State Machine

A State Machine (or Finite State Machine, FSM) is a mathematical model of computation that can be in exactly one of a finite number of states at any given time. It transitions between states based on inputs and conditions.

Why State Machines for Control?

Spacecraft missions have distinct phases with different control requirements. A state machine cleanly separates these phases and handles contingencies:

In AEGIS

The Mission Director is a hierarchical state machine managing mission phases:

STANDBY
  ↓ (activation)
ASCENT_COAST
  ↓
DEORBIT_BURN
  ↓
HYPERSONIC_COAST
  ↓
SENSOR_WARMUP (new)
  ↓
ESTIMATOR_WARMUP (new)
  ↓
POWERED_DESCENT
  ↓
HOVER_TARGETING
  ↓
TERMINAL_DESCENT
  ↓
LANDED

Contingency Transitions

Any state can transition to:

• HARD_ABORT — Catastrophic failure (multiple engine failures, degenerate allocation, vessel destroyed)
• DT_SPIKE handling — Skip KF predict, hold FDI, but guidance still runs

Smart Activation

Instead of starting from STANDBY, the Director evaluates current conditions and injects into the appropriate phase:

• Moving upward → ASCENT_COAST
• Falling from space → SENSOR_WARMUP
• On the ground → STANDBY

Advantages

• Clear phase separation — Each state has well-defined entry/exit behavior
• Explicit contingency handling — No hidden failure modes
• Testability — Each state can be tested in isolation
• Readability — Mission logic is easy to follow

Related Concepts

• Control Theory — The mathematical basis
• Mission Planning — Higher-level mission design
• Fault Detection and Isolation — Triggers state transitions
• Real-Time Systems — Timing constraints in state transitions

Sources

• AEGIS Project (src/main.py, src/mission/)