With the Cooperative Mobile Positioning (COMET) project we have started investi- gations in connection with the fourth gen- eration (4G) with the purposes of design- ing an "augmentation" solution that could overcome the issues regarding GPS and GPS-free technologies in order to o®er lo- calization to anyone, anytime and any- where. The idea behind the COMET orig- inates from the fact that more likely re- liable RSS measurements detected among neighboring mobiles can enhance the lo- calization accuracy also in environments where usual terrestrial-based localization techniques o®er a bad estimation. For these purposes we have implemented an hybrid Mobile WiMAX / WLAN sce- nario where a data fusion for the posi- tion estimate is performed by the Extended Kalman Filter. Results obtained from the static and mobile case show that cooper- ation can enhance the location accuracy, o®ering more than 26 % of gain.