Anders Nelander

Switched array concepts for 3-D radar imaging

A number of antenna array concepts are investigated for three-dimensional radar imaging. The concepts are based on switched arrays and wide-band signals to generate a virtual array or co-array to span a sector volume in the wave vector space to get image resolution. The proposed arrays can be used for short range imaging in penetrating radar applications. This approach is quite general and it can also be used in other system concepts. Virtual arrays have mostly been used in radio astronomy and ultrasonic imaging. The related synthetic arrays with antenna motion have been more common in radar systems.

Continuous coded waveforms for noise radar

This paper describes a new class of continuous waveforms for noise radar that can be processed using earlier developed methods for reducing pulse compression sidelobes and equalizing compressed pulses before Doppler filtering. The proposed waveforms are based on codes that modulate sub-pulses which define the bandwidth of the waveform. For constant amplitude waveforms the codes are phase codes and the sub-pulses can consist of rectangular pulses with constant phase or linear phase. The proposed waveforms use many short codes to produce a code with a length that is the product of the shorter code lengths. The resulting long code can be arbitrarily long by introducing new shorter codes iteratively. The codes can be random or deterministic with low mismatch losses in the processing at the expense of a quasi-periodic but unpredictable waveform.

Comparison of wideband phased array concepts for achieving frequency independent radiation characteristics

A comparison of three phased array concepts that can be used to reduce the frequency dependence of the radiation characteristics of very wideband phased arrays is presented. The three concepts consist of a multiband array, a frequency tapered array and an array with varying element sizes and element distances. Parameters such as aperture area, number of elements, gain, effective radiated power (ERP) and half-power beamwidth (HPBW) are presented. It is shown that for two of the studied concepts, the frequency dependence can be drastically reduced compared to a conventional wideband phased array. Application of the array concepts in radar and electronic support measure (ESM) are discussed.

Deconvolution approach to terrain scattered interference mitigation

Terrain scattered interference or hot clutter is a problem in radar ECCM, especially for airborne radar with low sidelobe antennas and conventional adaptive sidelobe cancellation. A deconvolution approach is proposed to mitigate terrain scattered interference. This approach is based on obtaining an estimate of the complex multipath impulse response from a short time interval in the received signals. The impulse response estimate is then convolved with a direct path reference signal to generate an estimate of the received terrain scattered interference signal. This interference signal estimate is then subtracted from the received main beam signal to generate a main beam signal with mitigated terrain scattered interference.

Inverse Filtering for Noise Radar Processing

This paper proposes pulsed noise-like or noise waveforms to be able to perform pulse compression with inverse filtering operations to get high resolution, low sidelobes and possibly super resolution. The processing is based on the earlier developed methods to reduce sidelobes and to equalize the response from each processed pulse. This impacts the clutter filtering and allows better clutter suppression. The proposed method can also handle pulse to pulse agility with different pulse lengths and different pulse repetition intervals.

Clutter properties for STAP with smooth and faceted cylindrical conformal antennas

Conformal antennas, which assume the shape of the platform, have several advantages; like reduced weight and space, aerodynamic design and increased field of view. We are interested in detection of moving ground targets with air-borne radar with faceted or smooth vertical half-cylinder or planar antennas with different subarray sizes. We simulate radar systems and study clutter properties which are important for suppressing the clutter with STAP (Space-Time Adaptive Processing), properties by which we can compare the antennas. We use old analysis tools and propose some new which are easy to interpret and draw conclusions from. We find that the faceted and smooth half-cylinder antennas have no significant differences in clutter suppression performance. The plane antenna has poorer performance. The subarray division is more important than the antenna geometry. The number of antenna channels is related to the clutter rank and the clutter fraction of the signal space.

Refined method for terrain scattered interference mitigation

Terrain scattered interference mitigation methods for airborne radar have been described in previous papers and deconvolution methods for interference suppression have been proposed. This paper describes a refined method for terrain scattered interference mitigation and investigates performance for different conditions and limitations. An important part of the method is a threshold function to enable least squares estimation of the multipath impulse response. Least squares estimation can be applied if the impulse response is time limited or if a limited number of impulse response coefficients are required for sufficient interference suppression. The impulse response estimation can be performed either before or within the coherent processing interval. It is assumed that the direct path signal can be accurately estimated without strong multipath errors. Small relative Doppler shift errors are required between the direct path and the terrain scattered paths. Several strong jamming signal sources cannot be resolved and suppressed. Receiver blocking gives errors in the impulse response estimate and jamming signal residues in the main beam signal. Clutter signals might have to be suppressed to get accurate impulse response estimates. Bandwidth effects must be considered to get numerical stability in the inverse filtering and deconvolution operations. The refined method can suppress interference down to the receiver noise level in simple terrain scattering scenarios.

Measurement and analysis of multipath by a rough surface reflector using a digital array antenna

A receiving digital array antenna and a rough surface reflector have been used in multipath measurements, whose purpose is to learn more about undesired multipath surface reflections (surface clutter) in radar and how to suppress the reflected signals. The multipath and direct signals were analyzed by Capon power direction-of-arrival spectrum and by the eigenvalues of the spatial correlation matrix. Motion of radar, targets and jammers was simulated by moving the reflector, which also resulted in decorrelation of the multipath signals. The analysis showed that the number of large eigenvalues, and accordingly the required adaptive suppression performance, is dependent on the correlation between the signals and on the number of transmitter signals.

Analysis of terrain scattered interference-mitigation

Terrain scattered interference mitigation methods for airborne radar have been studied and deconvolution methods for interference suppression have been proposed. This paper describes an analysis of the terrain scattered interference mitigation performance for different conditions and limitations. The direct path signal must be accurately estimated without strong multipath errors. A time-limited impulse response is required to avoid too much suppression of target signals. Small relative Doppler shift errors are required between the direct path and the terrain scattered paths. Several strong jamming signal sources cannot be resolved and suppressed. Receiver blocking gives errors in the impulse response estimate. Clutter signals must be suppressed for accurate impulse responses. Numerical stability must be ensured in the inverse filtering and deconvolution operations.

SAR/MTI from helicopters

MTI/SAR is a well established technique for airborne surveillance. In a helicopter these two modes of operation can be combined as well. The helicopter can also easily move in the vertical direction, making SAR mapping of the height profile of the ground surface possible. In this paper, MTI/SAR from a helicopter is studied in more detail. Relationships are developed between the involved parameters, and the performance is displayed in some examples. Phase error compensation is needed by IMU or autofocusing due to the non-linear movement of the antenna along the SAR path. Highly improved mapping performance can be achieved by using an ESA with multiple beam generation, or high speed scanning. For high resolution mapping in three dimensions interferometric SAR is needed.