Abstract

This paper discusses approaches for using standard Internet technologies to meet the communication needs of future space missions. It summarizes work done by the Operating Missions as Nodes on the Internet (OMNI) project at NASA/GSFC since 1997. That project arose from a small group of engineers who had been involved with building NASA communication systems for over 20 years. Since NASA needed communication systems for space long before the Internet evolved, NASA developed many custom protocols and communication techniques to meet its “space specific” communication needs. However, as the Internet evolved, it needed to address all of the same communication issues of errors, delays, and intermittent links. Those challenges may not have seemed space related, but the solutions developed can be used to address space communication issues. The key is to select the appropriate Internet Protocols that can support space communication while also providing direct interoperability with the terrestrial Internet.

This paper uses a layered approach to discuss all aspects of using Internet technologies in space. It starts with the low-level physical, data link and data routing issues related to using Internet Protocols to support basic spacecraft communications. After identifying options for supporting basic datagram delivery in space, the paper describes issues for selecting transport protocols and applications to meet various mission data delivery needs. Information is provided throughout the paper to identify key implementation issues and provide information on the current status of products in each area. Finally, current implementation and usage of these protocols in both spacecraft and ground systems are discussed.

Keywords: Spacecraft networking; Internet in space; Space communication protocols; Internet space missions; Space shuttle STS-107

Article Outline

1. Introduction

2. IP in space architecture overview

2.1. End-to-end network concept

2.2. ISO protocol layer model

3. Proposed architecture

3.1. Onboard spacecraft IP LAN

3.2. Spacecraft router function

4. Physical layer issues

4.1. Bit delivery

4.2. Modulation and coding

4.3. Forward-error-correction coding

4.4. Separation of framing and coding

5. Link layer issues

5.1. Onboard Ethernet framing

5.2. RF link HDLC framing

5.3. High-rate RF link framing

5.4. Framing overhead

6. Network layer issues

6.1. Internet Protocol

6.2. Datagram routing

6.3. Mobile IP

6.4. Data prioritization

6.5. Network protocol overhead

7. Transport layer

7.1. UDP

7.2. RTP

7.3. TCP

8. Application layer

8.1. UDP-based applications

8.1.1. Simple data delivery

8.1.2. Reliable file transfer with UDP

8.1.3. Time synchronization

8.2. TCP-based applications

8.2.1. Reliable simple data delivery

8.2.2. Reliable file transfer with TCP

8.2.3. E-mail

8.3. Upper layer protocols and applications

9. Space vs terrestrial issues

9.1. Long delay

9.2. Noise

9.3. Power, CPU, and bandwidth constraints

9.4. Intermittent connectivity and variable routing

9.5. Forward/return path asymmetry

10. IP-based operations scenarios

11. Ground-based demonstrations

12. Space-based demonstration

12.1. Ground station implementation

12.2. Flight tests

13. UoSAT-12 IP test results

13.1. On-orbit clock synchronization with NTP

13.2. Error-free downloads with FTP

14. Missions using standard IP communication

15. U0SAT-12 (Surrey Satellite Technology Ltd.)

15.1. UoSAT-12 summary

15.2. AlSAT-1—Surrey Satellite Technology Ltd.

15.3. AlSAT-1—Summary

15.4. CHIPSat—University of California Berkeley/NASA

15.5. CHIPSat Summary

15.6. CANDOS—NASA GSFC

15.7. CANDOS Summary

15.8. Additional missions lessons learned

16. IP mission lessons learned summary

16.1. Data link protocols (framing, frame error detection, virtual channels)

16.2. Network protocols (packet addressing, packet routing)

16.3. Transport Protocols (subchannels (ports), unreliable/reliable delivery)

16.4. Applications

16.5. Test and analysis equipment

17. Future work

17.1. Status of onboard LAN hardware

17.2. Status of onboard WAN hardware

18. Conclusions

Acknowledgements

References

Vitae