Sune R. J. Axelsson

Noise radar using random phase and frequency modulation

Pulse compression radar is used in a great number of applications. Excellent range resolution and high electronic counter-countermeasures performance is achieved by wideband long pulses, which spread out the transmitted energy in frequency and time. By using a random noise waveform, the range ambiguity is suppressed as well. In most applications, the random signal is transmitted directly from a noise-generating microwave source. A sine wave, which is phase or frequency modulated by random noise, is an alternative, and in this paper, the ambiguity function and the statistical characteristics of the correlation output for the latter configuration are further analyzed. Range resolution is then improved because the noise bandwidth of the modulated carrier is wider than that of the modulating signal, and the range sidelobes are also further suppressed. Random biphase modulation gives a 4-dB (/spl pi//sup 2//4) improvement, but much higher sidelobe suppression could be achieved using continuous phase/frequency modulation. Due to the randomness of the waveform, the output correlation integral is accompanied by a noise floor, which limits the possible sidelobe suppression as determined by the time-bandwidth product. In synthetic aperture radar (SAR) applications with distributed targets, this product should be large compared with the number of resolution elements inside the antenna main beam. The advantages of low range sidelobes and enhanced range resolution make frequency/phase-modulated noise radar attractive for many applications, including SAR mapping, surveillance, altimetry, and scatterometry. Computer algorithms for reference signal delay and compression are discussed as replacements for the classical delay line implementation.

Beam characteristics of three-dimensional SAR in curved or random paths

Interferometric synthetic aperture radar (InSAR) provides average height information by combining data from two parallel paths. True three-dimensional (3-D) SAR also detects the height distribution, which is of significant interest in airborne reconnaissance, forest inventory, and subsurface or wall-penetrating sensing applications as examples. In this paper, the beam performance of 3-D SAR is studied and compared for different curved line paths, such as circles, ellipses, and spirals ending up with random sampling. Curved path geometry reduces the ambiguity in height angle of traditional multipass SAR, and random path variation further improves the sidelobe suppression. The poor sidelobe suppression of a single circle path is significantly improved in near-range geometry in combination with high range resolution. By introducing a window function dependent on focus point and path position, high sidelobe suppression was achieved in an extended ground area below the circle path.

Generalized Ambiguity Functions for Ultra Wide Band Random Waveforms

The generalized ambiguity function of a random ultra wide waveform can be defined from the cross-correlation between the waveform and a time-scaled and delayed version of it, where the time scaling is related to the relative velocity of the target and the delay to the range. As a result of the randomness, the ambiguity function fluctuates between different realizations. A full characteerization includes its statistical distribution but often second orders staatistics are sufficient. The ambiguity function is then described as a sum of its average and a noise component defining a noise floor. The average value includes in the integrand a time-scaled version of the autocorrelation function of the random waveform, while the variance of the ambiguity function defines the noise floor of the side lobes. Particularly useful in characterizing the ambiguity function of a random waveform is the second moment statistics describing the sum of the squared aveerage and the variance. Formulations are developed analytically for Gaussian waveforms, phase-modulation by continuous Gaussian noise and randomized step frequency radar.

Suppression of Noise Floor and Dominant Reflectors in Random Noise Radar

Noise radar can be used in a great number of applications. Wide bandwidth gives high range resolution, and the non-periodic waveform suppresses the range ambiguity. Due to the randomness of the waveform, a noise floor limiting the possible side lobe suppression accompanies the correlation integral. In strong clutter with dominant reflectors the induced noise floor can be too high and further suppression is needed. Improved clutter cancellation methods have mainly been discussed in connection to pseudo-random bi-phase pulse compression radar. In this paper, mismatched filtering is applied to suppress the side lobes of random noise radar. An iterative subrtraction algorithm for cancellation of noise floor due to dominating reflectors is analysed and is successfully tested on both random step frequency radar data and noise sodar data.

Suppressed ambiguity in range by phase-coded waveforms

In Doppler radar for surveillance and remote sensing, the ambiguities in range/Doppler are usually solved by varying the pulse repetition frequency. In this paper, two alternative methods are discussed based on phase-coding and orthogonal waveforms. The first one distributes a phase code over a pulse sequence with only one phase step per pulse. The received pulses are correlated with the phase code sequence, and by varying the delay of the phase code, the subintervals in range are scanned and reflector responses outside the focused range interval become highly suppressed. The alternative method that was studied applies a set of near-orthogonal phase codes, which modulate the pulses transmitted. In the receiver, the different subintervals in range are scanned, or detected in parallel, by correlating the signal from the scene by the delayed code sequence of the pulse transmitted. Because orthogonal codes are used, a strong suppression is achieved for signals originating outside the focused subinterval in range.

Analysis of Ultra Wide Band Noise Radar with Randomized Stepped Frequency

Step frequency radar has been used for a long time in wide-band radar applications, such as turntable ISAR, airborne VHF SAR and ground penetration radar. The frequency is stepped linearly with constant frequency change, and range cells are formed after FFT-processing of the received frequency samples. The total covered bandwidth defines the range resolution, and the length of the frequency step restricts the non-ambiguous range interval. Random choice of the transmitted frequencies suppresses the range ambiguity and improves the ECCM performance. In this paper, the ambiguity function and the noise floor limiting the possible side lobe suppression are analyzed. Experimental results are compared with theoretical predictions using sets of randomly distributed radar raw data obtained from linear step frequency radar measurements.

Estimation of target position and velocity using data from multiple radar stations

By using high resolution range information from multiple radar stations in monostatic or bistatic modes, the position estimates can be highly improved compared with conventional measurements from a single radar. In this paper, fundamental geometry relationships and the possible accuracy of target position and velocity estimates are analyzed when data from several radars are combined. The velocity error is proportional to the wavelength and the Doppler resolution, which is limited by the available time of measurement. The estimation error increases when the radars are close, or when the target is close to the radar level surface. For ground stations, the accuracy in estimated target altitude is usually significantly lower than the position data in the horizontal plane. Examples and simulation results are presented, displaying the performance of three radars working in bistatic and monostatic modes. The analysis is generalized to an arbitrary number of radar stations.

Acoustic Random Noise Radar Using Ultra Wide Band Waveforms

Ultra wideband noise radar gives high range resolution and the range ambiguity is suppressed as a result of the non-periodic waveform. Random noise waveforms could also be applied in acoustic radar (Sodar/Sonar). Both wave generation and signal processing can then be performed on a PC. As a result, some signal processing algorithms used in random noise radar/sodar can be tested simpler using sound waves. This paper presents basic relationships and algorithms for signal processing in random noise radar/sodar, and the noise floor generated by the randomness of the transmitted signal is defined. A PC-controlled acoustic radar with ultra wide band random noise waveform (1-8 kHz) was implemented to test some of the algorithms described Measurements were carried out on both moving objects and stationary scenes. Moving target indication using stretched time processing (Doppler) and change detection algorithms were tested as well. Recording from a bridge shows the potential use of the technique for water level indication as an example.

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.

SAR/MTI radar mapping from ships and ground vehicles

In this paper, the basic performance of MTI/SAR radar in ship and ground vehicle applications is analysed. If a single antenna beam scans a sector, significant degradations in MTI sensitivity and SAR resolution occur due to the reduced dwell time on target. Improved performance can be achieved by digital array beamforming with multiple beams, or high speed scanning. SAR needs accurate phase error compensation by inertial measurements or autofocus due to the non-linear movement path of the antenna.