MAVLink message structure in Drone Programming - Step-by-Step Execution

Concept Flow - MAVLink message structure

Start: Prepare message

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Add Header: Start sign, length, seq, sysid, compid, msgid

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Add Payload: Data fields

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Calculate CRC

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Append CRC to message

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Send complete MAVLink message

This flow shows how a MAVLink message is built step-by-step from header to payload to checksum before sending.

Execution Sample

Drone Programming

header = [0xFE, 2, 1, 1, 1, 24]
payload = [100, 200]
crc = calculate_crc(header + payload)
message = header + payload + crc
send(message)

Builds a MAVLink message with header, payload, calculates CRC, then sends it.

Execution Table

Step	Action	Data Added	Message State	Notes
1	Start message preparation	None	[]	Empty message buffer
2	Add header fields	[0xFE, 2, 1, 1, 1, 24]	[0xFE, 2, 1, 1, 1, 24]	Header includes start sign, length, seq, sysid, compid, msgid
3	Add payload data	[100, 200]	[0xFE, 2, 1, 1, 1, 24, 100, 200]	Payload contains message-specific data
4	Calculate CRC	CRC bytes	[0xFE, 2, 1, 1, 1, 24, 100, 200, crc1, crc2]	CRC ensures message integrity
5	Append CRC to message	[crc1, crc2]	[0xFE, 2, 1, 1, 1, 24, 100, 200, crc1, crc2]	CRC appended to message
6	Send message	Full message	[0xFE, 2, 1, 1, 1, 24, 100, 200, crc1, crc2]	Message sent over communication link
7	End	None	Message sent	Process complete

💡 Message fully constructed and sent; process ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
header	[]	[0xFE, 2, 1, 1, 1, 24]	[0xFE, 2, 1, 1, 1, 24]	[0xFE, 2, 1, 1, 1, 24]	[0xFE, 2, 1, 1, 1, 24]	[0xFE, 2, 1, 1, 1, 24]
payload	[]	[]	[100, 200]	[100, 200]	[100, 200]	[100, 200]
crc	None	None	None	[crc1, crc2]	[crc1, crc2]	[crc1, crc2]
message	[]	[0xFE, 2, 1, 1, 1, 24]	[0xFE, 2, 1, 1, 1, 24, 100, 200]	[0xFE, 2, 1, 1, 1, 24, 100, 200]	[0xFE, 2, 1, 1, 1, 24, 100, 200, crc1, crc2]	[0xFE, 2, 1, 1, 1, 24, 100, 200, crc1, crc2]

Key Moments - 3 Insights

Why do we add a CRC at the end of the MAVLink message?

The CRC is added to check if the message was received correctly without errors, as shown in step 4 of the execution_table.

What does the header part of the MAVLink message contain?

The header contains fixed fields like start sign, length, sequence number, system ID, component ID, and message ID, as seen in step 2 of the execution_table.

Why do we add the payload after the header and before the CRC?

Payload holds the actual data for the message and must be between header and CRC to be included in the CRC calculation, as shown in steps 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what does the message state include?

A Only the header fields

B Header fields plus CRC

C Header fields plus payload data

D Only the payload data

💡 Hint

Check the 'Message State' column at step 3 in the execution_table.

At which step is the CRC calculated and added to the message?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look for the step mentioning 'Calculate CRC' in the execution_table.

If the payload length changes, which part of the message must also be updated?

A Length field in header

B Sequence number

C Start sign in header

D CRC only

💡 Hint

Refer to the header fields in step 2 and how length relates to payload size.

Concept Snapshot

MAVLink message structure:
- Header: start sign, length, seq, sysid, compid, msgid
- Payload: message data
- CRC: checksum for error detection
Build message by adding header, payload, then CRC
Send complete message over link

Full Transcript

This visual execution shows how a MAVLink message is built step-by-step. First, the message preparation starts with an empty buffer. Then the header fields are added, including start sign, length, sequence number, system ID, component ID, and message ID. Next, the payload data is appended, which contains the actual message information. After that, the CRC checksum is calculated over the header and payload to ensure message integrity and appended to the message. Finally, the complete message is sent over the communication link. Variables like header, payload, crc, and message change their values as the message is constructed. Key points include the purpose of the CRC for error checking, the contents of the header, and the order of adding payload before CRC. The quizzes test understanding of message state at each step, when CRC is added, and how payload length affects the header length field.