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'.