An_Airborne_X-band_Synthetic_Aperture_Radar

Declaration

I declare that this dissertation is my own, unaided work. It is being submitted for the degree of Master of Science in Engineering at the University of Cape Town. It has not been submitted before for any degree or examination in any other university.

Signature of Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Cape Town

January 21, 2004

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Abstract

This dissertation focuses on the design and implementation of an X-band receiver for use in the South Afican Synthetic Aperture Radar (SASAR II) project. The SAR will be used to demonstrate the capability of building a high resolution X-band imaging radar in South Africa. The design starts by investigating the maximum power return from different targets over a swath width with changing incidence angles. A receiver-power-level table and diagram were constructed, with the power return from a trihedral corner reflector as maximum input power and thermal noise as the minimum input power to the receiver. The output of the receiver, which has to be fed to the input of an analogue-to-digital converter (ADC), is limited by the ADC’s maximum operating input power. Amplifiers, attenuators and mixers were chosen to implement a dual-stage downconversion from a radio frequency (RF) of 9300 MHz to a 2nd IF of 1300 MHz and then to a 1st IF of 158 MHz. A sensitivity time control (STC) is implemented in the receiver to cater for the limited dynamic range of the ADC. The power return varies with range and hence, time. Thus, an STC will correct for low return power, at far range, by boosting the received signal and attenuating large return power, at close range, ideally providing a fairly constant power return at the receiver output. A manual gain control (MGC) is also needed in the receiver, such that none of the components are driven into saturation. The gain control is switched on when large targets are expected to fall in the swath width, otherwise it is switched to a minimum for targets with low backscattered power. The tests that were carried out on the receiver components showed that all the components operated very close to their specifications. The cascaded filters work well in tailoring the front-end 3-dB bandwidth to close to the required 3-dB bandwidth. The receiver was designed to have enough gain to boost the maximum power received to within the operating range of the ADC, without saturating any components in the receiver. The noise figure test showed a noise figure of 4.20 dB. This is 1.73 dB higher than the calculated noise figure of 2.47 dB which is a result of an underestimation of the losses in the system.

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Acknowledgements

I would like to express my sincere gratitude to God Almighty for making this dissertation possible.

I am especially thankful to my supervisor, Professor Mike Inggs, for all his guidance, support and numerous technical input in shaping up this project from its embryonic stage to its final stage.

I am also indebted to Mr. Hannes Koetzer, from Reutech Radar System, for providing equipment, time and advice in the testing of the noise figure of the radar receiver and improving the design.

In addition, I wish to thank all the graduates in the Radar and Remote Sensing Group (RRSG) at UCT, especially Richard Lord and Thomas Bennett, for all their help and assistance in writing up this dissertation.

Finally, many thanks to my father Taleb, my mother Rookshanah and my sister Somayya for their love, support and encouragement during my studies.

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List of Figures

2.1Radiation Pattern of an Antenna with 3 dB Beamwidth of 6◦. . . . . . . . 10

2.2Clutter Area in Azimuth Beamwidth and Pulse Duration Projected on the Ground. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3Radar Backscatter From a Target. . . . . . . . . . . . . . . . . . . . . . . 12

2.4Geometry of stripmap SAR. . . . . . . . . . . . . . . . . . . . . . . . . 13

2.5Stationary Point Targets in a Moving Radar Beam. . . . . . . . . . . . . 14

2.6Resolution of Two Pulses [6]. . . . . . . . . . . . . . . . . . . . . . . . . 15

2.7 Radar Scene Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1Receiver Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2Resolution with 3-dB Receiver Bandwidth. . . . . . . . . . . . . . . . . 20

3.3Waveguide Antenna Dimensions. . . . . . . . . . . . . . . . . . . . . . . 21

3.4Elevation Antenna Pattern. . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.5Target in Cosecant-Squared Antenna Beam. . . . . . . . . . . . . . . . . 24

3.6Cosecant-Squared Antenna Pattern. . . . . . . . . . . . . . . . . . . . . 25

3.7Peak Power Return of Point-Like Targets at X-Band. . . . . . . . . . . . 29

3.8Boresight Axis Position on Swath Width . . . . . . . . . . . . . . . . . . 30

3.9Peak Power Return with Boresight Axis Close To Near-Swath. . . . . . . 31

3.10Peak Power Return With Boresight Axis On Swath Centre. . . . . . . . . 32

3.11Peak Power Return With Boresight Axis Close To Far-Swath. . . . . . . . 32

4.1Radar Receiver Block Diagram. . . . . . . . . . . . . . . . . . . . . . . 34

4.2Receiver Equivalent Circuit. . . . . . . . . . . . . . . . . . . . . . . . . 35

4.3Cascaded Circuit Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . 36

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4.42nd Nyquist Spectrum. . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.5Single Stage Downconversion. . . . . . . . . . . . . . . . . . . . . . . . 37

4.6Dual-stage Downconversion. . . . . . . . . . . . . . . . . . . . . . . . . 37

4.7RF and Image Output from a Mixer [18]. . . . . . . . . . . . . . . . . . . 39

4.81-dB compression point [14]. . . . . . . . . . . . . . . . . . . . . . . . . 40

4.9Third Order Intercept Point [14]. . . . . . . . . . . . . . . . . . . . . . . 41

4.10Spurious Reponse Levels at the X-band to L-band mixer. . . . . . . . . . 41

4.11Spurious Response Levels at the Mixer Output for the 1st Downconversion Stage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.12Filter Transfer Function. . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.13STC Curve with Ground Range. . . . . . . . . . . . . . . . . . . . . . . 44

5.5Final MGC Setup with Power Level Indicated . . . . . . . . . . . . . . . 58

5.152nd IF Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . 67

5.16Relative Gain of 2nd IF Output Signal. . . . . . . . . . . . . . . . . . . . 68

5.17 X-Band Test Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

5.18Filter Transfer Function of 3rd IF . . . . . . . . . . . . . . . . . . . . . . 70

5.19Noise Figure Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

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