Inter satellite communications

Constellation Operations: Inter-Satellite Communications

8th Annual AIAA SOSTC

Improving Space Operations WorkshopApril 24-25, 2002

Naval Satellite Operations Center (NAVSOC)Point Mugu, California

April 25-26, 2002 Improving Space Operations Workshop - Intersatellite Communications

Slide 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


Slide 3

Space Mission Architecture - Today

Science Processing Center Science Processing Center

Bent pipe communication

s

• 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


Slide 4

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


Slide 5

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 there’s 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


Slide 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


Slide 7

Information that Needs to be Exchanged

Spacecraft and Instrument H&S 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


Slide 8

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


Slide 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


Slide 10

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 H&S engineering telemetry

Routine, emergency out-of-limits Receive science instrument data

Potential bandwidth problem if high rate, high volume


Slide 11

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


Slide 12

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 H&S 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


Slide 13


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.


Slide 14


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?


Slide 15

Last Years Recommendations and Results

Key Driver for Use of this Technology is System ResponsivenessInter-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


Slide 16

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

Backup Slides


Slide 18

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


Slide 19

LEO Aggregations

Constellation String of Pearls Cluster


Slide 20

Elliptical Orbit Aggregations

Constellation String of Pearls Cluster


Slide 21

Lissajous Orbits

Sun-Earth Line

L2

1.5 million km

L1

1.5 million km


Slide 22

Interspacecraft Communications TopologiesConstellations

Adjacent planes


Slide 23

Interspacecraft Communications TopologiesClusters

CentralizedMothership

Drone

DroneDrone

DistributedTopology


Slide 24

Interspacecraft Communications TopologiesString of Pearls


Slide 25

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


Slide 26

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.


Slide 27

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 solutionsRegardless of the details, mission operations ground resources and especially mission operations staff workload will be impacted without the luxury of increased mission operations budgetsGreater on-board autonomy and more effective ground-based automation will be beneficial and contribute to alleviate the impact to mission operationsSimulation 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 phasesIntroduce advanced concepts into control centers and ground systems

Goal-oriented commanding Mothership may serve as central relay for drones Automated TT&C and mission operations systems (e.g., “expert” or “rule-

based” systems)

Documents

Inter satellite communications