Distributed Airborne RF Teams (DART)

The potential performance benefits of Distributed Coherent Radar (DCR) systems have been well-known for more than two decades. Foremost, signal-to-noise ratio (SNR) gains approaching N2 are possible for N independent apertures employing Cohere-On-Receive (COR) techniques. COR systems benefit from Multi-Input-Multi-Output (MIMO) orthogonal waveforms and matched filter detection. When Cohere-On- Transmit (COT) is employed, SNR gains approaching N3 may be achieved. SNR gain translates directly to increased target detection range, or conversely, detection of smaller targets at the same range. Airborne and satellite DCR systems offer game-changing new capabilities that will revolutionize persistent surveillance, covert communications, and foster a new generation of electronic warfare (EW) techniques. At the system level, significant challenges exist in precisely measuring and maintaining constant awareness of key system parameters. These include the relative positions, angular orientations, RF propagation time delays, RF waveform phases, and independent motions of all N members in the team. System challenges also include limited data lifetimes of the spatial, temporal, and phase Coherence Parameters (CP) that are critical to achieving COR and COT SNR gains. At the hardware level, on-board instrumentation that measures system parameters are affected by noise and systemic errors. IMUs use accelerometers to measure linear motion in a Cartesian frame. IMUs integrate measured acceleration to get velocity, and integrate again to get position. Accelerometer errors compound rapidly during integration. IMUs use gyroscopes to measure angular velocities in the yaw, pitch, and roll planes. All IMUs and clocks exhibit offsets from true zero, as well as skew as a function of time, and random walk noise called drift. These greatly impact the lifetime of measured relative positions, orientations, delays, phases, and CPs in airborne DCR systems.  A very specialized suite of tightly integrated algorithms is needed to perform a wide range of DCR functionality, ranging from basic hardware monitoring to highly sophisticated system level techniques. Algorithms operating at the hardware level must measure and track offset and skew errors in the IMU and clock on its own aircraft. System level algorithms must control the tempo and scheduling of system parameter measurements, and monitor the validity of measured data as a function of time. System level algorithms use the data to detect relative differences in the system parameters across the team, and must compensate for the differences when computing CPs to achieve COR and COT. When system parameter errors grow large enough to impact coherence, new measurements must be performed, and the corresponding clock and IMU errors characterized and recorded, until the next measurement cycle. Algorithms at the system level also must perform all distributed coherent RF sensing, communication, and EW modes.