The global positioning system (GPS) is the primary enabling technology of the communication, navigation, surveillance and air traffic management (CNS/ATM) systems that are used to support heavy air traffic. One of the key components of the CNS/ATM system is the automatic dependent surveillance broadcast (ADS-B) data link, and this provides an aircraft's position derived from GPS at 1090 MHz. However, radio frequency interference (RFI) or unpredictable factors related to the ground environment might degrade or interrupt GPS service. In order to maintain the normal operations of aviation surveillance and navigation during GPS outages, the alternative positioning, navigation and timing (APNT) concept is proposed. One of our APNT approaches is wide area multilateration (WAM) using on air ADS-B signals. For a WAM system using a pseudo range measurement of 1090 MHz ADS-B signals, the positioning performance depends on the quality of pseudo range measurements, the geometric distribution of the ground stations, and the positioning algorithms. In earlier works, we presented the post-processing results for an ADS-B WAM test bed, as well as different time synchronization designs, in order to gain improvements in ranging performance. Additionally, we showed the positioning performance of different positioning algorithms and the effects of the geometric distribution of the ground stations on positioning performance. Additional ranging source of the universal access transceiver (UAT) signal at 978 MHz was used to evaluate the resulting WAM positioning performance in our previous works. The conclusions suggest: 1) that a more effective time synchronization scheme is needed, and 2) the separation distance between the ground WAM stations should be greater than 50 nm. In this paper, a GPS disciplined oscillator (GPSDO) is integrated into our universal software radio peripheral (USRP) based ADS-B receiver. The GPSDO has been shown in our earlier works to have adequate performance for 20~30 minutes to bridge short periods of GPS outage. We will evaluate the synchronization error of our ADS-B WAM test bed with GPSDO for an extended period of time in this work, and the effects of this synchronization error on ranging performance are investigated in this paper. Additional signal processing or filtering techniques are studied in this work to maintain the timing stability of GPSDO for an extended period of time (without GPS). Some external and internal error sources of our current ADS-B WAM test bed, such as signal conditioning and data loss, are also studied to gain improvements in ranging performance. Importantly, the convergence of the positioning method should be investigated, and the hybrid method of using iterative and non-iterative positioning algorithms is presented in this work to enhance the availability of the position solution of our WAM test bed. Finally, the results of experiment using actual 1090 MHz ADS-B signals and the improved WAM test bed are also presented in this paper.