Central to future radar technology will be the application of array antennas, and this book is dedicated to the application of these antennas which offer great flexibility for system design in combination with suitable control and signal processing. The design work can be imagined roughly in the following way: on the basis of a required radar function we look for a signal and data processing concept with the aid of statistical signal theory or filter theory. For a developed concept the efficiency is examined analytically or by simulation studies. The antenna concept, suitable to the required function, can then be developed using basic knowledge about the antenna array. Within this book we will find this implicit procedure at several points. Statistical signal theory forms an important fundamental tool, and is presented as simply as possible and limited to the most important aspects. In addition, we need some basic relations for the most important characteristics of array antennas. Then we have to choose between the available array-antenna architectures.

In this chapter we summarised the basic tools for considering problems of signal processing from an engineering viewpoint. After the introduction of basic statistics we discussed the likelihood ratio test for deriving optimal processing rules for signal or target detection. Some important examples have been treated.

Beamforming with all antenna elements of an array for transmitting or receiving means reproducing a desired beam pattern as closely as possible. Generally, a narrow main beam with high gain at the centre in the desired direction combined with low sidelobes for all other directions is required. The fundamental procedure for beamforming is weighting the individual signals of all antenna elements and then summing all these weighted signals.

Pulse compression with sufficient Doppler tolerance may be achieved using polyphase codes derived from linear and nonlinear frequency modulation. Low sidelobes in range and Doppler are required, especially for the radar search function. These may be achieved by an LFM-derived phase code together with Hamming weighting or by applying a phase code from nonlinear frequency modulation. A loss in resolution has to be taken into account in both cases. For a discrete and known Doppler frequency with an expanded and mismatched filter vector a sidelobe reduction is possible. The compression is then achieved without a loss in resolution. A set up for the expanded filter vector results in a least-square minimisation for all range elements. This version may be useful for search in clutter or for target tracking. Complementary phase codes show increasing sidelobes and responses from unambiguous range from targets with Doppler-shifted echoes. Pulse eclipsing and the corresponding blind ranges can be tolerated to a certain extent in the search mode. Reduced resolution and signal peak power can be sufficient for target detection at shorter ranges. Range resolution may be improved by a factor of about four by oversampling and a least-mean-square filter, if the subpulse matched filter is not strictly bandlimiting.

This chapter presents sequential detection applied for the search mode of phased-array radars. A range dependent weighting has to be used to combine the test function from range cells for forming one test function for the respective beam position. Then the highest gain, expressed in SNR saving, is achieved at maximum range.

After the detection of targets in the search mode there follows target location with range and direction estimation which is as precise as possible. This procedure may in principle be performed with the same receiving data as that already used for detection. Multifunction radar systems with phased arrays may apply an additional acquisition mode after target detection to confirm the first target detection and thereby cancel to a large extent false alarms produced by noise and interference. This radar task may be performed with increased transmit energy compared to that used for the search mode, because there are relatively fewer acquisition orders. Because of the higher SNR for the acquisition mode the location accuracy will be improved. The waveform will be chosen for improved range resolution. After target acquisition the location function will also be applied to the target tracking process.

To detect slowly moving targets, which always have their spectrum near the clutter spectrum, the ditch of the processing gain function, necessary for clutter suppression, must be as narrow as possible, matched to the clutter spectral width. This is best accomplished by STAP processing with sufficient degrees of freedom. These spatial and temporal degrees of freedom can be provided for future airborne phased-array radars by an adequate number of subarrays or super-subarrays and an FIR filter in the time domain. The final Doppler filter bank then follows.

Imaging of tracked moving targets is possible by applying the principles of synthetic aperture. The relative motion between the target and the fixed radar system creates this aperture. The necessary rotation of the target with respect to the line of sight amounts to only few degrees. The movement of the target has to be known exactly for creating this aperture by processing; target tracking alone is not sufficient. Additional focusing techniques must be applied, and frequency and phase estimation from the Wigner Ville distribution seems to be an advantageous procedure. A one-dimensional image in the cross-range dimension is thus achieved in a plane which is orthogonal to the rotation axis. A two-dimensional image requires additionally a high range resolution. This can be achieved by a synthetic high bandwidth by applying a frequency-stepped pulse series instead of one pulse. If a target shows motions around several rotation axes, for example a ship, two dimensional images are also possible.

The ELRA system was developed with active array antennas, signal and data processing subsystems for the demonstration of a multifunction radar system. The antenna components have been developed and built within the labs of FGAN-FFM electronics department (EL). During all phases this project has stimulated theoretical studies, especially in the field of sequential detection and array processing.

This chapter discuss some aspects of the choice of some main systems' parameters and their relationships for multifunction operation. We consider the observation of the air space to detect, locate and track flying targets, the classical task of a surveillance radar. The aim is to establish a track for each target as early as possible. We have discussed in several preceding chapters the arbitrary movement of the agile beam, which is generally a pencil beam. There is the desire to use this freedom in a most intelligent and effective way, which means to achieve reliable tracking of incoming targets with adequate location accuracy. The tracks should be established at a maximum range with a minimum of mean power. Thereafter, the tracks should be maintained with an adequate tracking rate and power.