UAVs share many components with UGVs and Internet of Things. In particular, they use a number of sensors and techniques known from robotics, autonomous cars, sensor networks, sometimes just changing its application. Here we focus on those components that differ from UGV aforementioned in other sections.

Sensors deliver to the FC and operator all necessary information about the current state of the UAV. Depending on the airframe used, the purpose of the drone and flight modes available, some of them are optional and some necessary to operate. Most of the flight controllers include at least IMU, frequently magnetometer and pressure sensor. In case they're located as external components, communication is held using popular embedded systems protocols: I2C, SPI and Serial, less often CAN.

IMU is an integrated mems sensor that contains accelerometer and gyroscope. Nowadays IMUs deliver 3D (3 axes) information instead of 1, so it is all integrated into a single chip. Old constructions may contain separated devices, one per axis but it is almost impossible to find a commercial UAV using this technology.

The IMU is necessary to operate stably any multirotor. A human operator is unable to manually control directly multirotor motors and requires a flight controller (FC) to operate it. FC, on the other hand, requires reference to the current rotation and its acceleration and that is the purpose, each multirotor drone requires gyros and accelerometers.
IMUs may also integrate barometer and compass (magnetometer) to deliver full orientation to the FC. The common marking is so-called “DOF” (Degree of Freedom) telling exactly, what is integrated within the chip:

• 6 DOF - a basic IMU, usually for racing drones. It is composed of 3 DOF (3 axes) accelerometer + 3 DOF gyroscope, delivering to the drone relative information about its position regarding gravity. This kind of solution, if not assisted with additional sensors, is unable to orientate geographically and unable to hold altitude.
• 7 DOF - an extension to the 6 DOF with a barometer. A common approach is to assembly 6 DOF IMU + external pressure sensor.
• 9 DOF - an extension to the 6 DOF with a magnetometer (compass, also 3 DOF). See remarks on quality in the Magnetometer section, below.
• 10 DOF, a full set integrated (Figure 1). It includes 9 DOF + barometer.

Many FCs integrate IMU (and other chips) onboard. Some use communication protocols and external breakout boards.

The barometer is a pressure sensor used to monitor drone flight altitude (vertical position) thus enabling the drone to keep its altitude constant and help the operator to keep them hovering stable. A flight mode where the vertical altitude is kept constant is frequently referenced as “Alt hold” mode and drones without a pressure sensor do not offer this function. Most of the commercial drones are equipped with at least one pressure sensor, the only exception is racing drones class, where this function is absent or limited and usually disabled during a race if present at all.
The barometric pressure sensor is essential for autonomous operations.

In case of most drones, flight time is less than an hour, and their range is limited to few kilometres, so it is common to assume that air pressure does not change importantly and do not affect altitude calculations over this period and distance/area. For drones that operate for a long-distance/long time, it is necessary to introduce pressure correction systems that will provide the ability to re-calculate altitude, as in case of large scale planes. This is usually done with reference ground stations that deliver current readings and their changes over time and space (i.e. airport or ground station located in the landing area). In lightweight constructions pressure sensor is usually covered with ventilating foam to avoid rapid pressure changes, coming i.e. from the wind and propellers that may affect UAV vertical stability.

Differential pressure sensors are used to check true airspeed. Any flying object's move, in case there is an airflow (wind), can be described by at least two, distinguishable velocities:

• Ground speed, obtained usually from the navigation system such as GNSS (satellite-based) positioning.
• Airspeed, a relative speed within the air.

Above can be equal only if there is no air movements at all (rarely happens). In the case of multirotors, the difference between airspeed and ground speed is not very critical for its operation, while, it can be critical in case of fixed wings. Wind direction drives, in particular, take-off and landing direction of the fixed-wing drones and planes: you always take-off and land towards the direction where the wind blows from, or at least as much as possible, so “against” the wind. It is airspeed that is meaningful when landing, not the ground speed. Meaning can be anything from saving energy to ability to take-off and land at all.

Airspeed is measured using a differential pressure sensor that measures the difference between statical pressure and dynamic one, coming from the pipe located towards the flight direction (Figure ##REF:pitotplane##). This device is usually referenced as Pitot, eventually Prandtl tube and is an essential device in any plane (also its failure historically was the reason for the serious and deadly plane accidents, like Air France flight 447 in 2009). Pitot / Prandtl tubes tend to freeze and block with ice on the higher altitudes, so in full-scale planes, they're usually heated to prevent such accidents (Figure 4). Also, for this reason, this component is usually at least duplicated and located in different locations on the fuselage, sometimes even on the wings.

In the figure 5 there is a Prandtl pipe module for drones, that uses popular MPXV7002DP sensor.

A digital compass is a MEMS sensor able to detect a magnetic field. This enables drones to perform “smart” operation i.e. to rotate relatively to the magnetic North and keep flight direction. Obviously, this information is also delivered by the satellite navigation systems yet working well for moving objects, while not so precise for a hovering multirotor. Most of the commercial drones include 3 axes (3 DOF) magnetometer. This sensor is very sensitive for the environmental conditions, i.e. indoor building construction, electrical cables, power lines and so on can disturb readings. For this reason, it is pretty common, that operator may require “compass calibration” before the flight. Calibration is as simple as rotating drone horizontal and vertical, to let the FC read maximum and minimum values returned by the magnetometer. The magnetometer can be integrated with IMU or can be a separate module and usually communicates with FC using I2C or SPI protocols. Note, magnetometer (as well as IMU) is physically oriented, so it is common that the breakout board contains markings presenting chip orientation and thus axes (Figure 6).

The purpose to use a thermometer in case of drones is threefold:

• Environment monitoring to react for specific flight conditions, i.e. enable de-icing (i.e. Pitot tube heating) or warn operator about dangerous conditions - that usually happens on high altitude flights or in low temperature and high humidity and is related to the dew point.
• Monitoring of internal components state, mostly to avoid overheating and related electronics and battery failure.
• Using thermometer as a part of the mission target, i.e. temperature scan or payload monitoring, etc.

The digital thermometers on the low-level hardware are based on NTCs and PTCs, however, the most frequent case is to use sensors integrated with other sensors like i.e. barometers are frequently accompanied with a thermometer to ensure temperature compensation, but on can read temperature separately, as well. There are thermometers integrated with microcontrollers as well.

Satellite navigation is in no doubt a choice number one in drones while flying outdoors. It is for both autonomous flight as well as for remote control ones, even for experienced operators. GNSS positioning is able to keep drone horizontally stable and thus i.e. compensate wind drift.
We discuss navigation principals in a separate chapter, here just focusing on its sensors. So far, the only drones that do not benefit from GNSS positioning are FPV racing drones but still many of them contain receiver to hold their position in other flight modes than racing.

GNSS receiver requires an external antenna (usually ceramic), and because of the signal suppression, it is common to keep the connection between antenna and receiver as short as possible. For this reason, receiver and antenna are frequently in a form of a single block and located in the upper part of the fuselage. The connection between GNSS receiver and FC is usually a serial, pretty frequent one-directional. As GNSS receiver requires an Almanac data to operate, and this one can be received from the satellites (long time) or from the web (short time), some receivers are able to obtain it from the FC. And it is pretty frequent that FC obtains it from the ground station via telemetry/communication link or directly i.e. via the cellular connection or other Internet connection available. As downloading an Almanac is time-consuming and it has a direct impact on “getting a fix” on a cold start delay, some receivers contain a coin battery and battery-backup memory, to keep an Almanac ready and not to download it on each cold start (i.e. when main drone battery is replaced).

Nowadays most of the modern GNSS receivers are multi-channel and multi-constellation ones. It means that they use different satellite constellations to obtain position and can benefit from a statistical approach to get even better and more accurate positioning. Additionally, some of the receivers are able to use WAAS (Wide Area Augmentation System) and SBAS (Satellite Based Augmentation System) to introduce corrections live, both transferred via satellites as well as via ground radio stations. In most cases, the basic constellation used it is GPS (GPS Navstar). In general, the majority of GNSS receivers use at least one more of the following list:

• Glonass (Russia),
• BeiDou (China),
• Gallileo (EU).

A leader of GNSS receivers in drones is Ublox and you will find their receivers in many amateur and commercial solutions. Advanced models with high precision positioning offer centimetre accuracy (according to the manufacturer) as i.e. in NEO-D9S series. Standard precision receivers offer some 1m accuracy (static), i.e. popular NEO M8 series (Figure 7).

As the majority of the drone power sources are electrical batteries, it is essential to monitor their capacity and use. Typical Li-Po battery has known discharge curve, and one of the most useful sensors is to observe battery voltage and this way predict necessity to terminate mission and land to re-charge or swap the power source. The other approach is to use a current sensor to estimate power consumption and calculate its total use. In practice, both techniques are used as power source down usually equals instant fall of the drone to the ground.

A wast number of different sensors, measuring physical phenomena is present in drones. I.e. drones with fossil fuel engines (motors) may benefit from liquid fuel level sensor, measuring remaining fuel capacity, rotation sensor can be used to monitor rotation speed and so on.

There are no UAVs without a single actuator. Any device moving around controllable way requires usually at least one actuator per single degree of freedom (usually much more). In terms of drones, we usually talk about servos and motors.

In many constructions, where motor and propeller attached is primary (or the only) source of lift generated, it is important to use appropriate propulsion, able to deliver thrust necessary for operation. It is not so simple in case of fixed-wings but pretty straightforward in case of helicopters and multirotors: total thrust is a sum of the thrust of all propulsion.
A general rule of thumb says that in any case, thrust to MTOM (maximum take-off mass) should be at least 2:1. The lower the ratio, the less responsible the drone is and in particular if it falls below 1:1, UAV is unable to ascend and to hover. On the other hand, too high ratio causes the drone to be hard to control and may lead to instability.
A typical drone for aerial photography has thrust to MTOM ratio around 3:1 and 4:1.
A racing drone is at least 5:1 and it is not unusual to see 13:1 and more for advanced 3D pilots.

Servos (short from servomotor) are used for various utilities, like i.e. driving control surfaces, retracting landing gear, changing propeller's angle and many, many more. There is a vast number of different sizes of servos, starting from miniature ones, weighting grams to large ones with some couple of kilograms of its weight, but in any case, servo contains an electric motor inside and a decoder able to provide a current rotation angle (note for 360-degree servos, below). It also contains some electronics to control and correct its rotation. Summarising, controlling servo is as simple as “telling” the servo to rotate to the desired angle and it does it for you, including corrections, if the external force (i.e. pendulum) causes to overshot or undershot the target. A miniature servo is present in figure 8.

Servos are connected with 3 cables, power (+/-) and control. The last one uses PWM (Pulse Width Modulation) to control the angle of the servo. PWM frequency is constant but it is the duty cycle, that controls the servo rotation. We distinguish 2 types of servos: analogue (standard) and digital. In any case, they're controlled with PWM. Digital may be misleading here: standard “analogue” servo PWM signal is 50Hz while digital is 300Hz (or other, but more than 50). As basic, “analogue” PWM standard is 50Hz, so the period is 20ms, the benefit of the “digital” over “analogue” is rather theoretical in most applications: it is a simply quicker response.
There are also special “slow” servos used i.e. to deploy flaps and thus change wing's lift force slowly rather than rapidly. A 0-degree rotation angle is equivalent to the 1ms high/19ms low digital control signal duty cycle, while 180 degrees is for 2ms duty cycle. Naturally, this scale tends to be linear, so 90 degree is equivalent to 1.5ms: see figure 9 for graphical representation of the control signal.

As one can see from the above, the most common case is a servo operating on 0..180 degrees range. There are others, however: i.e. 270 degrees.

A servo is typically described by the following set of parameters:

• Physical dimensions and weight.
• Power voltage range.
• Gear material: plastic/metal, number of teeth and shaft parameters (i.e. diameter, fixing screw and other).
• Torque (kg/cm), frequently provided as related to the powering voltage, it is a maximum mass your servo can lift using 1cm arm.
• Digital / Analogue.
• Speed measured as the time necessary to rotate servo by 60 degrees under full load. It is also related to the power voltage.
• Others.

There are two most common colour coding for servo cables:

• Futaba: black/red/white: (-GND/+power/control signal);
• Hitec: black/red/yellow: (-GND/+power/control signal);
• JR Radios: brown/red/orange: (-GND/+power/control signal);

In most cases, the plug is a female 2.54, 3 pole connector, JR standard or Futaba. The difference is Futaba connector contains, additionally, side plastic guide, so it is impossible to connect the servo wrong way.

You may also find servos with 5 connectors: 2 of them are driving DC motor inside, while 3 others are connected to the potentiometer (decoder) that you can read current rotation angle. This kind of servos requires external control logic, however.

Electric DC motors and in particular their lightweight versions are most common propulsion systems in UAVs.
There are two classes of electric motors:

• brushed,
• brushless.

Electric DC motors vary in diameter from a couple of mm to 15cm with a power consumption of some mA to 200A.

Brushed motors use an internal switching system to alternate current direction thus changing magnetic field. It is pretty easy to recognize the brushed motor as it has just two wires (brushless has three). Speed can be controlled via control of the energy delivered, i.e. changing voltage (directly or rather via PWM duty cycle). Brushed, coreless motors are designed to rotate in one direction. This is the reason, why brushed motors are marked CW (Clock-wise) and CCW (Counter-wise). While some of them can operate other direction, it is not very efficient. Because brushed motors use the brushed switch inside, named commutator, that uses friction. It wears out over time thus brushed DC motors popular only in smallest, miniature drones. Brushed motor construction is not scalable in terms above some diameter, weight to torque ratio is rapidly decreasing. Because of the mechanical, friction-based, commutator construction, brushed DC motors used to be considered less reliable than brushless ones. The advantage is simplicity on powering and speed control, usually using a single MOS-FET transistor and PWM.
Miniature DC brushed motors are marked pretty frequently with their external sizes: i.e. 8520 means 8.5mm diameter, 2cm length. A common maximum voltage is 1s (up to 4.2V) on most of the miniature drone brushed DC motors. While some report motors can operate on higher voltage (even 2S that is equal to 8.4V max), they tend to overheat then and break quickly. Sample brushed motor is present in Figure 10.

There is a class of brushed motors for UGVs that are much bigger and support higher voltage but we do not consider them in drone's section.

Brushless motors used to be designed for not so small drones, as their internal construction is pretty complex. Recently, however, brushless motors range was extended with miniature and super-miniature motors along with assembling technology development and they tend to replace brushed motors even in miniature UAVs. Still, they are more expensive comparing to brushed motors and require complex control electronics (ESC, Electronic Speed Controller). Brushless motors can operate in both directions. Brushless motors connect with 3 cables to the ESC. Changing rotation direction is as simple as swapping two of three wires (any pair).

Brushless motors do not contain commutator that wears out over time: they are more reliable and lasting longer than brushed ones.

Brushless motor is composed of the stator with coils, connected permanently to the wired terminals and rotating rotor with permanent magnets (Figure 11).

Universal (non-proprietary) motors have usually marked the way one can read its features, i.e. HK-4015-1450KV means the motor is:

• HK - manufacturer marking (here HobbyKing)
• 40 - motor diameter,
• 15 - motor height,
• 1450KV - 1450 rotations per 1 Volt (see remark below).

Motor's electrical features are defined via maximum voltage it can handle, maximum current and most of all, its rotation speed given as the number of revolutions per 1 Volt of power. Increasing voltage (within maximum limit given) speeds up the motor. This is usually rated under no-load (without propeller) so may differ in real-world scenarios. A rule of thumb is higher KV, faster the motor rotates. It does not necessarily mean it is more energy-efficient, as faster rotation is usually for smaller motors thus it delivers lower torque.

Some motors also contain a note on its internal, electromagnetic construction, i.e. 12N14P means:

• 12N number of permanent magnets in the stator,
• 14P number of coils in the rotor.

In general, the lower the N and P are, the more powerful engine is, but on the other hand, higher N and P means smoother and more precise rotation (i.e. necessary for gimbals). Typical for multirotor is 12N14P.

Motor's windings (cable diameter and wiring) has a direct impact on its resistance, thus on motor's KV. In short, the thicker and shorter the cable (fewer turns), the lower resistance, the more KV the motor has.
The winding (wiring) can be single strained and multi strained (wired using single or parallel cables, where the parallel is usually three).
Single strained wiring tends to have better heat management thus are used for higher voltage, i.e. 5-6S. Because of the bigger diameter, you cannot pack it very well, thus single strained motors are bigger than multi-strained ones.
Multi strained wiring can be better packed because of smaller empty spaces between wires, thus such coil creates a higher magnetic field than single strained wiring, that means multi strained motors are more energy-efficient and smaller.

Physical properties of the motor include also:

• Maximum thrust (eventually a list of thrust generated, regarding voltage and propeller size/type).
• Weight.

There are types of mechanical constructions:

• Inrunner: The external body (can like) is static while the rotor is inside of the motor. There is a shaft and construction mimics brushed motors. In such construction, the rotor is called the core.
• Outrunner: The stator with coils is inside while the rotor with magnets is outside: most of the engine is rotating, including external housing. In such construction, the rotor is called the motor bell.

Each construction has some features comparing inrunner to outrunner. In particular, the following is to consider when juxtaposing features of comparable two:

• Inrunner has a smaller diameter than outrunner.
• Outrunner has a lower profile (height) than inrunner.
• Outrunner body rotates.
• Inrunner has better heat dispersion (coils are located outside, magnets inside) than outrunner (this is partially true, cause modern outrunner's shell is constructed like a kind of fan, to ventilate interior).
• Outrunner generates larger torque than inrunner.
• Inrunner has higher KV (rotations per volt) than outrunner.
• Inrunner has better energy efficiency.

In the table below, there are proposed applications with respect to the inrunner and outrunner motors (Table 1).

In the Table 2 there is a juxtaposition of a UAV quadcopter frames and corresponding motors and propellers. One may use them as a starting point when planning a new construction.

Please note, it is very individual to construct a drone, so above values are on average.

The ESC (Electronic Speed Controller) is necessary to control a three-phase brushless motor, so we discuss them in this section.

ESC accepts power input usually directly from the drone battery and there is one ESC per motor (Figure 12). Its purpose is to control brushless motor rotation speed and also direction. ESC is controlled using one of the RC protocols directly from the RC receiver or (most common as for now) indirectly from the FC. There are various protocols that include bi-directional ones but in any case, the output of the ESC is simply three wires that drive directly brushless motor. It is ESC's duty to generate an appropriate sequence to let the motor spin correctly.

The major features of ESC are:

• input voltage range (provided in “S”)
• maximum current,
• size and mass,
• communication protocol used (most ESCs can handle at least a bunch of them, including simplest PWM signal),
• set of additional features, like i.e. ability to deliver stabilised power to the flight controller or RC receiver (so-called built-in BEC),
• programmability.

Regarding brushless motor, ESC is not only responsible for speed control but also for speed-up / slow-down characteristics, behaviour on zero throttles (if the motor is kept in position or let it float freely) and so on. As ESC generates PWM on its outputs, its frequency impacts motor torque, temperature and smoothness of rotation. Some ESCs deliver capability to force constant rotation speed (useful for variable pitch propellers) and most of them offer programmable change of the rotation direction (without need to swap wiring that may cause re-soldering).

Programming (changing of the parameters/features/behaviour) of the ESCs is possible threefold (depending on particular model and manufacturer):

• via so-called programming cards - they have nothing to do with cards actually, it is simply an external device you connect an ESC to and choose a configuration that is then stored in the non-volatile memory of the microcontroller that manages ESC;
• via a sequence of the input signal changes (operator using throttle enables programming mode and then in sequence changes all parameters);
• via FC or PC computer - some of the ESCs provide programming input, usually serial or USB, while others let the FC program it.

In the communication section, we discuss ESC communication protocols.

In the case of miniature drones and also FPV in class 250 or smaller, as total energy consumption is rather low or average, it is common to see 4 in 1 (for quadcopters) or 6 in 1 (for hexacopters) integrated ESCs that you connect motors directly to a single board. It is only a physical construction, integrating separate ESCs into the single board to minimise space. Some of them are in a form of a stack (along with flight controller and other communication modules), usually mounted at the bottom of the stack.

Piston engines are miniature versions of the motors as we use in full-size cars and planes. In the case of RC scale, they are usually single-piston ones (Figure 13), but there are constructions that mimic radial engines (Figure ##REF:rcradialengine##) as we know them from 1st and 2nd world war warplanes (radial engine was invented in 1901, however).
It is not very common to see piston engine in multirotors, because of their construction complexity, weight and control challenges. Rather, they take their place in scale UAVs in a form of fixed-wing ones and in larger construction, usually exceeding some 25kg.

There are two common types of power type, regarding fuel they use:

• gas - similar like in case of passenger cars, using a spark plug to ignite fuel. Those use regular, eventually high octane aviation fuel.
• glow with glow plug that uses heat from compression of the methanol fuel along with the heat of the glow plug.

For construction simplicity, scale engines are air-cooled.

Indeed there are scale jet engines, turbo-jets in fact, so far there are no turbo-fan constructions available. Those engines are used in top of high speed and high-performance UAVs and require complex construction (Figure 15). There are used mostly in fixed-wing drones as a replacement for propeller-based propulsion and due to the high cost and complex maintenance, are not very popular. Yet drones using those engines are the ones that hold records for speed of their flight, reaching 700km/h.
Interestingly, in 2019 there was a first successful presentation of a jet quad drone that uses 4 jet engines, instead of propellers.

EDFs are not a class of propulsion themselves, but we mention them here as they mimic turbofan engines that are not represented in UAV class so far. EDFs have nothing common with real turbofan engines other than the way they look from outside (Figure 16). The need for this construction came from the need to mimic passenger plane engines that are turbofans (as i.e. in popular Boeing 737 or Airbus A320). EDF is short from Electric Duct Flow and uses electric inrunner motor, mounted coaxially, that spins the large ventilator which generates the thrust.

Along with motors, propellers are the most notable component of most (all but soarers) UAVs. Propeller's cross-section looks like wing's cross-section and it used to generate lift or thrust (eventually both). Depending on the drone purpose, its size, motors used, one need to choose appropriate propeller type, size and material to ensure performance, efficiency and safety. Popular UAV propellers can be as small as one inch of diameter and as big as some 40 inches. The material used to construct propeller is wood, plastic (mostly nylon and polycarbonate), carbon fibre (Figure ##REF:prcpropeller##). Each propeller has at least 2 blades that can be fixed or rotating parallel to its length (so-called variable pitch propeller).

Each propeller is characterised with the following set of parameters:

• diameter,
• rotation direction (CW/CCW),
• a number of blades (typical is 2 to 5),
• fixed/variable pitch,
• pitch,
• hub diameter (shaft diameter, sometime delivered with a set of adapters),
• material,
• foldability.

Choosing an appropriate propeller is state of the art: first of all, in most cases, a first-hand choice is provided by the motor manufacturer. Note, the higher the diameter, and the higher the pitch, there is a bigger load to the motor, thus it also means higher current. And higher current impacts directly ESC and battery, not to overstress both. When using ready UAV the choice is usually a replacement 1:1 as delivered by the UAV manufacturer. When designing your own drone, there is a variety of choices and usually, the motor manufacturer delivers a parameter table where propeller sizes are juxtaposed along with expected construction (rated under full load).

In the case of multirotors, each pair of motors rotate opposite, so propellers are usually sold in sets (pair, 4 pieces), as CW + CCW.

Most of the propellers have their elementary data printed on their hub or close to it. Markings are provided in inches, so i.e. 1045 means 10 inches of the diameter and 4.5 inches of pitch.

What exactly is propeller's pitch? It is the theoretical distance the propeller would move in a solid environment during one full turn. You can consider it as in case of a screw going through the wood. Obviously, the higher the pitch the bigger force it generates but similar way to the wing, if pitch it to high, laminar flow may break and thus lower the performance. The higher the pitch, the more torque it requires to operate thus the motor load is bigger.

The bigger the propeller is in its size, the more important it is to keep it balanced. Vibrations disturb IMU and cause bearings and shafts to wear out quicker. Serious vibration may lead to airframe destruction. Balancing is done using propeller balancer device (Figure 18).

To change propeller weight (balance it) you may use a piece of self adhesible thin tape (to add weight) or use water paper to polish the blade to remove weight. In no case should you modify the upper part of the blade, but bottom one.

In many constructions, propellers are foldable. Popular DJI Mavic series use foldable propellers to decrease drone size in transportation. Propellers are folded on the operation planar (Figure 19). Once motors spin, blades unfold because of the centrifugal force.

In the case of motorized soarers (fixed-wing UAVs that benefit from energy-less soaring over the sky), however, fixed pitch propellers would cause resistance, dramatically lowering soaring performance. In such a case, we use foldable propellers as present in Figure 20. Propellers unwind automatically, once motor spins and flow of the air force them to fold over fuselage when not in use, to limit air resistance. There are two versions of such propellers for motors located in the front of the aircraft (so-called pullers) and those that fold outwards when using a motor located in the back of the flying wing airframe (so-called pushers).

Flight controllers (FCs) are necessary to implement flight logic and in particular autonomous flight. Their features vary from a simple quadcopter stability control to advanced autonomous navigation with collision avoidance using sensor fusion with visual data. Obviously, features are limited with hardware capabilities of the microcontroller used to implement FC, but in case of most modern microcontrollers are equipped with decent core and large memory. The whole logic is based on the firmware used.

There are three approaches to the FCs and firmware:

• closed model - where FC is bound to a particular drone model and cannot be re-configured or adapted to the other airframes;
• open but proprietary - where one can use proprietary FC and firmware (i.e. DJI), but along with aerial part, there is a configuration tool that enables you to fine-tune or even adapt FC to your needs;
• open source - virtually ^[1], you can reprogram the FC as you wish, up to its hardware limits;

Here we focus on the third approach and we present a list of most popular firmware available. A time ago, first open-source FC hardware was developed as a natural extension of Arduino (ATMega) microcontrollers. 8-bit ATMegas, as pretty good for early UAVs, mostly fixed-wing ones, nowadays wouldn't handle increasing demand on advanced features for modern operations. There is still support for one of the first available solutions named Ardupilot (actually, its 8-bit version) but the project is considered to be obsolete and rather frozen.

Multicopters required much more powerful microcontrollers to use IMU and RTOS (Real-Time Operating System) to handle flight logic. Thus the most popular microcontrollers include Arm core-based ones, in particular, low power, low voltage 32-bit STM32 family.

The general schema of the drone with FC as a heart of it is present in Figure 21. Most of FCs include a variety of extension ports with serial, I2C, SPI and CAN protocols. Some of them have integrated RF module to directly communicate with selected remote controllers and telemetry systems. Some also offer image processing and video overlay features, i.e. for FPV racing.

The most popular microcontrollers for FCs are:

• Historically, ATMega 2560 (Ardupilot, 8 bit).
• STM32 F1, F3, F4, F7 - in no doubt, the most popular at the moment: most of the open-source and proprietary FC use one of those chips. STM32 is 32 bit one. The first choice for FPV racing, video filming drones, and so on.
• Broadcom BCM series with ARM core, not so low-power, well known from Raspberry Pi (and clones). Rather for larger drones, with some at least 45cm body.
• Intel Movidius, a niche yet very powerful microcontroller, capable i.e. to implement live video stream processing for optical stabilisation. Implemented in Ryze/DJI Tello.
• LPC1768 with ARM core, also niche but used by DJI (i.e. Naza M).
• Intel Atom, x86 and x64 architectures - for large drones.

Some FC boards integrate programming circuit, voltage stabilisers and even brushed speed controllers (usually in a form of MOS-FET transistors), i.e. F3 EVO brushed for miniature drones (Figure 22). FC usually integrates at least 6DOF IMU (usually more DOF) and more complex and more expensive constructions include barometer and accelerometer on-board. STM32 is powerful enough to decode most of the RC protocols in real-time (parallel to other duties), so there is no need to use an external decoder, just the high-frequency radio to extract a digital signal from FM transmission.

Many manufacturers deliver “stackable” flight controllers for 25cm and bigger drones class. One of the boards (usually mounted as second from the bottom) is FC itself, while the bottom board is usually integrated speed controller (brushed or brushless). Optionally, the top board is an extension board, i.e. handling on-screen display overlays for FPV racing, GPS receiver, and so on (Figure 23).

An interesting initiative is Pixhawk, open standard hardware for drones ^[2]. This project originated from the Ardupilot, and at the moment constitutes de-facto standardisation in professional drones. Pixhawk and its clones implemented according to the open standard are available from various vendors and use common software solution that includes FC firmware, ground station software and other components (Figure 24). This initiative integrates much well-known hardware and software companies, including 3D Robotics, Microsoft, Yuneec, Flir and NXP (among others). DJI is not a member of this consortium, however.

FC hardware is nothing without the firmware that implements various features. Open source FC firmware is available via Github (in most cases), and is extended on daily basis and periodical deploys. Everyone can download repository, and “cook” its own version of the firmware. As there are many different hardware solutions even for same microcontrollers, most of the repositories contain “config” files for a compilation, that prepares binary firmware packages for specific boards. Construction of this firmware on the code level is modular so i.e. it is easy to disable (remove) SD card logging function if your board does not provide one or you're not willing to use it and you need to free resources for other modules.

Here we present a list of common FC firmware:

• Ardupilot - historically the oldest one and also the most popular even nowadays (currently for 32-bit microcontrollers, formerly for 8-bit microcontrollers). For UAV and also for UGV. Provides wide autonomous flight features including ground station software and telemetry.

A bunch of related projects (non-exhaustive) that originate from one source (former OpenPilot ^[3]):

• Cleanflight - stable, changing slowly, new release once a quarter.
• Betaflight (a fork of the Cleanflight) - Development area for CleanFlight. If you have most modern hardware, look for firmware here (CleanFlight may be delayed). Updates appear weekly.
• Baseflight - also related to the above, but currently outdated, do not use if not necessary. Provides firmware for some old, resource-limited FCs.
• INav (a fork of the Cleanflight) - oriented for autonomous flights and video filming.
• Raceflight (a clone of CleanFlight) - racing drones and performance-oriented. Lacks such features like i.e. GPS navigation.
• PX4 Flight Stack - similar to Ardupilot - firmware for open hardware initiative (i.e. Pixhawk FC).
• LibrePilot (a clone of former OpenPilot), niche. You can find it in many cheap CC3D (STM32F3) FCs.

Updating of the firmware may be tricky, as requires connecting FC in bootloader mode via USB cable. Note, as firmware changes the way it stores parameters from version to version, it is pretty common that re-flashing cleans your configuration and requires you to configure controller from scratch (i.e. PID parameters, additional sensors, and so on). Always do a backup and configuration snapshot before updating firmware.

DJI offers a series of large FCs, designed for professional utilities, as i.e. A3 FC. While the solution is closed and proprietary, due to the large adaptivity capabilities, many people decide to use it, mostly because of high quality and legend reliability. In fact, the firmware is upgradeable, but there is no access to the source code. Along with FC there comes a configuration software that let you adapt controller features for your specific needs and your airframe.

RC control is a must for the majority of drones. Whether it is a fully manual flight or just remote monitoring and ability to take over in case of unforeseen situation, RC connection should be reliable. Even most advanced military UAVs present ability to let the human operator take manual control.
The remote control can be considered as controlling directly actuators remote way using radio communication from the ground controller (Transmitter) to the aerial unit (Receiver) and this is the way early RC models were implemented. For simplicity, the aerial unit delivered directly PWM signal, able to control servomotor, without a need to translate it. Nowadays, RC communication is usually bi-directional, where control signals are sent from the RC ground Transmitter to the aerial unit Receiver, while telemetry data is sent opposite. Sometime telemetry data is sent via separate channel and hardware, sometimes it is integrated with the Transmitter-Receiver combo solution. In any case, we consider a Transmitter to have a number of “channels” where one channel is equivalent to control one remote actuator. Basic channels drive control surfaces (for fixed wings) or indirectly motors via FC, those are throttle, rudder, aileron and elevator (as in fixed wings). Eventually, similar control applies to multirotors and helicopters. By the main channels, there are auxiliary ones, used for various operations: camera gimbal control, gear retracing, flaps control and so on.

Ground RC controller uses two sticks (to operate basic channels) whether it is a physical device or i.e. a touch screen of the mobile phone or tablet. Some RC controllers contain built-in LCD display ((Figure 25), while others use phone or tablet (Figure 26), or are “bare” style, without any observable controls, that is a case for FPV racing, where the operator wears FPV glasses (Figure 27).

Ground controller is using one of the assignments of control sticks called “modes”. The most popular is Mode 2, where left stick operates the throttle and rudder (yaw) while the right stick controls elevator (pitch) and ailerons (roll) (Figure 29).

The modern approach used in drone construction assumes that user interface (RC controller) should be separated from the transmitter to let the user decide, which radio standard to use in their scenario. For this reason, there is a vast number of RC controllers that have exchangeable transmitters. While this approach causes some more work while implementing solution, ability to use one controller to exchangeably control many drones sound interesting because operators simply “used to” use a particular device.

Modes presented in Figures 28, 29, 30 and 31 define four, basic channel, necessary to control a drone. More channels are usually bound to the switches and even if transmission resolution is somehow linear (i.e. mapped to the range 0..1023 in the receiver), binary switches usually send two values: 0 or max. There are also potentiometers and three-position switches (i.e. to deploy flaps to pre-defined positions), sliders and +- switches, changing channel value up and down. They're usually located on the top of the RC controller, eventually at the back to let the operator use fingers to access it easily (Figure 32). Channels bound to those control switches are usually referenced as AUX and numbered starting with 5 and up.

Modern RC controllers use a microcontroller and are able to mix channels as necessary: i.e. flying wing requires mixing of aileron and elevator, while the rudder is not in use at all.

Microprocessor-based RC controllers are usually able to freely bound UI components with channels, even with complex scenarios, where i.e. full throttle can set some other channel to trigger booster in the jet engine to provide additional thrust. For this reason, RC controllers provide a menu to configure its parameters and bind UI controls to channels. It is also common that “out-of-the-box” linear characteristic of the main channels can be altered i.e. to sigmoidal or limited once some maximum deflection of the control sticks has been reached. It provides an easy way to change drone responsiveness or limit its parameters for i.e. inexperienced operator.

RC Transmitter is a radio section connected to the RC controller (or eventually ground station i.e. PC computer, mobile phone and any other device dedicated for remote control of the UAV). The majority of the RC solutions are incompatible between manufacturers, and within the manufacturers, there are incompatible series as well. It is common to buy a Transmitter+Receiver combo (Figure 33). Eventually, when the Transmitter is physically integrated with the Controller, there used to be delivered Transmitter+Controller+Receiver. It is worthful to mention that many series are backwards compatible, but i.e. provide a limited number of features (i.e. lower number of channels transmitted or a one-, instead of a two-directional transmission, and some standards last for decades even if considered obsolete.

Transmitter and Receiver both have to be “bound”.\\Binding brings the possibility to own only one transmitter and many receivers (for many UAVs) obviously controlling one at a time.
Nowadays, binding fixes communication between particular devices using digital IDs, but historically analogue system required both radios had exchangeable “quartz” (oscillators) thus enabling multiple operators to operate only their drone even if sharing common space and fix it via setting same frequency. Naturally, two operators sharing the same frequency were causing interference and usually lack of communication, leading to broken connections (and possibly crashes). It is no longer a case, as digital transmission and frequency hopping used by many manufacturers, enables multiple operators to share the same radio space, obviously within reasonable limits.

Modern Transmitters and Receivers are two-directional ones:

• uplink: transmit commands from ground Transmitter (Controller, Ground Station) to the air Receiver,
• downlink: transmits in the reverse direction than uplink, sending telemetry data.

Two directional transmissions have great features, not to mention, it is possible to forecast “out of range” as Receiver transmits at least RSSI (Received Signal Strength Indicator) presenting current connection quality.

The most important features defining Transmitter and Receiver capabilities are listed below:

• a number of channels: at least 4, usually 6 or more;
• one-directional or two-directional communication;
• radio frequency and modulation / protocol / standard used;
• connector standards and size for Transmitter (i.e. compatibility with particular series of Controllers when considering universal modules);
• Receiver's size and weight (important for small and miniature drones);
• antenna connector standards;
• Receiver's telemetry ports and other communication ports;
• Receiver's output protocol (see communication protocols between Receiver and FC);
• other;

By the manual Controller and Transmitter-Receiver channel, professional solutions (but also amateur ones) use Ground Stations. They are in the form of PC/Mac computer and dedicated software, eventually using Android/IOS mobile or tablet.

Sometimes ground station and software is integrated with RC Controller, as i.e. in case of Yuneec ST 16 (Figure 34). This is the proprietary and closed solution.

Open source solutions for Ground Station (software) related to the ArduPilot ecosystem, and include (among others):

• Mission Planner (Figure 35),
• APM Planner,
• MAVProxy,
• QGroundControl,
• Tower (DroidPlanner),
• many other.

There are proprietary solutions that correspond with open source hardware and firmware, thanks to the open MAVlink protocol (see communication section).

Video cameras play important role in UAVs ecosystem, in particular in the aerial section (UAV itself).
Their purpose is a dual one:

• used to implement main task (purpose) of the drone device, i.e. aerial photography and video recording (perhaps the most common use), surveillance, monitoring, inspection and photogrammetry;
• flight monitoring and control, in the particular image based stabilisation, FPV flights, autonomous and manual navigation;

In any case, it requires a vast number of different cameras, lenses, mounting methods and wireless transmission solutions.

Drone cameras vary in size and optical capabilities. While some 480p camera is pretty enough for FPV racing, it is useless in case of professional cinematography, where 8k cameras are required, following user's demand on video quality.
For professional filming, drones used to be equipped with more than one camera. Amateur solutions share one, eventually two cameras, between UAV operator and camera operator (movie maker). In most cases and amateur solutions, UAV operator and camera operator is a single person.
Aerial photography cameras can be delivered by drone manufacturer or, if UAV's MTOM is huge enough, they can carry professional movie-making equipment, DSLR camera and so on (Figure 36).
Some manufacturers offer an interchangeable range of cameras, including video cameras, multispectral ones and thermal, sometimes integrating them in one body (Figure 37). Those are used in thermal inspections but also in SAR (Search and Rescue) activities.
In any case, the camera has to be stabilised and it is desired it can rotate (pan, tilt) in any direction, drone independent. For this reason, there are gimbals: they provide the ability to stabilise the camera and keep filming direction stable, even if drone rolls, pans or yaws, due to the manoeuvring or i.e. windy conditions and vibrations coming from the propulsion system. Obviously we consider here mostly multirotor airframes, but it also applies to the fixed-wing and helicopters. Majority of the movie recording drones are multirotors, however. Fixed wings ones are used when there is a need to record on long distance / long flight time and availability od such drones is rather limited - they are more popular in military solutions as reconnaissance and surveillance UAVs.

FPV systems on the other hand are least demanding in terms of optics, quality and peripherals. They are usually analogue ones because analogue transmission is almost zero latency and low video latency is essential for performance racing flights. For this reason, cheap CMOS and TTL cameras are used, delivering 480 / 575 lines, usually interlaced. Recently there started to appear digital FPV systems that introduced new video quality (i.e. 720p) with low latency (low as 28-30ms as i.e. DJI FPV system). As it is good enough for beginner racers, professionals still use analogue systems as unbeatable at the moment. It is also pretty common, that FPV cameras transmit low-quality signal (analogue or digital) to the FPV operator, but locally record high-quality video stream to the flash memory (usually microSD card) with at least 720p and even 1080p or 4K resolution (camera depending). The purpose is for a presentation and post-factum conflict resolution that may appear during racing and cannot be noticed in low-quality analogue transmission.

Depth cameras, i.e. Intel Realsense, are used to detect obstacles and avoid collisions with them. Many drones introduce these features now to help the operator. Such cameras are usually front-facing but recently, there appear new solutions capable to process multiple video streams onboard in real-time (as i.e. Nvidia Jetson Nano and Intel Movidius), so new UAVs have cameras facing backwards, down and even to the sides, to deliver 360-degree protection sphere around the drone. Additionally, depth cameras along with regular ones are able to provide position stabilisation when GNSS or other systems are unavailable i.e. in case of indoor flying.

Stabilisation cameras provide the ability to keep drone horizontally in one place, thanks to the optical stabilisation and image processing. It can be as simple as using sensors known from optical mouses that obviously have many limitations and as complex as advanced image processing with the detection of the characteristic objects and feature extraction like corners, lines and similar. The modern image processing also delivers the ability to let drones perform optical-based SLAM: Simultaneous Location And Mapping, generating 3D environment scene ad-hoc while flying. Obviously, it is rather for larger drones as requires additional energy thus larger battery, however i.e. miniature brushed drone DJI Tello, successfully uses front and down-facing cameras for indoor stabilisation (Figures 38 and 39) - the integrated image processing and flight control is implemented using Intel Movidius Myriad chipset. Optical stabilization requires an adequate level of light.

A good gimbal solution can stabilise in 6 DOF (3 rotation axes + 3 planar movements) and professional and semi-professional drones have the gimbal with a camera hanging under their fuselage (Figure 40). Cheaper solutions offer cameras that cannot yaw because they are mounted in front of the drone: this is in case of the popular DJI Mavic series (Figure 41), still, there is a stabilising gimbal and ability at least to tilt in some limited angles. Some proprietary camera solutions sold with drones introduce the ability to zoom, but the majority of drones provide a wide lens: some 70-120 degrees FOV (Field of View). In case of heavy lifting cinematic drones, all parameters are camera dependent but it is important to mention, that usually with such gimbal (or drone) there is delivered a set of extra motors and servos, that need to be attached to the camera, to remotely operate it (i.e. rotate zoom ring on the lens or press the photo button), as many cameras do not provide a remote wired/wireless interface to control it.

FPV cameras in racing drones do not require gimbals at all. They are fixed to the drone body, usually pointing some 20-40 degrees up, as most of the flight time drone is tilted (Figure 42). The faster the drone is, the bigger is the tilt angle.

Transmission of the video signal between an aerial unit and a ground station is strictly related to the protocols used. We discuss it more in the communication section but here it is just to mention that video transmission requires wide bandwidth. Obviously, in the case of digital transmission, compression may help to fit into the available bandwidth with a cost of quality. In general, we distinguish two types of transmission:

• analogue, almost zero latency, used mostly in FPV racing,
• digital, using coding and decoding, thus introducing notifiable latency that disqualifies it from the FPV and real-time tasks.

Mixed models include recording of the high-quality video in the aerial unit and transmission via downlink lower quality stream. As there do exist professional video links that let you broadcast high-resolution video stream live (i.e. for live reporting on TV) it is rare to use them in amateur and semi-professional drones as they are heavy units, that require large drones and also cost a fortune. This kind of downlinks use multi-channel transmission and usually operate on licenced radio frequencies, to avoid interference, so requires special equipment.

I case of the amateur and semi-professional solutions, transmission channels operate on popular, “free” radio frequencies and in most countries transmitter power is limited by law. Note, there are different frequencies in different countries, i.e. North America can freely use 915 MHz while it is forbidden to use in Europe as overlaps with cell-phone bands, and opposite, 868MHz is an open frequency in Europe, that is limited in the USA. For this reason, some solutions and hardware may work only locally and their use can be prohibited in other regions. Many manufacturers deliver EU and US versions of their devices.
Anyway, the most popular frequency for video transmission is WiFi, open 2.4GHz and most of all 5.8 GHz. Note, even 2.4GHz WiFi has slightly different regulations regarding bandwidth in different countries but the core remains common for the whole world. The majority of amateur and semi-professional solutions operate on 5.8 GHz, i.e. popular Boscam system (Figure 43). It is also pretty common that cameras transmit data simply via WiFi: you bind a device to the drone camera separate link from RC and receive video stream on a computer, mobile phone, or tablet. In case of analogue transmission, the standards refer to the analogue TV and it is PAL and NTSC. In the case of digital ones, it is usually a MPEG stream, and resolution is limited to some 720p. Miniature indoor drones use simply WiFi for both control and video transmission, eventually Bluetooth that additionally limits image quality.

Using WiFi and 2.4GHz for video transmission causes frequent video quality glitches in radio-noisy environments, so flight range is drastically limited in such areas.

Good antennas in both transmitter and receiver is worth more than extra transmission power. As drone (transmitter) changes its position against ground station (receiver), we usually use omnidirectional antennas for transmission (Figure 44). For long-range video downlinks, there are directional antennas that automatically point towards the drone, based on ground station and drone position.

Every antenna is intended to work with some particular frequency (or its limited range). Note, using inappropriate antennas drastically limits transmission range! To increase transmission quality and range, it is much better to use antennas with higher gain and suitable for the frequency, than increase power transmission while using the wrong one.

There are three approaches to present live video transmission:

• external monitors (popular in professional aerial cinematography),
• FPV googles used for racing (Figure 45),
• presentation on the mobile/tablet/dedicated device, separate or integrated with the controller, for the operator (Figure 46).

The last one is the most popular and used in the majority of amateur, professional and semi-professional drones. Obviously, image quality is limited, to some maximum 720p, eventually 1080p. As live transmission is used for monitoring mostly, it is common that cameras mounted on the drone record high-quality video stream.

By the aforementioned, there is a number of additional components, accompanying drone ecosystem, i.e. antennas and trackers, mechanical components, power distribution boards, batteries (we discuss them in depth in another chapter) and so on.