apsis-rs

A flight controller and physics simulator written in Rust. Built from first principles, following aerospace industry conventions (NED/FRD coordinate frames, PID cascaded architecture).

This is a learning project. The goal is to deeply understand flight controller design and industry conventions while writing idiomatic, well-typed Rust.

AI Disclosure

No "vibe-coding" has been used in this project, and AI has never had access to the full source code. AI was used as a learning aid and general guidance tool to understand aerospace conventions and flight controller theory. AI-generated content is limited to: unit tests and documentation/clarifying comments, all of which have been reviewed and most modified to fit.

Build & Run

# Run the simulator with real-time telemetry UI
cargo run -p telemetry_ui

# Run tests
cargo test

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Crates

flight_core

The heart of the project. Contains:

• Simulator — physics engine integrating quaternion kinematics, thrust model, drag, and angular dynamics at ~130 Hz
• Controller — full cascaded PID pipeline: TrajectoryGenerator → VelocityController → AttitudeController → RateController → Inputs

primitives

Strongly-typed unit system (Meters, Velocity, Acceleration, Jerk, Radians, Quaternion, ...) with unit algebra enforced at compile time.

let distance: Meters = 2.mps() * 1.seconds(); // d = v * t

telemetry_ui

Real-time telemetry display built with egui.

Pilot input (W/A/S/D + arrow keys) is body-frame. W: Forward S: Backward A: Left D: Right ↑: + Altitude ↓: - Altitude ←: Rotate counter-clock wise →: Rotate clock wise. R: Reset commands.

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Controller architecture

Pilot Input (body frame)
        ↓  rotate_body_to_world()
  FlightTarget (AxisTarget per axis)
        ↓
  TrajectoryGenerator   ← jerk/accel/velocity limits derived from specs
        ↓
  PositionController (PID - target from trajectory)
        ↓
  (*) VelocityController (P + ARW integral + D on acceleration + ff)
        ↓
  q_target_from_acceleration()
  throttle_from_acceleration() - tilt-compensated
        ↓
  AttitudeController (PID)
        ↓  
  RateController (PID)
        ↓
  Inputs { throttle, pitch, roll, yaw }
        ↓
  Simulator

(*) VelocityController note: Not a standard PID. The derivative term acts on measured acceleration (not velocity error derivative) to avoid setpoint-kick noise. The integral uses Anti-Reset Windup (ARW / tracking). The saturation error is fed back to modify the integrated error before accumulation, keeping the integrator from winding up when output is clamped. The vertical axis uses a conditional integrator instead of ARW: it only accumulates when the saturation error and velocity error have opposite signs (i.e. the output is not already pushing in the right direction).

Coordinate conventions follow aerospace standard throughout: NED world frame, FRD body frame, positive pitch = nose up.

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References

• Quaternion Algebra
• Trajectory Generator