Dual Polarization Implementation Approaches for Weather Radar

Introduction

              Radar systems in the nation's civilian network of radars for weather surveillance operated by the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service (NWS) were completed nearly 25 years ago, while many systems used by the Federal Aviation Agency (FAA) for aircraft surveillance were developed 45 or more years ago.  The radar surveillance networks that are used for the surveillance of air traffic and meteorological phenomena nationally are aging and costly to maintain.  These aging radar systems could benefit greatly from the advances in performance, capabilities, and scalability that are offered by modern phased array radar technologies. 

The scalability and multi-mission functionality of modern phased array radar systems offers the opportunity to replace several variants of aging legacy radars with a common phased array based radar system that is scalable to the mission needs of the installation location.  A national implementation of approximately 350 MPAR radars could replace the existing NWS and FAA radars, offering enhanced capabilities from a common scalable implementation supporting multi-mission functionality. This abstract summarizes points from a study commissioned by the FAA into certain engineering analyses and cost trades for a dual polarization implementation strategy for a new Multifunction Phased Array Radar (MPAR) that might address weather and air surveillance needs. 

              The candidate systems whose capabilities are to be replaced or consolidated include the ASR-11, TDWR (Terminal Doppler Weather Radar), and dual-polarized WSR-88D Next Generation Radar (NEXRAD) radars, among others. Particular attention was paid to comparing and contrasting various dual-pol implementation approaches, including Simultaneous, Alternating, and Simultaneous-with-Waveform-Diversity (SWD) modes. With each implementation, three major classes of array geometry—a single rotating face, a four faced truncated pyramid, and a cylindrical commutating array—are also studied, leading to a comprehensive matrix of dual-pol approaches and geometry options. In compliance with the contractual specifications, the study presents a scalable system that ranges from a basic configuration intended to perform the core air surveillance mission of the ASR-11 with the TDWR weather observation mission, up to a full MPAR system that can also perform the precision weather observations of the dual-polarized WSR-88D.

Benefits of Polarimetric Measurements

           The benefits of polarimetric measurements have long been known to the weather observation community, and the currently deployed national network of WSR-88D radar systems operated by NOAA and the National Weather Service has recently been upgraded to perform polarimetric measurements utilizing horizontally (H) and vertically (V) polarized signals. These systems collect co-polar echoes HH and VV, from which a quantity known as differential reflectivity Z_dr is formed. Z_dr can be used to detect hydrometeors, to provide some classification of their type (rain, snow, hail, etc.), and to determine rates of rainfall. Any new system must provide, at the least, the same polarimetric capabilities as the existing WSR-88D systems.

           Additional polarimetric measurements include the cross-polar echoes HV and VH, which are not presently collected by the WSR-88D. These measurements, which are used to form a quantity called linear depolarization ratio (LDR), can increase the detection sensitivity to various forms of frozen and partially frozen precipitation, and to more accurately determine whether they are snow, hail, sleet and graupel (soft hail). The ability to make co-polar measurements is likely to be important in future weather observation and weather forecasting radars. In research studies, the measurement of LDR has also indicated the presence of supercooled water, which is a factor in the icing of aircraft wings. Co-polar measurements may therefore have potential value to the air surveillance community, as well as to the weather community. For these reasons, it may be favorable to use array hardware and polarimetric approaches that can collect the full matrix of polarimetric data (HH, VV, HV and VH).

 

Dual Polarization Benefits and Approaches

           The WSR-88D performs polarimetry by passively splitting the transmit power between the H and V feeds to the dish antenna, producing a linearly polarized wave that is, nominally, 45° slant polarized. It then collects co-polar information through dual channels that independently receive and process H and V echo returns. Of the three polarimetric approaches, this Simultaneous mode is the most cost effective for a system with high peak power; Alternating mode would require a megawatt-class switch to toggle the transmit power between H and V, while the SWD mode would require two parallel and well-matched megawatt-class transmitters. The coupling of H and V upon transmit, however, imposes rigorous purity requirements on receive performance (45 dB of cross-polarization isolation or XPI) that are tolerable on a dish but that would significantly drive up the manufacturing and test costs of an active electronically scanned phased array (AESA).

           This problem is circumvented by noting that the cost calculus for a solid-state phased array antenna is quite different from that of a dish in other, offsetting, respects. For a phased array, it is straightforward to implement Alternating mode by putting a polarization selection switch at each element, since they operate at relatively low power (typically 10's to 100 watts). Doubling up power amplifier chips for SWD mode to transmit a different waveform on H and V ports is also straightforward on a phased array. XPI requirements for these modes are in the 20-30 dB range, which can be achieved even during electronic scan without the need for rigorous and expensive manufacturing and test processes. These modes have the further advantage of measuring the full polarimetric matrix, including the cross-polar information needed to derive the LDR parameter.

           The notional system proposed in the present study consists of a 4x4m “core” array of 6,400 elements that can perform air traffic and basic weather surveillance (including wind shear). A simple geometric restacking produces a 2x8m ASR-variant of same range, sensitivity and performance, but that mimics the beam shape of existing ASR radar systems. Adding thinned array panels to the 4x4 core array completes the full 10x10m MPAR array that provides complete functionality for air surveillance and precision weather observations.

Note: The opinions documented in this paper are those of the authors, and not necessarily those of the FAA.