Interferometric Radar System including a Photonics-based Architecture for Remote Sensing Applications

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Scuola Superiore Sant’Anna

A thesis submitted for the degree of

Doctor of Philosphy

Interferometric Radar System including a

Photonics-based Architecture for Remote Sensing

Applications

Suzanne Assis de S. Melo

Supervisors: Prof. Giancarlo Prati Prof. Antonella Bogoni

Prof. Arismar Cerqueira Sodr´e Junior

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Abstract

This thesis presents the development of different systems for next genera-tion radar transceivers and applicagenera-tions. In particular, three main works are described in the context of radar systems: automotive applications, dual-use radar/communication system and environmental monitoring.

The first work regards the use of interferometric radar techniques for automotive applications. This work is the first one that uses interferometric measurements in the context of automotive applications to measure the height of road speed bumps. Currently, this task is usually performed by other types of systems (e.g., video cameras, GPS, Lidar systems). The interferometric techniques can prevail over traditional methods in many ways, specially due to the fact that radar based techniques can provide this kind of obstacle detection even at night or under adverse weather conditions. Here, an interferometric radar system at 24 GHz is used to detect small road objects, in this case speed bumps. Experimental results confirm the effectiveness of the method employed to estimate the bumps height, and it can be used in the context of driver assistance or even driverless cars in a future.

The second work is the implementation of a dual-use radar/communication (rad/comm) system using a photonics-based radar transceiver. Firstly, we

intro-duce the concept of photonics and how it enabled the enhancement of purely electronic radars, resulting in the development the first dual-band photonics-based radar system. Afterwards, the dual-band functionality was tested within the scope of this thesis, for the first time in a dual-use rad/comm scenario. The analysis presents no penalty induced by the coexistence of the two operations, thus proving the effectiveness of the photonic-based transceiver as dual-use simultaneous system. Future radar transceivers are likely to communicate with each other and with the surrounding environment while performing traditional radar tasks. Although multifunctional radar transceivers can be found in the literature, it is the first time that a dual-band photonics-based radar unit is used for a simultaneous transmission and reception of a dual-use radar/comm system.

Finally, the third work comprises for the first time the development of a dual-band photonics-based transceiver for environmental monitoring, which is divided in two stages: single and multiple scatterer case. The system performs monitoring of ground displacements, as landslides, based on the principle of differential interferometry, which translates the phase variations into small displacement measures. The combination of several sinusoidal waveforms allows to synthesize a large signal bandwidth, which provides improved range cell resolution. Moreover, the dual-band capability allows to enhance the displacement measure accuracy, due to the possibility of applying the advanced interferometric techniques among the different frequency bands. The system was tested in a single and in a multiple

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scatterer scenario reaching a sub-mm resolution without correction algorithms, proving the effectiveness of the system for the target applications. The use of two bands also confers an improved robustness to the environmental conditions, and allow to use different system parameters simultaneously. Moreover, it also allows a reduction of SWaP and footprint of the whole system, greatly benefiting future radar transceivers, which will render more information, be more flexible while providing a reduction of dimensions and costs.

Although there are three different works apparently uncorrelated, the main scientific novelty lies in the third project, i.e., the use of interferometric techniques using a dual-band photonics-based architecture for environmental monitoring. The idea was to exploit both interferometry and the dual-band capability of the photonics system to show their broad potentiality even in different contexts. For this reason, interferometry has been applied for automotive applications, showing its effectiveness to measure small objects for future driverless cars. On the other hand, the dual-band photonics system has been properly verified in a dual-use radar/comm scenario. Finally, both features are explored in the third project to bring up a new paradigm in the context of remote sensing applications for environmental monitoring, due to its characteristics, i.e. robustness to environmental conditions, combination of multiple bands to make the system more precise, software-defined radio architecture, reduced dimensions and power, etc. For so, it is not only more versatile compared to ground-based interferometric radar systems already present in the literature, but also is a strong candidate for fulfilling the requirements of next generation radar systems.

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Acknowledgements

This work was partially supported by the following projects:

• The National Project PREVENTION (with the contribution of Ministry of Foreign Affairs, Directorate General for the Country Promotion).

• The ERC-POC project “Photonic Environment moniToring & Risk Assess-ment” PETRA (number 641388).

• CAPES-Brazil in the context of the Inatel-CNIT joint Project number 88887.125018/2014-00, entitled “Integra¸cão Sistêmica de um Transceptor Multiprotocolo Baseado em Tecnologia Fotônica com Arranjos de Antenas Reconfiguráveis Controlados Opticamente”.

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List of Publications

1. S. Melo, A. Bogoni, E. Marchetti, S. Cassidy, E. Hoare, M. Gashinova, M. Cherniakov, “24 GHz Interferometric Radar for Road Hump Detections in Front of a Vehicle”, accepted as oral presentation to the International Radar Symposium (IRS) 2018, Bonn, Germany.

2. T. H. Brand˜ao, F. Scotti, H. R. D. Filgueiras, A. A. C. Alves, S. Melo, D. Onori, A. Bogoni and Arismar Cerqueira S. Jr., “Dual-Band Radar System Based on a Unique Antenna and Photonics Processing”, submitted to IET Radar, Sonar and Navigation.

3. S. Melo, S. Maresca, S. Pinna, F. Scotti, M. Khosravanian, A. Cerqueira S. Jr., F. Giannetti, A. Das Barman and A. Bogoni, “Photonics-based Dual-band Radar for Landslides Monitoring in Presence of Multiple Scatterers”, accepted for publication in the IEEE/OSA Journal of Lightwave Technology. Pre-print available doi: 10.1109/JLT.2018.2814638.

4. S. Melo, S. Pinna, S. Maresca, F. Scotti, M. Khosravanian, A. Cerqueira S. Jr., A. Bogoni, “High precision displacement measurements in presence of multiple scatterers using a photonics-based dual-band radar”, in proceedings of the IET Radar Conference, Belfast, UK, 23-26 October 2017. (Prizewin-ning: 3rd place for “Best Paper Student Award” in the IET Radar 2017).

5. S. Pinna, S. Melo, F. Laghezza, F. Scotti, E. Lazzeri, M. Scaffardi, P. Ghelfi, A. Bogoni, “Photonics-Based Radar for Sub-mm Displacement Sensing,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 23, no. 2, pp. 168-175, March-April 2017.

6. S. Melo, S. Pinna, F. Laghezza, F. Scotti, I. F. da Costa, D. H. Spadoti, Arismar C. S. Jr., and A. Bogoni, “Photonics-based dual-use Transceiver based on a single dual-band Antenna Array”, in Telecommunications Brazil-ian Symposium (SBrT), Santarem, PA, Brazil, 2016. (Received honorary distinction among the papers of the XXXIV SBrT).

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7. S. Pinna, S. Melo, E. Lazzeri, A. Bogoni, F. Scotti, and F. Laghezza, “Sub-mm Displacement Measure via Multi-band Phase estimation in a

Photonics-based Radar System”, in proceedings of 13th European Radar Conference (EuRAD) 2016, London, United Kingdom.

8. S. Melo, S. Pinna, A. Bogoni, F. Laghezza, F. Scotti, I. F. da Costa, D. H. Spadoti, A. Cerqueira S. Jr., “Dual-use System Combining Simultaneous Active Radar & Communication, Based on a Single Photonics-Assisted Transceiver”, in proceedings of 17th International Radar Symposium (IRS) 2016, Krakow, Poland.

9. Arismar Cerqueira S. Jr., I. F. da Costa, S. Pinna, S. Melo, F. Laghezza, F. Scotti, P. Ghelfi, D. H. Sapdoti, and A. Bogoni, “A Novel Dual-polarization and Dual-band Slotted Waveguide Antenna Array for Dual-use Radars”, in proceedings of EuCAP (European Conference on Antennas and Propagation) 2016, Davos, Switzerland.

10. A. Malacarne, V. Sorianello, A. Daly, B. Kogel, M. Ortsiefer, S. Melo, C. Neumeyr, M. Romagnoli, and A. Bogoni, “High-Speed Long-Wavelength VCSELs for Energy-Efficient 40 Gbps Links up to 1 km Without Error Cor-rection,” in proceedings of OFC (Optical Fiber Communication Conference) 2015, Los Angeles, CA, USA, paper Tu2H.1.

11. Cerqueira S. Jr., Arismar; Ca˜nas-Estrada, Natalia; Noque, Dionisio F.; Borges, Ramon M.; Melo, Suzanne A.S.; Gonzlez, Neil G.; Oliveira, Julio C.R.F.: “Photonic-assisted microwave amplification using four-wave mixing”, IET Optoelectronics, 6 pp., 2015.

12. S. A. S. Melo; A. R. do Nascimento Jr.; Arismar Cerqueira S. Jr., Ph.D.; L. H. H. Carvalho; D. M. Pataca; J. C. R. F. Oliveira; H. L Fragnito, “Frequency Comb Expansion based on Optical Feedback, Highly Nonlinear

and Erbium-doped Fibers”, Optics Communications v. 312, pp. 287-291, 2014.

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List of Figures

2.1 Elements involved in a radar transmission and reception process Richards et al. (2010).. . . 23

2.2 Block diagram of a superheterodyne receiver Richards et al. (2010). 25

2.3 Representation of the EM fields and velocity vector Richards et al. (2010). . . 28

2.4 Types of electromagnetic waves Richards et al. (2010). . . 28

2.5 The pulsed radar waveform with its parameters Richards et al. (2010). 30

2.6 Directional beam antenna over a surface at range R Richards et al. (2010). . . 33

2.7 Representation of two-dimensional search radar system Richards et al. (2010).. . . 38

2.8 Representation of three-dimensional search radar system Richards et al. (2010).. . . 38

3.1 Transmit frequency and deviation from the ideal SFCW curve. . . 42

3.2 Backscattering geometry for interferometric data acquisition. . . . 44

3.3 Block diagram of the experimental setup. . . 45

3.4 Experimental setup: (left) front view of the system, (right) side perspective view of the experiment. . . 47

3.5 Targets used in the experiments: (left) two spheres; (right) speed bump. . . 47

3.6 Range Profile for (a) two spheres (b) speed bump. . . 48

3.7 Height estimation for: (a) spheres; (b) speed bump. . . 50

4.1 Spectrum of a mode-locked laser (Tr is the pulse repetition time). 53

4.2 Scheme of the RFS technique Pataca et al. (2013).. . . 54

4.3 Optoelectronic comb generator block diagram Metcalf et al. (2013). 55

4.4 All-pass ring resonator Bogaerts et al. (2012). . . 57

4.5 Scheme for enhancing multiple Four-Wave Mixing for comb gener-ation (CW - continuous wave, OSA - optical spectrum analyzer, EDFA - erbium dopped fiber amplifier, P - power) Melo et al. (2014). 58

4.6 Block diagram of the principle of generation of a microwave signal from a MLL.. . . 60

4.7 Block diagram of a photonic-based analog-to-digital converter. . . 61

4.8 Scheme of a photonics-based transceiver. . . 62

4.9 Dual-band antenna model. . . 65

4.10 Reflection coefficient of the dual-band SWAA: the red and black curve with circles correspond to the simulated and measured results respectively. . . 66

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4.11 Reference scenario and photonic-based transceiver basic concept, DDS direct digital synthesizer, DPS digital signal processor, ADC analog-to-digital converter. . . 66

4.12 (left) Experimental setup (MLL mode-locked laser, DDS direct digital synthesizer, MZM Mach-Zehnder modulator, BPF band-pass filter, LPF low band-pass filter, ADC analog-to-digital converter, OFDM orthogonal frequency division multiplexing, SWAA slotted waveguide antenna array), (right) (A) optical spectrum of the MLL; (B) electrical spectrum of the applied signals; (C) optical spectrum

of the modulated signal after the MZM; (D) electrical spectrum at the photodiodes output, with the following RF filters highlighted; (E) electrical spectrum of the filtered RF signals before the wideband

antenna; (F) electrical spectrum of the received signals after the receiver PD with the LPF highlighted. . . 67

4.13 Power spectrum of the downconverted received signal. . . 68

4.14 Photo of the in-field experiment with visible the dual-band slotted waveguide antenna array. . . 69

4.15 a) Range Doppler map of a radar trace b) Velocity profile fitting of the target for 1.8 seconds of observation. . . 70

4.16 Graph of the error vector magnitude (EVM) versus received power for the OFDM transmission (PDC photonic downconversion). The insets show the constellation for different values of EVM for the gray dashed curve. . . 70

5.1 Scheme of the photonics-based displacement measure experiment (MLL- Mode-Locked Laser, DDS - Direct Digital Synthesizer, MZM - Mach-Zehnder Modulator, ADC - Analog-to-Digital Converter). 76

5.2 Moving antenna setup. . . 76

5.3 Electrical spectrum of the dual-band RF signal (a) S-band signal, b) X-band signal. . . 77

5.4 IF-spectrum of the received SFCW signal. . . 78

5.5 IF-spectrum of the generated SFCW signal. . . 79

5.6 (A) Measured displacement vs. real displacement; (B) Accuracy as a function of the displacement.. . . 80

5.7 Experimental cross-correlation function of the received echo for 0 km (blue), 1.5 km (red) and 3 km (green) distant targets.. . . 81

5.8 Systems precision as a function of the range. . . 81

5.9 Displacement precision vs signal SNR. . . 82

5.10 Illustration of the concept of the photonics-based dual-band radar system for environment monitoring. . . 84

5.11 Displacement trends for the five simulated targets. . . 85

5.12 Typical processing for extracting the phase information from the range cells.. . . 86

5.13 Displacement error estimates from computer simulations of the S-band radar data with five targets. . . 87

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5.14 Absolute values in log-scale of the displacement estimation error for the target at 145 m distance, for different values of SNR. The error curves are obtained by differential phase measurement among the S and X band. . . 88

5.15 Received SFCW signal after downconversion to the IF frequencies. 89

5.16 a) Real data displacement, b) Accuracy as a function of the dis-placement. . . 90

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List of Tables

1.1 Performance of the photonics-based radar transceiver . . . 18

3.1 Parameters used in the experiments. . . 46

5.1 Summary of the main parameters of the photonics-based radar system and transmitted SFCW signal. . . 85

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Chapter 1 Motivations and Background

In this chapter a general overview of the work developed in this thesis will be presented. First, a background of the main subjects will be discussed, i.e. current radar systems and interferometric radar systems, followed by a discussion of the actual limitations of electronic radar systems. Then, an introduction on microwave photonics will be presented and how its concept enabled the development of the first photonics-based radar system.

Finally, this chapter also presents the aim of the research, future perspectives and the thesis outline.

1.1 Background

1.1.1 Current Radar Systems

RADAR (RAdio Detecting And Ranging) refers to a device that operates by radiating electromagnetic energy and detecting the echo returned from reflecting objects Skolnik (2007). The returned signal can provide different information about the target: the range (or distance), the angular location, the velocity if it is moving, and even providing information about the target size and shape Skolnik

(2007).

Different systems that preceded radar devices were able to measure the distance to a target between 1903 and 1925Peebles (1998). However, Radar systems have evolved a lot since World War II (WWII). It went from a largely military detection system into a complex three-dimensional imaging tool, with applications in many different areas, from commercial aviation to fundamental research in the earth and planetary sciences. Behind the concept of Radar, as we know today, lays more than a century of radio development.

In the early 1800s, Michael Faraday demonstrated that electric current produces a magnetic field and the energy in this field returns to the circuit, when the current is stopped. In 1864, James Maxwell formulated the general equations of the electromagnetic field, being validated experimentally 22 years later by the German physicist Heinrich Hertz. He was able to prove that electromagnetic waves traveled in straight lines and that they can be reflected from a metal object Cheney and Borden (2009).

In 1904, Christian Hulsmeyer developed a device capable of detecting ships. However, it was only in 1922 that attention was drew to the work of Hertz,

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thanks to Guglielmo Marconi, who proposed in principle what we know today as marine radar. Since then, many experiments were conducted in order to refine Marconi’s ideas. The radar importance for detection and tracking was finally consolidated when scientists and engineers discovered how to use a single antenna for transmitting and receiving Cheney and Borden (2009).

During 1930s, there were many efforts to promote aircraft detection. The main reason was the imminent burst of WWII. Germany was the first in line in the development of radar units on the ground, and in the air for defense against allied aircraft. As the war progressed, radar went from an early warning device to an offensive machine employed to automatically track aircrafts and attack them Cheney and Borden (2009).

The beginning of modern radar started with the invention in 1939 of the cavity magnetron oscillator, which allowed radar signals to operate at higher frequencies. Before that, radar systems operated at low frequencies, usually in the VHF band (from 30 to 300 MHz) Cheney and Borden (2009).

The progresses in radar slowed down after the WWII until 1950. As civil mariner became more important, new and better radar systems began to emerge. Nowadays, radar has many applications. It is widely used in aviation and trans-portation, for navigation, for collision avoidance, and for low-altitude flight. It can be used for monitoring vehicle speed, monitor weather, including Doppler measurements of precipitation and wind velocity. Moreover, radar is also a pow-erful imaging tool, being used for land-use, agricultural and for environmental monitoring. Radar systems can also be used to map surface topography and measure crustal change. Furthermore, it is current under research for medical microwave tomography applicationsCheney and Borden (2009).

Radar history has witnessed a gradual and continuous migration from analog to digital systems. The digital approach is by now consolidated, leading to the paradigms of software-defined radar (SDR) and cognitive radar Debatty(2010). The development of SDR has given a preeminent role to processing in remote sensing systems, allowing to mitigate undesired phenomena ,such as unwanted echoes, propagation impairments, distortions and noise introduced by the electronic devices employed in signal transmission and reception Laghezza et al. (2015b).

There are many advantages of using radar system for remote sensing. For example, it can be used 24 hours a day, in any light conditions unlike most optical systems. Radar systems can also be used in all weather condition, since the long wavelengths can penetrate clouds, smoke, sand, etc. Some types of radar can even go through foliage, buildings, soils and another vast range of materials Cheney and Borden (2009).

1.1.2 Interferometric Radar Systems

Interferometry is a technique in which electromagnetic waves are superimposed causing the phenomenon of interference in order to extract information Hariha-ran (2010). Interferometry is applied in a wide range of applications, such as: astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy (and its applications to chemistry), quantum mechanics, nuclear and particle physics, plasma physics, remote sensing, biomolecular inter-actions, surface profiling, microfluidics, mechanical stress/strain measurement,

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velocimetry, and optometry Hariharan(2010).

When it comes to radar systems, interferometry was born with the aim of distinguishing two or more objects at the same range Hanssen (2001). It takes advantage of the phase information, using either two different antennas or repeated acquisitions in order to obtain not only the range information, but also the angular measurements Hanssen (2001). In this way, it is possible to observe relative distances as a fraction of the radar wavelength. Moreover, the observation of the angular differences is enabled by exploiting the difference in the sensors locations. These characteristics are of vital importance for example, for topographic mapping Hanssen (2001).

It was not until the end of the WWII that radio interferometry was devel-oped Hanssen (2001). In 1946, a radio interferometer was built in order to find new cosmic radio sources Hanssen (2001). In the field of geodesy, Evans and HagforsBracewell (1968) were able to map the radar reflections from iso-range and iso-Doppler lines. The problem of ambiguity between reflections from the northern and southern hemispheres of the planet remained until 1969 whenRogers and Ingalls (1969) solved this problem using two antennas. Zisk (1972) used interferometry to measure elevation differences of the moon. The technique was refined through the years, reaching new levels of horizontal and height resolution, for example, for topographic mapping of the lunar poles Margot et al. (1999);

Margot (1999).

Graham was the first to publish experimental results with airborne radar interferometry for topographic mapping using two antennasGraham (1974). The method patented in 1971Richman (1982) enabled to obtain elevation informa-tion from the phase difference between two different antennas Henderson and Lewis (1998). Dual antenna airborne interferometry (single-pass architecture) matured in the 80s using digital processing techniques and coherent multiplicative interferometry Goldstein et al.(1985).Zebker and Goldstein (1986) were able by then to produce interferograms. It means that for every resolution element the amplitudes of two images are multiplied and their phases are differenced. The same authors discovered that along-track interferometry (i.e. using two antennas mounted in the flight direction of the aircraft) results in a sensitivity to motion of scatterers, relative to the stationary backgroundGoldstein and Zebker (1987). Li and Goldstein (1987, 1990) showed for the first time interferometry with satellite data, using the repeat-pass method (where the satellite revisits an acquisition area after a certain period) using historic Seasat data, and byGabriel and Goldstein

(1988) and Goldstein et al. (1988) using the shuttles SIR-B data.

Radar interferometry has also find utility in automotive applications among others. Recently, interferometric radar systems have been proposed for unmanned ground vehicle (UGV) applications. Among them, we can highlight vehicle collision avoidance and braking systems Ross and Robbins (1979), traffic monitoring Jung et al. (2013), obstacle detection through high-resolution radar images Sun et al.

(2011), etc.

Jiang et al. (2017) presented an Interferometric Synthetic Aperture Radar (InSAR) system using forward-looking technique to tackle the problem of road object detection. It can give more information of harsh off-road environments than existing AGV based radars, by providing a scattering image, a coherence image and a digital terrain model (DTM) of the scene under observation. Additionally,

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this kind of radars can provide information on the road in all-weather condition and even penetrate vegetation Sun et al. (2011) allowing a complete mapping of the environment for vehicles to navigate autonomously, or for driving assistance systems.

Besides the applications using classical interferometry described above, an-other useful technique is the differential interferometry. It regards the real-time monitoring of sub-millimeter displacements usually for applications related to the environment monitoring Hariharan (2010). More into details, this method is used for many applications related to early warning and mapping displacements of natural hazards, as earthquakesMassonnet et al. (1993); Gabriel et al. (1989); Ze-bker et al. (1994), volcano deformation monitoringAmelung et al. (2000); Hooper et al. (2004), land subsidence, for example caused by mining activities Carnec and Delacourt(2000); Galloway et al.(1998); Massonnet et al.(1997), glacier and ice motionGoldstein et al. (1993); Gray(2005), landslides monitoring Pieraccini et al. (2003);Luzi et al. (2004); Kimura et al. (2000) etc.

1.1.3 Actual Limitations of Electronic Radar Systems

The highly versatile current and future radar systems demand cutting-edge electronic technologies. As mentioned before, radar systems are evolving towards new multifunctional and multiband sensing devices, in contrast to the traditional ranging and surveillance systems that it used once to be.

They now require higher performance in terms of spatial and velocity resolution, very stable radio-frequency (RF) sources and very precise signal detection and digitization Ghelfi et al. (2015). In addition, it would be desirable the use of multiple simultaneous coherent radio signals at different frequencies enabling a multifunctional system capable of producing waveforms up to the millimeter waveband (mmW). The critical point is the maintenance of the phase stability over the frequencies necessary for coherent pulse-Doppler processing, target imaging and clutter rejectionSkolnik (2007); Richards et al. (2010).

Furthermore, this reconfigurable system requires a software-defined RF signal generator. It is also well known that software-defined radio systems, which use analog-to- digital converters (ADC), digital-to-analog converters (DAC) and digital signal processors (DSP) to elaborate signals in the software domain, have many advantages with respect to the analog counterparts, as flexibility, stability, reliability, etc.Arslan(2007) They allow to employ techniques that are impractical to implement in hardware (e.g. compressed sensing methods) Tian and Giannakis

(2007).

Actual electronic technologies do not allow a full digital SDR system due to the high frequencies of employed RF signals and, for these reasons, analog stages are used to perform up and downconversions into the bandwidth of interest

Skolnik(2007). Such processes usually employ analog mixers, amplifiers and filters, which can be a source of noise and often suffer from electromagnetic interferences, causing degradation of the sensitivity and dynamic range. In addition, the use of electronic ADCs represents a major problem regarding the signal bandwidth, since its precision drops with increasing input bandwidth and sampling speedJuodawlkis et al. (2001); Valley (2007); Walden (2008). Indeed, electronic subsystems can guarantee high performance over a given bandwidth and, as a rule of thumb, as

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the frequency increases the performance worsensRichards et al. (2010);Walden

(2008);Laghezza et al. (2015b).

Therefore, currently software-defined radars are a major challenge in terms of technical requirements, specially for high-speed Direct Digital Synthesizers (DDSs) and ADCs.

1.1.4 Microwave Photonics

Microwave photonics (MWP) emerged as an interdisciplinary area that studies the interaction between microwave and optical signals. It has a wide range of applications, such as broadband wireless access networks, sensor networks, radar, satellite communications, instrumentation and warfare systems Yao (2009).

Microwave photonics take advantage of the unique features of photonics, including wide bandwidth, immunity to electromagnetic interference (EMI), low loss and low distortion propagation, low phase noise of optical clocks, and extremely high frequency flexibility Ghelfi et al. (2015); Capmany and Novak (2007).

The aim of this research field is to use photonics technologies to overcome the major issues of electronics systems, as described in the subsection 1.1.3. Its major functions are: photonic generation, processing, control and distribution of microwave and millimeter-wave signalsYao (2009). The critical elements for microwave photonics systems are optical sources capable of fast modulation, highly stable optical clocks, suitable transmission media, and fast detectors or optically controlled microwave devices Seeds and Williams(2006).

A microwave or mm-wave signal can be generated by means of photonic techniques Goldberg et al.(1992); Li et al.(2011); Ghelfi et al. (2012);Lin et al.

(2005); Khan et al. (2010). A low phase noise and frequency-tunable signal is desirable for many applications, such as in modern radar, wireless communications, software defined radio and instrumentation Yao (2009). Usually, this process is achieved by optical heterodyning, in which two optical waves of different wavelengths beat at a photodetector Gliese et al. (1992). The newly generated electrical signal frequency at the photodetector output will correspond to the wavelength spacing of the two optical waves Gliese et al. (1998). The photonic generation is capable of generating a wide range of carrier frequencies (up to the mmW), with high phase stabilityFortier et al. (2011). High-quality microwave or mm-wave signals can be generated by using optical injection lockingGoldberg et al.

(1983), optical phase locked loop Rideout et al. (2006), optical injection phase locking Bordonalli et al. (1999) or based on optical clocks, as the electro-optical oscillatorsMaleki (2011) or mode-locked lasers (MLLs) Lin et al.(2005); Chou et al. (2003). Moreover, microwave photonics allow the simultaneous generation of multiple RF carriersGhelfi et al. (2015).

The distribution of RF signals is prohibitive in the electrical domain due to high losses of RF lines, as coaxial cables Yao (2009). When using optical fibers instead, this task becomes practically ideal. State-of-the-art optical fibers present an extremely broad bandwidth and low loss, which benefits the microwave or mm-wave signals distribution Yao (2009). Specifically for radar systems, the distribution of RF signals takes place from the transceiver to the antenna, where the conversion to electronic domain occurs Ghelfi et al.(2015).

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speed performance of receivers as mentioned before. In the last few decades, the use of optical technologies to accomplish fast analog-to-digital conversion has achieved significant enhancement respect to its electronic counterpart, thanks to the reduced timing jitter of optical sources Yao (2009); Valley (2007). The photonic-based ADC guarantees a large input bandwidth, high sampling rates with extremely low jitter, a fully digital approach, independence from the RF carrier and the capacity to simultaneously receive multiple signals Ghelfi et al.

(2015); Valley (2007); Laghezza et al. (2013); Chou et al. (2009); Khilo et al.

(2012).

Microwave photonics can also be used for all-optical microwave signal pro-cessing, photonic true-time delay beamforming and radio-over-fiber systems Yao

(2009);Seeds and Williams(2006);Capmany and Novak(2007);Cox and Ackerman

(2008).

1.1.5 Photonics-based Radar System

The first photonics-based coherent radar system has been realized under the ERC-funded project PHODIRGhelfi et al.(2014). Moreover, the potential of using photonics in radar systems has been highlighted in several recent works Ghelfi et al. (2016); Scotti et al. (2015a, 2017, 2016). It was proved the suitability of the photonics approach in terms of flexibility and high performance, demonstrating its effectiveness in operative scenarios, as aerial Scotti et al.(2015d) and maritime field trials Scotti et al.(2017).

Furthermore, the concept of a multi-band photonic transceiver has also been investigatedGhelfi et al. (2015).The capability of working in different bands is one of the essential requirements of SDR systems. It is an important feature in order to take advantage of the strengths of each carrier frequency. For example, to trade off long distance detection and target tracking. In fact, S-band radars, i.e., radio waves with frequencies that range from 2 to 4 GHz, present a strong immunity against weather clutter due to their frequencies, which are not easily attenuated. For this reason, they are preferable for early-warning applications. On the other hand, X-band radars (8 to 12 GHz) are usually used for target tracking, since this kind of frequencies are better to generate narrower beams Ghelfi et al.(2015). C-band radars operate on the frequency range from 4 to 8 GHz. Because of its wavelength, the dish size does not need to be very large. This makes C-band radars affordable for TV stations. The signal is more easily attenuated, so this type of radar is best used for short range weather observation as well.

As we can see, different types of frequency bands present different features, and therefore adapting the radar carrier frequency would allow optimizing the performance based on the operative conditions (weather, target distance, target material, required precision, etc.) Ghelfi et al. (2015). Moreover, a multiband system also allows to merge data from different bands, expanding the total bandwidth in order to improve the system range resolutionVan Dorp et al.(2010);

Scotti et al.(2015a).

Additionally, another advantage of the multiband transceiver includes imple-menting different functionalities in a single system. For example, to create a high capacity networked sensors. It would allow radars to communicate among them for optimizing the sensor configuration and increase the situation awareness in

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security contexts, or able to merge in the same hardware independent sensing and communication operations.

Therefore, since photonics allow implementing coherent multiband systems with a single photonics-based transceiver, gathering in the system many functionalities, it holds the potential for simplifying the multiband radars implementation. For this reason, it enhances the system size, weight and power (SWaP), reducing costs and improving the system performance.

The key component that allows the flexible production of RF carriers with tunable frequency is a multifrequency optical oscillator. The intrinsic phase-locking condition of the employed oscillator reported byGhelfi et al. (2014), enables the generation and reception of the radar signal with a high degree of coherence. The performance of the photonics-based radar system is summarized in table1.1.

Parameter Photonics-based transceiver State-of-the-art electronics transceiver

Transmitter

Carrier frequency Flexible direct generation up to 40 GHz

Direct generation below 2 GHz; upconversions above 2 GHz Signal jitter Typical > 15 fs; integrated

from 10 kHz to 10MHz

Typical >20 fs, integrated from 10 kHz to 10 MHz Signal-to-noise ratio > 73 dBM Hz−1 > 80 dBM Hz−1

Spurious-free dynamic range > 70 dBc > 70 dBc

Instantaneous bandwidth 200 MHz, easily extendable

with MLL at higher repetition rate < 2 GHz Receiver

Input carrier frequency Up to 40 GHz, with direct RF undersampling

<2 GHz; downconversions at higher frequencies Instantaneous bandwidth 200 MHz, easily extendable

with MLL at higher repetition rate < 2 GHz Sampling jitter <10 fs, integrated

from 10 kHz to 10MHz

Typical >20 fs, integrated from 10 kHz to 10 MHz

Spurious-free dynamic range 50 dB > 70 dB

Effective number of bits >7 for carrier

frequency up to 40 GHz < 8 f or carrier f requency < 2 GHz

Test results of the photonics-based transceiver are shown versus state-of-the-art electronic radar transceivers.

Table 1.1: Performance of the photonics-based radar transceiver

1.2 Aim of the Research

The aim of this research is to present innovative developments within the context of next generation radar systems for different current trendy applications. The three main applications in which this work is focused on are: automotive applications, dual-use radar/communication system and environmental monitoring.

First of all, this thesis reports interferometric techniques being used for radar systems for automotive applications, specifically for detection and height estimation of small road objects, as speed humps. Detecting this kind of objects is a challenging task due to its low height. Up to now, this kind of monitoring has been accomplished mostly based on other types of technologies, e.g., video cameras, GPS, Lidar systems. However, this kind of technologies can fail under difficult visibility conditions. For this reason, radar based techniques can overcome traditional methods since it can provide this kind of obstacle detection even at

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night or under adverse weather conditions. A real aperture radar system allied with interferometric technique will be proposed for detecting small obstacles, such as speed humps in road scenes for automotive 24 GHz radar. The use of this carrier frequency with interferometric technique for road object height estimation is explored for the first time to the best of our knowledge. The high sensitivity of phase information to small height variations was explored in order to obtain a high resolution, even in complex environments.

Furthermore, we explore the benefits of photonics being applied to radar sys-tems. A radar based on photonic architecture allows to have a system operating in a coherent multi-band configuration. The possibility of exploiting more than one different band confers improved robustness to the environmental conditions. Moreover, it also confers flexibility due to the systems capacity of dynamically tune the RF operative carrier in a software defined manner. The dual-band capability, which has already been investigated byScotti et al. (2015d,b,a), for a radar operating in the S- and X-bands being used for a maritime and aerial scenario. The main advantage of the proposed system is the capability of sharing the same hardware for generating and receiving, with a single frequency conver-sion stage, coherent multi-band highly stable microwave signals. Compared to an equivalent multi-band electronic architecture, the photonics-based approach enables to strongly reduce system weight and size by reducing the total number of local oscillators and up-down conversion stages.

The dual-band capability will be further investigated within this thesis. Here, a dual-use radar-communication system based on a single dual-band photonics-based transceiver and antenna, in the S- and C-bands, will be presented. The system is completely software defined and will be evaluated in terms of surveillance and communication. The experiments implementation aim to analyze the correct simultaneous transmission and reception of the two different signals, one for radar detection and the other for communication purposes. Future radar transceivers are likely to communicate with each other and with the surrounding environment while performing traditional radar tasks. Although multifunctional radar transceivers can be found in the literature, it is the first time that a dual-band photonics-based radar unit is used for a simultaneous transmission and reception of a dual-use radar/comm system based on a single dual-band antenna element.

Finally, we gather the two main topics of the other two activities described above, to develop the main contribution of this work: exploit for the first time interferometric techniques using the dual-band photonics based system for environ-mental monitoring applications. This project is divided in two stages: for single and multiple scatterer scenario. In this project, the photonics-based transceiver is used to perform monitoring of ground displacements, as landslides, based on the principle of differential interferometry, which translates the phase variations into small displacement measures. The aim of this third project is to prove that the high phase coherence among the RF carriers, provided by photonics, strongly benefits the final measurements accuracy, as will be discussed later. The possibility of using different frequency bands (two bands in this case) allied with a software defined radio architecture allows to tune the operative RF carriers for adapting the system to the environment (as weather condition or observed scenario) and to the range of interest Taylor(2001). When it comes to differential interferometry, the combination of these characteristics would greatly enhance the

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final accuracy contributing to a reliable early risk detection. The proposed system, suitable for the monitoring of landslides or civil structures (buildings, dams, etc.), demonstrates the capability to reach displacement accuracy in the sub-millimeter scale. Furthermore, it will be investigated the system capabilities in a multiple scatterer scenario, and evaluate how it can affect the precision of the displacement estimates. This activity aims to prove the system potentiality and versatility for fulfilling the requirements for next generation radar systems.

1.3 Future Perspectives

The projects developed within the scope of this thesis, i.e., interferometric radar system for automotive applications, dual-use radar/comm photonics-based system and interferometric radar system using a dual-band photonics-based transceiver for environmental monitoring are quite new fields of study.

Over the years, all three subjects have led to great advances in the research field. Obstacle sensing using radar interferometry for driving assistance and autonomous navigation was introduced inYamawaki et al. (2000). The frequency modulated continuous waveform (FMCW) roadside 60GHz radar system had however some restrictions because of short detection range. Since then, it has reached the potential of detecting obstacles with a maximum range of up to 500 m in the Ka-band (26.5 GHz to 40 GHz)Jung et al. (2013), and detecting positive and negative obstaclesJiang et al.(2017), providing abundant information with forward 3D imaging techniques with an stepped frequency waveform in the frequency range of 9 to 10 GHz.

In the field of dual-band transceivers, we can highlight the recent achieve-ments in this field, as the implementation of the first dual-band millimeter-wave transceiver operating in the 22-29-GHz and 77-81-GHz for short-range automotive radar bands Jain et al. (2009). Moreover, a multi-purpose radar system suitable for applications with different requirements on dynamic range, resolution, and miniaturization degree has been presented in Ng et al. (2017). The proposed ar-chitecture enables the cascading of multiple TRXs and allows the implementation of MIMO radar (multiple transmit and receive antennas) systems in two different frequency bands by using a single voltage controller oscillator (VCO) circuit.

Related to the third project, the differential InSAR for deformation monitoring using GB interferometric systems have reached long distances ranges, up to 4 km, with displacement accuracy on the order of mm or sub-mm Atzeni et al.

(2015). Kieffer et al. (2016) have reported a 200 MHz GB-InSAR system with an accuracy of ± 0.1mm working in the Ku band (12 GHz to 14 GHz) up to 4 km of distance range.

Still a lot of work can be done in all fields described. The new paradigms in drive safety towards autonomously vehicles, in multi-band transceivers capable of performing different tasks simultaneously and environmental monitoring to prevent and reduce natural hazards produced by ground failures are pushing towards the edge of the current employed technologies. Progresses in remote sensing techniques led to high quality information, with sufficient details and cost-effective for many engineering applications Wasowski et al. (2017).

Regards to automotive applications, radar sensors are just one of the many technologies employed towards autonomously driven systemsBengler et al.(2014).

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In future, it is likely to see more and more combined techniques in order to obtain even more accurate and precise measurements. Radar technology can be used allied with vision sensors and GPS for example, since visual information can be very important in a number of related applications, such as lane detection, traffic sign recognition, or object identification Sun et al. (2006b); Lundquist (2011). For example, it is possible to improve the measurement of the speed humps height, towards the estimation of the object’s shape. In this direction, radar based measurements, as the one that will be presented in this thesis, combined with a camera-based approach can be an efficient solution in this sense.

Moreover, one of the currently trends is to use higher automotive frequencies, as 77-79 GHz or even higher, in order to decrease the size of the hardware components of the system, specially the antennas. With a system that is smaller in size and weights less, it is possible to implement it in the vehicle with no disturbance to the driver.

The next generation of radar systems, capable of overcoming the performance of traditional radars, are expected to perform many different tasks and substitute them in the next future. Among the new tasks transceivers will present are: intelligent signal coding, e.g. OFDM, CDMA, MIMO radar, digital beamforming for higher angular resolution avoiding mechanical steering, array imaging for efficient systems, and a combination in a single unit of radar and communication features Wiesbeck et al. (2015). The innovative system technologies will allow entirely new functions and applications, replacing most of the current systems. Moreover, the integration of most functions in a single device allied with the software defined radio concept, where most of the systems configuration and processing are performed by a remote computer station, will render the future radars much more versatile, flexible and will render more information. They will also be smaller and significantly cheaper, reducing the overall system’s SWaP and outperforming traditional radar systems.

For deformation monitoring of natural hazards, a strong recent research trend is the multi-temporal interferometry (MTI). In this technique the radar sensor periodically re-visits the same area, providing information on distance changes between the on-board radar sensor and the targets on the groundWasowski et al.

(2017). Until now, this technique was relatively little explored by engineers. The capability of providing long-term ground deformation monitoring, offers an unprecedented opportunity for early detection and warning of potential slope instability hazards, but further research is required focusing on the integration of data from MTI and ground-based geotechnical monitoring Wasowski et al.(2017). Specifically related to this thesis, i.e., landslides monitoring, the system could be combined with MTI technique to perform geomechanical modeling of the deforma-tion in order to improve the knowledge of the phenomenon. Another interesting future trend, is the combination of InSAR and ground-based measurement for monitoring subsidence Raucoules et al. (2003).

1.4 Thesis Outline

This thesis is divided as follows. Chapter2 is going to present the theory of radar systems, including its functions, antennas basics, the radar range equation, system parameters, the theory of electromagnetic waves, scattering mechanisms,

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radar cross section and types of radar systems. This chapter will be essential for understanding the mechanisms behind the working principle of radar and how it can be applied within the scope of this work.

Chapter3will examine interferometric radar systems. First, it will be presented the theory of interferometry followed by the explanation of the considered waveform within this work: the Stepped-Frequency Continuous Wave. The next section will present the theory and experiments of interferometric radar systems for automotive applications, with a prior introduction, the state-of-the-art for small road object detection, and then the principle of operation, experiments and results for a 24 GHz radar system being used for detection of speed humps in front of a vehicle. It will be followed by the respective conclusions. This work was developed in collaboration with the University of Birmingham, in the “Microwave Integrated Systems Laboratory” Group.

Chapter4will discuss in details the photonics-based radar systems. The topics described will be multiple frequency optical sources, transmitter and receiver architecture, how its possible to guarantee the coherence in the photonics-based radar system, the multi-band configuration, and the dual-band antenna array. The last section will present the operational principle, experimental setup and results for a photonics-based radar system being used as a dual-use transceiver for radar and communication purposes. The work reported in this last section was developed within the scope of this thesis in the “Photonics Network Laboratory”, Pisa in collaboration with the “Wireless and Optical Convergent Access Laboratory”, Inatel, Brazil. Finally, conclusions will be presented.

Chapter5is going to present interferometric techniques using a photonics-based radar system for sub-mm displacement sensing. A discussion on environmental displacement monitoring will be firstly presented, followed by an overview of current radar systems for remote sensing landslides applications (satellites and ground-based InSAR). Moreover, the simulations, experiments and results will be shown for sub-mm displacement sensing using a photonics-based radar system in two cases: for single and multiple scatterers. All the work reported in Chapter 5

was developed inside the “Photonics Network Laboratory”, Pisa for the completion of this thesis.

Finally, the conclusions are going to be addressed in Chapter 6, with a general discussion regarding all the research done and what can still be improved in future.

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Chapter 2 Theory of Radar Systems

2.1 Radar Subsystems

Radar is originally an electrical system that uses electromagnetic (EM) waves to transmit and receive radiofrequency (RF) signals towards a region of inter-est Richards et al. (2010). In Fig. 2.1 Richards et al. (2010), we can see the main elements involved in a radar transmission and reception process, which basically involves a transmitter, an antenna, a receiver and a signal processor unit, with the possibility of being more or less complex.

Figure 2.1: Elements involved in a radar transmission and reception processRichards et al. (2010).

Referred to Fig. 2.1, the transmitter is responsible for generating the EM waves, which are going to be transmitted through the antenna subsystem Richards et al.(2010). It is composed by a waveform generator, a power amplifier and a circulator or switch. The high-power amplifier can be a Klystron, a traveling-wave tube, a crossed-field amplifier or a solid-state device. A high-power oscillator such as a magnetron can also be used as the transmitter. Due to practical concerns, such as average transmitting power and stability, high-power amplifiers

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are preferred over high-power oscillators Skolnik (2007). In coherent systems, the transmitted signal is generated from a set of stable oscillators, and its initial phase is known. Afterwards, the coherent radar has the capability of maintaining the phase relationship between the transmitted and received signals. On the other hand, in noncoherent systems the initial pulse phase is not known, and can be generated, for example, by free-running oscillators Richards et al. (2010).

A circulator or switch is usually responsible for simultaneously connecting the transmitter and receiver to the antenna, providing isolation to protect the receiver from the high power signal coming from the transmitter. When pulsed waveforms configuration is employed, the transmitter is off during the reception, and the echo signal is directed to the receiver chain Skolnik(2007).

The antenna is the subsystem that radiates and receives the signal. Modern radars use directive antennas, which possess a narrow beam (typical beamwidth for tracking radars is about 1 or 2◦) Skolnik (2007). The target can be detected only if it is located within the antenna field of view, which is defined as the angular region that the main beam can be scanned. In order to scan a larger area, the antenna can be steered mechanically or electronically, depending on the required scanning speed. For example, electronically steered phased array antennas can reach less than one microsecond speed Skolnik (2007). Due to this agility, phased-array antennas are able to carry out functions that conventional antennas can not, improving significantly the number of targets that can be seen and tracked Stark (1974).

The antenna size is inversely proportional to frequency. The higher the frequency, the smaller will be the antenna dish. Usually, the antenna requirements (such as frequency selection and aperture size) depend upon the application. For example, search radars operate ideally in lower frequencies, while tracking radars tend to employ higher frequencies, with electrically large apertures and relatively low power Richards et al. (2010);Skolnik (2007).

Afterwards, the EM signal propagates through the atmosphere, reaching the target and other unwanted surfaces present in the area illuminated by the antenna called clutter. The signal reflected from these objects, propagates back to the antenna and is applied to the receiver circuits Richards et al. (2010).

In the receiver chain, the signal will be separated from interferences sources (as white noise), properly amplified and downconverted in order to be detectedRichards et al.(2010); Skolnik (2007). Since great part of the noise is introduced by the receiver itself, the first stage of the receiver (which is also the most significant por-tion of the noise inserted) must present the lowest noise figure as possible Skolnik

(2007).

The receiving process in the majority of modern radars goes as follows. The echo signal captured from the antenna is coupled to the mixer through the duplexer. In some configurations, the received signal goes first through an attenuator in order to protect the receiver from saturation and provide enhanced dynamic range, in a process called Sensitivity Time Control (STC). Then, the EM signal is amplified, usually by a low-noise amplifier (LNA). Afterwards, band-pass filter is used to eliminate the out-of-band frequency components. A mixer is used to converted the signal from RF to an intermediate frequency (IF), in a process called downconversion, making use of a local oscillator (LO). The IF amplifier applies a gain to the receiver signal and maximizes the output signal-to-noise ratio, which is

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a function of the matched filter. This feature helps to maximize the detectability of the signal Skolnik (2007). Posteriorly, another filter is used in IF for rejecting the unwanted intermodulation products. After this, detection takes place, and the signal is applied to an ADC. Here, the signal will be digitized and then a DSP unit will be responsible for processing the signalSkolnik (2007). The detector is the device responsible for removing the carrier from the modulated signal in order to make the data available to be analyzed by the DSP Richards et al.(2010). In a noncoherent processing a threshold is established at the ADC output and the summed signal is compared with this threshold: if the signal surpasses it, a target is assumed to be presentSkolnik (2007).

Modern radar receivers are required to perform many different tasks, including change of frequency, bandwidth and gain function in order to adapt the system to the application and environment. Therefore, these more complex functions are performed by digital control circuits, which choose the optimal radar mode and its parameters and often include automate detection of receiver faultsRichards et al. (2010).

The great majority of modern radar systems employs the type of receiver described above, known as superheterodyne receivers. Fig.2.2 shows the block diagram of such receivers Richards et al. (2010). Other types of receivers also present, such as: crystal video, superregenerative, and homodyneRichards et al.

(2010).

The crystal video receiver is the simplest one. It contains a detector that directly converts the RF signal to video and then forwards it to the processor. Its disadvantages are the low sensitivity due to the broadband noise that is not filtered out and, therefore, is applied to the detector input. Another disadvantage is that the signal shape is often distorted due to the amplification, which is performed all at once by the video amplifierRichards et al. (2010). For these reasons, this receiver is not used frequently in radar systems, being limited to short-range applications.

Superregenerative receivers are based on the positive feedback, which cause them to oscillate at the desired RF frequency. It achieves a extremely high-gain through the RF amplifier, which is the first element of the receiver chain, followed by a detector and a video amplifier. The drawbacks of this receiver are the poor sensitivity, inferior selectivity and stability compared to the superheterodyne receiver. Moreover, it presents high noise introduced by the regeneration process,

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which is predominant at the oscillation frequency Richards et al. (2010). It is mainly used in radar systems that requires simple configurations or noncritical low-cost applications.

In homodyne receivers, a portion of the transmitted signal is used as the local oscillator for the receiver mixerRichards et al. (2010). A circulator is responsible for isolating the transmitted signal, while coupling it to the antenna, and the received signal to the mixer. Therefore, it works as a duplexer for this radar. Because the transmit signal is a reference for the receiver, the system is said to be coherent. Homodyne receivers radars can be used for low-cost continuous wave (CW) radars and radars using FMCW Richards et al. (2010).

Heterodyne receivers are almost the same as superheterodyne receivers, already discussed in this subsection. While heterodyne receivers possess a LO frequency fixed, the superheterodyne uses a tunable local oscillator signal, which presents the capability of dynamically changing the operative RF frequency. This feature allows to employ frequency diversity or to make use of specialized processing waveforms, in which the LO frequency is tuned to follow the RF frequency Richards et al.

(2010).

The received signal suffers from varied noise sources. These noises can appear in different forms: internal and external electronic noise, reflection from unwanted surfaces, the so called clutter, unintentional external EM waves coming from other electronic sources, called electromagnetic interference, and finally intentional jam-ming in the form of noise or false targets. The signal processor unit is responsible for detecting and extracting information about the environment from raw radar signals, as target, clutter and jammingRichards et al.(2010). Modern systems use DSP units, which have the advantage of being completely programmable. This feature allows the system to implement a vast range of functions in a single unit, while enhancing the overall radar performance Richards et al.(2010).

2.2 Electromagnetic Waves

The electromagnetic waves are governed by the four Maxwell’s equations. These equations describe how electric and magnetic fields propagate and interactRichards et al. (2010). The first law, called Gauss’ law for electricity states that charged particles, for example electrons, produce electric fields. The electrical field flux through any closed surface is equal to the net charge inside the surface divided by the permittivity of free space, as mathematically described below Richards et al.

(2010): I s E dA = q ε0 (2.1)

where E is the vector electric field, q is the charge of the charged particle, and ε0 is the permittivity of free space Richards et al. (2010).

The second law, known as Ampere’s law, states that moving charged particles, named currents produce magnetic fields. The flux of the magnetic field through any closed surface is equal to zero, as Richards et al. (2010):

I

s

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where B is the vector magnetic field. It should be pointed out that E and B fields are orthogonal to each other Richards et al. (2010).

In the presence of electric field, a charged particle will experience an electric force, and in the presence of a magnetic field a moving charged particle will suffer a magnetic force. These two fields can, therefore, be visualized by the forces they act on the charged particles. The equation below describe the sum of these two forcesRichards et al. (2010):

F = q(E + υ × B) (2.3)

where F is the EM force vector, υ is the velocity vector of the charged particle, and × denotes the vector crossproductRichards et al. (2010).

Moreover, Faraday’s law says that electric fields are produced by time-varying magnetic fields and vice-versa. It states that the circulation of E around any closed loop is equal to the negative time derivative of the flux of the magnetic field through any area bounded by the loopRichards et al. (2010).

I loop E ds = − Z area ∂B ∂t dA (2.4)

Finally, the fourth Maxwell equation states that magnetic fields are produced by time-varying electric fields. It is known as the Ampere-Maxwell law. It states that the circulation of B around any closed loop, is equal to the permeability of the vacuum times the electric current flowing through any area bounded by that loop, plus the permeability of the vacuum times the permittivity of free space, times the time derivative of the flux of E through that areaRichards et al. (2010).

I loop B ds = µ0i + µ0ε0 Z area ∂E ∂t dA (2.5)

This interaction described above, produces EM waves that can propagate over long distances Richards et al. (2010).

In conclusion, the nature of the EM waves is described by the Maxwell’s equations. The coordinate system that represents the E and B fields is composed by three planes, orthogonal to each other , and also to the direction of propagation of the EM wave, as depicted in Fig2.3 Richards et al. (2010). As we can see, the electric field is aligned along the y-axis, the magnetic field is aligned along the x-axis, an the direction of propagation is represented by the z-axis. The amplitude of an EM wave, which propagates along the z-axis is Richards et al. (2010):

E = E0.cos(kz − ωt + φ) (2.6)

where E0 is the peak amplitude and φ is the initial phase Richards et al.

(2010).

The angular frequency is ω = 2πf radians/sec, and the wave number is k = 2π/λ radians/meter, where λ is the wavelength in meters and f is the carrier frequency in hertz Richards et al. (2010).

Fig. 2.4shows the types of EM waves according to the frequency and identifies the RF radar bands Richards et al. (2010). Each frequency band possesses its own characteristics, which makes each of them suitable for different applications. The High Frequency (HF) band, which comprises frequencies from 3 to 30 MHz,

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Figure 2.3: Representation of the EM fields and velocity vectorRichards et al.(2010).

Figure 2.4: Types of electromagnetic waves Richards et al. (2010).

was mainly adopted during the World War II Skolnik (2007). However, the many disadvantages of this frequency band, as large antennas, high natural ambient

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noise level, narrow bandwidths and restricted electromagnetic spectrum, make it unattractive for most modern applications. Nevertheless, long over-the-horizon detection of aircrafts makes use of the HF region, since the observation of large areas are not practical with conventional microwave radars Skolnik (2007).

The Very High Frequency (VHF) band comprises frequencies from 30 to 300 MHz. This band was employed in the majority of the early radars in the 1930s. The disadvantages of this band are quite the same of those for the HF band. In the other hand, the necessary technology is easy and cheap to implement, specially compared to microwave frequencies. The VHF band can find applications in lower-cost radars and for long-range radars such as those for the detection of satellitesSkolnik (2007).

Ultra High Frequency (UHF) band comprises frequencies from 300 to 1000 MHz. It can be suitable for applications as long-range surveillance radar, especially for extraterrestrial targets such as spacecraft and ballistic missiles, and it can also be used for airborne early warning. Still there are some disadvantages similar to VHF and HF, but here external noise is much less, and beamwidths are narrower than at VHF Skolnik (2007).

Frequencies ranging from 1 to 2 GHz belong to the L-band. In this band the external noise is low and it is possible to obtain high power with narrow beamwidth antennas. Applications include land-based long-range air surveillance radars, military 3D radars and long-range radars that must detect extraterrestrial targetsSkolnik (2007).

The S-band comprises frequencies from 2 to 4 GHz. The long range capability is more difficult to achieve at this frequency and also moving target indication is less suitable at this band compared to lower bands. Attenuation from rain can greatly reduce the range of these radars. The applications suitable for S-band radars are long-range weather radars that must make accurate estimates of rainfall rate, medium-range air surveillance applications such as the airport surveillance radar, military 3D and height finding radars, long-range airborne air surveillance pulse Doppler radars and also can be used for both air surveillance and precision tracking, as in phased array multifunction radarsSkolnik (2007).

Frequencies from 4 to 8 GHz compose the C-band. This band is a compromise between S and X-bands. Though, long-range air surveillance radars at this or higher frequencies is difficult to achieve. The C-band radars find applications as long-range precision instrumentation radars used for the accurate tracking of missiles, multifunction phased array air defense radars and for medium-range weather radars Skolnik (2007).

X-band comprises the frequency range from 8 to 12.5 GHz. In this frequency band, long range capability is a drawback. Rain and weather conditions can also influence negatively this band. However, X-band radars are generally of convenient size and are thus of interest for applications where mobility and light weight are important. Its wide bandwidth makes it suitable for high-resolution radars and the narrow beamwidths that can be obtained with relatively small-size antennas. These radars find applications in military weapon control (tracking) radar and for civil applications. Moreover, it can be suitable for shipboard navigation and piloting, weather avoidance, Doppler navigation, and the police speed meter Skolnik (2007).

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K-band. Later on, the band was subdivided into Ku, K and Ka bands. The

choice of subdividing the K-band was based on the fact that the center of the prior K-band was too close to the water-vapor absorption frequency, and a good compromise was to divide the spectrum in two bands on either side of it. The advantages of these bands are the wide bandwidths they can provide, and also he narrow beamwidths using small apertures. In the other hand, the drawbacks are the limitations due to rain clutter and attenuation, which are increasingly an issue at higher frequencies. An interesting application is airport surface detection radar for the location and control of ground traffic at airports at Ku band Skolnik

(2007).

Above 40 GHz until 300GHz, we have the so-called millimeter (mm) wave-lengths. Applications around 60 GHz are practically precluded due to the high attenuation caused by the atmospheric oxygen. This zone is further subdivided into letter bands in the IEEE standard, as represented in Fig. 2.4. The high attenuations are the major drawback of the mm frequencies, and are of major interest for operation in space, where there is no atmospheric attenuation. It might also be considered for short-range applications within the atmosphere where the total attenuation is not large and can be toleratedSkolnik (2007).

2.3 Radar Configurations and Waveforms

The waveforms commonly used in radar transmissions can be of two different types: continuous wave or pulsed signal. The CW waveform represents the config-uration in which the transmitter is continuously transmitting the signal, without interruption. Pulsed signals, differently, is represented by the transmitter that sends the waveform in finite time intervals. The receiver in the CW configuration is always operational, meanwhile in the pulsed signal, the receiver is on when the transmitter is “off”, so the transmitted signal can be detectedRichards et al.

(2010).

Figure 2.5: The pulsed radar waveform with its parametersRichards et al. (2010).

In the pulsed waveform, the time in which the signal is transmitted is known as pulse width τ , usually measured in µs. During this time the receiver is isolated (turned off) from the transmitter in order to protect its components from the high