This paper presents the RFID-based monitoring system for operators working in outdoor scenario and sharing the worksites with Remotely-Controlled Farm Mahcinery (RCFM). The system is developed within the SMARTGRID Project, "Sistema smart per la gestione della sicurezza degli operatori in ambienti di lavoro con macchine mobili operatrici comandate a distanza", funded by the Italian National Institute for Insurance against Accidents at Work (INAIL, Italy). The method to determine the radial sector of workers with respect to the RCFM or obstacles is firstly presented and the proof-of-concept is realized through an Unmanned Grounded Vehicle (UGV) equipped with commercial-off-the-shelf UHF-RFID hardware. An experimental analysis is conducted in outdoor scenario to assess preliminary performance of the method.