explain

name	explain
description	Explain ArduPilot code, architecture, or subsystems in context. Use when the user asks how something works, what a file does, or wants to understand a code path.
argument-hint	<file, function, or subsystem>
allowed-tools	Read, Grep, Glob

Explain ArduPilot Code

Explain $ARGUMENTS in the context of ArduPilot's architecture.

Approach

1. Find the code — locate the relevant files, classes, and functions
2. Read the code — read the actual implementation, don't guess
3. Trace the call chain — understand how it's invoked (scheduler, main loop, interrupt)
4. Identify the subsystem — where it fits in ArduPilot's architecture
5. Explain clearly — use plain language, reference specific line numbers

ArduPilot architecture context

Execution model

• Main loop runs in AP_Vehicle::loop() calling the scheduler
• AP_Scheduler runs tasks at defined rates (400Hz, 50Hz, 10Hz, 1Hz, etc.)
• Each library's update() is called from scheduler tasks
• SITL runs the same code path as real hardware via HAL abstraction

Key singletons

• AP::ahrs() — attitude/heading (wraps EKF)
• AP::gps() — GPS data
• AP::baro() — barometer
• AP::ins() — inertial sensors (IMU)
• AP::logger() — DataFlash logging
• AP::vehicle() — current vehicle instance
• gcs() — MAVLink GCS communication

Data flow patterns

• Sensors → EKF → AHRS → Controllers → Actuators
• Sensor drivers read hardware via HAL, publish to frontend
• EKF fuses sensor data into state estimate
• Vehicle mode code reads AHRS, commands controllers
• Controllers output to motors/servos via HAL

Common patterns to explain

• Frontend/Backend split: Public API (frontend) delegates to driver (backend)
• Parameter system: AP_Param with AP_GROUPINFO tables
• Scheduler tasks: Registered in vehicle's scheduler_tasks[] array
• MAVLink handlers: GCS_MAVLINK::handle_message() dispatch
• Flight modes: Mode base class with run() called each loop iteration
• Logging: AP_Logger::Write() with format strings matching LogStructure.h

When explaining, always

• Reference specific file paths and line numbers
• Show the actual code, don't paraphrase
• Trace the execution path from entry point
• Note any compile-time guards (#if FEATURE_ENABLED)
• Identify the coordinate frame (NED, body, etc.) for navigation code
• Mention relevant parameters that control behavior