MAVLink message structure in Drone Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MAVLink message header parsing code?

Consider a MAVLink message header represented as a byte array. The first byte is the start sign (0xFE), the second byte is the payload length, the third byte is the sequence number, the fourth byte is the system ID, and the fifth byte is the component ID.

What will be the printed output of the following code?

Drone Programming

header = bytes([0xFE, 10, 5, 1, 20])
start_sign = header[0]
payload_len = header[1]
seq_num = header[2]
sys_id = header[3]
comp_id = header[4]
print(f"Start: {start_sign}, Payload Length: {payload_len}, Seq: {seq_num}, SysID: {sys_id}, CompID: {comp_id}")

A Start: 0xFE, Payload Length: 10, Seq: 5, SysID: 1, CompID: 20

B Start: 254, Payload Length: 16, Seq: 5, SysID: 1, CompID: 20

C Start: 254, Payload Length: 10, Seq: 5, SysID: 20, CompID: 1

D Start: 254, Payload Length: 10, Seq: 5, SysID: 1, CompID: 20

💡 Hint

Remember that bytes are integers in Python and 0xFE is 254 in decimal.

The byte 0xFE is 254 in decimal. The payload length is the second byte, which is 10. The sequence number is 5, system ID is 1, and component ID is 20 as per the byte positions.

🧠 Conceptual

intermediate

Which field in a MAVLink message ensures message ordering?

MAVLink messages include several fields in their header. Which field is primarily used to keep track of the order of messages sent?

A Sequence Number

B Component ID

C Message ID

D System ID

💡 Hint

Think about which field increments with each message sent.

The sequence number increments with each message sent and helps the receiver detect lost or out-of-order messages.

❓ Predict Output

advanced

What error does this MAVLink message checksum calculation code raise?

Given the following Python code snippet that attempts to calculate a MAVLink message checksum, what error will it raise when run?

Drone Programming

def checksum(data):
    crc = 0xFFFF
    for b in data:
        crc ^= b << 8
        for _ in range(8):
            if crc & 0x8000:
                crc = (crc << 1) ^ 0x1021
            else:
                crc <<= 1
    return crc & 0xFFFF

msg = [0xFE, 10, 5, 1, 20]
print(checksum(msg))

A TypeError: unsupported operand type(s) for <<: 'str' and 'int'

B No error, prints an integer checksum

C IndexError: list index out of range

D AttributeError: 'list' object has no attribute 'encode'

💡 Hint

Check the data type of elements in the list and how bitwise operations work on integers.

The list contains integers, so bitwise operations like << and ^ work fine. The code will run and print an integer checksum value.

📝 Syntax

advanced

Which option correctly defines a MAVLink message dictionary with message ID as key and payload length as value?

Choose the option that creates a dictionary mapping MAVLink message IDs to their payload lengths correctly.

A {msg_id: payload_len for msg_id, payload_len in [(1, 10), (2, 20), (3, 30)] if payload_len > 15}

B {msg_id: payload_len if payload_len > 15 for msg_id, payload_len in [(1, 10), (2, 20), (3, 30)]}

C {msg_id: payload_len for msg_id, payload_len in [(1, 10), (2, 20), (3, 30)]}

D {msg_id: payload_len for (msg_id, payload_len) in [(1, 10), (2, 20), (3, 30)] if payload_len > 15 else 0}

💡 Hint

Remember the syntax for dictionary comprehensions with conditions.

Option A correctly filters pairs where payload length is greater than 15. Option A has invalid syntax. Option A uses else incorrectly in comprehension. Option A includes all pairs without filtering.

🚀 Application

expert

What is the value of the 'payload' variable after parsing this MAVLink message byte stream?

Given a MAVLink message byte stream where the payload length is 3, and the payload bytes are [0x01, 0x02, 0x03], what will be the value of the payload variable after running the code below?

Drone Programming

message = bytes([0xFE, 3, 10, 1, 20, 0x01, 0x02, 0x03, 0xAA, 0xBB])
payload_len = message[1]
payload = message[5:5+payload_len]
print(payload)

A b'\x03\x10\x01'

B [1, 2, 3]

C b'\x01\x02\x03'

D b'\xFE\x03\x0A'

💡 Hint

Remember that slicing bytes returns a bytes object with the selected bytes.

The payload length is 3, so bytes from index 5 to 7 (inclusive) are extracted. These bytes are 0x01, 0x02, 0x03, which in bytes notation is b'\x01\x02\x03'.

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