Global navigation satellite system (GNSS) is essential to communication, navigation, surveillance, and air traffic management. This continuous accurate positioning service allows a GNSS-supported surveillance system to keep up with the growing requirements of civil aviation. However, many studies have shown that GPS signals are susceptible to interference. To continue accurate positioning service in the case of GNSS failure, various alternative position, navigation, and timing (APNT) systems have been proposed. The objective of APNT systems is to provide positioning service to ensure flight safety when GNSS fails. Our previous paper developed a wide area multilateration (WAM)-based APNT testbed that used only commercial off-the-shelf (COTS) components, USRP and GPSDO, for a 1090-MHz automatic dependent surveillance-broadcast (ADS-B) ground station. Another previous study investigated the differential time of arrival (DTOA) positioning algorithms for this WAM-based APNT testbed. The results indicated that the iterative positioning algorithm suffered from divergence and that the non-iterative positioning algorithm gave multiple possible correct solutions. A positioning method was thus proposed to give more position fixes and more accurate fixes. The objectives of this paper are 1) to continue the evaluation of the DTOA positioning algorithms and 2) to seek an additional signal of opportunity for this WAM-based APNT testbed. This paper first determines the conditions or parameter settings that most impact the performance of DTOA positioning algorithms in terms of the position fix availability and accuracy. In addition to improving DTOA positioning algorithms, this paper also searches for another signal of opportunity that is 1) independent of DTOA measurements and 2) uses the existing WAM-based APNT testbed. The Doppler measurement of the 1090-MHz ADS-B signal is thus of interest. Doppler measurement can provide the relative bearing angles between the target aircraft and an ADS-B ground station. The position of the target aircraft can be determined if there are sufficient bearing observations. Also, this bearing observation is considered as independent information to the range observation, which is beneficial for positioning performance if both observations are used in a complementary way. Accordingly, this paper provides improved positioning results based on the DTOA observation alone with improved ground station distribution with the previously proposed algorithm. The assessment of the Doppler measurement of the actual 1090-MHz ADS-B signal acquired from the developed WAM-based APNT testbed is presented. The method of acquiring bearing observations from the Doppler measurement is analyzed in detail. Finally, the benefit of combined positioning method that utilizes both relative bearing and differential range observations is analyzed by comparing dilution of precision (DOP) value with the original DTOA algorithm.