With the growing availability of civil Unmanned Aerial Vehicles (UAVs), a lot of diversified ways of using such systems appear everyday. No matter the use, positioning and navigation must be reliable for an effective use, positioning and navigation must be reliable for an effective use. Global Navigation Satellite System (GNSS) have proved to be accurate enough for most uses. But with the high number of GNSS applications and the diversity of environments of use, GNSS users are often exposed to potentially dangerous interferences such a spoofing. This paper studies the effects of spoofing attacks on the clock of a GPS receiver. After a short background study on interference detection and spoofing, four scenarios of spoofing attacks have been tested. The experiments were carried out on a ublox EVK-6T and a ublox EVK-M8T with the use of a Spirent GSS6560 and e Labsat 2, two GPS simulators.