In non-destructive testing and evaluation, the microwave-based synthetic aperture radar (SAR) imaging have shown great potential in the detection of defects on buried objects such as pipes. However, due to the pipe curvature and high stand-off distance when inspecting an insulated pipe, the useful defect information used to characterize the pipe image is lost as a result of low signal-to-noise ratio (SNR), resulting in a blurred and unfocused image. In this paper, we proposed a robust microwave-based SAR imaging using autofocus range-Doppler algorithm (RDA) for the inspection of an insulated pipe. Singular value decomposition is used to mitigate the effect of the insulation layer by removing dominant singular values that characterize the insulation layer, and the autofocus RDA is designed to refocus the SAR image using RDA residual refocusing. The SNR, improvement factor (IF), and squared error are used to evaluate the qualitative image information of the defect on the pipe. Experimental results showed the efficacy of the method in detecting defects on an insulated pipe, in particular, a significant reduction in the noise content of the image compared to the known SAR Omega-k algorithm. It was found that the autofocus RDA gave higher values of SNR and IF (3 dB and 6 dB) compared to the Omega-k algorithm (-1 dB and 2 dB), respectively.

The ionosphere is a dispersive medium of charged particles between the satellite and the user on Earth. These dispersive ionized media play a vital role in the various applications of GPS (Global Positioning Systems) because the ionosphere directly influences transionospheric radio waves propagating from the satellite to the receiver. Solar flares affect the ionization state of the ionosphere with their high intensity. Sometimes the intensity is so severe that it accelerates the rate of ionization, resulting in ionospheric storms; during the ionospheric storms the concentration of charged particles varies. Among the various phenomena in the ionosphere, TEC (Total Electron Content) is responsible for range error which produces a time delay in the radio signal. The rate of change of TEC with respect to time is abbreviated as ROT. It is one of the parameters that express the ionospheric irregularities with respect to time. This work investigates the effect of ROT fluctuation on the precise positioning of GPS receivers during low solar activity periods in the equatorial anomaly region. Good geometry and a sufficient number of locked satellites provide more accuracy within the centimeter level, but the case may be different when there are any ionospheric storms. Even a few satellite signals passing through the ionospheric irregularities can cause a significant error in positioning. Thus, it is important to understand the ionospheric irregularities observed by GPS receivers in order to correct them. The ROT (TEC/Minute) parameter is used here to study the occurrence of TEC fluctuation and its potential effect on GPS, such as a horizontal positional error or the satellite geometry of the GPS receiver. This investigation is based on the analysis of a one-year observation of a fixed GPS receiver installed at Bhopal (23.202⁰N, 77.452₀E), India during low solar active period in 2005. The GPS receiver used here is a GISTM-based dual frequency NovAtel OEM4 GPS receiver.

Ionospheric scintillation occurs for transionospheric radio waves propagating through random ionospheric irregularities, which affect the phase and/or amplitude observations made by the receiver. Generally, the scintillation induces excess carrier phase jitter in the phase lock loop (PLL) of the GPS receiver, and strong scintillation can cause a conventional PLL (ATAN [arctangent method], constant bandwidth Bn = 10 Hz) to lose phase lock resulting in no GNSS signal available at that time from the satellite path(s) affected. A PLL with a larger bandwidth is one solution to mitigate this but at the expense of extra phase noise, and this may not be an optimal solution during weak scintillation conditions. This study uses a novel WBMod (Wide Band Modeling) assisted PLL for robustness of availability of GPS services with lower introduction of extra phase noise. At the initial stage, an optimal PLL bandwidth is predicted using WBMod to stabilize the PLL during strong phase scintillation. A FAB (Fast Adaptive Bandwidth) PLL is used to minimize the phase error. To investigate this approach, a realistic scintillated signal is produced using 50 Hz raw GPS signal observations (carrier phase and intensity) collected at Yellowknife (Yell, 64.48° N, -114.52° E) in a Matlab-based GPS software receiver.

In synthetic aperture radar (SAR) applications, high-resolution images and effective estimation processes are vital for the reconstruction of any targets. This can be achieved using multicarrier waveforms such as orthogonal frequency division multiplexing (OFDM) with the help of appropriate signal processing algorithms. However, the quality of the reconstructed image degrades in low signal-tonoise ratio (SNR) environments during SAR data acquisition. In this paper, an integrated multiple signal classification (MUSIC) assisted least square estimation (LSE) algorithm (MUSIC-LSE) is proposed to enhance the quality of the reconstructed SAR image in a low-SNR environment. Simulation results measured and evaluated the quality of the reconstructed image using three performance indicators of root mean square error (RMSE), main lobe width and cumulative side lobe levels. These indicators are also used to investigate the effect of OFDM subcarrier selection on the reconstructed image for a different number of subcarriers. Experimental validation of the approach is carried out using two steel pipes to image and detect the curvature of the steel pipes. The results show that the proposed MUSICLSE approach produces better-reconstructed images compared to the existing linear frequency modulated (LFM) chirp and OFDM-LSE approaches in low-SNR (–10 dB) environments and enables the radar to distinguish and detect the curvature of the pipes even below the radar range and cross-range resolution.

The use of wireless sensor networks (WSNs) for the purposes of border surveillance is a very hot topic at the moment. Such a network comprises large numbers of sensor nodes, sparsely located over a wide terrain where the generated data are collected by multiple sinks located in secure head quarters. Due to its importance and criticality, data disseminated through such a network is vulnerable to outsider attacks. Therefore, a location dependent key management scheme could be employed in order to provide security in terms of confidentiality and authenticity. This paper presents a novel location dependent key management protocol for multiple sinks (LKMP-MS) whose design is based on a cell reporters scheme. In this protocol, the generated report regarding any event contains three security levels. In particular, the third level is implemented as an endorsement of the set of cell reporters allocated inside each cell. Accordingly, the generated event report is accepted by the sink. As a result, the adversary would have to compromise all endorsement nodes and all cell reporters selected by all sinks in order to generate a fake report inside a particular spot. This paper introduces a novel algorithm to implement the required revocation for both compromised nodes and cells in order to overcome the possible consequences of these entities being compromised. In order to evaluate and compare the LKMP-MS with recent schemes, a mathematical analysis is implemented and a simulation environment is built based on Contiki and MATLAB to validate the mathematical analyses. Finally, an extensive mathematical analysis is presented in order to calculate the optimum number of cell reporters that gives the highest level of security.