In the field of airborne earth observation there is special attention to compact, cost effective, high resolution imaging sensors. Such sensors are foreseen to play an important role in small-scale remote sensing applications, such as the monitoring of dikes, watercourses, or highways. Furthermore, such sensors are of military interest; reconnaissance tasks could be performed with small unmanned aerial vehicles (UAVs), reducing in this way the risk for one¿s own troops. In order to be operated from small, even unmanned, aircrafts, such systems must consume little power and be small enough to fulfill the usually strict payload requirements. Moreover, to be of interest for the civil market, cost effectiveness is mandatory. Frequency Modulated Continuous Wave (FMCW) radar systems are generally compact and relatively cheap to purchase and to exploit. They consume little power and, due to the fact that they are continuously operating, they can transmit a modest power, which makes them very interesting for military operations. Consequently, FMCW radar technology is of interest for civil and military airborne earth observation applications, specially in combination with high resolution Synthetic Aperture Radar (SAR) techniques. The novel combination of FMCW technology and SAR techniques leads to the development of a small, lightweight and cost-effective high resolution imaging sensor. SAR techniques have been successfully applied in combination with coherent pulse radars. Also the concept of synthetic aperture with FMCW sensors has already been put forward in literature, and some experimental systems have been described. However, the practical feasibility of an airborne FMCW SAR was not evident; the experimental sensors described in literature were, in fact, radars mounted on rail supports operating in ground SAR configurations and at short distances. The FMCW radars could perform measurements in each position of the synthetic aperture and then be moved to the next one. As in conventional pulse SAR systems, the stop-and-go approximation could be used; such an approximation assumes the radar platform stationary during the transmission of the electromagnetic pulses and the reception of the corresponding echoes. For airborne FMCW radars, however, the stop-and-go approximation can be not valid anymore because the platform is actually moving while continuously transmitting. A complete model for the deramped FMCW SAR signal was missing in the literature. In addition to the particular signal aspects relative to the combination of FMCW technology and SAR techniques, the use of FMCW radars for long range high resolution applications was not evident. In practical FMCW sensors, specially when using cheap components, the presence of unwanted non-linearities in the frequency modulation severely degrades the radar performances for long distances. Again, proper processing methods to overcome such limitation due to frequency non-linearities were not available to the scientific community. Therefore, the area of FMCW SAR airborne observation and related signal processing aspects was a very novel field of research. At the International Research Centre for Telecommunications and Radar (IRCTR) of the Delft University of Technology, a project was initiated to investigate the feasibility of FMCW SAR in the field of airborne earth observation and to develop proper processing algorithms to fully exploit the capability of such sensors. Within the framework of the project, the following novelties and main results have been reached and are presented in the thesis: Non-linearities correction. The author has developed a very novel processing solution, which completely solves the problem of the presence of frequency non-linearities in FMCW SAR. It corrects for the non-linearity effects for the whole range profile at once, and it allows a perfect range focussing, independently of the looking angle. The proposed method operates directly on the deramped data and it is very computationally efficient. A complete FMCW SAR signal model. The author has proposed a detailed analytical model for the FMCW SAR signal in the two-dimensional frequency domain. Based on this model, proper algorithms have been developed which guarantee the best performance when processing FMCW SAR data. Moving Target Indication (MTI) with frequency modulated CW SAR. Two SAR MTI methods are proposed. The first is based on the frequency slope diversity in the transmitted modulation by using linear triangular FMCW SAR, while the second makes use of the Doppler filtering properties of randomized SFCW modulations. First demonstration of an X-band FMCW SAR. A flight test campaign has been organized during the last part of 2005. The results were very successful. The feasibility of an operational FMCW SAR based on cheap components has been proved under practical circumstances. Thanks to the special algorithms developed, FMCW SAR images with a measured 45 cm times 25 cm resolution (including windowing) were obtained for the first time.@en

Airborne synthetic aperture radar is a promising new application for FM-CW radars. At the IRCTR, an operational demonstrator system has been developed in order to investigate practicability of FM-CW SAR and to prove that and FM-CW SAR an FM-CW SAR system can be operated in an efficient manner from a small platform. The platform used to fly the demonstrator system and the integrated navigation system will be introduced. In addition, the digitalization and the synchronization regarding the navigation system will be addressed. Subsequently, the motion compensation details, especially the squint angle compensation, are explained. The working stautus of the demonstrator system and the integrated navigation system is proven by the results of an airborne FM-CW SAR campaign. The results obtained during the airborne campaign are presented and the effect of squint angle compensation is shown. The airborne results could not be further improved by applying more sophisticated motion compensation algorithms or autofocussing techniques. Therefore, it was decided to develop an improved FM-CW front end in order to enhance the FM-CW SAR image quality.@en

A new technique to enhance the indication of moving targets is proposed, which makes use of randomized Stepped Frequency Continuous Wave (SFCW) modulations, where the frequency order is changed from pulse to pulse. This kind of signals gives the possibility to discriminate target responses whose Doppler spectrum is folded back into the clutter region. The discrimination is performed directly during the range compression, so avoiding spectrum aliasing in the Doppler domain. For the range compression, the deramping technique, commonly used with linear SFCW, is extended to the case of non linear stepped modulations. In this way, the sampling constraints can be relaxed also when using randomized SFCW signals, allowing the same sampling frequency as with deramped linear SFCW, and reducing therefore the system complexity. Compared to linear modulations, randomized SFCW signals give also the advantage to suppress range ambiguities and therefore the capability to look at further range.@en

This paper presents a novel approach for processing data acquired with Frequency Modulated Continuous Wave (FMCW) dechirp-on-receive systmes by using a non-linear frequency scaling algorithm. The range frequency non-linearity correction, the Doppler shift induced by the continuous motion and the range migration removal are performed at the same time in the wavenumber domain with a very efficient solution. The cross-range focusing is then achieved with conventional matched filtering. Real data have been acquired with the FMCW SAR demonstrator system built at the Delft University of Technology. Stripmap images processed with the proposed method are presented in order to show the validity of the non-linear frequency scaling algorithm.@en

The combination of Frequency Modulated Continuous Wave (FMCW) technology and Syntehetic Aperture Radar (SAR) leads to lightweight, cost-effective imaging sensors of high resolution. In rithms cannot be considered valid anymore, so the motion within the sweep needs to be taken into account. Analytical development of an FMCW SAR algorithm starting from the deramped signal and whithout using the stap-and-go approximation is presented in this paper; it is then validated processing simulated and real data. Furthermore, the effects of a moving target as tehy appear in an FMCW SAR image are described and the results can be used to assist Moving Target Indicator (MTI) capabilities.@en

A new technique to enhance the indication of moving targets is proposed, which makes use of randomized Stepped Frequency Continuous Wave (SFCW) modulations, where the frequency order is changed from pulse to pulse. This kind of signals gives the possibility to discriminate target responses whose Doppler spectrum is folded back into the clutter region. The discrimination is performed directly during the range compression, the deramping technique, commonly used with linear SFCW, is extended to the cased of non linear stepped modulations. In this way, the sampling constraints can be relaxed also when using randomized SFCW signals, allowing the same sampling frequency as with deramped linear SFCW, and reducing therefore the system complexity. Compared to linear modulations, randomized SFCW signals give also the advantage to suppress range ambiguities and therefore the capability to look at further range.@en

In some earth observation applications there is a requirement for low cost, high performance imaging radar systems small enough to be operated from small, even unmanned, aircraft. The existing pulse radars are non-cost effective complex systems. Therefore they are not suited for these applications. Frequency Modulated Continuous Wave (FM-CW) radar systems are on the other hand generally very compact and relatively cheap to purchase and to use. IRCTR, together with TNO, is investigating the feasibility of combining FM-CW technology and high resolution Synthetic Aperture Radar (SAR) techniques. This research has led to the development of two different systems. An FM-CW millimetre wave SAR sensor (35 GHz) was built and is being tested. The system is battery powered and uses a PC to control the radar and store the data, including GPS and data from motion sensors. The system is mounted in a pod attached to a light motor glider. Furthermore, a detailed system model has been developed in order to estimate and analyze the performance of the demonstrator system. The measuerementsw made with the demonstrator system are evaluated and the findings are used to improve the system model. This improved model will be a strong aid in the design of future higher performance FM-CW SAR systems. The second development regards a miniature P-band (450 MHz) polarimetric FM-CW SAR. This system is intended for use with an ultra-light aircraft. It is designed for tropical forest monitoring studies but could be used in foliage penetration applications as well. This system will be battery powered and uses a small memory stick to store the raw data. Data processing is now foreseen as an off-line activity. The design of the system is still under study and the realization is expected to start in the second half of 2005. This system will employ a Direct Digital Synthesizer to generate the waveform that is transmitted at P-band. Two separate dual polarized patch antennas will be used for transmit and receive.@en