Richard Lord's Homepage




Radar Remote Sensing Group
Department of Electrical Engineering
University of Cape Town
Private Bag
Rondebosch 7701
South Africa
Tel: +27 (0)21 - 650 2792
Fax: +27 (0)21 - 650 3465


BSc (Elec Eng) Project

My 4th-year BSc project investigated the feasibility of interpolating wind field measurements obtained from the ERS-1 satellite.

The ERS-1 satellite is equipped with a wind scatterometer, which produces estimates of ocean surface wind speed and direction over a 500 km wide swath or strip across the Earth's surface. However, because of orbital constraints, the swaths do not overlap, resulting in large areas between swaths where no wind field measurements are available. Since wind field data is important for a number of oceanographic, meteorological and climate studies, this project investigated the feasibility of interpolating wind field measurements between swaths.

The following interpolation and smoothing techniques for spatial data were investigated and compared:

Both kriging and spline based methods seemed to be appropriate techniques to interpolate wind field vectors. It was decided to use Ordinary Kriging for this project, because kriging was specifically developed for the prediction of random variables that exhibit spatial autocorrelation, and the wind field data was expected to exhibit a high degree of autocorrelation. Furthermore, Ordinary Kriging has been successfully applied in a number of areas such as soil mapping, mining, rainfall modelling and hydrology.

From the results obtained the conclusion was drawn that it is feasible to interpolate and extrapolate wind field measurements obtained from satellite scatterometers using Ordinary Kriging.

This project culminated in a journal publication with the following reference:

M.R. Inggs and R.T. Lord, "Interpolating Satellite Derived Wind Field Data Using Ordinary Kriging, with Application to the Nadir Gap," IEEE Transactions on Geoscience and Remote Sensing, vol. 34, no. 1, pp. 250-256, January 1996. tgrs96.pdf (315K)

PhD Thesis

Ultra-wideband synthetic aperture radar (SAR) systems operating in the VHF/UHF region are becoming increasingly popular because of their growing number of applications in the areas of foliage penetration radar (FOPEN) and ground-penetrating radar (GPR). The objective of this thesis is to investigate the following two aspects of low-frequency (VHF/UHF-band) SAR processing:
  1. The use of stepped-frequency waveforms to increase the total radar bandwidth, thereby increasing the range resolution, and
  2. Radio frequency interference (RFI) suppression.
A stepped-frequency system owes its wide bandwidth to the transmission of a group of narrow-bandwidth pulses, which are then combined using a signal processing technique to achieve the wide bandwidth. Apart from providing an economically viable path for the upgrading of an existing single frequency system, stepped-frequency waveforms also offer opportunities for RFI suppression.

This thesis describes three methods to process stepped-frequency waveforms, namely

  1. an IFFT method,
  2. a time-domain method, and a
  3. frequency-domain method (originally developed and described by A.J. Wilkinson ).
Both the IFFT method and the time-domain method have been found to be unsuitable for SAR processing applications. The IFFT method produces multiple "ghost targets" in the high resolution range profile due to the spill-over effect of energy into consecutive coarse range bins, and the time-domain technique is computationally inefficient on account of the upsampling requirement of the narrow-bandwidth pulses prior to the frequency shift. The frequency-domain technique, however, efficiently uses all the information in the narrowband pulses to obtain high-resolution range profiles which do not contain any "ghost targets", and is therefore well suited for SAR processing applications. This technique involves the reconstruction of a wider portion of the target's reflectivity spectrum by combining the individual spectra of the transmitted narrow-bandwidth pulses in the frequency domain. It is shown how this method may be used to avoid spectral regions that are heavily contaminated with RFI, thereby alleviating the problem of receiver saturation due to RFI. Stepped-frequency waveforms also enable the A/D converter to sample the received narrow-bandwidth waveform with a larger number of bits, which increases the receiver dynamic range, thereby further alleviating the problem of receiver saturation during the presence of RFI.

In addition to using stepped-frequency waveforms for RFI suppression, a number of other techniques have been investigated to suppress RFI. Of these, the notch filter and the least mean squares (LMS) adaptive filter have been implemented and applied on real P-band data obtained from the E-SAR system of the German Aerospace Center (DLR), Oberpfaffenhofen, and on real VHF-band data obtained from the South African SAR (SASAR) system. Both methods significantly suppressed the RFI in the real images investigated.

It was found that the number of range lines upon which the LMS adaptive filter could operate without adaptively changing the filter tap weights was often well above 100. This facilitated the re-writing of the LMS adaptive filter in terms of an equivalent transfer function, which was then integrated with the range-compression stage of the range-Doppler SAR processing algorithm. Since the range-compression and the interference suppression could then be performed simultaneously, large computational savings were achieved.

A technique was derived for suppressing the sidelobes which arise as a result of the interference suppression of the LMS adaptive filter. This method was also integrated with the range-compression stage of the range-Doppler processor, leading to a very efficient implementation of the entire RFI suppression routine.

A pdf copy of the thesis can be obtained here (11MB). Other research outputs can be viewed in the list of Published Papers.

Postdoctoral Research

During 2002 I conducted postdoctoral research at the Remote Sensing Technology Institute of the German Aerospace Center DLR, Oberpfaffenhofen, Germany. This experience was made possible by a postdoctoral fellowship, which was kindly awarded to me by the National Research Foundation (NRF).

During my stay at the DLR I was involved with the development of synthetic aperture radar (SAR) processing algorithms for the future German TerraSAR-X remote sensing satellite, which will launch in 2006. TerraSAR-X is a high-resolution X-band SAR based on active phased array technology, which allows operation in StripMap, ScanSAR and SpotLight modes in different polarisations, with scientific and commercial applications.

My work concentrated on the development of a spaceborne SpotLight SAR simulator, which can accurately simulate raw SpotLight SAR data. This simulated data was used to analyse the characteristics of a SpotLight SAR processor, which was developed by a colleague at the DLR. Any approximations, which are made by this SpotLight SAR processor, could accurately be analysed with the simulated data, regarding their effects on the impulse response function (IRF) and the resulting processing accuracy. Even the effects of satellite steering errors (for example inaccurate yaw-steering) could be analysed. Simulation results of worst-case scenarios were produced in terms of satellite position (latitude and longitude) and antenna pointing direction (off-nadir angle).

During my stay I wrote a number of technical notes, or was co-author of technical notes. These notes summarise most of the work I was involved with, including a study on the absolute orbit accuracy requirements for the TerraSAR-X satellite, calculation of Kepler orbits to simulate the satellite trajectory, calculation of the Earth ellipsoid intersection given the antenna pointing vector, and an investigation of the SpotLight processor approximations.

I gained significant work experience at the DLR in the field of radar remote sensing, and it was a pleasure working together with a highly professional team on the TerraSAR-X project.


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