The global positioning system (GPS) is the primary enabling technology for the modern communication, navigation, surveillance, and air traffic management (CNS/ATM) systems. However, many studies show that GPS services may be degraded or interrupted by radio frequency interference (RFI). In order to maintain normal positioning, navigation and timing services during GPS outages, the alternative positioning, navigation, and timing (APNT) system is proposed. In our previous studies, we developed a wide area multilateration (WAM) test-bed using the existing ADS-B data link at 1090 MHz to perform the differential time of arrival (DTOA) positioning algorithms. We found that the positioning performance was adequate when an aircraft is near the ground WAM stations, but insufficient when the aircraft is a certain distance away from the ground WAM stations network. The main reason is that the geometric distribution of ground WAM stations is not sufficient when the aircraft is far away. In order to enhance the positioning performance of the WAM system, we proposed in our previous paper using the relative bearing angle measurement between the aircraft and ADS-B ground station, and the relative bearing angle algorithm can be considered as an independent positioning opportunity to the DTOA algorithm. Our preliminary simulation results indicated about a 30% improvement in the dilution of precision (DOP) if one could combine the DTOA measurement and the relative bearing angle measurement for positioning. In this paper we investigate techniques to obtain the actual angle of arrival (AOA) measurement from the existing ADS-B data link at 1090 MHz. Our approach is to use USRPs based ADS-B software receivers to measure the phase difference of the incoming ADS-B signal at the each receiver. The phase difference could be utilized to compute the corresponding AOA measurement. The first task is to identify multiple aircraft from the received ADS-B signals at the same 1090 MHz in order to associate the phase difference measurements with the corresponding aircraft. Therefore, our USRP-based ADS- B receiver first decodes ADS-B messages to identify the aircraft ICAO number and records the I/Q data at the same time. A coherent phase detection process is required to successfully estimate the phase difference between two receivers, and the GPS disciplined oscillator (GPSDO) is applied in the experiment. One issue that also needs to be resolved is the initial phase offset between two synchronized receivers, and the resolution techniques are also discussed in this paper. With the phase difference observations of a receiver pair, a transformation computation is needed to convert the phase difference to the AOA estimation. We are interested in the horizontal positioning, as the one-dimensional AOA measurement is projected onto a horizontal plane. In the last part of the paper, there is a simulation of the 2-D multiple signal classification (MUSIC) algorithm to demonstrate the feasibility of this method.