Multilateration in the absence of GPS and reference transmitter synchronization

Surface and air multilateration (MLAT) systems currently deployed in the National Airspace System (NAS) employ either Global Positioning System (GPS) or reference transmitter (RefTran) line-of sight communications to maintain radio synchronization. In systems where GPS is the primary source of timing, fallback synchronization is maintained through either RefTran processing, or for isolated radios, through specially-deployed high-precision clocks that maintain time independently for a specified period. In these systems, system-wide position accuracy can be maintained only for as long as the high-precision clocks can maintain the specified synchronization, typically a few days. The new über MLAT algorithm presented in this paper builds on traditional maximum likelihood MLAT processing in such a way that ground stations receiving RF squitters are synchronized through previous squitters from other targets or previous squitters from the same target. Signals received at ground stations from a particular subject aircraft (target A) squitter are grouped into clusters using the timestamp applied by the central processing station. Each cluster is then paired with a cluster from an earlier squitter, say squitter B. Provided A and B clusters have at least five ground station radios in common, target position estimates can be formed for both squitters using the proposed new algorithm. Position accuracy performance prediction through geometric dilution of precision (GDOP) is derived via the Cramer-Rao Lower Bound (CRLB) and is presented along with simulations that verify the GDOP accuracy calculations. In addition, processing actual Surveillance and Broadcast Services System (SBSS) target data illustrates that not only is the predicted accuracy achieved with no a priori radio bias removal (tuning), the number of available clusters to pair with a subject cluster is typically quite large during peak en route traffic periods, particularly for high altitude aircraft. Applications of this algorithm include (1) continuation of MLAT processing in the absence of GPS timing and (2) ground station synchronization in the absence of GPS.