Changes to Hail PDU as a Result of a Single Lander Communicating With Multiple Orbiters

1. Changes to Hail PDU as a Result of a Single Lander Communicating With Multiple Orbiters Matthew Cosby Space Link Protocols Working Group Berlin 2011 16th – 20th May 2011

2. Contents slide • 1 Issue and Scenario • 2 Proposed Changes • 3 Proposed Changes to the Standard

3. 1Issue and Scenario

4. 1 Issue and Scenario • Issue: • Forward and Return sequence numbers are not reset at hail • The Standard relies on the sequence numbers being the same at the start of a pass • This synchronisation is performed outside the protocol (either zeroed at power up or set by the vehicle controller) • There is only a Set V(R) directive, not a Set V(S) • Scenario : • A Lander on the surface with two or more Orbiters acting as a communications relay • The Lander is not power cycled during the passes of the different Orbiters • The Lander does not have its sequence numbers (N(R) and N(S)) reset by the vehicle controller between the 2 passes

5. N(R) = 0, N(S) = 0 • N(R) = n, N(S) = n 1 Pass of the First Orbiter • Scenario : • For the first pass the Lander and Orbiter have been powered up before the start of the link session. Both the Lander’s and Orbiter’s sequence numbers are set to zero • In this scenario the link session is established and continues with the transfer of data • Once the link session has ended, then the Lander’s sequence numbers are now the last values used during this pass Start Communication Finish n = between 0-255

6. N(R) = n, N(S) = n • N(R) = n, N(S) = n 1 Pass of the Second Orbiter • Scenario : • For the second pass the new Orbiter will have been powered up before the start of the link session with its sequenced numbers set to zero • The Lander has the sequence numbers set to the values at the end of the last pass • The Lander’s and Orbiter’s sequence numbers will not be in sequence and as a result no Sequenced Communications will be able to be performed • There is only a 1 in 256 chance that the sequence numbers will be the same Start No Communication Finish n = between 0-255

7. 2Proposed Changes

8. 2 Proposed Changes • Required Changes: • Include a directive in the Hail PDU that synchronises the sequence numbers • There are 3 Implementation options : • Either, include the “Set V(R)” directive in the Hail PDU. This directive meaning needs to change to allow the resetting of both N(R) & N(S) on the remote side. • Or, include the “Set V(R)” directive in the Hail PDU and a new “Set V(S)” directive. This new directive can be a modification of one of the spare bits of the current Set V(R) directive. • Or, include the “Set V(R)” directive in the Hail PDU and the remote system initiates a Set V(R) directive as the first response to the Hail. After conducting a document review, Option 3 requires less of a change to the standard

9. 3Proposed Document Changes

10. 3 Proposed Document Changes • Required Document Changes (Option 3): • Table 6-3: S33 • Table 6-4: S13 • Table 6-7: E3 • Table 6-10: E30