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Synchronization of active radar target simulator for SAR tests
Authors
	Lepekhina T.A.
	Nikolaev V.I.
Date of publication
	2016
Abstract
	Active radar target simulators are used for synthetic aperture radar calibration and tests. In general, the simulator should convert the received impulse of radar sounding signal into a computed superposition of its scatterings from a set of terrestrial scene bins considering amplitudes, delays and phase shifts, the scene being simulated before the test. A simple active transponder is designed to simulate a singular point target. Its principle of operation is based on analog or digital recording of received radar signal and its playing back after a stable delay. One-point active transponders are used for SAR spatial resolution and polarimetric tests, and for radiometric calibration [8]. A multiple point target can be simulated using a multi-channel transceiver, each hardware or software channel being responsible for a single virtual point. Signal processing in real time provides a stable delay regardless exact knowledge of input signal receive moment, but the simulated pattern can be composed of just a few discrete pixels. However, if SAR radiometric resolution or on-board imaging performance in high resolution mode is to be tested, a simulated test target should be composed of millions of pixels, this exceeding available productivity of signal processors for digital signal conversion to be performed in real time. Instead, echo signal data array based on test pattern model, trajectory and known sounding signal envelope can be computed preliminarily and stored, then played back synchronously with radar-under-test sounding signal reception. In flight tests, aircraft trajectory disturbances lead to sounding pulse reception moment fluctuation. Once there is a rigid constraint of return signal data sampling and its generation timing with DAC clocking signal, the delay stability cannot be obtained better than timing granularity. If trajectory fluctuations are tracked by a micro-navigation system and compensated in the imaging algorithm, this function being verified in the flight tests, ignoring delay errors would lead to image defocusing. Similar effect is probably to be encountered if remote SAR sounding period is not multiple of simulator integer clock period. Once echo signal simulation instead of direct sounding pulse transceiving is considered, the requirements of SAR flight tests imply the effective relative noise added by the transceiver to be less than minus 40 dB. It corresponds to initial phase mismatch between the received pulse and the transmitted echo simulation to be limited by l.4, or, thereafter, delay variation to be of order minus 2…3 with respect to the clock period. For this requirement to be satisfied, special schematic should be applied. A principle of fine delay and phase adjustment is introduced below. Taking into account the approach of preliminary output signal data simulation because of limited real time signal processor throughput, delay and phase correction methods are also oriented to minimize real-time output data processing. An available correction technique is based on combination of a switched delay line in the circuit of DAC clock signal and a high-precision controlled phase shifter to adjust up-converter LO signal phase. The structure of the proposed active simulator is presented in Fig. 1. The simulator contains symmetrical channels for wideband signal reception and generation, digital conversions being executed at zero IF. Input data flow is redirected to the RAID storage for future processing. The same storage contains output signal data array, the data being read in segmented mode and played back by I, Q-channel DACs. In addition, every impulse is processed by a computing module based on FPGA as stated below. The essential difference of the proposed active target simulator from real-time active transponders is the presence of the above final adjusting elements controlled with proper codes generated by the signal processor on base of input sounding signal processing results. The possibility of input sounding signal analysis is based on considered simulator destination for SAR flight tests and calibration. Unlike warfare operation circumstances, a radar-under-test is self-designed, or the tests are ordered by SAR operator. Therefore, the data array for simulation of echo signal according to the test scenery can be simulated on base of known sounding signal envelope, sounding time diagram and carrier trajectory. The source data and the resultant array should be stored in the database before the test. The known sounding pulse modulation can be used for its correlation processing executed immediately after input signal reception to measure received pulse delay and initial phase. The processing is based on correlation algorithms known from automatic control and navigation theory [3]. A software module of correlation analysis executes two convolutions of received sounding signal envelope with appropriate reference functions downloaded preliminarily from the database. The first one is the conjugated and inverted complex envelope of the expected sounding pulse, considering its initial phase, and the complex convolution should look like the response function of pulse envelope subject to the delay λ and the phase shift ψ of the received pulse. The second convolution is the discrimination function vanishing at the argument of the first convolution maximum. The delay is found as the equation root for the discrimination function, and then the initial phase shift is determined as the phase of the response function interpolated in the neighborhood of maximum. The suggested simulator is designed to operate with high-resolution SARs (0.25 – 1 m) with sounding signal spectrum bandwidth 150 … 600 MHz. Playback signal delay correction should be implemented by means of clocking signal delay in the range of sampling period with the granularity of 0.05 – 0.1. An available hardware solution of controlled discrete delay line can be based on a set of coaxial lines with the corresponding length step connected between two logical multiplexers. For example, 16 lines differing by 0.1 m step are required for sampling period 5.33 ns and 4-bit delay code. Transmit signal initial phase correction should compensate the sum of two phase errors resulting from the uncertainty of sounding signal reception moment and the delay of transmitted signal at carrier frequency. The range of phase correction is 0 … 360, and code length is determined according to the specification of relative phase noise added by the simulator in SAR test hologram. The power of this noise component depends on code length n as (5.2–6n) dB. Therefore, in order to obtain relative noise level minus 40 dB, a 8-bit phase shifter with 1.4 resolution is required. In consideration that standard phase shifters are mostly 6-bit, a continuous voltage-controlled phase shifter is to be implemented, the control voltage being supplied from a 12 bit DAC. An alternative schematic solution for fine phase adjustment would be a vector modulator controlled by a pair of DC signals. Delay and phase switching in the pauses of transmit pulses enables avoiding control voltage instability during transmission. Delay and phase control codes are chosen from a calibration table. A passive calibration loopback circuit should be provided in the simulator to supply the transmit signal at carrier frequency to the receiver input. The calibration implies test sounding impulse generation and transmission with zero phase shift and sequential setup of all delay steps, the received signal being processed by the correlation analyzer to measure the resultant delay and initial phase of the response. A similar procedure should be executed to obtain the code-to-phase characteristic, and a set of calibrations is to be made for a number of carrier frequencies in the operation range. The results are stored in the calibration table. The suggested principle of active arbitrary test target simulator synchronization enables to provide SAR tests including the modes requiring complex scenery analysis if signal processor throughput isn’t sufficient for full-scale data conversion in real time. The technique may be implemented for SAR through characteristic experimental estimation both in flight tests and in hardware-in-the-loop simulation.
Keywords
	Synthetic Aperture Radar, active simulator, synchronization.
Library reference
	Lepekhina T.A., Nikolaev V.I. Synchronization of active radar target simulator for SAR tests // Problems of Perspective Micro- and Nanoelectronic Systems Development - 2016. Proceedings / edited by A. Stempkovsky, Moscow, IPPM RAS, 2016. Part 1. P. 236-241.
URL of paper
	http://www.mes-conference.ru/data/year2016/pdf/D178.pdf