Communications Infrastructure for Fractionated Spacecraft

Transcription

1 Communications Infrastructure for Fractionated Spacecraft Michael A. Koets, Mark Tapley, Buddy Walls, Jennifer Alvarez Southwest Research Institute

2 Fractionated Spacecraft Replace monolithic satellite with multiple freeflying modules Capabilities Extend and enhance ongoing missions Replace failed components Survive module failure Spatially distributed sensing Requirements Coordinated Navigation Distributed Operations Inter-module Communication

3 Communications for Fractionated Spacecraft Challenges Wide variation in distances between modules Limitations on size, weight, and power Space-qualified, radiation-tolerant hardware Regulatory approval for use of radio spectrum Desirable Characteristics Predictable Performance: Throughput, Latency Efficient use of data communications capacity Robust tolerance to loss of any module Inter-module distance estimation Distribution of a common precision timing reference

4 F6WICS: TDMA Burst Communications System TDMA Channel Access Efficient use of capacity D Frame Ownership Predictable latency and throughput No contention for channel Fully Distributed Link Coordination C D Network Management B A C Network Management B Data Slots D A C D Network Management B A Robust to loss of any node On-demand resource allocation Three data transfer mechanisms C Network Management B A Frame Initiation Frame Management Frame Management Data Slot Allocation Data Data

5 Generic Transfers Best effort transfer Utilizes all capacity not allocated to other transfer mechanisms Multicast at physical layer Generally highest throughput transfer mechanism No latency guarantees Bulk data transfers User Data Generic Transfer

6 Isochronous Transfers Long-term capacity reservation Guaranteed latency Precisely scheduled data delivery Streaming data Distributed control loops Periodic data with strict latency requirements User Data Isochronous Transfer

7 Critical Transfers Minimum latency transfers Infrequent, mission-critical communications Guaranteed access to first data slot of frame Prioritized access if multiple requests Critical navigation data Hazard avoidance Sensor cueing User Data Critical Transfer

8 Physical Layer Waveform Wideband Direct Sequence Spread Spectrum (DSSS) Burst Waveform Scalable data rate changes with distance Requires precision time synchronization between transmitter and receiver Specialized code synchronization algorithm enables rapid synchronization and extended tracking between frames Capabilities Enabled by Code Synchronization Estimation of range between radios Precision time transfer

9 Supporting Services Inter-module Range Estimation Utilize physical layer mechanisms to estimate distance between modules Independent of GPS Resolution of ±6 meters sufficient for navigation Ranges disseminated throughout network Common Timing Reference Distribution Utilizes physical layer mechanisms to establish coordinated, distributed timing reference Timestamp range measurements, data transfers Link Layer Topology Monitoring Monitor and report local inter-module connectivity to two hops

10 Overhead Analysis Configuration Parameters Population Data slots per frame Latency / Throughput Max operational range Range / Overhead 8.9 km operational range for this data TDMA implies deterministic overhead Overhead is all time not spent sending user payload data Overhead, % slots/frame 16 slots/frame 32 slots/frame 64 slots/frame Network Population, modules

11 Throughput Analysis User data rate 6000 automatically scales with distance 5000 between slots/frame communicating 16 slots/frame 32 slots/frame 64 slots/frame modules 3000 Data rate set by most 2000 distant addressee in multicast 1000 Throughput is total user data capacity Inter-Module Range, km Throughput, kbps

12 Iterative Software Development

13 Conclusions and Future Work F6WICS is a communications system tailored to the specific needs of fractionated spacecraft Three distinct data transfer mechanism provide guaranteed communications characteristics at the data link and physical layers Physical layer mechanisms provide supporting services for navigation and distributed operations Design is now being implemented on terrestrial prototype hardware On-orbit demo mission planned

14 Acknowledgements This work was supported by the Defense Advanced Research Projects Agency (DARPA) The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government. Distribution Statement A : Approved for Public Release, Distribution Unlimited.