Transponder-Aided Joint Calibration and Synchronization Compensation for Distributed Radar Systems
March
Transponder-Aided Joint Calibration and Synchronization Compensation for Distributed Radar Systems
Wen-Qin Wang 0 1 2
0 School of Communication and Information Engineering, University of Electronic Science and Technology of China , Chengdu , P. R. China
1 Funding: The work described in this study was supported by the National Natural Science Foundation of China under grant 41101317 and the Program for New Century Excellent Talents in University under grant NCET-12-0095. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
2 Academic Editor: Ke Lu, University of Chinese Academy of Sciences , CHINA
High-precision radiometric calibration and synchronization compensation must be provided for distributed radar system due to separate transmitters and receivers. This paper proposes a transponder-aided joint radiometric calibration, motion compensation and synchronization for distributed radar remote sensing. As the transponder signal can be separated from the normal radar returns, it is used to calibrate the distributed radar for radiometry. Meanwhile, the distributed radar motion compensation and synchronization compensation algorithms are presented by utilizing the transponder signals. This method requires no hardware modifications to both the normal radar transmitter and receiver and no change to the operating pulse repetition frequency (PRF). The distributed radar radiometric calibration and synchronization compensation require only one transponder, but the motion compensation requires six transponders because there are six independent variables in the distributed radar geometry. Furthermore, a maximum likelihood method is used to estimate the transponder signal parameters. The proposed methods are verified by simulation results.
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Competing Interests: The authors have declared
that no competing interests exist.
Distributed radar system operating with separated transmitters and receivers offers many
operational advantages [14] to conventional monostatic and multi-frequency or multi-polarized
radars [58], like the exploitation of additional information contained in bistatic reflectivity of
targets [9], reduced vulnerability [10], and forward-looking imaging [11]. Distributed radar
may offer reduced vulnerability to countermeasures such as jamming, as well as increased
slow-moving target detection and identification capability via clutter tuning, in which the
receiver maneuvers so that its motion compensates for the motion of the illuminator to create a
zero Doppler shift for the area being searched. This could be worthwhile, e.g., for topographic
features and drainage, to show the relationships that occur between forest, vegetation, and
soils. This also provides important information for land classification and land-use
management such as agriculture monitoring, soil mapping, and archaeological investigation. Attracted
by these special advantages, various spaceborne and airborne distributed radar missions have
been suggested or developed [12].
However, in a distributed radar the receiver uses an oscillator that is spatially displaced
from that of the transmitter; hence, the phase noise of two independent oscillators cannot be
canceled out. This superposed phase noise corrupts the received radar signal over the whole
coherent integration time, and may significantly degrade subsequent imaging performance. Even
when low-frequency or quadratic phase errors as large as 45 degree in a coherent processing
interval can be tolerated, the requirement of frequency stability is only achieved by using ultra
high-quality oscillators [13]. In the example of the bistatic spaceborne radar system
TanDEMX [2, 14], the relative phase has to be measured with at least 1 Hz sampling frequency in order
to follow, unwrap and compensate the oscillator phase drifts within the acquisition [15, 16].
Furthermore, aggravating circumstances are often accompanied for airborne platforms due to
different platform motions, the frequency stability will be further degraded. Thus, frequency
synchronization compensation is required for distributed radar systems.
There is relative lack of practical synchronization technique for distributed radar systems.
Since distributed radar is of great scientific and technological interest, several potential
synchronization techniques have been suggested. The use of duplex links for oscillator frequency
drift compensation was proposed in [14]. This concept is similar to the microwave ranging
technique. However, this two-way operation is too complex to be applied for multistatic radar
systems. We have investigated a direct-path signal-based phase synchronization technique in
[17]. To receive the direct-path signal, the receiver must fly with a sufficient altitude and
position to maintain line-of-sight contact with the transmitter/illuminator. In [18], we propose a
time and phase synchronization method via global positioning systems (...truncated)