Antarctic Ice Shelf 3D Cross-sectional Imaging using MIMO Radar

A. Hari Narayanan UCL Electronic and Electrical Engineering, UKP. Brennan UCL Electronic and Electrical Engineering, UKR. Benjamin UCL Electronic and Electrical Engineering,

UKF. Gillet-Chaulet Laboratoire de Glaciologie et Géophysique de

l’Environment, FranceK.W. Nicholls British Antarctic Survey, UK

IGARSS 2011

Outline

•Introduction and research aim

•Trial radar system

•MIMO antenna configuration

•Signal processing Single channel processing 2D Digital beamforming I/Q mixer imbalance correction

•Trial results

•Conclusion and further work2

Introduction – Motivation

3

•Monitoring of melting rate of Antarctic ice shelves for predicting rising sea levels

•Cross-section profile/image of ice shelf provides details of changes to ice shelf thickness

•Periodic and accurate measurements of ice shelf thickness using ground based radar

Introduction – Current & Alternative Methods

•Current ground based ice shelf imaging uses monostatic radar moved along ice plane

•Phased array radar can provide alternative ‘stationary’ depth and cross-range imaging

4

•Large number of elements while closely spaced required for angular resolution

•Multiple In Multiple Out concept reduces number of elements for desired aperture

•Arrangement in 2D can provide 2D cross-range details of ice shelf

5

Introduction – Phased Array vs. MIMO

Research Aim

•Investigate the benefits of 2D ice shelf imaging using MIMO radar:

Trial conducted by BAS on Ronne Ice Shelf

Field data analysed by UCL to produce radar image

6

Trial Radar System

•Step Frequency Radar using Vector Network Analyser (VNA)

•Radar specification in free space:

Operating frequency 305 MHz

Bandwidth 160 MHz

Frequency steps 5001

Total transmit time 250 s

Range/Depth resolution 0.937 m

7

Trial Radar System – Revised specifications

8

•Relative permittivity of ice must be considered for radar imaging:

Wavelength in ice, λ 0.5324 m with εr=3.1 [1]

Revised depth resolution 0.532 mMaximum depth 2.6 km

P. Kanagaratnam, S.P Gogineni, N. Gundestrup, and L. Larsen, “High-resolution radar mapping of internal layers at the North Greenland Ice core Project,” Journal of Geophysical Research, vol. 106, no. D24, pp. 799 –811, Dec. 2001.

[1]

MIMO Antenna - configuration

•MIMO concept - 12 physical antennas form 36 ‘virtual’ elements

•Based on ‘phase centres’ from a transmitter and receiver pair

•Single pair of transmitter and receiver used for trial

•36 sets of data collected from trial 9

•Element spacing were too large (>λ/4) resulting in grating lobes

•Grating lobes produces ‘ghostings’ in radar image 10

MIMO Antenna - beampattern

Signal Processing

•VNA provides radar matched filtered signal

•Blackman window function reduces side-lobes and enhances reflections at greater depths

•Inverse Discrete Fourier Transform produces depth plot 11

•Depth plot from position T1 and R6

•Reflection from base of ice shelf at 1.6km detected

•Off-nadir reflections at 1.3 km also seen

12

•Beamforming performed on 36 depth plots to obtain cross-range information

•2D beamsteering vector for array:

•Angular resolution for θ and φ is 5.2o

•Steering increments of 1.87o used for θ and φ for scanning range of ±45o

36...1

sincossin4

,

n

bananr yxjed

13

2D Digital Beamforming

•Rising peak at maximum range is usually due to imbalance in I/Q mixer of VNA

•Amplitude and phase correction factor applied to either In-phase or Quadrature signal:

•Only 2 dB reduction at maximum range achieved from best combination of A and α

•Depth plot truncated up to 2.1km

jQIc AenZjnZnZ )()(

14

I/Q mixer imbalance correction

15

I/Q mixer imbalance correction – T1 R6

Trial Results•Beamforming gives 3 spatial dimensions (Depth,θ and φ)

•Set φ to a fixed value to produce cross profile image:

16

Observations

•Grating lobes made multiple copies of reflections in angular/cross-range domain

•Basal reflection is inaccurate; location of off-nadir reflections are undetermined 17

Conclusions

•2D MIMO array enables 3D ice shelf imaging without mobile platform

•Correctly spaced antenna elements can produce cross-section radar image of ice shelf

•Grating lobes from sparse antenna spacing will degrade radar image

•I/Q mixer correction provides minimal reduction in mean power at maximum range in depth plot 18

Further work

•2D array maybe suitable at certain locations, linear array is preferred at trial site

•Further trial to be carried out using linear MIMO array with appropriate antenna spacing

19

Acknowledgement

Field data for analysis provided by British Antarctic Survey

20

Arvind Hari Narayanan a.narayanan@ee.ucl.ac.uk

Appendix - 2D Array Geometry

21