Constellation Operations: InterSatellite Communications

1
Constellation Operations Inter-Satellite
Communications

• 8th Annual
• AIAA SOSTC
• Improving Space Operations Workshop
• April 24-25, 2002
• Naval Satellite Operations Center (NAVSOC)
• Point Mugu, California

2
What are the Drivers for Interspacecraft
Communications?

• NASA Near- mid- and long-term strategic plans
  (2000-2025 timeframe)
• HQ, Earth Science Enterprise, Space Science
  Enterprise
• Innovative Science Observing Concepts
• Formation Flying Missions
• Collaborative Earth- and Space-Science
  Observations
• Autonomous Event Recognition, Reconfiguration,
  and Response
• Sensor Webs
• Evolving Technologies
• MEMS microelectronics
• Electron beam lithography systems will contribute
  to the development of nanospacecraft components
  with extremely small mass
• The challenge nanospacecraft transmitter/receiver
  mass vs. on-board communications infrastructure
  and power for effective RF or optical link
  closure
• RF, optical, and digital communications
  technologies
• Communications protocols standards
• Mature terrestrial protocols Network (IP, IPv6),
  Transport (UDP, TCP), Application (FTP)
• NASA space communications protocols CCSDS suite,
  CCSDS Proximity-1, SCPS

3
Space Mission Architecture - Today
Bent pipe communications
Science Processing Center
Science Processing Center

• Classic stovepipe science data collection and
  mission operations
• Single or separate spacecraft missions with
  little or no dynamic planning for opportunistic
  science observations
• No real time collaborative information sharing
  between sensors, spacecraft, or investigators
• Bent pipe interspacecraft communications
• via TDRSS in support of command uplinks,
  telemetry downlinks

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Space Mission Architecture A Future Sensor Web

• High degree of synergy between a diverse suite
  of platforms
• Space-based
• Atmospheric (e.g., aircraft, balloons)
• Land (e.g., river gauges)
• Sea (e.g., buoys)
• Automated science data collection and mission
  operations
• On-board spectral signature detection algorithms
• Multiple spacecraft and platforms perform
  dynamic planning for targets of opportunity
• Real time collaborative information sharing
  between sensors, spacecraft, or investigators
• Interspacecraft communications becomes an
  intrinsic characteristic of space platforms

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Architectural Implications for Interspacecraft
Communications

• Constellations
• Knowledge of the whereabouts of member
  spacecraft within their orbits is reasonably well
  constrained.
• Spacecraft immediately ahead of or behind
  another in the same orbital plane
• Phasing relationships between spacecraft in
  adjacent planes
• Homogeneous Constellations
• Communications infrastructure is inherently the
  same
• Simplifies communications architecture since
  theres only one solution set implemented for the
  protocol stack (e.g., ISO/OSI 7 layer model
  components)
• Heterogeneous Constellations
• Drives need for standard communications protocol
  stacks
• Facilitate interoperability between S/C and
  ground segment
• Reduce mission implementation and ops costs
• Mitigate implementation risk

6
Architectural Implications for Interspacecraft
Communications

• Formation Fliers
• Knowledge of relationship between S/C that
  comprise the formation may simplify
  communications architecture
• Point-to-point
• Broadcast
• Proximity
• May permit low power communications especially
  important for low mass nanospacecraft
• Accretionary Formations
• Since they are not a priori known to come into
  being, standards are a must for communications
  protocol layers 1-4, 7 if these S/C might
  eventually communicate among one another

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Information that Needs to be Exchanged

• Spacecraft and Instrument HS Telemetry Data
• Characterized by relatively low data rates, low
  volumes
• Spacecraft operational status messages
• S/C orbit and attitude information
• Instrument(s) mode(s) of operation
• Instrument Pointing information
• Spacecraft Instrument Data
• Can be characterized by relatively high volumes
  and high data rates
• Typically unidirectional
• Collaborative missions may require bi-directional
  science data exchange
• May be used to facilitate distributed space-based
  computing
• On-board spectral (signal) signature processing
• Event recognition software
• Event response software
• Duty cycle will depend upon mission needs

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Information that May be Exchanged

• Ancillary information
• Most likely characterized by low rate, low volume
• Interspacecraft range and range-rate
• Status messages that facilitate or help to
  coordinate science observations, on-board
  processing status, etc.
• Science instrument calibration coefficients/tables
  
• Rate of data exchange and duty cycle of link
  utilization will depend upon individual mission
  needs

9
Mission Needs Ops Concepts will Drive Protocol
Issues

• Differences between space terrestrial
  communications environments
• Spatial relationship between two communicating
  S/C is continually changing
• In and out of RF range
• In and out of line-of-sight
• Changing pointing angles
• Available (on-board) communications transmitter
  power to close the link
• Directional (RF,Optical) less transmit power
  pointing knowledge required
• Omnidirectional more transmit power required
  broadcast can create duplicate packets in network
• Handling lost packets
• Terrestrial networks assume congestion slow down
  packet traffic to compensate
• Space networks assume noisy link re-transmit
  packet as soon as practicable
• Propagation delays can be (but are not
  necessarily) longer

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Interspacecraft Comms Potential Uses/Benefits

• For S/C presently not within view of a ground
  station
• Route all uplinks to the S/C that is within view
  of ground station
• Ground station antenna and support equipment
• S/C contact activity planning scheduling
  independent of ground station
• GEO-like nearly-continuous contacts may be
  possible with any S/C
• An increase of the uplink data rate may be
  required to serve multiple S/C
• Multiple S/C yield aggregate downlink data rates
  that may necessitate wider bandwidth (i.e.,
  higher data rate) to the ground
• Uplink route commands to one, some, all
  spacecraft
• Routine, emergency
• Receive HS engineering telemetry
• Routine, emergency out-of-limits
• Receive science instrument data
• Potential bandwidth problem if high rate, high
  volume

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Interspacecraft Comms Potential Uses/Benefits

• For S/C presently within view of a ground station
• Formation flying or cluster missions
• Contact with just one S/C in the cluster may
  eliminate multiple, successive uplink contacts
  for each S/C in cluster
• Uplink one set of commands to mothership which
  serves as a router-in-space for all drone
  spacecraft
• Independent of ground station view
• Unplanned science events, opportunistic science
• Automated identification (e.g., autonomous
  spectral/signal detection)
• Autonomous mission reconfiguration
• Notify or cue other spacecraft to conduct
  coordinated observations
• Event notification to mission operations
• Especially when S/C is not in view of ground
  station for long times (e.g., highly elliptical
  orbits)
• Anomaly identification and resolution

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Potential Impacts to Mission Operations

• If interspacecraft communications requires
  pointing and if it is not performed autonomously
  on-orbit
• Plan and schedule contact times and pointing
  angles for communicating S/C
• Additional mission ops responsibility and ground
  resources to plan, schedule, and upload
  communications activity commands and data
• Times when S/C can communicate
• On-board resources required
• Pointing information
• Monitoring system performance, especially when
  things go wrong
• Additional engineering HS telemetry data
  relative to comms subsystems to monitor and
  interpret
• Transmitter/receiver status
• On-board data buffer utilization (e.g.,
  packets/files sent/received)
• Communications traffic volume, duty cycle
• Communications error rates
• Reconfigure the communications network between
  spacecraft to facilitate work-arounds,
  degradations, failures

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Potential Impacts to Mission Operations

• Data routing to the ground from a S/C not in view
  of a ground station
• Are ground equipment resources available?
• Antennas and front-end electronics
• Front-end processors
• Ground data storage
• Communications networks
• Planning science observations becomes
  intrinsically more complex and more than one
  observation scenario may be available due to
  multiple S/C.
• Need robust science observation activity
  planning, scheduling, resource utilization, and
  conflict resolution tools
• Simulators may be used to better identify and
  evaluate several alternative what if scenarios
• Rule-based assistants may evaluate and
  recommend optimal performance criteria depending
  upon mission complexity
• On-board recorder management becomes more complex
• Navigation planning
• Tight formations will likely require high
  fidelity simulations to ensure collision
  avoidance and to test various what if
  navigation alternatives.

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Potential Impacts to Mission Operations

• Commanding
• Increases in complexity if the mission permits
  commands to be routed to S/C other than those in
  view of the ground station.
• Protocols such as IP (and IPv6 with multicasting)
  could be beneficial if suited to mission
  parameters
• Telemetry monitoring
• If routed through the constellation, telemetry
  data may be available nearly continuously from
  all S/C not just during those periods when a
  pass occurs.
• Impacts ground system resource utilization and
  mission ops personnel utilization.
• Today after loss-of-signal, ground resources are
  often released, and reconfigured. Mission ops
  personnel perform other functions when no S/C
  contact is in progress.
• Tomorrow But what if spacecraft contacts were
  effectively continuous from multiple spacecraft?

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Last Years Recommendations and Results

• Key Driver for Use of this Technology is System
  Responsiveness
• Inter-spacecraft Communications provides
  Information Exchange between vehicles to Enhance
  Autonomy to meet response time (latency) required
  to accommodate specific mission payload
  objectives
• Telemetry
• Commands
• Timing
• Ancillary Information
• Alerts/Event collaboration
• Provides Data Relay for (near) Global Coverage
• 100 Duty Cycle would be possible
• Relay information from one point to another
• Faster delivery to end-user
• Information presentation of data from multiple
  sources needs study
• Impact on operations staff
• Impact on Ground System performance

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Last Years Results Key Issues

• Validation and/or verification that an activity
  is complete and correct when out of view of
  ground operations (eg commanded maneuver)
• NEED TO BUILD TRUST IN AUTONOMY
• Management of multiple spacecraft with transition
  from sequential operations to potentially
  continuous view of all vehicles simultaneously
• No more concept of post-pass analysis as
  everything is potentially received in real-time
• System loading on real-time system

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Backup Slides

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The Future Space Mission Paradigm

• The long held paradigm of deploying and operating
  single spacecraft missions will be changed by the
  deployment and operation of Distributed Observing
  Systems.
• Constellations
• Formation Flyers
• Sensor Webs
• Interspacecraft communications can offer benefits
  to mission operations, however it will also
  impose other challenges that must be identified,
  understood, and resolved.
• Constellation orbits
• Will be a key driver relative to how
  interspacecraft communications may be conducted.
• Orbits and S/C configurations within orbits will
  impact the ground segment and mission operations
  support.
• Based upon JPL study
• Multiple Mission Platform Taxonomy A. Barrett
  JPL/CIT, Jan. 30, 2001

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LEO Aggregations
Constellation
String of Pearls
Cluster
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Elliptical Orbit Aggregations
Constellation
String of Pearls
Cluster
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Lissajous Orbits
L2
1.5 million km
1.5 million km
Sun-Earth Line
L1
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Interspacecraft Communications TopologiesConstell
ations
Adjacent planes
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Interspacecraft Communications TopologiesClusters
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Interspacecraft Communications TopologiesString
of Pearls
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Mission Operations Present and Future

• The present
• Mission operation are simple (e.g., SMEX, survey
  missions) to challenging (e.g., HST, AM-train)
  depending upon mission design and ops concept
• The future
• Challenging even for relatively simple (e.g.,
  survey) mission designs
• Multiple S/C for each mission
• More complex mission observation planning
  scenarios
• Potentially increased time to plan ground station
  contacts and create command loads
• Increased impact on ground station resources
  (e.g., antennas)
• Shorter duration between contacts for formation
  flyers or clusters
• Larger aggregate return link data volumes

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Mission Needs Ops Concepts will Drive Protocol
Issues

• IP, UDP/TCP, FTP
• Mature, robust, open layered protocol
  architecture
• In wide commercial use for terrestrial
  applications
• Promotes interoperability between space and
  terrestrial networks
• Widespread use promotes lower ground system
  implementation costs
• Mitigates implementation risk and shortens
  implementation schedule
• Familiarity (terminology, concepts, usage) with
  user community
• Out-of-the box implementation of TCP slow-start
  algorithm may not be suitable to every space
  mission
• CCSDS
• Mature and in wide use for NASA space missions
• Interoperability with other foreign space- and
  ground- networks
• Well adapted to noisy space communications
  environment
• SCPS and Proximity-1 emerging to address current
  protocol deficiencies vis-a-vis terrestrial
  protocols use in future constellation
  communications.

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Conclusions and Candidate Recommendations

• Alternative mission architectures, as well as
  functional and performance objectives for
  distributed space observing systems require a
  variety of interspacecraft communications
  solutions
• Regardless of the details, mission operations
  ground resources and especially mission
  operations staff workload will be impacted
  without the luxury of increased mission
  operations budgets
• Greater on-board autonomy and more effective
  ground-based automation will be beneficial and
  contribute to alleviate the impact to mission
  operations
• Simulation software will be highly desirable by
  helping to identify alternative mission scenarios
  and to objectively and quantitatively assess
  specific impacts upon science missions in the
  design and operational phases
• Introduce advanced concepts into control centers
  and ground systems
• Goal-oriented commanding
• Mothership may serve as central relay for drones
• Automated TTC and mission operations systems
  (e.g., expert or rule-based systems)