A Networked Radar Approach For Resolving Range/Velocity Ambiguity

A NETWORKED RADAR  APPROACH FOR RESOLVING RANGE /VELOCITY AMBIGUITY

Nitin Bharadwaj^* and V. Chandrasekar

Colorado State University, Fort Collins, Colorado

* Email: nitin@engr.colostate.edu

 

Doppler radar transmitting pulses with uniform pulse repetition frequency (PRF) have a fundamental limitation on maximum unambiguous range (r_a) and maximum unambiguous velocity (v_a) determined by the pulse repetition time and the wavelength. There is always a conflicting trade off between r_a and v_a as their product is fixed for a given wavelength. This trade off is more stringent for X-band radars due to the shorter wavelength. The first generation CASA (The Center for Collaborative Adaptive Sensing of the Atmosphere, an engineering research center established by the National Science Foundation)  radars are low cost X-band magnetron radars. Hence, there is a hardware limitation to implement time and phase coded waveforms, which are the existing or proposed waveforms to mitigate range ambiguity on a single radar.  In this paper a new approach to mitigate range ambiguity is presented. The new approach is a network based technique where spatially distributed monostatic radar are used to mitigate range ambiguities. In this paper the range ambiguity problem is formulated for a networked radar environment by using the principle that the underlying intrinsic properties of the medium such as reflectivity must remain consistent in a networked environment. The ambiguity in range is resolved by jointly processing the measurements from all the individual radars. Magnetron radars have random transmit phase and hence the waveform is naturally random phase coded. The transmit pulse phase is measured on a pulse-to-pulse basis. Random phase coding of transmit pulses at each node is used to identify the presence of stronger second trip echoes. The measured reflectivity at a given resolution volume will be contaminated by spatially independent second trip echoes in each radar. The problem is formulated to jointly detect the presence or absence of overlaid echoes in each radar node. The presence or absence of overlaid echoes is used to estimate the composite reflectivity at a given resolution volume.  A simulation study is carried out to evaluate the performance of the network based range ambiguity mitigation technique. In addition to the simulation study the networked based  approach is evaluated  with  measurements from the CASA testbed. The CASA testbed consist of a network of four X-band radars deployed in Oklahoma.