1. The choreography of pointing

In The Sirens of Titan the characters are moved by forces they do not fully understand. A spacecraft is moved by control laws it does understand, or at least had better understand before launch. The control law is the choreography that turns sensor data and actuator capability into a stable, pointed platform.

2. Pointing modes

The desktop will operate in different pointing modes depending on what it is doing:

  • Nadir pointing. The platform points at Earth’s center. Common for Earth observation and communications.
  • Sun pointing. Solar arrays face the sun; payloads may be pointed elsewhere or stowed.
  • Inertial pointing. The platform holds a fixed orientation relative to the stars. Useful for astronomy or stable observation targets.
  • Target tracking. The platform points at a specific ground site, satellite, or celestial object.
  • Ram pointing. The platform points along the velocity vector, sometimes used for drag makeup or payload geometry.

Each mode has its own control law, sensor priority, and operational constraints. The transitions between modes are also controlled events.

3. Control laws

The basic idea is simple: compare desired attitude to estimated attitude, compute the error, and command actuators to reduce the error. The implementation includes:

  • Proportional-derivative or proportional-integral-derivative control.
  • Feedforward for known disturbances or slews.
  • Momentum management to keep wheels from saturating.
  • Gimbal-rate limiting to avoid singularities in wheel configurations.

4. Safe mode

Safe mode is what the platform does when attitude knowledge is degraded, sensors fail, or the control system detects an anomaly. Typical safe-mode behavior:

  • Sun acquisition: rotate until sun sensors see the sun, then stabilize with that as the primary reference.
  • Spin stabilization: spin around a principal axis to passively maintain attitude.
  • Wait for ground: stop active maneuvers and broadcast telemetry until operators diagnose the problem.

A good safe mode is boring. It does not try to continue the mission. It tries to keep the platform power-positive and thermally stable until help arrives.

5. What the reading leaves unresolved

How autonomous the safe mode should be. A fully autonomous safe mode can recover faster but may make decisions ground disagrees with. A ground-dependent safe mode is simpler but leaves the platform vulnerable during outages. The desktop’s autonomy budget will be tested here.