2. Logs and MAVLink¶

What is a flight log?¶

During a flight, the flight controller reads its sensors hundreds of times a second. It estimates where it is, decides what to do and drives the motors. It also writes a record of all of this, and that record is the flight log.

Where a log comes from

There are two kinds, and AnaTele reads both:

Log	Where it is made	What it contains
Telemetry log `.tlog`	On the ground station (e.g. Mission Planner), from the radio link	MAVLink messages, each stamped with the time the ground received it
DataFlash log `.bin` / `.log`	On the drone's SD card	ArduPilot's own on-board records; usually higher rate and more detail

MAVLink is the language drones and ground stations use to talk. Each MAVLink message has a type (for example ATTITUDE or GPS_RAW_INT) and named fields (roll, pitch, lat, lon…).

A log is a stream of typed messages¶

Here is the start-to-finish stream of a simulated flight in which one motor fails. Each tick is one message.

Message stream

Things to notice:

ATTITUDE, GLOBAL_POSITION_INT and SERVO_OUTPUT_RAW are dense bands: they arrive many times a second.
STATUSTEXT (human-readable text such as "Crash: Disarming") shows up only a handful of times, but these messages are often the most important ones.
MISSION_ITEM_REACHED marks each waypoint the autopilot reports reaching.

Different messages, different rates¶

Sample rates

In this 102-second simulated .tlog, attitude, position, motor outputs and the controller's targets arrive at 10 Hz (10 times a second). RC input arrives at 5 Hz, and vibration, EKF status, GPS and battery at 2 Hz.

Rate limits what you can conclude

A rule that needs "above 30 for at least 0.5 s" can't be checked properly with data every 0.5 s. AnaTele knows this. When the data is too sparse, it marks the rule unassessable instead of pretending it's healthy (chapter 8).

What AnaTele keeps¶

When AnaTele opens a log it keeps every decoded record, with its byte offset in the file. The simulated motor-failure log has 5,387 records. Every chart, threshold and hypothesis in the report can therefore be traced back to the exact bytes it came from. This is the evidence viewer in the desktop app.