Description

The spacecraft reports its orientation (i.e., attitude) and time on a regular basis. These tasks record the information, for use by other tasks. The Slaves are responsible for distributing the data.

The Master receives seven messages per second from the spacecraft: 5 attitude, 1 time tone, 1 ancillary (containing orbit information and status info). The Master redistributes these messages to Slaves on all CPUs.

The Slaves use these messages to build time and attitude tables for interrogation by other tasks/functions. Attitude tables can be interpolated to give spacecraft attitude at a requested time.

Time tone messages, combined with time hacks (also distributed to all CPUs) provide wall clock time throughout the system, accurate to ~200 nanoseconds.

The Spacecraft Attitude/Time subsystem supports several data paths:

• Attitude

  The Spacecraft sends Attitude packets (five times per second) to the Attitude/Time Master. The Master forwards these (as messages) to the Slaves. The Slaves use this data to update their Attitude tables.

• Ancillary

  The Spacecraft sends Ancillary packets (once per second) to the Master. The Master forwards these to the Slaves, which retain the information in RAM.

• Time Tone

  The Spacecraft sends Time Tone packets (once per second) to the Master. The Master forwards these (as messages) to the Slaves. The Slaves use this data to update their Time tables.

• Time Hack

  The Spacecraft emits Time Hack interrupts. The 1 PPS interrupt routine handles these, updating the Time Tables.

Note: This animation requires QuickTime.