Research On Space Based GMTI And Ambiguity Mitigation Methods

Ground Moving Target Indicator (GMTI) using Space Based Radar (SBR) is attractive and the signal processing theories are studied by many researchers around the world. With the space based multi-channel and multi-static radar as the background, this dissertation focuses on the fundamentals of space based Space-Time Adaptive Processing (STAP), the effects of range, doppler and angle ambiguities on ground moving target indicator radar and the mitigation methods. The main achievements of this paper are as follows:Elementary problems for space based STAP are studied in chapter 2. Based on the analysis of formation configurations of distributed small satellites system and the very large aperture antenna geometries of single satellite based radar system, the antenna arrays of space based GMTI radar are modeled as sparse subarrays and sparse array with non-isotropic elements. Simple and accurate clutter rank estimating formulas are derived for these two sparse array models using space time equivalence theory. Basic constraints of space based GMTI radar are presented based on the formulas. Then efficient STAP algorithms and architectures are provided according to the limitations of SBR. The earth rotation effects on STAP performance are analyzed with a formula for clutter Doppler returns affected by the earth's rotation given. And a simple mitigation method based on range-doppler compensation is provided.Special effects of range, Doppler and angle ambiguities on the performance of space based STAP and GMTI are analyzed based on echo Doppler in chapter 3. Original conlusions are developed forward and described as follows: Range ambiguity obstructs range-doppler compensation of clutter returns, degrading the Minimum Detectable Velocity (MDV) and output Signal to Clutter plus Noise Ratio (SCNR) and increasing false alarm probability. Apriori clutter information and array geometry fluctuate the Doppler and angle ambiguities effects on GMTI performance.Angle ambiguity mitigation methods based on array signal processing theory are studied in chapter 4. Two random sparse array pattern synthesis approaches are lucubrated. Firstly the iterative linear constraints least square method are revised and made applicable to random sparse array. Then the problem is expressed as second order cone programming which is solved with interior point method. The approaches are used to analyse the influence of array geometries on array patterns, which gives a direction to suppress velocity bind zone induced by grating lobes. The angle ambiguity mitigation method is described as follows: The element positions of sparse uniform array are randomized to reduce blind speed zones, then the so called "nulling STAP" algorithm is used to suppress false targets. The linear constraints STAP method and the man-made interference method are provided to form nulls in the directions of ghosts. Experimental results illustrate that the ambiguity mitigation method is robust and effective.Range and Doppler ambiguity suppression methods using variable Pulse Repetition Frequency (PRF) are studied in chapter 5. When multiple PRF sets are used, a weighted clustering algorithm is provided to distinguish the real targets from ghosts if multiple targets detected. Simple evolutionary algorithms are used to select appropriate PRF sets. The range and velocity bind zones, ambiguities and the ghosts are eliminated effectively using this method. Two Pulse Repetition Interval (PRI) variation schemes, the random disturbance and the uniform step disturbance, are added to the constant PRI when staggered PRI waveform is used. Simulation results show that preferable velocity coverage is achieved at the price of about 2dB output SCNR degradation for both the schemes.Doppler and angle ambiguity mitigation methods using frequency orthogonal waveform are studied in chapter 6. Limitations of waveform parameters, such as frequency gap and the number of signals, are analyzed and their effects on the output SCNR of space time frequency adaptive processor are addressed. T hen a two-step detection algorithm is presented to reject Doppler and angle ambiguities, which collaborates the results of the space time frequency adaptive processor and space time adaptive processors of each frequency. Simulation results illustrate that the algorithm is effective in resolving Doppler and angle ambiguities, decreasing both the false alarm probability and the leak alarm probability.