A dual channel system ^{1 2}, which is based on the GPS and the GSM Network, is being developed to compensate the problem of the lost of GPS signals in providing location services to mobile users in urban areas. In this design, when GPS signals are being blocked in blind spot areas, GSM positioning algorithms would be used as an alterative method to provide location estimations. This research is an investigation in search of a set of location estimation algorithms based on signal attenuation to work with GPS, so as to develop a dual channel positioning system. With the technical support from a local mobile operator, we have constructed and conducted several real world experiments for our investigation and results are promising.