A Low-cost Mechanically-Steered Phased-Array Polarimetric Doppler Weather RADar (WRAD)


Joint project between
Fraunhofer FHR and University of Bonn

Fraunhofer FHR: Stefano Turso (PI)
University of Bonn: Silke Trömel (PI) and Clemens Simmer (PI)

Abstract
Dense networks of inexpensive short-range weather radars have great potential to yield a fundamental advancement in the understanding of weather phenomena and achieve a timely reaction to extreme events. Due to the Earth curvature, Figure 1a, about 70% of the troposphere below 1 km cannot be observed by radar means [1]. Consequently, traditional long-range weather radars (up to about 200 Km range) are unable to provide coverage of the lower part of the atmosphere where most of the human interaction with weather phenomena takes place.

Composition of short-range weather maps, Figure 1b, appears therefore a viable solution to improve sensing of the lower troposphere, enhance spatial resolution and achieve a revisit time shorter than one minute.

fig_1_abstract_clean.jpg

Figure 1: a) Earth curvature effect. b) Short-range weather maps composition concept.


Indeed, bi-axial mechanically steered weather radars usually require up to five minutes to complete a typical volume coverage pattern while many weather events evolve within tens of seconds and intense events change dynamics and structure within few minutes [2].

Phased array radars (PARs) are based on Active Electronically Scanned Array (AESA) antenna apertures allowing for instantaneous beam steering. A PAR weather radar is optimally suited to improve temporal resolution and reach the goal of servicing up to one volumetric weather map per minute. Extensive research has shown that faster updates yield increased warning lead time, improve forecast accuracy and enhance probability of detection [3].

With the goal of servicing weather maps with higher spatial and temporal resolution, the Fraunhofer Institute for High Frequency Physics and Radar Techniques (FHR) in partnership with the Institute for Geosciences, Department of Meteorology of the University Bonn (UBonn) is designing and developing a novel Phased-Array Radar (PAR) within the framework of PROM. The system will be deployed in range with the twin X-band Doppler radars at the JOYCE Core Facility and tested for data quality assessment.



References

  • “Atmospheric Sciences: Entering the Twenty-First Century,” p. 181, National Academy Press, 2101 Constitution Avenue, NW Washington, DC 20418 USA, 364 pp. 1998, ISBN 0-309-06415-5.
  • Bluestein, H. B., W.-C. Lee, M. Bell, C. C. Weiss, and A. L. Pazmany, 2003: Mobile Doppler radar observations of a tornado in a supercell near Bassett, Nebraska, on 5 June 1999. Part II: Tornado-vortex structure. Mon. Wea. Rev., 131, 2968–2984.° Carbone, R. E., M. J. Carpenter, and C. D. Burghart, 1985: Doppler radar sampling limitations in convective storms. J. Atmos. Oceanic Technol., 2, 357–361.
  • Wilson, K. A., P. L. Heinselman, C. M. Kuster, D. M. Kingfield, and Z. Kang, 2017: Forecaster performance and workload: Does radar update time matter? Wea. Forecasting, 32, 253–274, doi:10.1175/WAF-D-16-0157.1.


Curren status (summer 2021)

Reflector-based weather antenna concept Sustainable deployment of a dense network of weather radars strictly requires low unitary and ownership costs. To this end, electronic scan is limited to a single dimension (along the zenith angle) while a rotor provides mechanical steering in azimuth, as shown in Figure 1.

fig_1_2021.jpg

Figure 1: Typical antenna assembly for hybrid electronic steering (elevation) and mechanical rotation (azimuth).



While a simple mechanical rotation can provide full azimuth coverage, at least electronic beam steering in elevation is necessary to achieve timely volumetric sounding of the troposphere. This allows to overcome the major limitation of current operational polarimetric weather radars, which typically require up to five minutes to mechanically steer a large dish in two dimensions and provide complete sensing of the hemisphere. Technological breakthroughs like the availability of highly integrated MMICs promise to lower fabrication costs to a point of competition with mechanically steered weather radars. Distributed power generation removes the need of a costly power amplifier and rotary joint, reduces losses on transmit and grants graceful degradation. The entire back-end circuitry, including the first stage raw data processing and reduction, can rotate together with the antenna (as per the receiver over-elevation approach) to improve the receiver noise figure. Data reduction can be performed on-board before transmission of datasets across the slip-ring data lines [4]. Allowing for non-inertial beam steering, phased array radars (PARs) are optimal candidates to improve temporal resolution and reach the goal of servicing up to one volumetric weather map per minute. However, large apertures are physically required to generate narrow beams, which for active antennas translates into higher complexity and cost with respect to bi-axial mechanically steered solutions. To overcome this limitation, Fraunhofer FHR has envisioned a unique antenna concept (patent pending) to minimize complexity and cost with the ultimate goal of enabling mainstream use of AESA apertures within cost-sensible applications. Instead of directly using an AESA aperture to achieve both beam steering and gain generation, a simple parabolic cylinder is illuminated by an active feed for electronic steering in elevation while azimuthal focusing and beam symmetrization are obtained via the reflecting surface. This solution effectively decouples beam steering and gain generation, thus yielding minimization of complexity and cost [2]. Upon reflection, beam symmetrization and gain enhancement are observed. Most importantly, near conservation of XPD feed performance are predicted at broadside, with acceptable degradation at 45° steering angle.

fig_2_2021_clean.jpg

Figure 2: a) Radiation pattern, surface current density and (u,v) XPD plots. Broadside illumination, simulations. b) Radiation pattern, surface current density and (u,v) XPD plots. 45° beam steering, simulations.


Advanced AESA functions like dynamic tapering constitute prospective development to enhance beam and sidelobes symmetry at greater steering angles.

Outlook
Development of a suitable low-cost AESA front-end is a crucial step towards sustainable deployment of short-range netted weather radars able to generate composite maps with high space and time resolution, and finally improve monitoring of the lower troposphere. Initial results from the weather radar initiative at Fraunhofer FHR in the framework of PROM are profiled, showing a potential for novel concepts and designs to meet strict polarimetric requirements and generate valid weather radar observations within cost-constrains.

References

  • Turso, S., Bertuch, T., Stanko, S., Knott, P., Trömel S. and Simmer, C., 2018, April. A Low-Cost Mechanically-Steered Weather Radar Concept. In Radar Conference (RadarConf), 2018 IEEE.
  • Turso, S., Salzburg, C. G., Vizcarro, M. and Bertuch T., 2020, A Novel Antenna Concept for Weather Applications Based on a Cylindrical Parabolic Reflector, 2020 IEEE Radar Conference (RadarConf20), Florence, Italy, pp. 1-4, doi: 10.1109/RadarConf2043947.2020.9266385.

First year summary


Tile sub-array with enhanced cross-polarization discrimination
Tile sub-array with enhanced cross-polarization discrimination A new generation of RF chipsets featuring high level of functional integration allows for reduction of cost, emission levels, complexity and development risk at X-band. However, matching the same data quality of bi-axial mechanical solutions is mostly independent from the technological implementation and strictly requires unbiased estimation of polarimetric moments for consistent classification of hydrometeors over the entire scanning volume [6]. Consequently, one of the most stringent requirements specific to weather sensing is the achievement of a cross-polarization discrimination (XPD) in excess of 30 dB [7]. Investigation on methods to improve the polarization purity have been conducted to ensure feasibility within cost constraints. Specifically, an optimized design of a tile sub-array based on specular antenna excitations has been pursued and finely tuned to reach the required polarization performance [8]. Such scalable tile, Figure 1, can be further assembled to function as the sub-array of an electronically-steerable aperture.

fig_1_2020.jpg

Figure 1: Antenna feed tile sub-array.



Instrumental validation essentially supports theoretical expectations concerning polarimetric performance, also for beam directions off-broadside. Three-dimensional radiation patterns and XPD plots in (u,v) coordinates, Figures 2, show good agreement in between simulations and measurements. In particular, XPD levels better than 40 dB are reached at broadside. At 45° steering angle about 30 dB XPD are still observed.

fig_2_2020_clean.jpg

Figure 2: a) Tile sub-array radiation diagram and (u,v) XPD plots, simulations. b) Tile sub-array radiation diagram and (u,v) XPD plots, measurements.

References

  • D. S. Zrnic and R. J. Doviak, “System Requirements for Phased Array Weather Radar”
  • G. Zhang, R. J. Doviak, D. S. Zrnić, R. Palmer, L. Lei, and Y. Al-Rashid, “Polarimetric Phased-Array Radar for Weather Measurement: A Planar or Cylindrical Configuration?,” in J. Atmos. Oceanic Technol., 28, pp. 63–73, 2011, doi: 10.1175/2010JTECHA1470.1.
  • M. Vizcarro, S. Turso, C. G. Salzburg, and T. Bertuch, “A Dual-polarized X-band Patch Antenna Sub-array with Low Cross-polarization for Weather Radar Applications,” in 2019 20th International Radar Symposium (IRS), Ulm, Germany, Jun. 2019, pp. 1–6, doi: 10.23919/IRS.2019.8768175.




















A Low-cost Mechanically-Steered Phased-Array Polarimetric Doppler Weather Radar


Project based at the
Fraunhofer-Institut für Hochfrequenzphysik und Radartechnik FHR

Fraunhofer-Institut für Hochfrequenzphysik und Radartechnik FHR: Stefano Turso (PI)

Project Goals

Dense networks of inexpensive short-range weather radars might yield a fundamental advancement for the understanding of weather phenomena and the timely reaction to extreme events. Due to the Earth curvature, Figure 1a, about 70% of the troposphere below 1 km cannot be observed by radar means. Consequently, traditional long range weather radars (up to about 200 Km range) are unable to provide coverage of the lower part of the atmosphere where most of the interaction with weather phenomena actually takes place.

Composition of short-range weather maps, Figure 1b, appears therefore a viable solution to improve sensing of the lower troposphere, enhance spatial resolution and achieve a revisit time shorter than one minute [1].













Fig. 1a. Earth curvature effect Fig. 1b. Short-range weather maps composition concept.


The Fraunhofer Institute for High Frequency Physics and Radar Techniques (FHR) in partnership with the Institute for Geosciences and Meteorology at the University of Bonn is designing and developing a novel Phased-Array Radar (PAR) to be operated within the framework of PROM. The system will be deployed close to the twin X-band Doppler radars at the JOYCE Core Facility and extensively tested for data quality assessments. Sustainable deployment of a dense network of weather radars strictly requires low unitary and ownership costs. To this end, electronic steering is limited to a single dimension (to scan the zenith angle) while a rotor provides mechanical steering in azimuth, as shown in Figure 2.


Fig. 2a. Flat panel concept, front.


Fig. 2b. Flat panel concept, back.


While a simple mechanical rotation can provide full azimuth coverage, at least electronic beam steering in elevation is necessary to achieve timely volumetric sounding of the troposphere. This allows to overcome the major limitation of current operational polarimetric weather radars, which typically require up to five minutes to mechanically steer a large dish in two dimensions and provide complete sensing of the hemisphere.
Technological advancements like the availability of highly integrated MMICs promise to lower fabrication costs to a point of competition with mechanically scanned weather radars. Distributed power generation removes the need of a costly power amplifier and rotary joint, reduces losses on transmit and grants graceful degradation. The entire back-end circuitry, including the first stage raw data processing and reduction, can be located right behind the panel (following the receiver over-elevation approach) to improve the receiver noise figure. Pre-processed datasets can be transmitted to a server via radio-link for further processing.

• Current Status, Polarimetric Antenna

A new generation of RF chipsets featuring high level of functional integration allows for reduction of cost, emission levels, complexity and development risk at X-band. However, matching the same data quality of bi-axial mechanical solutions is mostly independent from the technological implementation and strictly requires the unbiased estimation of polarimetric moments for consistent classification of hydrometeors over the entire scanning volume [2]. Consequently, one of the most stringent requirements specific to weather radars is the achievement of a cross-polarization discrimination (XPD) in excess of 30 dB [3]. Investigations on methods to improve the polarization purity have been carried on to ensure feasibility within cost constraints. As an outcome, an antenna tile has been engineered in the form of a scalable sub-array, Figure 3, and finely tuned to reach the required polarization purity.




Fig. 3. Antenna sub-array.


Ongoing validation mostly supports theoretical expectations concerning the polarimetric performance, also for beam directions off-broadside. Three-dimensional radiation patterns and XPD plots in (u,v) coordinates, Figures 4, show good agreement in between simulations and measurements, with XPD levels above 30 dB at a beam steering angle of 45°.


Fig. 4a. Row module radiation diagram and (u,v) XPD plots, simulations. Fig. 4b. Row module radiation diagram and (u,v) XPD plots, measurements.


Development of a suitable low-cost AESA front-end is a crucial step towards sustainable deployment of short-range netted weather radars able to generate composite maps with high space and time resolution, and finally improve monitoring of the lower troposphere.
Early results from the weather radar project at Fraunhofer FHR are summarized, showing a potential for novel concepts and designs to meet strict polarimetric requirements and generate valid weather radar observations within cost-constrained solutions.


References
[1] “Atmospheric Sciences: Entering the Twenty-First Century,” p. 181, National Academy Press, 2101 Constitution Avenue, NW Washington, DC 20418 USA, 364 pp. 1998, ISBN 0-309- 06415-5.

[2] D. S. Zrnic and R. J. Doviak, “System Requirements for Phased Array Weather Radar”

[3] G. Zhang, R. J. Doviak, D. S. Zrnić, R. Palmer, L. Lei, and Y. Al-Rashid, “Polarimetric PhasedArray Radar for Weather Measurement: A Planar or Cylindrical Configuration?,” in J. Atmos. Oceanic Technol., 28, pp. 63–73, 2011, doi: 10.1175/2010JTECHA1470.1.