Unmanned Systems Protocol; MAVLINK

Unmanned systems required a method of transmitting data back and forth from the aircraft to the ground station. While there are multiple methods and protocols used to accomplish this transmission, one of the best examples is MAVLINK. MAVLINK stands for Micro Air Vehicle Link, and is the primary method of transmitting data on of the shelf consumer drones like the Parrot AR drone, as well as many of the do it yourself kits and collegiate robotics programs utilizing the PIXHAWK or Ardupilot flight controllers. The following will describe a commonly purchased DIY autopilot kit, the Arducopter, attached to an F450 style quadcopter frame.
              Mavlink was designed in 2009 by Lorenz Myer under LGPL License (Balasubramanian, 2013). Mavlink works by sending encoded messages. These messages are first encoded by the ground control software often called Mission Planner (MP). The encoded messages are sent either via USB serial connection, or through wireless telemetry, both can be used, but if a serial connection is made it takes priority (Balasubramanian, 2013).
Each of the Mavlink messages consists of 17 bytes which are made up of a 6-byte header, 9-byte payload message, and 2-byte check summery. The 9-byte payload message is the actual raw data that contains the information or command that the operator and the aircraft cares about (Balasubramanian, 2013). One example of these messages is the heartbeat. In this message, the GCS is asking the aircraft if it is still connected every single second. This is an important feature as it can direct the aircraft to perform preset failsafe maneuvers like a position hold, a return to home, or a land immediately. The communication is bidirectional and updates the GCS with real-time location, destination and various health and status indications(Balasubramanian, 2013).
            A DIY quadcopter has the ability to carry a wide verity of sensors, the most common sensor attache a lightweight action camera like the GoPro capable of capturing high-resolution footage. These cameras are often mounted underneath the aircraft from a gimbal stabilized carrier that will provide drastically smoother video. Power for the camera is given from rechargeable onboard battery. While the gimbal is driven from the aircraft's primary power supply. Data storage for the camera is also onboard and is recorded to a micro SD card. It is important to pair the camera with a suitable sd card what can write the large amounts of data at high speeds. Action cameras such as the GoPro can accept memory cards ranging from 4gb to 125gb in some instances and should be rated at class 10 for maximum performance (Gokey, 2018). Certain GoPros have the ability to transmit their live video to the GCS through a separate 5.8ghz transmitter, however, the latency is quite high, which could be disorienting when flying through a first-person view. The transmitted video, however, will lose some of its quality by the time it gets to the GCS. This is intentional as it decreased the latency to allow for a flyable picture. The data can then be recorded from the ground through a digital video recorder (DVR) and saved to a separate memory card, or hard drive.
            DIY drones are very capable of performing complicated tasks and managing large amounts of data. These homebuilt systems also offer the builder the option to customize and modify the drone however they see fit. The Mavlink protocol is a simple but robust communication link that enables users to safely command and monitor their aircraft from the ground and is accessible to all users.
  


References
Balasubramanian, S. (2013, November 15). MAVLink Tutorial for Absolute Dummies. Retrieved from https://diydrones.com/forum/topics/mavlink-tutorial-for-absolute-dummies-part-i?groupUrl=arducopterusergroup&

Gokey, M. (2018, April 18). The best microSD cards for your phone, GoPro, or Nintendo Switch. Retrieved from http://www.businessinsider.com/best-micro-sd-card

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