Jamming is the act of intentionally directing a disturbing electromagnetic wave towards a communication system in order to disrupt or prevent signal reception. Jamming is becoming a serious threat for several services including Global Navigation Satellite System (GNSS) where it is used to prevent the computation of the user position. This paper describes the joint efforts of the European Commission (EC) Joint Research Centre (JRC) and of the Faculty of Maritime Studies and Transport of the University of Ljubljana to experimentally evaluate the GNSS jamming threat. In particular several experiments have been conducted in order to build a library of scenarios for the evaluation of jamming detection and mitigation techniques. Data containing jamming signals have been collected in the JRC anechoic chamber and different approaches have been compared for the detection of jamming signals. The analysis shows a good coherence among the different detection metrics considered.