MAVLink message structure in Drone Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the MAVLink message header?

The MAVLink message header contains important information like the start sign, payload length, sequence number, system ID, and component ID. It helps the receiver understand and process the incoming message correctly.

beginner

List the main parts of a MAVLink message.

A MAVLink message has four main parts: header, payload, checksum, and optionally a signature. The header tells about the message, the payload carries the data, the checksum ensures data integrity, and the signature verifies authenticity.

beginner

What does the payload in a MAVLink message represent?

The payload is the part of the MAVLink message that contains the actual data or information being sent, such as sensor readings or commands.

beginner

Why is the checksum important in a MAVLink message?

The checksum helps detect errors in the message during transmission. If the checksum does not match, the message is considered corrupted and discarded.

intermediate

What role does the sequence number play in MAVLink messages?

The sequence number helps keep track of the order of messages sent. It allows the receiver to detect lost or out-of-order messages.

Which part of a MAVLink message contains the actual data being sent?

A Signature

B Header

C Checksum

D Payload

The payload carries the actual data or information in a MAVLink message.

What does the checksum in a MAVLink message help with?

A Detecting errors in the message

B Encrypting the message

C Identifying the sender

D Specifying message length

The checksum is used to detect errors that may have occurred during message transmission.

Which field in the MAVLink header helps identify the drone sending the message?

A System ID

B Sequence number

C Payload length

D Component ID

The System ID identifies the drone or system sending the message.

What is the purpose of the sequence number in MAVLink messages?

A To indicate message priority

B To track message order and detect lost messages

C To encrypt the message

D To specify payload size

The sequence number helps track the order of messages and detect if any are lost.

Which part of the MAVLink message ensures the message is from a trusted source?

A Payload

B Header

C Signature

D Checksum

The signature verifies the authenticity of the message source.

Explain the structure of a MAVLink message and the role of each part.

Think about how the message starts, what data it carries, how errors are checked, and how trust is verified.

Why is the sequence number important in MAVLink communication?

Consider how drones keep track of messages they receive.

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