Mars Odyssey Navigation
Moriba Jah
Jet Propulsion Laboratory
California Institute of Technology
Mars Odyssey Navigation at CU Boulder
April 2, 2002
2001 Mars Odyssey
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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.
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Spacecraft Mission constraints
To 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.
Mars Odyssey Navigation at CU Boulder
April 2, 2002
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Trajectory Selection: Pork-Chop plot
Courtesy of Rodney Anderson
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Interplanetary Trajectory
Earth at
Arrival
Vernal
Equinox
1.02 AU
1.38 AU
Launch:
07-APR-2001
45.9˚
TCM-5
Mars Arrival
24-Oct-2001
E-7 h
TCM-4
E-12 d
TCM-3
E - 37 d
TCM-1
L + 46 d
Mars at
Launch
10 day time ticks
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TCM-2
L + 86 d
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Collecting Navigation Data
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Collecting Navigation Data
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 turnaround time; typical frequencies are also at S- and X-band
– Used in nearly all interplanetary missions since late 1960s
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Range and Doppler Tracking
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Radial vs Angular Measurements
•
For most interplanetary missions, S/C position uncertainty is much smaller in Earthspacecraft (“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
Spacecraft Position
Uncertainty Ellipsoid
z
sDeclination
sRight Ascension
sRadial
Range
r
y
Declination
1999 Capability
Radial Error
Angular Error (at 1 AU)
Position
2m
3 km*
Velocity
0.1 mm/s
0.1 m/s
*Equivalent to angle subtended by quarter atop
Washington Monument as viewed from Chicago
x
Right Ascension
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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)
Mars Odyssey Navigation at CU Boulder
April 2, 2002
2001 Mars Odyssey
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Very Long Baseline Interferometry - ΔDOR
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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)
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DSN Viewperiods
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Navigation Processes
• Trajectory/Mission Design
• Orbit Determination
• Maneuver Design &
Analysis
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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
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Our Targeting Plane: B-plane
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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
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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
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Spacecraft Configuration (cruise)
+Z
Earth
+X
Sun
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Solar Array
Normal
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Thruster Configuration
TCM-2
TCM-3
RCS-2
RCS-3
AACS Cruise
Coordinate Frame
(Same as
Mechanical Frame)
RCS-4
TCM-4
Mars Odyssey Navigation at CU Boulder
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TCM-1
RCS-1
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Models That May Be Estimated
Trajectory 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
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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
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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
Mars Odyssey Navigation at CU Boulder
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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)
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Maneuver Design
Trajectory Correction Maneuvers
(TCMs)
• 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
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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
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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
Mars Odyssey Navigation at CU Boulder
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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
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Mars Odyssey Navigation
Navigation Major Events
•
•
•
•
•
•
•
•
Mars Odyssey Navigation at CU Boulder
April 2, 2002
Injection
TCM-1
TCM-2
TCM-3
TCM-4
TCM-5 (Contingency)
MOI
Period Reduction Maneuver
2001 Mars Odyssey
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Interplanetary Trajectory
Earth at
Arrival
Vernal
Equinox
1.02 AU
1.38 AU
Launch:
07-APR-2001
45.9˚
TCM-5
Mars Arrival
24-Oct-2001
E-7 h
TCM-4
E-12 d
TCM-3
E - 37 d
TCM-1
L + 46 d
Mars at
Launch
10 day time ticks
Mars Odyssey Navigation at CU Boulder
April 2, 2002
TCM-2
L + 86 d
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The B-plane
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Mars Odyssey Navigation
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Mars Odyssey Navigation
TCM-1 Execution Date: 23-May-01
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Mars Odyssey Navigation
TCM-2 Execution Date: 02-July-01
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Mars Odyssey Navigation
TCM-3 Execution Date: 17-Sept-01
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Mars Odyssey Navigation
TCM-4 Execution Date: 12-Oct-01
Target
Alt:
Inc:
300 km
93.47˚
Current Estimate (OD034)
Alt:
324.1±11 km
Inc:
94.10˚±0.2˚
Current Miss (Est-Target)
Alt:
+24 km
Inc:
+0.6˚
TCM-4 to Correct Miss
DV:
0.08 m/s
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Data Type Contributions to the Solution
OD Knowledge
at the time of
TCM-4 Design (3s)
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MOI Configuration
Thrust Vector
Velocity
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Mars Odyssey Navigation
MOI and PRM
MOI
Burn to Oxidizer depletion to minimize Capture Orbit Period
Main Engine Thrust: 694.7 N
Oxidizer mass available: 121.3 kg ==> 1183 sec burn
Design
Start time: 24-OCT-2001 02:26:19 UTC - ERT
Magnitude: 1426 m/s
Pitch rate: 0.03727 deg/sec (44.1 deg in 1183 sec)
Expected Capture Orbit
300 km post-MOI periapsis altitude
19.9 hour period
PRM
Period 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 PRM
If post-MOI orbit period > 22 hrs => PRM to reduce period to 20 hrs
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Mars Orbit Insertion
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MOI - View from Earth
Goal:
Altitude: 300 km ± 25 km
Inclination: 93.5° ± 0.2°
Achieved:
Altitude: 300.75 km
Inclination: 93.51°
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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
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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
Mars Odyssey Navigation at CU Boulder
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Aerobraking Navigation Process
Long Orbits
Drag
Pass
A2
A1
P1
P2
P3
Tp < 225 sec
Tp > 225 sec
Collect Tracking Data
Analysis
And
Uplink
Drag Pass
(No Comm)
P1
A1
Collect
Tracking
Data
Nav
Analysis
Mars Odyssey Navigation at CU Boulder
April 2, 2002
Sequence
Update &
Uplink
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Navigation Process
Short Orbits
A1
An
A2
….
P1
P2
Collect
Tracking
Data
Mars Odyssey Navigation at CU Boulder
April 2, 2002
Nav
Analysis
P3
Pn
Pn+1
Tp < 225 sec
Tp > 225 sec
Sequence
Update &
Uplink
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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)
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Conclusions
Questions, comments, etc.
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