Donny Danudirdjo

Anisotropic Phase Unwrapping for Synthetic Aperture Radar Interferometry

This paper presents a new phase unwrapping method for synthetic aperture radar interferometry (InSAR). Compared with other phase observations, InSAR data are unique due to the foreshortening effect in SAR images. This effect makes the interferogram phase anisotropic, i.e., the statistics of phase gradients along the ground-range axis are different from those in the azimuth direction. Furthermore, the distribution of phase gradients in the ground-range direction is not symmetric. The proposed method targets the most likely unwrapped phase by considering the foreshortening effect in SAR observation, noise characteristics in InSAR phase data, and fractional Brownian surface as a suitable model for natural topography. Experiments with simulated terrains and real InSAR data show that the method gives comparable or better digital elevation model results than the other tested methods.

Local Subpixel Coregistration of Interferometric Synthetic Aperture Radar Images Based on Fractal Models

This paper presents a fine coregistration method for synthetic aperture radar (SAR) image processing, such as in InSAR interferogram generation. Under the assumption that SAR images are properly modeled as fractional Brownian motion, relative subpixel offsets between two images can be derived from the statistics of their increments. The method does not require upsampling or cross-correlation, thus allowing for an accurate offset estimation with less computational load. Implemented as a local coregistration procedure, it also provides a nonrigid geometric alignment that nicely follows the topography of the area. Experimental results show that the method gives comparable results to the conventional method, in terms of the accuracy of the generated digital elevation models. In particular, it displays superior accuracy for images with near homogeneous fractal behavior.

InSAR Image Regularization and DEM Error Correction With Fractal Surface Scattering Model

This paper presents a method for removing spikes in digital elevation models (DEMs) caused by residues in interferometric synthetic aperture radar (InSAR) phase image. We consider that the scattering mechanism is properly modeled by the small perturbation method for fractal surfaces and present a model that relates the phase and magnitude in InSAR image. This data model provides the regularization term of the method, without directly enforcing smooth phase or magnitude. Noise models are given by additive Gaussian for the phase and multiplicative non-unit-mean gamma for the magnitude. Experiments with simulated and real L-band data show that the proposed method considerably improves DEM accuracy and simultaneously suppresses speckle and phase noise.

Synthesis of two-dimensional fractional brownian motion via circulant embedding

Fractional Brownian motion (fBm) is a useful model to represent various natural phenomena and its synthesis has been a topic of interest in literature. This paper presents an algorithm to generate two-dimensional fBm based on circulant embedding method. Although this method has been proven in simulating one-dimensional fBm, its extension into two-dimensional is not straightforward. To solve the problem, we use circulant embedding as an exact method to synthesize the second-order increments of fBm, and recover the desired fBm via integration in frequency domain. The advantage of using second-order increments is its direct extensibility to any dimension. Experimental results show that the proposed method works efficiently fast and offers acceptable results when compared to the theoretical statistics of fBm.

Magnetic imaging system in smartphones based on built-in magnetometer

Magnetometers have recently become a built-in feature in most tablets and smartphones around the globe.

Magnetic Subsurface Imaging Systems in a Smartphone Based on the Built-In Magnetometer

We present a magnetic subsurface imaging system on a smart-phone that employs the built-in magnetometer. The smart-phone’s sensor measures magnetic field strength at sparse locations on a user-defined grid of the surveyed area. Based on the collected data, magnetic field distribution of the entire area is then reconstructed by using an interpolation algorithm, which yields field values in all of three-axial directions. Since the field shows the influence of the buried objects within the surveyed area to the background magnetic fields, the shape and size of the objects can be derived from the image representing the field. An implementation of the system as an application software on Android smart-phone, which consists of data measurement and reconstruction subsystems, is also described. We demonstrate the capability of the proposed system by performing subsurface imaging of small hidden objects in a laboratory test-range.

A low cost design of Low Field NMR apparatus suitable for research and educational purposes

This paper is written to report the development of Low Field NMR system using inexpensive material and to simulate NMR signal acquisition. One of the prospects of conducting this research is the possibility of building low field MRI for neural activity imaging. Several parts needed for building the system are considerably low cost and affordable. Homogeneous magnetic field is generated by Helmholtz coil whose frame is built from non ferrous material (PVC) to minimize distortion to the homogeneity. This coil is able to provide magnetic field up to 3.35 mT at 7.4 A. In order to increase the amplitude of NMR signal, the sample is induced by magnetic field of 22 mT using prepolarization coil. Simulation of NMR signal acquisition is done by transmitting 100 computer-generated NMR signals using a coil. The transmitted signal is then acquired by receiver coil to be amplified and filtered. The test shows that the receiver system is able to detect 10 microvolt signal. Also, this paper shows that averaging all of acquired signals will increase the SNR of the acquired signal. By using affordable materials and the simplicity of the design, this apparatus is suitable for research and educational purposes.

Multispectral imaging with a modified DSLR camera

This paper describes a multispectral imaging system using a Canon EOS 1000D camera, photographic lenses, and a dispersive element. The system consists of three subsystems: a PVC holder to arrange the optical devices that can be connected to the camera, an acrylic frame to support the position of the PVC holder, and expectation maximization algorithm to reconstruct the spectral-spatial data cube. We perform a calibration process to output the reference matrix of the reconstruction using four LEDs with different wavelengths. The output of the system is a data cube in the form of 20 images that represent the observed intensity response from 455 nm to 645 nm.

Design and realization of multi-wide-band FMCW radar for educational purposes

We present a simple design and realization of a low-cost FMCW radar. The modular design, separated the system into baseband and RF system, makes the working frequency changeable and the bandwidth programmable. The baseband is built from low-cost microcontrollers, whereas the RF system consisting of basic components of an SFCW system. We present a realization of a S-Band UWB radar with a programmable PRF and bandwidth 1.0 GHz and shows experimental results of the implemented radar.

A prototype of compact digital holographic microscope with Mach-Zehnder configuration

Digital holographic microscope (DHM) is a non-destructive imaging device to obtain three-dimensional (3D) images of microscopic objects. In this research, we design and implement a prototype of compact digital holographic microscope system to obtain 3D images of multiple objects placed horizontally. The features include the use of Mach-Zehnder interferometer with vertical beam propagation, the addition of four mirror mounts and three micro shafts to provide accurate beam adjustment, a modified electronic circuit as intensity regulator to control laser intensity, and the inclusion of Canon EOS 1000D as the image detector that allows wider intensity adjustment by means of the ISO setting and shutter speed. We check the system performance using two types of test objects, namely multiple sugar grains and USAF 1951 resolution target. Experimental results show that the prototype can reconstruct 3D images of multiple objects and USAF 1951 line patterns up to 5.66 lines/mm.