GNSS civil jammers are small portable sets capable of broadcasting disruptive signals in the GNSS bands. In this research, the effects of different civil jammers on GPS receivers are investigated and some techniques for detection, characterization and mitigation are proposed. Firstly, the presence of jammer signals is investigated. The detection techniques are categorized into power content analysis, power spectral density techniques and normality analysis. After detecting the jammer signal, there is a need to characterize the hostile signal parameters. Herein, different methods are considered which are classified into linear, bilinear and adaptive time-frequency representation methods. Following this, by using the estimated features of the interference signal, a clean version of the received signal is extracted. The output of this section is used as an input to the receiver. The performances of the proposed methods are assessed with different metrics including Carrier-to-Noise spectral density (C/N0) and frequency and phase lock indicator.