Defence Research Agency

About: Defence Research Agency is a based out in . It is known for research contribution in the topics: Synthetic aperture radar & Radar. The organization has 1211 authors who have published 1109 publications receiving 31542 citations.

An overview of the development of the Earth Observation Data Centre and its processing and achiving facilities

Abstract: This paper presents an overview of the development of the Earth Observation Data Centre (EODC) and its facilities for processing and archiving ERS-1 data. The hardware systems of the EODC and the implementation of product processing chains are described. The paper concludes with a look at the achievements of the EODC and a discussion of the problems encountered with a view to the lessons learnt for future developments.

Search for chaotic output from a sigma-delta modulator: an archetypal retarded nonlinear system

Abstract: One of the most common methods by which a non-linear physical system can be determined to be stable, globally unstable, or in fact chaotic is by an investigation of its Lyapunov exponents. When the system contains a delay equation the correct co-ordinate space for the system is infinite dimensional. In this case special techniques must be used to make the problem solvable by numerical methods. We have investigated a practical example of such a retarded non-linear system: the continuous Sigma-Delta modulator (CSDM). The CSDM is an adaptation of the widely used digital Sigma-Delta modulator in which, now, the input and output are continuous functions of time, the quantizer has been replaced with a sharp hyperbolic tangent function and the feedback delay time can be varied continuously. This allows the system dynamics to be modeled by delay ordinary-differential equations. We have simulated such a device for various feedback configurations and delay times and have been able to show how a full calculation of the Lyapunov exponent spectrum allows a detailed analysis of stability conditions for the CSDM. Furthermore, we discuss the implications of this approach for retarded non-linear systems in general and a range of signal processing applications in particular.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Multiresolution analysis using adaptive wavelets

Abstract: A form of nonlinear multiresolution analysis is described in which, by appropriate scaling and choice of analyzing wavelet, information concerning the structure of a signal or image is derived from local maxima and minima in the data transformed to position-scale, or wavelet, space. There results a data coding as a discrete sum of 'signal wavelets' that are generated adaptively to give a good local fit to the data, rather than being specified a priori as in standard applications of the wavelet transform. Although the wavelets generated in this manner are not in general mutually orthogonal, an orthogonal set of basis functions is derived subsequently in the form of linear combinations of appropriately weighted wavelets. A further novel aspect of the method is the introduction of a non-Gaussian statistical model which is fitted to the data and used to categorize individual data samples and to focus attention upon unusual events. Statistical distributions at different scales are related by means of fractal exponents. Applications are described to remote- sensing imagery, where the 'unusual events' are typically man- made artifacts in natural environmental backgrounds, and to a medically oriented signal in the form of heart rate, where the method can be used to draw attention to unusual patterns of heart beats.

Prediction model updating using the Rose-200 oblique ionospheric sounder at mid and higher latitudes

Abstract: Differences between predicted and measured maximum usable frequency (MUF) values arise due to the short-term temporal variability of the ionosphere, particularly at high latitudes, and the inability of statistically based ionospheric models, used in HF prediction programs, to predict these variabilities. Techniques, however, are being investigated to improve ionospheric models by incorporating real-time ionospheric measurements, obtained using oblique or vertical incidence sounders. A technique, termed 'updating' (Uffelman et al. 1982, and Goodman and Daehler 1988) has been used with the MINIMUF prediction program at mid-latitudes to improve the predicted MUF. It has also been demonstrated by Uffelman et al. how the updating results obtained from a single path can be spatially extrapolated to improve MUF predictions on other spatially distributed mid-latitude paths. The present paper investigates the applicability of the updating technique to mid and higher-latitude propagation paths. Oblique soundings are recorded using ROSE-200 (Radio Oblique Sounding Equipment), and the MUF values measured over the mid and higher-latitude paths are compared with updated MUF predictions made using the APPLAB prediction model. The investigations are performed during a period of low auroral activity and approximately 48 hours of data are analysed with updates being performed at 6 hourly intervals. >