Advanced dEsign and testing of a polArimetric X-banD antenna for avionic weather radar

Final Report Summary - AHEAD (Advanced dEsign and testing of a polArimetric X-banD antenna for avionic weather radar)

Executive Summary:

Modern weather radars on-board commercial aircraft first arrived around in the 1950’s, with the intent to detect and avoid storms along the flight path. Since then, technology and performance have progressed tremendously. With the integration of wind-shear and turbulence detection, aircraft radars have grown into complex systems. At the same time, these systems have become extremely common.

Most of today’s airborne weather radars operate in the X-band, whereas ground-based weather systems usually operate in the S-band or C-band. In the realm of ground radars, dual-polarization technology is being introduced on these S-band and X-band systems, following extensive work by the NOAA and others. In 2013, all US-based NEXRAD ground weather radars were equipped with dual-polarization.

Dual-polarized weather radars offer clear advantages with respect to accuracy and versatility. With the merging of data from the two polarizations, the radar system will be able to establish differential reflectivity, correlation coefficients, linear depolarization and specific differential phase. These lead to better performance in detecting tornadoes, better rainfall prediction, precipitation types, icing detection and 3D-cloud build-up. In turn, these benefits will allow improved trajectory management.

However, airborne radar systems face much more stringent requirement and boundary conditions than their ground-based counterparts. This is particularly the case for the antenna. Further, dual-polarization has been demonstrated for other applications, but not for a nose-mounted set-up. The nose-mounted set-up for dual-polarized weather radar poses form-fit requirement that are extremely tight. Also, weight, power and cost concerns pose a very significant economic issue particularly for the aviation environment.

The antenna is one of the most critical components in the realization of avionic polarimetric weather radars for a number of reasons:

a) Stringent performance required in terms of polarization (co/cross coupling; polarization purity of each channel), while maintaining usually applied requirements in terms of beam width and side-lobe level; also considering the effects that the antenna radome can have on e.m. field polarization while changing the steering of the antenna beam

b) Weight and size

c) Mechanical/electrical interface to aircraft

d) Compatibility with other on-board electrical/electronic devices

e) Environmental constraints

While points (b-e) are more or less common also to currently applied non-polarimetric radars, point (a) is peculiar of polarimetric antennas

The AhEAD project aims at designing, breadboarding and validating a dual polarized antenna suited for polarimetric weather radar to be installed on aircraft.

The proposed antenna solution is the result of an articulate study and design process based on the following main activities

• Definition of Requirement
• General requirements of weather radar and also specific of polarimetric antennas were considered
• Review of state of art of polarimetric antenna technologies and assessment of candidate technologies
• Selection of two possible candidates and preliminary analysis
• Detailed design of the most promising solution
• Prototyping, measurement and validation
• Antenna evaluation and roadmap for full device development

Project Context and Objectives:

The AhEAD project aimed at developing an antenna to demonstrate the feasibility of a polarimetric antenna with the required electrical performances to be effectively applied in a polarimetric weather radar system for avionics

Several antenna technologies have been considered as potential candidates, mature and consolidated such as Slotted Wave guide and printed antennas as well more innovative solutions SIW and connected arrays.

A printed antenna was finally selected as baseline for a number of suitable features for the commercial aircraft field of application:

1. Low mass
2. Low cost
3. High flexibility and modularity
4. manufacturing process well-evolved
5. very large range of high-quality substrates to design high performance antenna
6. high degree of system integration
7. quite accurate translation from CAD model to the actual hardware up to Ka-band
8. Good electric performances

Slotted wave guide-based solutions represent surely a valid alternative, but were finally discarded principally because many patents exist specifically mentioning weather polarimetric radar.

The antenna design was mainly driven by the very demanding requirement in terms of electrical performances, particularly referring to the dual polarization feature.

In the following the main driving requirements have been summarized for the target antenna size of 28” that has been considered in the project:

1. Band: 9300 – 9500 MHz
2. RL<-17.7dB
3. SLL
4. HPBW< 3.7°
5. Power 300Watt
6. Losses 2dB
7. XPD 35dB
8. Port isolation -35dB

As regarding printed technology, one of the most critical item, was the ohmic loss. For this reason, a series fed u-strip beam forming network was applied.

Project Results:

The design and realization of a polArimetric X-band printed antenna is a very challenging task. In particular the dual polarization feature makes the design more complex than a single polarization antenna and at the same time requires more stringent and demanding electrical performances to effectively take advantage of a dual polarization radar unit. Also, the loss item, especially referring to printed technology is a very demanding one. The designed antenna is a technical demonstrator of the feasibility of such antenna concept and of the obtainable electrical performances.

Finally, due to operating frequency and antenna dimension (28 inch) also the electromagnetic modelling through numerical commercial e.m. tool is a very difficult and demanding aspect of the project.

The proposed antenna design seems very promising in terms of SWAP – C (“Size Weight and Power – Cost”) performances, due to the printed circuit technology selected

Electrical performance seems promising too, nevertheless further activities of tuning and optimization of the design are necessary

Radiation masks (HPBW and SLL) at band edges should be improved

The main design parameters and antenna physics have been identified, that are the basis for upgrading and improving the antenna.

Items for improvement have been identified and preliminary evaluated, particularly referring to:

a) Material selection

b) Feeding strategies

Potential Impact:

Polarimetric weather radar, especially from non-stationary platforms such as aircraft and ships, may prove to be an important additional performance factor in atmospheric phenomena that are important to the determination of best trajectory from the user economical perspective and thus for ‘cleanest’ operation. In that sense, AhEAD implementation will be very important, for the following reasons:

• Involving the results in the respective aerospace and maritime standardization committees, thus accelerating the process to minimum performance definition and contributing directly to the state of art in terms of future equipment performance.
• The individual partners in AhEAD operate in a network of specific users and technically associated forums. Rockwell Collins, for example, provides weather threat detection system to a very large number of aircraft users. Thus, advancement in the state of the art such as for example the addition of polarization technique, will potentially see introduction into similar numbers of operation possibly and dependent on the final technology roadmap, by way of upgrades. The company marketing network will be the “communication channel” for such innovations, basically expanding the reach of the project to those same numbers.
• The proliferation of technological understanding of polarimetric radar operation will increase awareness of this activity and its achievement, thus precluding the need for other similar and concurrent activities and thereby saving cost at the macro level. In addition, such dissemination will send a message relative to the CleanSky framework in general, so that indirectly a positive contribution to the overall initiative is achieved.
• A step forward in Polarimetric radar design may have a significant impact on the volcanic ash detection problem.

List of Websites:

http://www.ahead-cleansky.eu/