**5. Conclusions and Future Research Outlook**

This review covered the state-of-the-art thermal UAV RS technology and, to the best of our knowledge, is the first to deal with this topic. We also outlined an overview of the latest applications of thermal UAV RS in the framework of PA. Starting from a synthesis of the fundamental principles of thermography, necessary for the less experienced who approach this scientific field, this was followed by a brief description of the features of thermal cameras, from the operation of the sensor that allows the conversion of measurements into images to a hint about the cameras' cost, briefly covering the topics of field data acquisition, calibration, and data processing.

As far as the application of thermal UAV RS in agriculture is concerned, a literature review made it possible to verify the presence of numerous works devoted to the subject. To the best

of our knowledge, most of the applications of thermal UAV RS have concerned the detection of crops water stress and the management of irrigation resources—both being crucial aspects for crop development and agriculture [12,83,151]. The other applications, in a smaller number, have dealt with the identification of symptoms that are caused by pathologies, phenotyping, monitoring, yield estimation, and the identification of drainage networks in the fields.

Certainly, the use of UAV thermal sensors is not as widespread as that of other sensors, such as optical and multispectral sensors, and this is probably due to the characteristics of thermal sensors and the type of data that are derived from them. Although the first aspect is important, an important limitation of thermal cameras is their geometric resolution, which is low when compared to, for example, RGB sensors. The second concerns the data that are derived from the sensor, i.e., the temperature, which has proved to be fundamental in the detection of water stress in plants, given the natural mechanisms that regulate the temperature of plants. Excluding, perhaps, this type of application, which sees thermal sensors as protagonists and advantageous over other sensors, especially for the possibility of detecting water stress connections in advance, other types of applications that have provided the exclusive use of thermal cameras are not many, especially when considering the field of plant pathology. The real potential of cameras on UAVs can be exploited at high levels and with maximum profit in terms of utility, focusing on the feature that makes UAVs unique: that of being able to simultaneously mount and use multiple sensors [152]. The thermal camera used side-by-side with RGB and multispectral sensors can increase the importance of UAVs in PA and expand their possibilities in terms of use. In particular, when considering, for example, plant pathology, in order to improve the ability to detect diseases or parasitic attacks at an early stage, the use of different sensors, including thermal sensors, and the fusion/combination of their derived data with optical and multispectral sensors, should be considered [153]. The possibility to perform surveys with a centimeter resolution, coupling different sensors at any time, and with more affordable costs, puts UAVs ahead of aircraft platforms. When compared to satellites, on the other hand, the introduction of platforms, including nanosatellites, equipped with sensors, capable of offering high-resolution images of less than 3 m or the ultra-high resolution of less than 1 m makes satellites increasingly competitive with regard to drones in PA applications [152,154–157]. It would be interesting to combine thermal (and optical) satellite data with UAV data, together with data that were collected on the ground, as shown, for example, in [109]. As things stand, when considering the different platforms and sensors of RS, no one is probably able to offer a high resolution in all spatial, spectral, and temporal dimensions [153]. Therefore, it would be desirable to synergize UAV images with high-resolution satellite images to improve the quality of the final products, including thermal RS, in order to overcome these limits.

Naturally, new developments are also expected in the framework of thermal RS. Likewise, it is also expected that the current trend of increasing user-friendliness for all types of users will continue in the technological development of UAVs and sensors. Greater automation, where possible, of aspects that are related to both the field data acquisition phase (preparation of the optimal flight plan, configuration, and calibration of the sensors before and during flight) and the data processing phase (together with the reduction of the time that is needed for data processing) is necessary as the next steps to implement the use of UAVs in agriculture. As far as ground data acquisition is concerned, as explained in the previous paragraphs, it remains an essential step for the moment. Specifically for thermal RS, a simpler and more easy and immediate combination of the data that were collected by weather stations with data derived from UAVs would be useful. Indeed, there are still important practical difficulties in the correct collection of data, when considering the mitigation of atmospheric effects, calibration, climatic conditions, and the complex interactions between soil and plants [7]—particularly true in the case of thermal RS, whose raw data before the processing steps are far from offering true and accurate temperature measurements. In this respect, thermal RS requires accurate knowledge of thermography [152] in all its application phases, from the preparation of the surveys to the final product. If this aspect, on the one hand, does not fail to stimulate the world

of research to explore all aspects of thermal RS, then it might, on the other hand, constitute a limit for use outside this field.

However, it is fair to say that, given the important progress in the use of RS sensors in agriculture, in the short term, new solutions should also be able to simplify and expand the use of thermal RS in agriculture and PA, increasing its integration in decision making [7]. PA needs high-intensity procedures for the use of acquired images and it requires the presence of experienced and qualified personnel [126], which results in higher costs for companies. Therefore, the use of advanced technologies, including the use of UAVs, remains confined to those farmers with large agricultural areas available [158]. This aspect is more evident in the case of thermal UAV surveys, when considering that their operational costs per hectare are higher than those of multispectral surveys [159].

**Author Contributions:** Conceptualization, methodology, investigation, data curation, writing—review and editing, G.M. (Gaetano Messina) and G.M. (Giuseppe Modica). All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors are grateful to DR-One S.r.l. (http://www.dr-onesrl.com—Belmonte Calabro, Cosenza, Italy) for providing UAV thermal surveys. The authors would like to express their great appreciation to the three anonymous reviewers for their very constructive comments provided during the revision of the paper, and that contributes to improving it significantly.

**Conflicts of Interest:** The authors declare no conflicts of interest.
