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

Concept Flow - MAVLink message structure

Start: Prepare message

↓

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

↓

Add Payload: Data fields

↓

Calculate CRC

↓

Append CRC to message

↓

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.

Practice

1. Which part of a MAVLink message contains the actual data being sent between drone and controller?

easy

A. Payload

B. Header

C. Checksum

D. Footer

Solution

1. Step 1: Understand MAVLink message parts

   A MAVLink message has a header, payload, and checksum. The header contains metadata, the payload contains the actual data, and the checksum verifies integrity.

2. Step 2: Identify the data container

   The payload is the part that carries the actual information or data sent between devices.

3. Final Answer:

   Payload -> Option A

4. Quick Check:

   Payload = Data part [OK]

Hint: Payload always holds the message data [OK]

Common Mistakes:

• Confusing header with data
• Thinking checksum holds data
• Assuming footer exists in MAVLink

2. Which of the following is the correct order of parts in a MAVLink message?

easy

A. Header, Payload, Checksum

B. Payload, Header, Checksum

C. Checksum, Header, Payload

D. Header, Checksum, Payload

Solution

1. Step 1: Recall MAVLink message format

   The MAVLink message starts with a header that describes the message, followed by the payload which contains the data, and ends with a checksum to verify message integrity.

2. Step 2: Match the correct sequence

   The correct sequence is Header first, then Payload, and finally Checksum.

3. Final Answer:

   Header, Payload, Checksum -> Option A

4. Quick Check:

   Order = Header -> Payload -> Checksum [OK]

Hint: Header always comes before payload and checksum [OK]

Common Mistakes:

• Swapping payload and header order
• Placing checksum before payload
• Assuming checksum is in the middle

3. Given this simplified MAVLink message structure in code:

message = {"header": {"msg_id": 24}, "payload": {"lat": 12345678, "lon": 87654321}, "checksum": 0xABCD}

What is the value of message["payload"]["lon"]?

medium

A. 0xABCD

B. 12345678

C. 87654321

D. 24

Solution

1. Step 1: Locate the payload dictionary

   The message dictionary has a key "payload" which itself is a dictionary containing "lat" and "lon" keys.

2. Step 2: Access the longitude value

   Accessing message["payload"]["lon"] retrieves the value associated with "lon" inside the payload, which is 87654321.

3. Final Answer:

   87654321 -> Option C

4. Quick Check:

   payload["lon"] = 87654321 [OK]

Hint: Payload keys hold data values, access with payload[key] [OK]

Common Mistakes:

• Accessing header instead of payload
• Confusing checksum with data
• Using wrong key names

4. Identify the error in this MAVLink message snippet:

msg = {"header": {"msg_id": 30}, "payload": {"alt": 500}, "checksum": "1234"}

Assuming checksum must be an integer, what is wrong?

medium

A. Payload key 'alt' is missing

B. Checksum is a string, should be an integer

C. Header missing 'msg_id'

D. Payload should be a string, not a dictionary

Solution

1. Step 1: Check checksum data type

   The checksum is given as a string "1234" but it should be an integer value for proper validation.

2. Step 2: Verify other parts

   The header has a valid "msg_id" and payload has the "alt" key correctly as a dictionary, so no issues there.

3. Final Answer:

   Checksum is a string, should be an integer -> Option B

4. Quick Check:

   Checksum type must be integer [OK]

Hint: Checksum must be numeric, not string [OK]

Common Mistakes:

• Ignoring checksum type
• Assuming payload keys missing
• Confusing header fields

5. You want to create a MAVLink message that sends GPS coordinates with latitude and longitude. Which structure correctly represents this message including header, payload, and checksum?

hard

A. {"header": {"msg_id": 33}, "payload": "lat=34567890, lon=98765432", "checksum": 0x1A2B}

B. {"payload": {"lat": 34567890, "lon": 98765432}, "header": {"msg_id": 33}, "checksum": 0x1A2B}

C. {"header": {"msg_id": 33}, "payload": {"lat": "34567890", "lon": "98765432"}, "checksum": "0x1A2B"}

D. {"header": {"msg_id": 33}, "payload": {"lat": 34567890, "lon": 98765432}, "checksum": 0x1A2B}

Solution

1. Step 1: Check message part order and types

   {"header": {"msg_id": 33}, "payload": {"lat": 34567890, "lon": 98765432}, "checksum": 0x1A2B} has the correct order: header, payload as a dictionary with numeric lat/lon, and checksum as a hex integer.

2. Step 2: Identify errors in other options

   {"payload": {"lat": 34567890, "lon": 98765432}, "header": {"msg_id": 33}, "checksum": 0x1A2B} has wrong order (payload before header). {"header": {"msg_id": 33}, "payload": "lat=34567890, lon=98765432", "checksum": 0x1A2B} uses payload as a string, not dictionary. {"header": {"msg_id": 33}, "payload": {"lat": "34567890", "lon": "98765432"}, "checksum": "0x1A2B"} uses strings for lat/lon and checksum, which is incorrect.

3. Final Answer:

   Correct structure with header, numeric payload, and integer checksum -> Option D

4. Quick Check:

   Correct order and types = {"header": {"msg_id": 33}, "payload": {"lat": 34567890, "lon": 98765432}, "checksum": 0x1A2B} [OK]

Hint: Header first, payload dict with numbers, checksum integer [OK]

Common Mistakes:

• Wrong order of parts
• Payload as string instead of dict
• Checksum as string instead of int