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Constellation Operations

Lessons Learned For Future Exploration

Angelita C. Kelly / NASA Goddard Space Flight CenterWarren F. Case / SGT, Inc.

The Earth ScienceAfternoon Constellation

Space Operations 2006 Conference

Rome, Italy

June 2006

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Topics

• Purpose• Earth Observing Constellations

– Morning Constellation– Afternoon Constellation (“A-Train”)

• Unique Challenges• Lessons Learned• Summary

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Purpose

Describe the lessons learned by flying the first 5 missions of the Afternoon Constellation.

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Earth Observing ConstellationsWhy Fly Constellations?

• Constellations provide the opportunity to make coincident, co-registered, and near simultaneous science measurements.

– The satellites align their orbital positions so their instrument fields of views overlap.

– Earth science data from one satellite’s instrument can be correlated with data from another.

The whole is greater than the sum of its parts

The Earth science community has long advocated placing numerous instruments in space to study the Earth and its environment.

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Earth Observing ConstellationsMorning Constellation

• Four members with descending equator crossing near 10:00 Mean Local Time (MLT).

• All 4 satellites are currently on-orbit.

• Landsat-7 nominal

• Terra nominal

• EO-1 lowering its orbit to satisfy re-entry requirements

• SAC-C raised its orbit to avoid a close approach with EO-1 and Landsat-7 in 2005, extending it’s lifetime in the process

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Earth Observing ConstellationsAfternoon Constellation

• All 7 members have ascending equator crossing times near 13:30 MLT.

• All but Glory and OCO are on-orbit (these due in 2008)

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Earth Observing ConstellationsAfternoon Constellation Phasing

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Earth Observing ConstellationsUnique Challenges

• The Earth Observing Constellations are unlike other satellite constellations.

• They present a number of unique challenges.

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(from M. Schoeberl)

MODIS/ CERES IR Properties of Clouds

AIRS Temperature and H2O Sounding

AquaCloudSatPARASOL

CALIPSO- Aerosol and cloud heightsCloudsat - cloud dropletsPARASOL - aerosol and cloud polarizationOCO - CO2

CALIPSO OCO

OCO - CO2 column

Aura

OMI - Cloud heights

OMI & HIRLDS – Aerosols

MLS& TES - H2O & temp

profiles

MLS & HIRDLS – Cirrus clouds

The Earth Observing Constellations are not a homogenous mix of identical satellites. They comprise several satellites with diverse instruments that provide complementary observations.

Earth Observing Constellations Unique Challenges (cont’d)

Glory

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Washington DCUSGS Map

13.5 km AIRS IR; AMSU & HSB

wave

13.5 km AIRS IR; AMSU & HSB

wave

6x7 km POLDER 6x7 km POLDER

5.3 x 8.5 km TES 5.3 x 8.5 km TES

CloudCloud

0.5 km MODIS Band 3-70.5 km MODIS Band 3-7

0.09 km CALIPSO0.09 km CALIPSO

1. 4 km Cloudsat1. 4 km Cloudsat

OCO1x1.5 km

The Afternoon Constellation observational “footprints” vary greatly

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Morning and Afternoon Constellations

SAC-C

EO-1

Terra

Landsat

In contrast, the Earth Science Constellation satellites orbit in close proximity so that observations occur at about the same time over approximately the same region.

Due to the relative closeness of the satellites (as small as 10 seconds), safety is an issue.

Most constellations are spaced around the Earth to provide instantaneous, global coverage (e.g., GPS, communications, satellite radio, weather).

GPS Constellation

Earth Observing Constellations Unique Challenges (cont’d)

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are managed by multiple organizations (both U.S. and International Partners)

• The Control Centers are at widely distributed locations

Earth Observing Constellations Unique Challenges (cont’d)

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Based on these challenges . . .

There was a clear need for Constellation management and coordination.We need to keep the constellation safe, thus enabling constellation science.

Constellation Management and Coordination Needed

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Mission Operations Working Group (MOWG)

In response . . .• The Afternoon Constellation Mission Operations Working

Group (MOWG) was formed with representatives from each mission

• The MOWG has been effective at addressing constellation management and coordination concerns

– Agreed on basic constellation operations philosophy:

– Agreed on basic orbital configuration

– Agreed on WHEN and HOW we need to coordinate (during special/critical events, anomalies)

– Agreed on process for handling changes and conflict resolution

Each mission operates independently, but all missions are committed to keeping the constellation safe

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Mission Operations Working Group (MOWG) (cont’d)

• For the past 3 years we have learned a lot working together as a "constellation team”

– Aqua, Aura, and PARASOL  provided a learning experience, exercising some of the agreed-upon interfaces and procedures

– Preparing for the launch and early orbit phase of CALIPSO and CloudSat provided recent lessons.

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Constellation Operations Lessons Learned

1. Science teams need to communicate their constellation requirements early.

2. Ensure all missions are willing to coordinate constellation requirements with each other.

3. Start constellation discussions early enough to incorporate constellation requirements into the mission operations concept and spacecraft design.

4. Analyze each mission’s maneuver capabilities and strategy.

5. Implement a coordination system.

6. Minimize number and complexity of constellation interfaces.

7. Thoroughly test all constellation interfaces.

8. Be prepared for changes in planned mission order of launches.

9. Analyze each mission’s ascent plan.

10. Analyze constellation contingency scenarios.

11. Set up a mechanism to authorize constellation configuration changes and resolve conflicts.

12. Coordinate end-of-mission plans.

13. Maintain communications between teams.

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AURA

AQUA

Lessons Learned #1Science teams need to communicate their

Constellation requirements early.

• Science requirements drive the operations concepts for both the mission and the constellation design.

• To do coordinated observations, science teams must ensure their requirements are understood by the mission design team.

Example:

Science requirement:Aura’s Microwave Limb Sounder (MLS) instrument needs to view the same air mass on the horizon that Aqua observed 8 minutes earlier by looking down.

Solution: Aura orbits 8-15 minutes behind Aqua, offset 215 km West.

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• The benefits derived from flying in close proximity to other satellites come at a cost.

• Mission teams must understand that coordination with other teams will be required.

– In nominal operations, little interaction is required– It is usually only during special activities (e.g., inclination adjust

maneuvers) and contingency operations that teams must coordinate.

• Agreements must be reached with all teams prior to a satellite’s entry into the constellation.

Lessons Learned #2Ensure all missions are willing to coordinate constellation requirements with each other

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• Start discussions early enough to incorporate constellation requirements into the mission’s operations concept and spacecraft design.

– Fuel allocations– Staffing– Glint constraints (relative to science requirements)– “What-if” scenarios.

Lessons Learned #3Start constellation discussions early

Example:• Constellation-flying requires more fuel than free-flying.• Formation flying with another constellation satellite requires even more fuel.

CloudSat has enough fuel to do formation flying with CALIPSO.

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• Evaluate the on-orbit maneuver philosophy for each mission– Each mission needs to evaluate its on-orbit maneuver philosophy.– A maneuver plan that works for a free-flying satellite may not be

appropriate in a constellation environment.

Lessons Learned #4Analyze maneuver capabilities and strategy

Example:• CloudSat must match CALIPSO’s maneuvers in order to maintain their formation.• A dangerous situation can occur if a scheduled CALIPSO maneuver is delayed.

CloudSat must react immediately.• CloudSat changed its maneuver strategy to schedule an automatic “undo”

maneuver in case CALIPSO’s maneuver is delayed. Once CALIPSO has maneuvered successfully, CloudSat’s “undo” maneuver is cancelled.

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Lessons Learned #5Implement a Coordination System

• Develop a centralized coordination system to automate routine functions– Orbital product exchanges– Event notifications– Ephemeris displays

• One centralized system relieves all organizations from developing multiple systems performing similar functions and redundant interfaces

Example:• For both the Morning and Afternoon Constellations, NASA GSFC developed the

Centralized Coordination System (CCS) to fulfill this requirement.

5 interfaces

Mission 2

Mission 3

Mission 6

Mission 4

Mission 5

Mission 1

1 interfaceMission 2

Mission 3

Mission 6

Mission 4

Mission 5

Mission 1

versus

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so minimize the number of interfaces.

Lessons Learned #6Minimize number and complexity of

constellation interfaces

• Standardize formats as much as possible (e.g., STK).Example: Some of our products require format conversions, increasing the

complexity of the task and introducing a potential source of error.

Example: An ICD, signed by management, was not reviewed sufficiently by the people building and operating the system. This caused late-breaking changes to operational systems.

• Get “buy-in” and review from all interfacing organizations. Interface agreements must be reviewed and agreed-to by both management and operations organization.

Example: Some product transfers require 2 “hops”, when 1 hop could have sufficed. This increases the likelihood of facility or network problems.

NASA Goddard NASA Langley CNES

Products for CNES Products for CNES

Products for Goddard Products for Goddard

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Lessons Learned #7Thoroughly test all constellation interfaces

• Allocate time to conduct thorough interface testing prior to launch to identify any problems early.

• Where possible, incorporate constellation testing into existing mission testing to reduce additional impact to the mission.

• Agree on needed stand-alone constellation testing and simulations.

– These verify that agreed-upon constellation procedures are workable.

Example: Pre-mission simulations for CloudSat and CALIPSO identified some incompatible formats.

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Example:• CALIPSO/CloudSat were to launch before PARASOL, so they received more

attention from the Constellation MOWG. CALIPSO/CloudSat’s launch was delayed, so some Constellation testing with PARASOL did not happen.

• Fortunately,– The Constellation entry risk for PARASOL was lower since CALIPSO/CloudSat

were not yet on-orbit– PARASOL did not have any constellation-related anomalies during ascent, and– CNES did an excellent job of keeping the other mission teams informed.

Lessons Learned #8Be prepared for changes in the planned

mission order of launches

• Mission B may launch before Mission A, even though Mission A was to launch first. This may have consequences.

Jan Feb Mar Apr May Jun Jul AugSep Oct Nov DecApr May20052004

Jun Jul Aug

PARASOL

CALIPSO/CloudSat

Projected Launch Dates

2006

Launched

Application:• We benefit in preparing for the Glory and Orbiting Carbon Observatory (OCO)

missions in 2008

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Constellation to ensure safety.– The most risk to the Constellation occurs for the final injection burns,

although the entire ascent (including contingencies) needs examination.

• Have a third party evaluate ascent plans– This was done for the CALIPSO and CloudSat ascents

• Conduct ascent simulations, including recovery from an anomalous situation.

Application:• Glory and OCO ascents will be analyzed and coordinated with the rest of the

constellation teams.

Lessons Learned #9Analyze each mission’s ascent plan

Example:• Ascent simulations showed the need for teams to coordinate more via telecons

during the launch and early orbit (L&EO) phase.

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Lessons Learned #10Analyze constellation contingency scenarios

• Each mission team performs contingency analysis for their own satellite, but not those involving other satellites. “Credible” constellation contingency scenarios must be analyzed

– Identify the most likely contingencies.– Analyze the ability of each mission to react to contingencies– Develop procedures to mitigate the risks.– Get all teams to signoff on the procedures– Simulate the contingency procedures to verify their efficacy.

• If and when contingencies do occur, the response and resolution will be timely, efficient, and effective.

Example:• If one satellite goes into safe-hold and starts drifting, it eventually could threaten

another Constellation satellite. A collision between the two could create a debris field that could threaten all missions. Contingency procedures were developed and signed off by all teams so there will be no confusion over a mission’s actions in a contingency situation.

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Lessons Learned #11Set up a mechanisms to authorize constellation

configuration changes and resolve conflicts

• Establish an approval process for planned constellation configuration changes.

• Establish a process to resolve conflicts.

Examples:1. Aqua’s original ground track

control requirement was ±20 km.– CloudSat and CALIPSO science

teams asked Aqua to change this to ±10 km to improve the science.

2. To maintain its mean local time (MLT) requirement, Aqua originally planned to conduct inclination adjust maneuvers in Spring 2005.

– CloudSat and CALIPSO mission teams asked that Aqua perform these maneuvers earlier in order to save fuel for their missions.

In both cases, Aqua was able to accommodate the requests.

Coordination Process

No conflicts

Conflict

Notices sentto affectedmissions Conflict not resolved

Conflict resolved

Resolutiondecisions

NASAHQ

CNESHQ

ConstellationExecutive

Board

Unresolved conflict

No conflict

KEY:

• Provide orbit data• Analyze conflicts• Resolve conflicts• Implement resolution decisions

Resolved Notresolved

ResolvedNo action required

Analysis Orbit data

Mission Teams

PARASOL

CALIPSO

CloudSat

Glory

Aura

Aqua

UpdatedOrbit data

Start

Data flow

OCO

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Lessons Learned #12 Coordinate End-Of-Mission Plans

• A mission must ensure that its exit from the constellation does not present a close approach risk to nearby satellites.

• The end of mission plan must be reviewed by the other constellation members several months before the mission begins exiting the constellation.

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Lessons Learned #13 Maintain Communications Between Teams

Example:• Remember also that Points of Contact usually change after launch.

Be sure to identify points of contact for both the planning/development phase and the on-orbit phase, then involve both groups in the communications flow.

• Issue periodic updates based on personnel changes.

• Facilitate and encourage communication between the mission teams throughout the mission life cycle.

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1. Start early. Talk with science teams. Develop an operations concept for a constellation of diverse satellites and organizations.

2. Understand individual mission capabilities.

3. Get the mission teams to communicate and work together as one constellation team.

4. Get signed agreements for coordination, especially for the handling of contingencies.

5. Develop a coordination system to exchange data.

6. Minimize complexities, but always be prepared for changes and contingency situations.

We hope that these lessons prove useful for other constellations.