Kaichiang Chang and Mike Sarcione Radome Design for Minimizing focused reflection of Air-supported Radome (patent) Kaichiang Chang and Mike Sarcione

Outline Problem statement How the invention solves the problem Why you believe that the invention is new Conclusion

Background For missile defense, the SBX radar requires high system sensitivity for surveillance search for target detection and precision track for target discrimination. An air-supported radome is enclosed the SBX radar to provide environmental protection and high transmission efficiency of the radome due to its thin membrane and low seam scattering. This radome uses the prolate geometry to reduce radome size for cost reduction.

Radome Specification Shape Ideal Dimensions 60’ radius spherical top half 116’ (vertical), 60’ (horizontal at equator) radius prolate bottom half Figure 1. Radome Geometry Radome center 43 ft 4 in R=60 ft R=116 ft 103 ft 4 in 103 ft 2 in ILC Radome Envelope Hemispherical region Prolate region

Problem statement The existing radome membrane produces a -12 dB radome reflection. By geometric optics analysis, high focusing factor of around 22 dB is produced due to local curvature effect of the prolate geometry in the transition region which is sensitive to air pressure and wind loading. As a result, the total focused power of 10 dBo is reflected back to T/R modules in the centered of the array to produce permanent damage to the modules.

SBX radome with Prolate Geometry Prolate geometry is used to reduce the radome diamter from 134’ to 120’ .

Radome Surface Measurements Elevation surveys Radome surface measured with total station theodolite All data points measured at Stage 2 (6.4” H2O) and Stage 3 (11.1” H2O) radome pressure 164 measurement planes 80 points for each plane Camber gage measurement along 8 meridians in the transition region Approximately 14 ft of arc distance above and below the equator Azimuth surveys All data points measured at Stage 2 (6.4” H2O) radome pressure 6 measurement planes 364 points for each plane Elevation plane measurement (typical) Horizontal plane measurement (typical)

Elevation Plane Local Radii Results Stage 2 pressure – local radius all data Prolate radius to “base ring” radius occurs over a large region, -25° to -35° Transition region occurs over a large region, +10° to -15° El. Local radius of 60 ft.

TR Module Root Cause Root Cause Analysis: Status Date: 10/30/06 Root Cause Analysis: The failure tree reduction has been completed Backed up with failure signature analysis The root cause has been isolated to focused reflections from the radome, as a result of an improper shape of the radome Deviations from ideal shape of <~3 to 6 inches (<0.8%) in both horizontal and vertical profiles Sensitivity of reflected RF power to small deviations in local curvature not recognized during initial design phase As-built/as-is shape is within specified mechanical tolerances; not all required tolerances specified Mechanical design process focused on survivability under extreme conditions Action Plan: Radome Analysis Refining analytical tools and validating with recent platform measurements (Raytheon, Boeing, MIT/LL) Analyzing finite element model of radome shape based on mechanical properties of fabric (Boeing, Raytheon) Data Collection Limited (12% of plan) additional low-power RF validation measurements performed, correlate with predictive model Continue low-power RF measurements after vessel leaves port (Dec. 2006) Additional radome shape measurements performed in port to support corrective action development Failure Scenario: Array tests with 35% populated aperture had no central contiguous failure zone. After 65% populated array is tested (RF Safety & E3) near the XBR’s radar horizon, a large (530 TRMs) Rx failure zone is detected. Failures are limited to Rx PP port, no Tx port failures. Later diagnostics reveal that most failures occur with a single radar pulse, so duty cycle reduction is not an effective countermeasure. Two types of failure are found: vaporization (7.2 dBo) and fuzed wire bonds (5.3 dBo) based on forensics and on UOES SW limiters (from GBR-P analysis).

How the invention solves the problem By adding a foam/skin double layer on the existing radome membrane, the foam is spaced to produce 180 degree phase reversal between reflections from outer skin of the existing membrane and inner skin of the added membrane. A perfect cancellation between these reflections is obtained to reduce the reflection to be less than -25 dB over a 10% frequency bandwidth. As a result, focused radome reflected power is reduced to be -3 dBo which provides a good margin for T/R module protection from radome reflection.

Electrical performance Summary of New Radome design Performance improvement : - Reduce reflected power by more than 14 dB - Improve 2-way transmission loss by 0.6 dB RF Performance as function of design parameters: - Wide band frequency response - Superior performance for incidence angles of less than 20 degrees - Manufacturing tolerance : +- 10 mils suggested - Good performance in rain with 15 mm/hr rain rate ( lesser rain water below the equator in prolated geometry expected) Implementation Location : transition region in which local radii is around 90 feet

Radome return loss comparison vs frequency band

Radome return loss for parallel polarization

Radome return loss for perpendicular polarization

Core thickness tolerance of +-5 mils

Core thickness tolerance of +-10 mils

Core thickness tolerance of +-20 mils

Radome return loss comparison in rain

Radome return loss vs core thickness

Why you believe that the invention is new One of prior arts uses an additional layer for radome shaping to defocus radome reflection which is unfortunately not effective. Use of larger spherical air-supported radome is very expensive. Use of metal space frame radome produces high loss and high scattering level which can not meet SBX radar antenna performance requirements. By adding a foam/skin double layer to the existing air-supported radome membrane, this new radome design not only keeps the advantage of air-supported radome for environmental protection and SBX radar performance compliance but also reduces radome reflection without any T/R module damage.

Conclusion Sandwich radome design identified as the most cost-effective design by adding a foam/skin layer to the existing radome membrane. Superior electrical performance of Sandwich radome design obtained as function of wideband, incidence angle and mechanical tolerance. Sandwich radome can be used to mitigate local curvature effect due to manufacture tolerance, various stress conditions in wind or rain and relaxation of CONOPS limitation for the current radome. This new radome design can be applied for other radar systems in stress conditions.