2. Current Insights
This research has shown that different automatization algorithms, including DIP, DL and classification techniques, have been employed for automating different tasks of the aIRT procedure for inspecting PV power plants. Among the conclusions, this research showed that only a few among the selected studies have assessed two important aspects of the autonomous inspection procedure, namely, the optimization of the flight path (nine papers), and the detection of soiling (eight papers). These two topics are of great importance to increase time efficiency in aIRT and therefore should be further investigated. The latter goal of detecting soiling over PV modules and differentiating it from actual faults of the modules was investigated by some authors, e.g., Dunderdale et al. 
and Arosh et al. 
, together with the detection and classification of other faults.
For the task of performing the orthomosaicking of aIRT images to facilitate the localization of the faults in the field, four papers employed existing software to perform the task, while ten studies approached the development of algorithms to create the orthomosaic of the PV plant. However, most of the proposed methods are based on DIP techniques; therefore, the resulting mosaic consists of a simple image, without additional GPS information. The correlation of orthomosaic images with GPS coordinates and the identification of modules and strings according to the site nomenclature are areas that require further investigation.
Another approach to the challenge was developed by Wu et al. 
, with the development of an algorithm that detects PV arrays in power plants and performs the automatic correlation with their string identifiers. This is a promising strategy that could also be used to facilitate the localization of detected faults in the field through aIRT. Besides the study carried out by Wu et al. 
, another 20 studies among the selected literature focused on the development of algorithms to detect PV systems and panels in aerial imagery. However, only three of these studies focused on aerial IRT images of the PV plants, obtaining up to 93.16% precision in the results 
. On the one hand, 18 papers presented the results of developed algorithms for the detection of individual PV modules in aIRT images, of which three of them applied DL techniques. Although the methods are hardly comparable given their different structures for results (i.e., mask, box or line), their different dataset sizes and the different evaluation metrics used, a method that combined many algorithms (DIP, SVM and DL) for detecting PV modules in aIRT images and obtained an F1 score of 98.4% can be highlighted 
. On the other hand, the worst metrics were obtained with simple DIP filters 
, which although providing fast results with small datasets required for training, are characterized by a lack in generalization. This is important for the replication of the algorithm in images acquired in different conditions and with a different quality. The algorithms proposed by Carletti et al. 
, Xie et al. 
and Bommes et al. 
also performed the tracking of the modules in subsequent frames of an aIRT video. This task is of utter importance for the cross-correlation of detected modules and faults, as well as their location in PV plants.
Most of the selected studies have assessed autonomous fault detection and classification in PV plants through visual (12 papers), IRT (22 papers) and aIRT images (43 papers). Among these studies, 35% used DL techniques for the detection or classification of PV faults, with an increase in developed algorithms using CNNs in recent years. Still, DIP-based algorithms also presented high accuracy results, even though most of them use smaller datasets, and therefore their replication in other sets of data is possibly not feasible. The combination of DL or DIP techniques with classifier algorithms was a promising approach in recent studies. In the field, fault detection can either be processed on board, during the UAV flight, or subsequently through a post-processing procedure after the flight. For the first case, the high computational requirements and the processing time of DL are still a challenge, as even in high-performing computers, the processing of a set of images of a large-scale PV power plant (that consists of some gigabytes of data) can take hours when using a DL algorithm. In the same way as in the detection of PV systems and modules, many types of outputs for the algorithms are possible, namely the segmentation of the faults, the detection of damaged modules or even the classification of faults in separate classes. The classes also differ among authors, and these differences represent a great challenge not only for the comparison between studies, but also for the exchange of data, experiences and algorithms among researchers in PV community, which hinders the advancements in this area. The exchange of data to enable the development of larger and more generalized datasets that consider different environmental conditions is also deaccelerated by data protection clauses.
Besides the different result types, the different evaluation metrics (or the lack of them), dataset sizes and image resolutions of the inputs also make the comparison between studies difficult. However, in general, the algorithms with the highest metrics are the ones dedicated to detecting and classifying a few types of faults compared to those that carry out the classification of many classes of faults. This proves that detection and classification of multiple faults is a complex task and further investigation is required. On this subject, the algorithm developed by Bommes et al. 
can be highlighted for its encouraging results, with an accuracy of 90% in the detection and classification of faults in ten different anomaly types. In summary, to achieve the goal of an entirely autonomous aIRT procedure, advances in some of the tasks related to the technique must be achieved. Even tasks that were already the focus of many research studies, such as the detection and classification of faults, should be further explored to contemplate different types of datasets and conditions. The exchange of data and academic collaborations are fundamental to allow for a fully automatic procedure that not only detects modules and faults on PV modules but also provides information about the type and location of the faults, in a simple and accessible manner, to enable quick remediation measures.