized in a weighted least-squares sense (square-root information filter)– OD filter accounts for measurement and apriori state parameter accuracies

• OD products:– OPTG & SPK– P-file

2001 Mars Odyssey


What will our spacecraft experience?Satellite motion is determined by a number of forces that act on the spacecraft:

• Gravitational Forces– Central body force– Third-body force (other planets, moons)– Central body gravity field asymmetries– General relativistic effects

• Non-gravitational Forces– Thruster Firings

• Trajectory correction maneuvers (TCMs)• Attitude control thrusting• Angular Momentum Desaturations (AMDs)

– Solar Radiation Pressure– Aerodynamic Drag– Gas Leaks

2001 Mars Odyssey


Spacecraft Configuration (cruise)

Sun

Earth

Solar Array Normal

+X

+Z

2001 Mars Odyssey


Thruster Configuration

RCS-3 RCS-2

RCS-1RCS-4

TCM-3 TCM-2

TCM-1TCM-4

AACS CruiseCoordinate Frame

(Same asMechanical Frame)

2001 Mars Odyssey


Models That May Be EstimatedTrajectory Force Models

• Initial S/C position and velocity (State at Epoch)– 6 components of cartesian state

• Any S/C thrusting events– 3 components (DVx,DVy,DVz or |DV|, RA, DEC) for each discrete event– Many events over course of cruise trajectory: TCMs, AMDs

• Solar Radiation Pressure– Dependent on attitude profile and component orientation (solar panel)

• Specular• Diffuse

• Planet and Satellite Ephemerides and Gravity Fields– Gravity Field of Mars: MGS75C

• Atmospheric Density– Due to drag pass during aerobraking

2001 Mars Odyssey


Models That May Be Estimated

Measurement or Signal Path Models

• Earth Platform Parameters– Tracking Station Locations– Earth Rotation and Pole Nutation (Timing and Polar Motion)

• Tracking Data Calibration Parameters – Signal delays induced by Ionosphere and Troposphere

• Measurement Biases– Range Biases due to hardware delays – One-way doppler bias due to oscillator frequency drift

2001 Mars Odyssey


Trajectory Prediction

All planning is based on predictive capabilities, not real-time spacecraft location

• Trajectory Prediction involves accurately modeling and estimating all past events, as well as predicting all future events

• During the cruise to Mars, Nav must model all future events such as:– Solar Pressure - Attitude profile and component orientations– Thrusting - Angular Momentum Desaturations, or thruster slews

• Unmodeled forces must eventually be compensated with maneuvers– Solar pressure mismodeling can contribute ~ 10,000 km trajectory error– AMD mismodeling can contribute ~ 7,000 km trajectory error– These effects are inexpensive at TCMs-1,2, but can be costly at TCMs-3,4

2001 Mars Odyssey


Low-Torque Attitude

• Low-torque configuration starting at MOI - 50 days – Reduces desat frequency from ~1/day to ~1/week– Desat DV per event drops from ~ 8 mm/s to ~2 mm/s– Deterministic trajectory change per event decreases significantly

• Minimizes predict bias error

• At the time of TCM-4 Design (MOI-16 days) the deterministic altitude change remaining due to predicted AMDs :– Original Torque Profile: -80 km (Altitude Drop)– Low-Torque Profile: 5 km (Altitude Raise)

2001 Mars Odyssey


• Clean up Injection Errors from Upper Stage

• Remove Injection Bias

• Correct Targeting Errors– Maneuver execution errors– Orbit Determination errors

• Satisfy Planetary Quarantine (PQ) Requirements

• Achieve Injection Conditions

Maneuver Design

Trajectory Correction Maneuvers (TCMs)

2001 Mars Odyssey


Maneuver Analysis

• Statistical propellant usage calculated via Monte-Carlo analysis based on the nominal trajectory, and expected trajectory dispersions, due to– Launch vehicle injection dispersions– Orbit Determination errors– Maneuver execution errors

• Usually quoted as DV99 (99% of cases require no more than)

• PQ analysis is the calculation of aimpoint biases required to ensure that the probability of impacting a planetary body is sufficiently small– Probability of Impact calculated on each trajectory leg

• Includes probability of not being able to perform another maneuver– Based on expected trajectory dispersions– Generally presented in terms of B-plane aimpoints and dispersion ellipses

2001 Mars Odyssey


Planetary Protection Requirements

• COSPAR 1964:– “… a sterilization level such that the probability of a single viable organism aboard any

spacecraft intended for planetary landing or atmospheric penetration would be less than 1 x 10-4 … “

– “… a probability limit for accidental planetary impact by unsterilized fly-by or orbiting spacecraft of 3 x 10-5 or less … “

– At that time, it was thought that Mars had a life-harboring environment• Liquid water on the surface• Water ice caps• Atmospheric pressure ~ 85 mbar

– This led COSPAR to assign a probability of 1.0 that a terrestrial organism would grow on the planet

• NASA’s requirements for the Viking missions:– 10-3 or less of contaminating Mars. Combination of the following probabilities:

• Survival of organisms in space vacuum, temperature, and UV flux• Arrival of organisms at Mars• Survival or organisms through atmospheric entry• Release of organisms from the lander• Growth and proliferation of terrestrial organisms on Mars

2001 Mars Odyssey


Planetary Protection Requirements

• NASA’s Revisions 1988:– Category I: Spacecraft targets such as the Moon or Sun– Category II, III, IV: Flybys, orbiters, landers, and probes sent to planets or targets

with increasing exobiological interest– Category V: Sample return missions

• Specific Missions:– Viking 1 and 2 Landers: Substantial heating to produce P ~ 10-6 or less of

contamination– Mars Observer: Category III orbiter

• Launch aimpoint bias P ~ 10-5 or less• Spacecraft maneuvers P ~ 10-4 or less• Orbit maintained until Dec. 31, 2008; P > 0.95 of impact until Dec. 31, 2038

– Mars Global Surveyor: Category III orbiter– Mars Pathfinder: Category IV lander– Mars ’96: Category IV lander

2001 Mars Odyssey


Navigation Major Events

• Injection• TCM-1• TCM-2• TCM-3

• TCM-4• TCM-5 (Contingency)• MOI• Period Reduction

Maneuver


2001 Mars Odyssey


Interplanetary Trajectory

10 day time ticks

2001 Mars Odyssey


The B-plane

2001 Mars Odyssey



2001 Mars Odyssey


TCM-1 Execution Date: 23-May-01


2001 Mars Odyssey


TCM-2 Execution Date: 02-July-01


2001 Mars Odyssey


TCM-3 Execution Date: 17-Sept-01


2001 Mars Odyssey


TargetAlt: 300 kmInc: 93.47˚

Current Estimate (OD034)Alt: 324.1±11 kmInc: 94.10˚±0.2˚

Current Miss (Est-Target)Alt: +24 kmInc: +0.6˚

TCM-4 to Correct MissDV: 0.08 m/s

TCM-4 Execution Date: 12-Oct-01


2001 Mars Odyssey


Data Type Contributions to the Solution

OD Knowledgeat the time ofTCM-4 Design (3s)

2001 Mars Odyssey


MOI Configuration

Velocity

Thrust Vector

2001 Mars Odyssey


MOI and PRMMOI

Burn to Oxidizer depletion to minimize Capture Orbit PeriodMain Engine Thrust: 694.7 NOxidizer mass available: 121.3 kg ==> 1183 sec burnDesign

Start time: 24-OCT-2001 02:26:19 UTC - ERTMagnitude: 1426 m/sPitch rate: 0.03727 deg/sec (44.1 deg in 1183 sec)

Expected Capture Orbit300 km post-MOI periapsis altitude19.9 hour period

PRMPeriod Reduction Maneuver Scheduled for 3rd Periapsis after MOI (P4)Perform PRM (if necessary) to ensure completion of Aerobraking

If post-MOI orbit period < 22 hrs => No PRMIf post-MOI orbit period > 22 hrs => PRM to reduce period to 20 hrs


2001 Mars Odyssey


Mars Orbit Insertion

2001 Mars Odyssey


MOI - View from Earth

Goal:Altitude: 300 km ± 25 kmInclination: 93.5° ± 0.2°

Achieved:Altitude: 300.75 kmInclination: 93.51°

2001 Mars Odyssey


Aerobraking Nav Prediction Accuracy

• Requirement– Must predict Periapsis Time to within 225 sec– Must predict Periapsis Altitude to within 1.5 km

• Capability– Altitude requirement easily met with MGS gravity field (Nav Plan)– Timing requirement uncertainty dominated by assumption on future

drag pass atmospheric uncertainty

• Atmospheric Variability– Total Orbit-to-Orbit Atmospheric variability: 80% (MGS: 90%)

• Periapsis timing prediction– To first order, the expected change in orbit period per drag pass will

indicate how well future periapses can be predicted– This simplifying assumption is supported by OD covariance analysis

2001 Mars Odyssey


Nav Predict Capability• Example

– Total expected Period change for a given drag pass is 1000 seconds– Atmosphere could change density by 80%– Resulting Period change could be off by 80% = 800 sec– If orbit Period is different by 800 seconds, then the time of the next

periapsis will be different by 800 seconds– This fails to meet the 225 sec requirement

• Large Period Orbits– Period change per rev is large– Therefore can never predict more than 1 periapsis ahead within the

225 sec requirement with any confidence

• Small Period Orbits– Period change per rev is small (for example 30 seconds)– Therefore can predict several periapses in the future to within the 225

second requirement– Example: 80% uncertainty (24 sec) will allow a 9 rev predict

2001 Mars Odyssey


Aerobraking Navigation Process

Long Orbits

P1 P2 P3

Tp < 225 sec Tp > 225 secCollect Tracking Data

A1 A2

AnalysisAnd

Uplink

P1

A1Drag Pass(No Comm)

CollectTracking

Data

NavAnalysis

Sequence Update &

Uplink

DragPass

2001 Mars Odyssey


Navigation Process

Short Orbits

P2 P3

Tp < 225 sec Tp > 225 sec

A1 An

CollectTracking

Data

NavAnalysis

Sequence Update &

Uplink

P1….

Pn Pn+1

A2

2001 Mars Odyssey


What contributed to MOI success?

• A Baseline set of Navigation solution strategies were identified– Varied data arcs, data types, data weights, parameter estimates, a-prioris

• These solutions were regularly performed and trended– Built a time history of trajectory solutions– Trended evolution of parameter estimates and encounter conditions– Lessons learned from MCO and MPL

• Regularly demonstrate consistency to Project and NAG– Weekly Status Reports– Daily Status after TCM-4 (MOI-12 days) “Daily Show”

• Shadow navigators– Independent solutions run by Sec312 personnel (Bhaskaran, Portock)

2001 Mars Odyssey


Conclusions

Questions, comments, etc.

Mars Odyssey Navigation

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