A general idea of a UAV is to move in 3D airspace. It can be manually controlled via remote, usually, a human operator or it can be an autonomous flight with various levels of autonomy.
According to the Drone Industry Insights (2019. https://dronelife.com/2019/03/11/droneii-tech-talk-unraveling-5-levels-of-drone-autonomy/) there are 6 levels of drone operations autonomy:

Autonomy Level	0	1	2	3	4	5
Human Contribution to the Flight Control
Machine (Drone Systems) Contribution to the Flight Control
Flight Automation Degree	None	Low	Partial	Conditional	High	Full
Remarks	Remote Control (fully RC). UAVO controls the drone in 100% manually (i.e. operator directly drives control surfaces).	UAVO in Control but the drone has at least one function it controls independently to the human operator (i.e. flight stabilisation).	UAVO is responsible for operation safety. The drone can take over controls given by the operator and modify it (i.e. heading, altitude hold, position hold, “smart” flight modes).	UAVO acts as fall-back: the drone performs autonomous operation under given conditions (i.e. using preloaded flight plan). The Drone can introduce slight modifications to it. i.e. avoid collisions with detected objects.	UAVO is out of control here, the drone performs autonomous flight and is able to use its duplicated systems to remain safe and operable all time.	The drone performs fully autonomous decisions on the way they implement given task, using data and possibly AI to plan the flight and modify it.
Environment Interaction (i.e. Collision Avoidance)	None	Sense and Alert UAVO		Sense and Avoid, usually also Alert UAVO	Sense and Navigate

UAV ecosystem uses many levels of communication protocols. Starting from on-board communication between systems, through aerial-to-aerial and aerial-to-ground, finishing on satellite communication. Communication in UAV operations is essential to its safety, reliability and performance. Here we discuss the most popular communication protocols used in drones (Figure 1).

On-board communication protocols are used to exchange communication between the drone components, usually flight controller (FC), sensors and actuators. Those protocols are commonly known and shared with UGVs and IoT world, so we just briefly present their list here without in-depth review.
Actuators are specific for drones however, we discuss them in the following sub-chapter in-depth, along with remote control protocols (RC protocols).
The most common on-board, low-level communication interfaces and protocols are:

• I2C,
• SPI,
• Serial (COM),
• CAN (rather for larger drones),
• One-wire (rare).

The exact protocol use is usually driven by the set of sensors and components, that are present on-board of the UAV. Flight controller sometimes exposes set of dedicated ports (connectors), sometimes they are universal plugs that can be used as configured in the FC configuration.
In many cases, an elementary set of sensors is integrated with the FC, Additionally, for GPS positioning, NMEA protocol is frequently used.

Remote Control is an essential part of drones. While there do are fully automatic systems that take-off, implement the mission and then land 100% automatically, in any case, there is a backup solution using manual operation such as RC control. Additionally, following mission progress and current system conditions is essential thus telemetry is a natural part for all flying objects whether they perform autonomous or remote-controlled flight at the moment.

As from the beginning, RC was used to control actuators (usually control surfaces) so actuators communication protocols were and still are an essential part of the on-board communication.

ADS-B, TCAS, NMEA, others Many communication protocols are shared with IoT, computer networks, automotive industry, UGV, airborne systems and even space industry.

While it is possible to receive raw satellite signals over the radio and use it to decode the signal and obtain a lon/lat position using triangulation method (see the chapter on navigation for more details) it is common to use ready GPS receiver modules that simply communicate to the flight controller or other device, providing 2D/3D position (3D includes altitude), positioning accuracy, number of satellites in view (it directly impacts positioning quality) and so on. GPS modules use textual and binary communication, depending on the particular receiver chip and PCB board design. In particular, most GPS receivers are able to deliver information using NMEA protocol that is a standard communication protocol at the moment, usually in a textual form over the serial connection (the most common is 9600 bps). Sample NMEA data for Tallinn/Estonia old city central market square is present below:

$GPGGA,095531.290,5926.238,N,02444.715,E,1,12,1.0,0.0,M,0.0,M,,*6A
$GPGSA,A,3,01,02,03,04,05,06,07,08,09,10,11,12,1.0,1.0,1.0*30
$GPRMC,095531.290,A,5926.238,N,02444.715,E,,,100920,000.0,W*74