_Article_  
**Converting a Fixed-Wing Internal Combustion Engine RPAS into an Electric Lithium-Ion Battery-Driven RPAS**  
**[PERSON] 1,***2, [PERSON] 1, [PERSON] 1, [PERSON] 2 and [PERSON] 2**  
1 Instituto Nacional de Tecnica Aeroespacial (INTA), Area de Energia. Ctra, Huelva-Matalascanas, km. 34, 21130 Mazagon, Huelva, Spain; [EMAIL_ADDRESS] (A.F.B.); [EMAIL_ADDRESS] (J.A.B.)  
2 Departamento de Ingenieria Electronica, Universidad de Huelva, de Sistemas Informaticos y Automatica, 21007 Huelva, Spain; [EMAIL_ADDRESS] (F.S.); [EMAIL_ADDRESS] (J.M.A.)  
* Correspondence: [EMAIL_ADDRESS]; Tel.: +34-959208847 check for updates  
Received: 8 January 2020; Accepted: 20 February 2020; Published: 25 February 2020  
**Featured Application: The main application was to give a second life to an obsolete and declassified type of RPAS (remotely piloted aircraft system) for use in monitoring key parameters, in the cultivation of the olive tree and in other types of cultivation that occupy large areas of land. Similarly, it can be used to track and monitor endangered animal species. INTA (National Institute for Aerospace Applications) is located on the grounds of the Donana National Park, where the Iberian lynx lives, one of the most threatened fellies in the world, taking advantage of some of the main advantages of this RPA (remotely piloted aircraft) like autonomy, efficiency, low-noise level, and non-polluting.**  
**Abstract:** It is well proved that remotely piloted aircraft systems (RPASs) are very useful systems for remote sensing in precision agricultural labors. INTA (National Institute for Aerospace Applications) and the University of Huelva are involved in Tecnolivo Project that proposes the development of a marketable and easy-to-use technological solution that allows integrated, ecological, and optimized management of the olive grove through non-invasive monitoring of key agronomic parameters using RPASs. The information collected by the RPAS in regards to the state of the vegetation, such as hydric stress levels, plague detection, or maturation of the fruit, are very interesting for farmers when it comes to make decisions about their crops. Current RPAS applications for precision agriculture are mainly developed for small- to medium-sized crops using rotary-wing RPASs with small range and endurance operation, leaving aside large-sized crops. This work shows the conversion of a fully declassified and obsolete fixed-wing internal combustion engine (ICE) remotely piloted aircraft (RPA), used as aerial target for military applications and in reconnaissance and surveillance missions at low cost, into an electric lithium polymer (LiPo) battery-driven RPA that will be used for precision agriculture in large-sized crop applications, as well as other applications for tracking and monitoring of endangered animal species in national parks. This RPA, being over twenty years old, has undergone a deep change. The applied methodology consisted of the design of a new propulsion system, based on an electric motor and batteries, maintaining the main airworthiness characteristics of the aircraft. Some other novelties achieved in this study were: (1) Change to a more efficient engine, less heavy and bulky, with a greater ratio of torque vs. size. Modernization of the fly control system and geolocation system. (2) Modification of the type and material of the propeller, reaching a higher performance. (3) Replacement of a polluting fuel, such as gasoline, with electricity from renewable sources. (4) Development of a new control software, etc. Preliminary results indicate that the endurance achieved with the new energy and propulsion systems and the payload weight available in the RPA meet the expectations of the use of this type of RPAS in the study of large areas of crops and surveillance.  
**Keywords:** remotely piloted aircraft (RPA); remotely piloted aircraft system (RPAS); unmanned aerial system (UAS); unmanned aerial vehicle (UAV); battery; precision agriculture; crops; remote sensing