Applying a Composite Pattern Scheme to Clutter Cancellation

Applying a Composite Pattern Scheme to Clutter Cancellation with the Airborne POLARIS Ice Sounder Keith Morrison 1, John Bennett 2, Rolf Scheiber 3 k. morrison@cranfield. ac. uk 1 Department of Informatics & Systems Engineering Cranfield University, Shrivenham, UK. 2 Private Consultant, UK. 3 Microwaves and Radar Institute German Aerospace Research Center , Wessling, Germany.

COLLABORATORS Matteo Nannini - DLR Pau Prats - DLR Michelangelo Villano - DLR Hugh Corr - BAS ESA-ESTEC Contract: 104671/11/NL/CT Nico Gebert Chung-Chi Lin Florence Heliere

PRESENTATION • Problem • Composite Pattern - convolution - array polynomial • Application • Results

PROBLEM POLARIS R H dhr R air z ice bedrock

ANTENNA ARRAY Geometric alignment and dimensions of the 4 independent receive apertures of the POLARIS antenna

ICEGRAV 2011 Test-ID Bandwidth p 110219_m 155222_jsew 1 [MHz] 85 & 30 MHz p 110219_m 155222_jswe 1 85 & 30 MHz p 110219_m 155222_jsns 1 85 & 30 MHz p 110219_m 180339_jsns 2 85 & 6 MHz Remarks From east: grounded ice, then crossing the glacier tongue, frozen grounded ice in the middle, ice shelf in the west cross-track slopes with grounded ice profile along the glacier tongue

PROCESSING SCHEME

Rx Pattern Rx Array & Element Pattern

COMPOSITE ARRAY • Phased-array nulling traditionally optimizes performance by utilising available array elements to steer a single null in the required direction. • However, here we exploit the principle of pattern multiplication. • With different element excitations, nulls in differing angular directions are generated. • Composite array is produced by the convolution of two sub-arrays. • The angular response of the composite array is the product of those generated by the individual sub-arrays.

CONVOLUTION The building block is the 2 -element array. To generate a null at angle θA the excitation of the array is required to be: 1. 0 Similarly to generate a null at angle θB the excitation of the array is required to be: 1. 0

3 -ELEMENT : 2 -Null To preserve these nulls we must generate the product of the two patterns and this is achieved by convolving the two distributions to give the following 3 -element distribution: 1 a+b where: a = -exp[jkdsinϴA] b = -exp[jkdsinϴB] ab

4 -ELEMENT : 2 -Null 1 a where: a = -exp[jkdsinϴA] b = -exp[jkdsinϴB] b 0=-exp[jk 2 dsinϴB] b 0 ab 0

4 -ELEMENT : 3 -Null To generate a third null at angle, θC, requires convolution of the result from the 3 -element, 2 -null case with an additional 2 -element array, with the distribution: 1. 0

SIMULATION Wavelength Range bandwidth Noise power Chirp duration Clutter attenuation Range sampling frequency H=3000 m 0 m 150 m 300 m 500 m 700 m 1000 m

COMPOSITE – 3 3 NULL Double-null with 2° separation centred at left-hand clutter angle

COMPOSITE – 2 2 NULL

ARRAY POLYNOMIAL • This is done using Schelkunoff scheme. • Array excitation represented by the array polynomial and its representation as zeros on the unit circle. • Computationally straightforward because there are only three zeros for the four element array. θ Array factor: If substitute (where αd is a linear phase term to account for beam steering) then

Factorizing For the 4 -element case factorizes to (z-a) (z-b) (z-c) Providing coefficients 1, -(a+b+c), (ab+ac+bc), (-abc)

ARRAY POLY. – ALT. 3 -NULL • • Ensured maximum amplitude contribution from this zero in the nadir direction. Remaining two zeros were used to position the pair of nulls at the clutter angles.

NADIR RESPONSE

FINAL RECOMMEDATION a+b+1 1 a + b + 1 ab + (a+b) ab

RESULTS

COUNTERACTING NADIR NULLS

CONCLUSIONS • Considered two and three-nulling scenarios using convolution. • “Best Result” obtained from a modified 3 -null approach: » Array Polynomial: third null located 180° on the unit circle. • Nadir null can be avoided by allowing points to move off the unit circle.