Applying a Composite Pattern Scheme to Clutter Cancellation with the Airborne POLARIS Ice Sounder

1. Applying a Composite Pattern Scheme to Clutter Cancellation with the Airborne POLARIS Ice Sounder Keith Morrison1, John Bennett2, Rolf Scheiber3 k.morrison@cranfield.ac.uk 1Department of Informatics & Systems Engineering Cranfield University, Shrivenham, UK. 2Private Consultant, UK. 3Microwaves and Radar Institute German Aerospace Research Center, Wessling, Germany.

2. Collaborators MatteoNannini - DLR Pau Prats - DLR Michelangelo Villano - DLR Hugh Corr - BAS • ESA-ESTEC Contract: 104671/11/NL/CT • NicoGebert • Chung-Chi Lin • Florence Heliere

3. Presentation • Problem • Composite Pattern • - convolution • - array polynomial • Application • Results

4. Problem POLARIS R R H dhr air z ice bedrock

5. Antenna Array Geometric alignment and dimensions of the 4 independent receive apertures of the POLARIS antenna

6. IceGrav 2011

7. Processing Scheme

8. Rx Pattern Rx Array & Element Pattern

9. 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.

10. Convolution • The building block is the 2-element array. • To generate a null at angle θAthe excitation of the array is required to be: Similarly to generate a null at angle θBthe excitation of the array is required to be:

11. 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-elementdistribution: where: a = -exp[jkdsinϴA] b = -exp[jkdsinϴB]

12. 4-Element : 2-Null where: a = -exp[jkdsinϴA] b = -exp[jkdsinϴB] b0=-exp[jk2dsinϴB]

13. 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:

14. Simulation 0m 150m H=3000m 300m 500m 700m 1000m

15. Composite – 3 null Double-null with 2° separation centred at left-hand clutter angle

16. Composite – 2 null

17. 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

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

19. 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.

21. Nadir Response

22. Final Recommedation b c a+b+1

23. Results

24. Counteracting Nadir Nulls

25. 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.