  A new method for joint ranging and Phase Offset (PO) estimation of multiple
drones/aircrafts is proposed in this paper. The proposed method employs the
superimposed uncoordinated Automatic Dependent Surveillance Broadcast (ADS-B)
packets broadcasted by drones/aircrafts for joint range and PO estimation. It
jointly estimates range and PO prior to ADS-B packet decoding; thus, it can
improve air safety when packet decoding is infeasible due to packet collision.
Moreover, it enables coherent detection of ADS-B packets, which can result in
more reliable multiple target tracking in aviation systems using cooperative
sensors for detect and avoid (DAA). By minimizing the Kullback Leibler
Divergence (KLD) statistical distance measure, we show that the received
complex baseband signal coming from K uncoordinated drones corrupted by
Additive White Gaussian Noise (AWGN) at a single antenna receiver can be
approximated by an independent and identically distributed Gaussian Mixture
(GM) with 2 power K mixture components in the two dimensional (2D) plane. While
direct joint Maximum Likelihood Estimation (MLE) of range and PO from the
derived GM Probability Density Function (PDF) leads to an intractable
maximization, our proposed method employs the Expectation Maximization (EM)
algorithm to estimate the modes of the 2D Gaussian mixture followed by a
reordering estimation technique through combinatorial optimization to estimate
range and PO. An extension to a multiple antenna receiver is also investigated
in this paper. While the proposed estimator can estimate the range of multiple
drones with a single receive antenna, a larger number of drones can be
supported with higher accuracy by the use of multiple antennas at the receiver.
The effectiveness of the proposed estimator is supported by simulation results.
We show that the proposed estimator can jointly estimate the range of three
drones accurately.
