Evaluation of positioning algorithms for wide area multilateration based alternative positioning navigation and timing (APNT) using 1090 MHz ADS-B signals

GNSS is the major technology that supports the operation of the modern air traffic management (ATM) system. Also, the positioning performance of GNSS is enhanced by many augmentation systems by providing corrections to the error sources as well as the integrity messages on the post-correction error residuals. However, unpredictable factors on the ground user environment, such as the radio frequency interference (RFI) could severely degrade or interrupt the service of GNSS receivers. In order to maintain the normal operation of the ATM system during GNSS outages, the alternative position, navigation and timing (APNT) service is essential. One of the possible approaches to provide the APNT service is the use of existing ADS-B data link at 1090 MHz to perform wide area multilateration (WAM). In consideration to use the existing ADS-B data link for APNT, the current ground based transceiver (GBT) network that is originally deployed for surveillance purpose could be used as the WAM station to measure the signal time-of-flight to estimate the time of arrival (TOA). The attractive part of this 1090 MHz ADS-B WAM approach is that there infrastructure to support reception of the signal is already or being installed. However, these stations generally do not guarantee precise (~ 50 nanoseconds) or GNSS interference robust time synchronization. Hence a challenging issue to meeting the navigation and surveillance requirements of APNT is developing an appropriate time synchronization scheme for the ground stations. In ION GNSS+ 2013 conference, we presented the post processing results of ADS-B WAM test beds as well as the time synchronization designs in order to get adequate ranging performance. In this paper, we will first investigate the positioning algorithms for this approach, and the positioning algorithms under consideration are 1) the linear iterative least squares method, 2) the non-iterative quadratic equation solution method, and 3) a hybrid of the prior 2 methods. Also, the station geometry effects on the positioning algorithms (i.e., dilution of precision (DOP)) will be evaluated. In order to keep the time for extended period when GPS is outage, the filter design to model the clock will then be studied. That is, with the model of the clocks’ characteristics we would be able to predict the clock behavior to continue the normal operation of our ADS-B WAM test bed.