A Comparative Study on Application of Unmanned Aerial Vehicle Systems in Agriculture

Presently in agriculture, there is much ample scope for drone and UAS (Unmanned Aircraft System) development. Because of their low cost and small size, these devices have the ability to help many developing countries with economic prosperity. The entire aggregation of financial investments in the agricultural area has increased appreciably in recent years. Sooth to say, agriculture remains a massive part of the world’s commercial growth, and due to some complications, the agriculture fields withstand massive losses. Pets and destructive insects seem to be the primary reasons for certain degenerative diseases. It minimizes the potential productivity of the crops. For increasing the quality of the plants, fertilizers and pesticides are appropriately applied. Using UAVs (Unmanned Aerial Vehicles) for spraying pesticides and fertilizing materials is an exuberant contraption. It adequately reduces the rate of health dilemma and the number of workers, which is quite an impressive landmark. Willing producers are also adopting UAVs in agriculture to soil and field analysis, seed sowing, lessen the time and costs correlated with crop scouting, and field mapping. It is rapid, and it can sensibly diminish a farmer’s workload, which is significantly a part of the agricultural revolution. This article aims to proportionally represent the concept of agricultural purposed UAV clear to the neophytes. First, this paper outlines the harmonic framework of the agricultural UAV, and then it abundantly illustrates the methods and materials. Finally, the article portrays the outcome.


Introduction and Overview
The UAV is commonly known as UAS or, more precisely, as a drone, which can fly autonomously. UAV traditionally has a steadfast backdrop in aerodynamics, momentum arrangements, and theoretical physics' practical rules. In the recent few years, it has also been a band of smart agriculture [1]. Smart agriculture is data-driven; UAVs can correctly receive and equitably distribute this information, allowing producers to take steps based on individual soil circumstances. Precision farming, crop monitoring, geographical tracking, and field mapping are a few sectors where UAVs play a massive role [2,3]. Image analysis is the essence of a UAV. In image analysis, the photos are promptly captured by UAV camera, and it is then adjusted to enhance the quality using various software to explore typically and precisely the target area and correctly recognize the unconventional objects. Agricultural UAV properly utilizes image processing to save an accurate record of plants' growth, quality of the active area, pest control, etc. The dynamic UAV marketplace is presently strongly patchy, with diverse software providers and professionals competing fiercely in addition to leading manufacturers, and the predicted value of UAV Solutions in the agricultural industry is more than $32 billion [4]. A UAV can quickly survey an area of several kilometers and adequately provide long-range connections with or without payloads. UAVs come in various forms and exceptional abilities, and the aerodynamic designs differ remarkably from UAV's varieties. Different UAVs have a different speed and weight carrying capacitance. As a result, it is vital to understand which UAV is most suitable for agricultural monitoring and complex operation. Reliably using a UAV is a simple, speedy, and cost-effective alternative for agricultural activity [8]. It can also be serviceable in adverse weather conditions. UAVs can monitor crops more correctly, regularly, and affordably, accouching more eminent quality information that is modernized frequently to grant insight into crop improvement and highlight wasteful or unproductive practices. UAVs can also be utilized to produce multi-spectral pictures of crops, which are then examined to trace changes in form and maturity [9]. UAVs can contribute constructively valuable assistance to both producers and agronomists. It is reasonable to equip UAVs with infrared cameras to extract more information from the imagery [10]. Such as, it will undoubtedly help define the amount of chlorophyll in the specific crops properly. By universally adopting UAVs for agricultural mapping, producers can stay instantly updated on crops' health in a definite boundary and properly designate which area needs attention, plus having an assorted amount of actionable data available can prefer well-read choices scientifically based on factual data. Besides, in [11], a mobile software was divinely revealed for controlling the tracking data and storing conditions of the agricultural goods. This sort of modern system can be efficiently utilized in the monitoring field. Actually, positive cooperation is needed among the researchers, specialized companies, and producers to incorporate modern technological additions into efficient agricultural and forestry sectors' profitable solutions [12].
Moreover, UAVs for agriculture demonstrates a few limitations. Privacy risk, complex spraying environment, long-distanced positioning, and accurate data interpretation are some prime examples of its disadvantages [13]. As UAVs are remotely guided, they genuinely need a datalink with the ground control system, and the direct connection can be compromised by unwanted users. The essence of data is vital and must be the focus of UAV use judgments. However, the cons will not be the prime focus of this paper. If UAV technology continues to grow, the expenses and limitations of agriculture will definitely decrease [14].
This paper typically portrays the character of suitable UAVs for a specific agricultural purpose. Furthermore, it will be explicit about assuming which archetype of a UAV is imperative for particular farming works. This article's structured analysis is based precisely on the specific keyword and abstract searches in the Scopus, WOS (Web of Science), and Google Scholar databases. Plus, several reliable websites were also polled for subject-related information. Figure 2 shows the approximate number of articles published on Agricultural UAV from 2011-2020. Additionally, Table 1 profusely illustrates some of the published articles about UAVs' applications in recent years. This paper's remainder contains five key segments. Sections two, three, four, and five respectively picture the mythology and materials, crop monitoring, spraying system, and livestock farming. Moreover, the final part positively asserts the conclusion and future scope.    Budiharo [34] Multi-rotor (quad-rotor) Presented an outline of UAVs' study and practical application for agriculture, delivering things, and GIS. And typically described pesticide spraying and field mapping.

2019
Crop monitoring and agricultural spraying 30.
Jeziorska [35] Fixed-wing and multi-rotor Reviewed the operating system, enactment, scientific adaptability, and information processing methods, which are necessary for wetland monitoring and hydrological perspective.

Classifications of Unmanned Aerial Vehicle
The UAV itself is valuable as a recognized principle for transferring a decent amount of payload, which operates efficiently in particular agriculture functions. UAVs currently represent a suitable alternative to observe vast, remote, and challenging access areas accurately. Several modern kinds of UAVs exist scientifically based on the accompanying features. Naturally, depending on the ultimate purpose and intentions, one must adopt an appropriate UAV [22]. Comparably by size, there are nano [16] (Figure 3a), micro [17] (Figure 3b), mini [18] (Figure 3c), small [19] (Figure 3d), medium [20] (Figure 3e), and large [21] (Figure 3f) UAVs. Commonly, due to the miniature size, less battery capacitance, and limited weightload capacity, the nano and micro UAVs cannot monitor a vast area. As a result, it is until now not the worthiest choice for precision agriculture, but it can be typically implemented for Indore farming operations. Besides, from [19,20], it can be stated that the mini and small UAVs are playing quite an impressive role in crop monitoring purposes. Nevertheless, compared to the mini and small UAVs, the medium and the large UAVs are hard to maintain, control, and pretty high-priced. However, it is possible to sufficiently occupy the medium and large UAVs in the agrarian sector by utilizing them properly. It is also conceivable to typically classify the UAVs based on their specific type of wings. Fixedwing ( Figure 4a) represents a standard type of UAV where its arms are unmovable and controlled either remotely or autonomously. Another classic type of wing-based UAV is Hybrid (VTOL) (Figure 4b). Besides, it is also known as a fixed-wing jet or alleged as a hybrid UAV or VTOL aircraft [22]. A hybrid UAV or VTOL can be operated within almost any possible situation, and it ideally has a multi-rotor design that inseparably unites four or more propellers. Fixed-wing UAVs are already implemented tremendously for field mapping and livestock motive [18,23,25]. The uncomplicated design and control basis of the Fixed-wing UAV makes it more reliable, and it is not pricey compared to the other UAV models. On the other hand, the hybrid VTOL UAVs are enormously influential for its appropriate fight techniques, which can be handy for agricultural contemplation.
Rotary-wing UAVs are organized structures of two or three rotating mechanism blades that spin around a fixed pole, likewise, recognized as a rotor. The blades of the rotor operate a similar method as a fixed wing. Nevertheless, the settings of the rotor have specific benefits and limitations. Rotary-wing UAV is also divided into two other sections. One is single-dual rotors (Figure 5a), and the other is multi-rotor. The most reliable wing-based UAV is the multi-rotor UAV. Multi-rotors are quick and agile platforms and can satisfactorily perform demanding maneuvers. It is the most significant technological rise in the last decade. There are equally numerous models of Multi-rotor UAVs i.e., Tricopter (Figure 5b  Single-dual rotors and Tricopter UAVs are not involved in modern agriculture extensively due to its trite aerodynamics and controlling perspective. Whereas the Quadcopters are popularly employed for agricultural persistence because of their excellence in aeronautics. Plus, this is the most fitting prototype for the agricultural enclosure according to the number of researches that appeared regarding it. Consequently, it is noteworthy to discern briefly regarding the Quadcopters. A Quadcopter, also recognized as a quad-rotor, correctly is a UAV with four rotors and can fly without a pilot [31]. It is permissible to construct a Quadcopter using a wireless communication system meticulously. It is the most suitable designed UAV for an agricultural perspective. In a Quadcopter, every rotor plays a vital role in the desired direction and proper balance of vehicles and lifting payloads. A Quadcopter genuinely has two unique designs; (i) plus model (Figure 6a), and (ii) cross model (Figure 6b). The Quadcopter with cross configuration is more stable than the plus configuration Quadcopter, for the equitable distribution of rotor force during hover. Two counter-rotating propellers are there for each particular side's pitch and roll axis for the cross configuration. However, plus configuration has only one on each side. Pitching ahead in this practical form establishes minor stability in the roll axis [32]. In short, the four rotors sufficiently advance a Quadcopter to a very well-built flying machine. A pair of the rotors turns clockwise (CW), and the other pair rotates counterclockwise (CCW). Furthermore, that progressively eliminates the proportional torque naturally generated by the rotating-rotors. It also assists in excluding the boom requirement, which properly accommodates the Quadcopter, enhancing stability. Hexacopter and Octocopter UAVs are also exploited in various agricultural and monitoring operations [7,9,29,42,55]. Because of traditionally having a greater number of rotors, these UAVs acquire the potential advantage to convey a desirable amount of payload. Hence, producers can efficiently utilize these specific kinds of UAVs for spraying perspective because these UAVs can transport considerably more amount of fluid or sprinkling substances. But the aerodynamics and flight control of the Hexacopter and Octocopter UAVs are immensely complicated compared to the Quadcopters.

Unmanned Aerial Vehicle charging
The most significant inconvenience of a UAV is the weight of its systems, and it allows a small amount of weight capacity for the camera, sensors, and other essential stuff [67]. All the standard UAV models assuredly have a limited flight time and endurance. An ordinary UAV can typically operate with impunity for around 10-20min [68], and it is necessary to recharge a UAV if the battery has drained beyond 10 percent of its capacity [69]. Charging indeed indicates a capacitor attached to a circuit with a DC source and these two functions. The more substantial the mAhs, the better long-drawn the UAV may rest and recharge. There, in general, are many alternative ways to power up a UAV based on the charging type shown in Figure 7. Battery charging UAVs are far more substantial than fuel-cell based UAV at lessening carbon emissions. Additionally, fuel-cell based UAVs are extremely expensive to install correctly than battery charging UAVs. Battery charging UAVs are also more convenient to recharge or refill. Presently, most UAVs are battery-powered (89%), and significantly fewer developed UAV models are Fuel-powered (11%) [46]. USB-charging realistically is the more expeditious way of charging batteries and convenient ways of direct charging. It is withal the most frequently used method to properly charge a UAV, where the UAV takes the charges directly from the power source through a flexible wire, and the UAV contains a charging port like other daily used devices. Recently, the wireless charging system is getting exoteric, and researchers are working strenuously on this theme. In [70], an effective UAV wireless charging process has been proposed, where a charging station, including some transmission coils, was utilized to convey power to charge the UAV cell. Similarly, the researchers developed two wireless charging methods (stationary and dynamic) in [71]. A wireless power transmission system is used for a VTOL UAV based on magnetic resonant coupling in [72]. And a wireless power transmission system is used for a small-sized UAV based on the effectiveness of a mechanical part of the UAV in [73], portrayed in ( Figure  8a). Whereas, in [74], by using target detection and tracking algorithm, the researchers have strongly developed an autonomous wireless charging system. In this technology, the UAV detects, tracks, and lands correctly on the charging station, shown in (Figure 8b). A solar-powered UAV was sufficiently developed using PPMS in [75] to realistically achieve UAV's continuous surveillance. Comparably, a low-cost solar-powered UAV with enhanced control efficiency is highlighted in [76], exactly shown in (Figure 8c). Actually, the sun's power is inexhaustible, clean, and free, which produces green and low-cost electricity from direct sunlight by efficiently utilizing the photovoltaic component [77]. Therefore, it's evident to have electricity from sunlight, and the high-tech is becoming pre-eminent day by day. Other charging methods are still in the experimentation process and not vividly used. In [78], the researchers properly examined and conferred an unprecedented process intimately associated with operating a small capacity and less expensive fuel-cell on a UAV portrayed in (Figure 8d). Likewise, a study about the laser beam UAV charging is presented in [79], where the laser photovoltaic element was assembled to produce the utmost energy from a particular wavelength laser light origin. The effective implementation of the UAV and choosing the proper UAV definitely depends on the producer's point of view. Long-lasting or fuel-powered UAVs are costly and comparably hard to maintain. The producers need to wisely decide which version or which type of UAV and UAV charging method is suitable for them and employ it positively for the benefit of their own. Every specific type of UAV has its own restrictions and functional limitations but overcoming them is equally possible. Researchers are still working hard on this key issue and the improvement is continuing.

Methodology
UAVs are ostensibly a heterogeneous kneading of mechanics, hardware, and software. In brief, it is a unique combination of some intricate parts, where all the separate parts work together to make the UAV susceptible, and the components can be huge and sophisticated. Several essential features of the UAV are chosen precisely based on the configuration parameters and constructed as per the possible necessities. A Model UAV is purportedly shown in (Figure 9a). Some fundamental ingredients of a UAV are: • Camera: UAV camera or imaging system is applied precisely in UAV applications to collect visual images, where certain elements may also be in motion [36]. It may consolidate modern RGB cameras, thermal cameras, IR cameras, etc. The coherence of the camera is to search and rescue methodically or to monitor. Furthermore, the camera is a fundamental tool for crop monitoring. For various operations, different kinds of cameras are required. Basically, the application of images captured with a UAV in agriculture requires a differentiated workflow, such as Photogrammetric aspects and the Estimation of variables related to agronomic aspects, for which more complex analysis is required. • Landing Gears: Landing gears are indispensable parts of the safer landing. A UAV landing gear works in tacitly supporting the ideal weight of the whole aircraft. It also assists in rolling chassis and shock consumption throughout landing or takeoff, and appreciably reduces the impact pressures conveyed to the airframe. Besides, it prevents the UAV from being thrown off the deck. It's recently getting utilized for UAV wireless charging, and the method is to recharge a UAV battery by making electrical connections on the UAV landing gear and the landing place [80]. • Motors: An efficient motor's primary responsibility is to typically rotate the propellers of multirotor UAVs to operate them to fly. UAV demands thrust to fly in the atmosphere and to make the UAV perform, robust, and small motors are required. Furthermore, the most reliable mechanism which can be applied universally is BLDC (Brushless Direct Current) motors. BLDC Motors are the potential source of flight power during the operation of UAVs [81]. The motors are accurately controlled by an ESC (Electronic Speed Controller), which is employed to drive the propellers [24]. Especially, ESC is a miniature device, and it can rightly interpret the pilot's explicit instructions, which professionally manages the motors to regulate the movements properly. • Rotors: Rotors are the principal key for sufficient momentum, thrust, and complete control of a UAV. Genuinely, Rotors enable possible modifications of its pitch angle, yaw rate and also make alterations in the motor accelerators to properly handle the roll and vertical velocity [33]. It efficiently converts motion into velocity and seamlessly allows the UAV to balance accurately. The extended surface region of the rotors naturally generates fiction and supports UAVs drifting sidewise also. Rotors have a various number of blades and twist angles for different operations. • Accelerometer and Altimeter: Accelerometers are active devices combined with gyroscopes and magnetometers that measure acceleration and implement the expedition force, in which the UAV is subjected to all three axes [82]. It is additionally a necessary sensor for a UAV because it conveniently provides crucial inputs in a stationary point and accurately determines the UAV's position and adjustment. Moreover, an altimeter is efficiently utilized for altitude measurement by calculating the specific location's air pressure [83]. The exact altitude information is vital for the navigation parameter. • Controller and Connectivity: According to the UAV's smooth lead and control, the centralized UAV state must be accessible to the controller at a reasonable bandwidth. UAVs can be remotely controlled or flown autonomously, and it wholly relies on appropriate distance, data terminal, and complex topography. The controlling system on top adequately fulfills a requisite role in the UAV sprinkling system, prominently shown in (Figure 9b). GPS and battery are two crucial portions of the connectivity orderliness [2]. UAV navigation and air traffic management are two principal responsibilities for the UAV connectivity system. Nowadays, the practical applications of UAVs are quite scattered. While choosing the right UAV model, it is invariably required to determine which responsibilities must be undertaken using the UAV and whatever is genuinely needed. Some UAVs are suitable for astonishing speed and excellent durability, and some are great for long-range flying and monitoring accuracy. In sum, every specific purpose entails a specific kind of cognitive process, which additionally includes various types of UAVs, and all perspectives should be painstakingly analyzed before flying a UAV. UAVs can be uniquely specified in three significant categories based on the agricultural surveilling purpose, and Table 2 generally highlights the different types, speeds, and loads of these kinds of UAVs [3,10,84,85].

Hardware and Software Elements
UAVs are a motley composite of specialized hardware, software, and modern techniques. Usually, depending on the startling varieties of UAVs, multiple software and hardware elements are employed to regulate the UAV appropriately. Hardware typically denotes the unified body or the structural elements. As enormously complicated technological devices, UAVs assuredly have some chief ingredients, and it can be comparatively large and sophisticated. Indeed, UAV hardware sufficiently indicates the mechanical parts of a UAV, and it takes instructions from software applications. Further, the software can be proclaimed as the brain of a UAV. Software is ostensibly designed to direct the UAVs on whether to go and whatever to do. As a possible way to conjecture and combine every essential data of the UAV, the software component positively enhances an intricate system. UAV software contains a comprehensive collection of specific programs, methods, and operations. Plus, it performs a particular responsibility, as objected to its hardware parts. The unique combination of hardware and software regulates the UAV properly. Some of the underlying hardware and software elements are respectively presented in Tables 3 and 4.  Table 4. Some commonly used UAV Software elements and applications.

Crop Monitoring
UAV based crop monitoring is a high-tech that expedites crop vegetation index through high-resolution photos for various crops and intimates the crop's condition [37]. Monitoring crops' production is essential to predict and plan for disruptions in food stocks and shifts in global crop market rates. UAV imagery is the medium for accurate crop monitoring, and Wireless communication is the system to gain data. MAVLink (Micro Air Vehicle Link) communication protocol is one of the primary mechanisms for UAV's wireless communication [95]. Capturing the diverse range of wavelengths is the basis of the entire monitoring method. There are five principal wavelength bands, which the camera of the UAV professionally captures, i.e.,
After typically collecting the complete wavelength information, the accurate data is then analyzed by remote sensing software and converted into a vegetation index map. The identification of stressed areas in crops through multispectral indices requires a series of particular works with the consequent field validation, and NDVI (Normalized Difference Vegetation Index) is one of the vastly used vegetation indices [41]. By correctly applying the standard NDVI formula, it is exceptionally unproblematic to identify the green field's possible location instantly. As a result, spraying in the designated area becomes unequivocal. Additionally, the UAVs are autonomous, and there is no need for controlling it manually. The NDVI equation is presented in Equation (1). And NDVI is also characteristically mentioned in [1,3,4,8,9,11,14,35,36,41,44,[47][48][49][50][52][53][54][59][60][61]64,81,89,93,96,97,100,101].
Here RED is Reflectance of the Red-band or Red-light. The NDVI values are perpetually in the interlude of (−1; +1). If this equation's result is near 0, it ostensibly means the area's vegetation is low. Inadequate vegetation genuinely has average NDVI values of 0.2 to 0.5, and if the outcome is close to +1, it intimates the greenish density is high. Therefore, it is straightforward for a prominent farmer to perceive his field's condition. In [53], some model vegetation indices map were manifested ideally.The article also highlighted another similar formula like NDVI, which also ranges between (−1; +1), but the desired result tends to be higher than the NDVI values. This particular formula is recognized as the GNDVI (Green NDVI) formula, shown in Equation (2). The GNDVI values are similar to the NDVI, although the procedure typically uses the reflectance of Green-light (GREEN) instead of the Red-light (RED). Plus, other wavelength ranges rather than the Red-light should be examined to strengthen the high-biomass crop aptness while the Red-light is saturated [101]. So, the GNDVI formula is an eminent option, which is also highlighted in [4,8,36,47,50,64,96] GNDVI = (NIR − GREEN)/(NIR + GREEN) (2) It is imperative to vigilantly monitor a vast farming land in a comparatively short time, and the UAV-based report must be highly detailed, accurate, and swift. Therefore, to explicitly distinguish what kind of UAVs are more fitting for the desired field is vital. Furthermore, an ideal UAV based crop monitoring system is prominently displayed in Figure 10.
From [96], we can accurately perceive the Difference Vegetation Index (DVI), which is also calculated with the Red-light (RED) to decrease the direct impact of soil reflectance. Similarly, by applying the Green-light (GREEN), the GDVI (Green DVI) can be determined. The DVI and GDVI equations are respectively presented in Equations (3) and (4).
VARI (Visible Atmospherically Resistant Index) formula can also be employed based on the NDVI and Near-infrared camera to determine the crop's health [97], shown in Equation (5). In this equation, the Blue-light (BLUE) is also quite essential.  Table 5 represents some more well-known vegetation indices related to crop monitoring [4,8,36,41,50,54,96], where NIR, red, green, and blue wavelength data can be utilized according to the conditions.

Spraying System
The accelerated extension of this high-tech can be observed clearly, and the primary rationalization UAV has the unique features of full control accuracy and fast response speed to the sprinkling system. In [51], five vital parts of a spraying UAV are well highlighted, and the principal features are;
The fluid inside the medicine box instantly conveys to the nozzles and splits into tiny droplets under pressure. The Hall-flow sensor can be efficiently utilized to estimate the fluid flow inside the system, and the flow rate data can be fed-back to the sprinkling controller.
In summary, for spraying pesticides or fertilizers, a sprayer is connected technically to that tank of the UAV. The specific substance comes out from the tank by a nozzle. The controller correctly handles the whole process. It initiates the nozzle of the sprayer. A spray motor with a pump is employed to reliably produce suitable pressure to spray the fluid [42]. In [89], a distinctive way of calculating the optimum position of spray points needed for a specific stressed area has been displayed. And in [62], the researchers proposed a way of identifying the classifiers of spraying and non-spraying fields to accomplish the autonomous UAV spraying method in real-time. A suitable method for proper and secure payload transfer in site-specific crop arrangement is precisely shown in [90]. Differ-ent sprayers UAVs have a varying number of nozzles, substances flow speed, and weight capacitance. However, the quantity of nozzles realistically is not the primary reason for a fast flow speed. The analysis of several spraying nozzles and speeds is purportedly illustrated in Table 6, highlighting the upgraded applications and the performance of different types of UAVs delicately [7,30,42,43,45,55,58,98,99].

Livestock Farming
Livestock farming organically is where somebody can progressively increase the livestock for preferred food and sell for profit. Livestock highlights animals grown remarkably in a rustic environment to contribute constructively to industry and productive assets such as meat, eggs, milk, leather, etc. Livestock farming primarily involves breeding, cultivating, and fattening. This innovative technology graciously helps industrious farmers vigilantly monitor and control their animals and the desired field. The technology can positively enhance animal health by passively allowing the producers to respond instantly before an enigma appears. Actually, the principal method of UAV based livestock farming is uncomplicated. Firstly, the field circs are efficiently captured by an appropriate UAV camera, and then the evident result gathers in the private server. Finally, the producers can vigilantly monitor everything from anywhere by reliably using his phone or computer.
Nonetheless, the technology can also be utilized to meaningfully improve the field's production ability, which could be prioritized above the animals' well-being. The livestock farming methods sufficiently demonstrate the potential to develop animal welfare by efficiently implementing a constant image of well-being situations in real-time and facilitating immediate intrusions that avail the active congregation [88]. A UAV can naturally play a massive role in the livestock farming system. As an example, [50] accurately describes that the COVID-19 pandemic has undoubtedly added intensity on food production, demand, and supply chain arrangements, primarily striking grain food producers and importer nations. A short food supply chain traditionally based on the economic output from regional farms is less receptive to move and export bans and trades as a stable way in these strains. Local UAV-based data clarifications can grant an opportunity to approach these difficulties. The assignments of the UAV are very manageable in this high tech. UAVs typically picture an aerial overview of the livestock area employing thermal imaging and high-definition cameras. In [86], the researchers exuberantly reviewed the UAV imaginary system for surveying wild animals, reflecting the vivid future of livestock farming. Thus, for this sort of monitoring purpose, the perfect UAV landing is vital, and in [65], an applicable UAV landing control technique was explained and examined for the agricultural purpose. This unique technology is still expanding in the research phase, and leading researchers are quaintly adding tons of modern features in this theme placidly. Various prolific setups based on the image processing and machine learning system for UAV livestock monitoring of the sheep is well presented in [102]. Similarly, a UAV based convolutional neural networks for recognizing and counting Sheep is resolutely demonstrated in [103], and likewise for calves in [92] (Figure 11a). Moreover in [104] (Figure 11b), a numeration network for real-time counting and localization of infrequent targets in high-resolution UAV imagery was proposed. On the other hand, in [93], the extensive survey purportedly shows that UAV based wildlife conservation and management is progressing because of the versatility in generating real-time data toward convenient decision making. What's more, there are still some limitations of UAV controlled livestock farming, such as: Personal privacy attack remains a substantial concern for the producers.

Conclusions and Future Scope
There is a tremendous potential for a gradual increase in the field of agricultural UAVs. Firstly, this paper systematically highlights the UAVs' primary classifications based on their comparable size, rotors, and wings. Then it typically represents the Quadcopter's flying configuration, which is more fitting for agricultural purposes. After that, the paper sufficiently demonstrates the UAV charging methods and the possible consequence of it. Moreover, the exposition also reflects the conventional methodology of a UAV, and the essential parts. Next, the study underlines the necessary hardware and software elements of a UAV. The substantive crop monitoring system, UAV sprinkling method, and livestock farming techniques are also manifested in the latter part. Finally, the paper draws the conclusion and future scopes of the high-tech.
UAVs are lessening the incredible amount of wastage of fertilizer and pesticides. It is resolutely keeping the laborers aside from chemical side-effects and making their job comfortable and fast. The gist is, by properly utilizing the GPS and NDVI formula, an agricultural area that can be easily monitored and can detect the prime spots to sprinkle the effective pesticides or fertilizer. The NDVI rule adequately provides reliable information on a reasonable scale. As a result, Producers will be able to accurately determine the shortage of exact volumes of water, pesticide, and fertilizer. The eternity of the UAV market for agricultural purposes is exceptional. The requirement to sustain the stability between expense and quality has undoubtedly made UAVs mainly engaging among the agricultural technology developers, and the prominent roles of the agrarian UAVs are constantly expanding without imperiling the characteristic. These UAVs are proficient in accumulating information, and nimble production as advancing the standard. However, environmental changes occur pretty swiftly and make it troublesome for the producers to monitor vast fields or livestock. Hence, a few local producers utilize UAVs with 3-D mapping ability for field analysis information and thoroughly monitor the crops and livestock, and this technology is also applying remote sensing equipment for spraying pesticides and crops. The operational costs of the agricultural UAVs are yet considerably high, and there is much room for advancement. Notably, the battery capacitance, Nozzle types, image capturing, and processing system are some points that can be progressively improved. It can also help to conserve some of the world's most severely threatened wildlife. Environmental change, territory destruction, and ruthless exploitation have led to the extinction of half of the earth's wild creatures. Researchers are currently applying the UAV thermal image acquisition system to gather accurate species location and population data required for conservation strategies. Furthermore, innovative approaches will undoubtedly benefit wild animals. Some animals outlive in the colossal game reserves, and with a UAV, it is possible to instantly reach a considerably vaster range than one can by foot. A thermal camera can appropriately carry out an extensive role because it makes it more manageable to hint out the animals. Plus, the animals are less disturbed by this modern system. It is imperative to carefully preserve these animals and make sure they do not experience any poaching effects [105].
Sooth to say, Using UAVs for precision agriculture is not only delivering agricultural benefits but also shaping the UAVs' revolution. This review will provide a brief knowledge to the new researchers about the agricultural UAVs, and it can play a positive role for future agrarian purposes. That pleasant day is not far-off when the farmers will likely sit on their UAV and roam across the farming land.

Conflicts of Interest:
The authors declare no conflict of interest.