Signal design and processing for noise radar

An efficient and secure use of the electromagnetic spectrum by different telecommunications and radar systems represents, today, a focal research point, as the coexistence of different radio-frequency sources at the same time and in the same frequency band requires the solution of a non-trivial interference problem. Normally, this is addressed with diversity in frequency, space, time, polarization, or code. In some radar applications, a secure use of the spectrum calls for the design of a set of transmitted waveforms highly resilient to interception and exploitation, i.e., with low probability of intercept/ exploitation capability. In this frame, the noise radar technology (NRT) transmits noise-like waveforms and uses correlation processing of radar echoes for their optimal reception. After a review of the NRT as developed in the last decades, the aim of this paper is to show that NRT can represent a valid solution to the aforesaid problems

Waveform Design for Ground-Penetrating Radar

A ground-penetrating radar is being designed to find subterranean structures. This is difficult to do because of varying mediums. Having more bandwidth can help mitigate this problem. Because the frequency spectrum is so cluttered, one method to do this is to use non-contiguous orthogonal frequency division multiplexing (NC-OFDM) to occupy several free areas of the spectrum. An NC-OFDM waveform was designed and optimized with respect to peak-to-average-power ratio, orthogonality, spectral leakage and autocorrelation sidelobes. Techniques such as the use of a Zadoff-Chu sequence and a gap filling algorithm were implemented to do this. The waveform was tested in simulation to show that while computationally expensive, this may be a viable waveform for ground-penetrating radar

Investigation of Non-coherent Discrete Target Range Estimation Techniques for High-precision Location

Ranging is an essential and crucial task for radar systems. How to solve the range-detection problem effectively and precisely is massively important. Meanwhile, unambiguity and high resolution are the points of interest as well. Coherent and non-coherent techniques can be applied to achieve range estimation, and both of them have advantages and disadvantages. Coherent estimates offer higher precision but are more vulnerable to noise and clutter and phase wrap errors, particularly in a complex or harsh environment, while the non-coherent approaches are simpler but provide lower precision. With the purpose of mitigating inaccuracy and perturbation in range estimation, miscellaneous techniques are employed to achieve optimally precise detection. Numerous elegant processing solutions stemming from non-coherent estimate are now introduced into the coherent realm, and vice versa. This thesis describes two non-coherent ranging estimate techniques with novel algorithms to mitigate the instinct deficit of non-coherent ranging approaches. One technique is based on peak detection and realised by Kth-order Polynomial Interpolation, while another is based on Z-transform and realised by Most-likelihood Chirp Z-transform. A two-stage approach for the fine ranging estimate is applied to the Discrete Fourier transform domain of both algorithms. An N-point Discrete Fourier transform is implemented to attain a coarse estimation; an accurate process around the point of interest determined in the first stage is conducted. For KPI technique, it interpolates around the peak of Discrete Fourier transform profiles of the chirp signal to achieve accurate interpolation and optimum precision. For Most-likelihood Chirp Z-transform technique, the Chirp Z-transform accurately implements the periodogram where only a narrow band spectrum is processed. Furthermore, the concept of most-likelihood estimator is introduced to combine with Chirp Z-transform to acquire better ranging performance. Cramer-Rao lower bound is presented to evaluate the performance of these two techniques from the perspective of statistical signal processing. Mathematical derivation, simulation modelling, theoretical analysis and experimental validation are conducted to assess technique performance. Further research will be pushed forward to algorithm optimisation and system development of a location system using non-coherent techniques and make a comparison to a coherent approach

Waveform Design with Time and Frequency Constraints for Optimal Detection of Elastic Objects

In active sonar, the goal is to learn about an object or environment by transmitting a sound and processing the echo. The sound we choose to transmit will determine what we learn about the object, much like the choice of question we ask a person will determine what we learn from them. Thus, designing the best (i.e. optimal) transmit waveform is a longstanding area of research that remains active since different environments and ever evolving operational objectives weigh heavily on how we define optimality.In this work we extend a recent result by Kay that gives the optimal transmit signal that maximizes the probability of detecting an elastic object in the presence of Gaussian reverber- ation and additive Gaussian interference. Kay's solution specifies the spectral magnitude for the optimal transmit waveform, and hence there is an unlimited number of "optimal" wave- forms that can be transmitted, all with the same spectral magnitude but differing in terms of time domain characteristics such as duration and peak power. We extend Kay's approach in order to obtain a unique optimal waveform by incorporating time-domain constraints into two optimization-based problem formulations. These two problem formulations lead to new and complementary signal design approaches that impose temporal duration constraints while preserving, to varying degrees, the optimality inherent in the spectral magnitude

FMCW radar prototype development for detection and classification of nano-targets

Detection and classification of nano-targets (less than 5 cm in size) are becoming important technical challenges as nano-targets are largely invisible to conventional radar. Nano-drones, for example, may soon become a tangible threat capable of providing short-range stealthy surveillance. Similarly, insect pests are posing a significant agricultural risk by causing crop losses and subsequently reducing the yields. Frequency Modulated Continuous Wave (FMCW) radar is a technology that can provide short-range detection, with no blind range and very high resolution, at a relatively low cost. This paper presents the latest results of an ongoing project aiming at designing and developing a low-cost and bespoke 24 GHz FMCW radar prototype to enable detection of nano-targets and extract their Doppler signatures. A home-brew S-band FMCW radar prototype has been initially designed and developed, using off-the-shelf components, to demonstrate the feasibility of our proposed design solution and inform all future activities at 24 GHz. Several experiments have been carried out to test the S-band prototype and assess its performance against larger drones and cars. Results have shown targets could be successfully detected and their micro-Doppler signatures extracted using Short-Time Fourier Transform (STFT) techniques

Time and Frequency Transfer in a Coherent Multistatic Radar using a White Rabbit Network

Networks of coherent multistatic radars require accurate and stable time and frequency transfer (TFT) for range and Doppler estimation. TFT techniques based on global navigation satellite systems (GNSS), have been favoured for several reasons, such as enabling node mobility through wireless operation, geospatial referencing, and atomic clock level time and frequency stability. However, such systems are liable to GNSS-denial, where the GNSS carrier is temporarily or permanently removed. A denial-resilient system should consider alternative TFT techniques, such as the White Rabbit (WR) project. WR is an Ethernet based protocol, that is able to synchronise thousands of nodes on a fibre-optic based network with sub-nanosecond accuracy and picoseconds of jitter. This thesis evaluates WR as the TFT network for a coherent multistatic pulse-Doppler radar – NeXtRAD. To test the hypothesis that WR is suitable for TFT in a coherent multistatic radar, the time and frequency performance of a WR network was evaluated under laboratory conditions, comparing the results against a network of multi-channel GPS-disciplined oscillators (GPSDO). A WR-disciplined oscillator (WRDO) is introduced, which has the short-term stability of an ovenised crystal (OCXO), and long-term stability of the WR network. The radar references were measured using a dual mixer time difference technique (DMTD), which allows the phase to be measured with femtosecond level resolution. All references achieved the stringent time and frequency requirements for short-term coherent bistatic operation, however the GPSDOs and WRDOs had the best short-term frequency stability. The GPSDOs had the highest amount of long-term phase drift, with a peak-peak time error of 9.6 ns, whilst the WRDOs were typically stable to within 0.4 ns, but encountered transient phase excursions to 1.5 ns. The TFT networks were then used on the NeXtRAD radar, where a lighthouse, Roman Rock, was used as a static target to evaluate the time and frequency performance of the references on a real system. The results conform well to the laboratory measurements, and therefore, WR can be used for TFT in coherent radar