Airborne Polarimetric Doppler Weather Radar: Possible Beam-Forming Architectures

The Airborne Phased Array Radar (APAR) with dual-polarimetric and dual-Doppler capability enables concurrent estimates of microphysical properties (e.g., precipitation types and sizes, quantitative precipitation estimation) and 3-D winds in precipitation (Vivekanandan et. al. 2014). At present, no other airborne instrument has the potential to estimate 3-D winds and microphysics concurrently. The APAR will capitalize on Active Electronic Scanning Array (AESA) technology to incorporate beam multiplexing (BMX) mode of operation which is capable of collecting the desired number of samples, allowing more accurate radar measurements, in less time than a continuous sampling mode (Vivekanandan et al., 2018).

In the case of AESA with dual-polarimetric configuration, cross-coupling between horizontally and vertically polarized signals biases polarimetric radar estimates. This effect is widely known and is mitigated in APAR by limiting the scan range close to the principal planes when acquiring dual-polarized data. Additional flexibility can be obtained by independently weighting the phase and amplitude of AESA elements on transmit and receive to realizing an adaptive beam. The adaptive beam feature could be used for suppressing unwanted interference and minimizing surface clutter on weak weather echo. One of the undesired features of the AESA is that the gain and beamwidth of the antenna changes as the beam is steered away from broadside. Radar calibration procedures must take into account these variations. Beamforming architectures can have a significant effect on the accuracy and stability of the calibration when applied under actual operating conditions. This is especially true of an airborne radar where vibration and pressure and temperature variation can be extreme.

Phased array radar beamformers could be distinctly configured in three types of architecture: (i) analog, (ii) hybrid, i.e., a combination of analog and digital, and (iii) digital. In an analog array RF phase shifters and attenuators are used for steering and shaping the beam (Herd and Convey, 2016). The performance of RF components is sensitive to temperature, and their precision is limited by the quantization or number of bits that are used to represent phase and attenuation. Beamforming is achieved by summing signals from individual receive elements by an analog combiner. The front-end analog combiner fixes antenna beamwidth and sidelobes, and they cannot be modified.

Conversely, in a fully digital array, RF phase shifters and attenuators are replaced by complex multiplication using digital electronics. As digital electronics based complex multiplication is relatively immune to quantization effects, precise control of phase and amplitude is realized. This also enables element level digital pre-distortion (DPD) of the transmit waveform to be performed, thereby improving both antenna and range-time sidelobes. From an overall architecture perspective, element level digitization of T/R module versus digital sub-array has to be carefully considered with respect to flexibility in adaptive beamforming, polarimetric performance, calibration, and imaging of rapidly moving weather system. In addition power consumption, cooling and weight of a digital architecture must be considered, especially for an airborne system. The digital aspect of the architecture reinforces scalability and allows the arrays to be developed using commercial off-the-shelf (COTS) components. This paper describes possible beamforming architectures of the APAR taking into consideration data rates at various radar subsystem interfaces.

REFERENCES:

Herd, J. S, and M.D. Convey, 2016: The evolution of modern phased array architecture. Proc. of the IEEE, Vol. 104, No.3, 519-529.

Vivekanandan, J., Lee, W.-C., Loew, E., Salazar, J. L., Grubišić, V., Moore, J., and Tsai, P.: The next generation airborne polarimetric Doppler weather radar, Geosci. Instrum. Method. Data Syst., 3, 111-126, https://doi.org/10.5194/gi-3-111-2014, 2014.

Vivekanandan, J. and Loew, E.: Airborne polarimetric Doppler weather radar: trade-offs between various engineering specifications, Geosci. Instrum. Method. Data Syst., 7, 21- 37, https://doi.org/10.5194/gi-7-21-2018, 2018.