In our previous research into an alternative navigation system, we examined the development of a wide area multilateration (WAM) system based on the automatic dependent surveillance-broadcast (ADS-B) as one of the primary backup sources to mitigate the effects of global navigation satellite system (GNSS) outages. The ADS-B WAM is a cost-effective solution because ADS-B radio is the main aviation surveillance system, and both the aircraft and ground infrastructure to support ADS-B being installed now. Therefore, a multilateration algorithm based on the differential time of arrival (DTOA) estimations of the 1090 MHz ADS-B Mode S ES signal from several ground stations was both developed and implemented in earlier research. Specifically, we presented the post-processing results of the ADS-B WAM test bed as well as different time synchronization designs in order to gain improvements in ranging performance. Additionally, we analyzed the effects of different positioning algorithms as well as the geometric distribution of the ground stations on positioning performance. We validated that the DTOA passive positioning performance could meet the aviation surveillance requirements (i.e., 95% position accuracy < 92.6 m) when the aircraft is within the vicinity of the WAM ground stations. However, the positioning performance of the developed ADS-B DTOA based WAM system suffers if the geometry between the aircraft and the WAM ground stations is insufficient. The geometry between aircraft and the WAM ground stations can be evaluated by the dilution of precision (DOP) calculation, with a low DOP value representing the good geometry for positioning. The positioning accuracy will decrease rapidly when the aircraft is away from the vicinity of WAM ground stations. For island countries, the distribution of WAM ground stations is limited to the available land. In order to resolve this limitation, we propose using the altitude information reported in an aircraft's ADS-B data to enhance the developed ADS-B DTOA based WAM system's positioning accuracy and availability. The benefits of the use of aircraft reported altitude to the DTOA positioning algorithm are twofold: 1) one could reduce the number of WAM ground stations required to provide surveillance service, and 2) one could simultaneously apply more than three DTOA measurements to the DTOA positioning algorithm. Therefore, this paper first determines the proper altitude model that could be incorporated into the DTOA positioning algorithm in the Earth-Centered Earth-Fixed (ECEF) coordinate system, and then the effects of altitude error between the mean sea-level (MSL) and ellipsoid on the DTOA positioning algorithm are analyzed. A new improved DTOA positioning algorithm with altitude aiding is proposed, and the proposed algorithm is to generate an error-free altitude observation model to accurately assess the positioning performance. Finally, the positioning accuracy and availability improvement of the enhanced ADS-B DTOA based WAM with altitude aiding information for island countries are also evaluated in this.