# A Novel Channel Calibration Method for Bistatic ISAR Imaging System
## 1 Introduction  
The bistatic radar, wherein the transmitting station and the receiving station are spatially separated, and the baseline length is comparable to target distance, has prominent advantages against the \"Four Threats\" [1; 2; 3]. The bistatic ISAR using the non-backscatter echo [4; 5], can provide more adequate look-angle diversity than the monostatic ISAR [6; 7; 8; 9]. Research on bistatic ISAR including the imaging principle, algorithm and application has attracted much attention recently [10; 11; 12; 13; 14; 15; 16; 17]. In practical ISAR imaging systems using LFM waveform, a radar high frequency signal is generated, transmitted and received through a series of steps, such as low noise amplification, feeder transmission, antenna radiation and reception. Through these steps, the baseband echo signal is no longer the ideal LFM waveform. The mismatch with the ideal reference signal deteriorates the pulse compression performance and results in a blurred ISAR image [18]. Hence, channel calibration should be conducted to eliminate the influence of non-ideal channels, including non-ideal amplitude and phase characteristics in both monostatic imaging systems and bistatic ISAR imaging systems [19; 20]. Channel calibration for monostatic radar imaging systems can be realized by calibration tower with a transponder [21; 22; 23; 24; 25; 26]. The calibration tower is located at a few kilometers away from the radar. The calibration tower is illuminated, and the transponder is responsible for transmitting the received signal to the radar with a fixed time delay to obtain the channel characteristics. The calibration method based on the transponder can achieve good results, since the calibration tower is close tothe radar and the high SNR of received signals can be obtained. However, the baseline between the transmitting station and receiving station is up to hundreds of kilometers in bistatic ISAR systems, which results in failure of the aforementioned method. In [27] a channel calibration method for bistatic stepped-frequency (SF), ISAR imaging systems based on accumulated spherical satellite echoes are studied to compensate the non-ideal channel characteristic and keep the coherence of the sub-pulses within a burst required by the system. However, the accumulation process of corresponding sub-pulse data in each burst needs to know in advance the precise orbital information and requires strict full-phase-coherence of the system. The phase variation is caused by the radial motion between each burst. In fact, the orbital information is not precise enough to fulfill the phase compensation of corresponding sub-pulse signals. Therefore, after coarse compensation based on the orbital information, it still needs to search the range and radial motion parameters of the satellite for corresponding sub-pulses in all bursts, excluding the first one. It is cumbersome to obtain the optimal parameters. Additionally, it is not easy to get access to the best accumulation result, since the parameters estimation result depends on the designed cost function. For bistatic ISAR imaging systems using a LFM waveform, the SNR of pulse compression result is much higher than the result of each sub-pulse signal using SF waveform, because the LFM waveform offers more bandwidth than the sub-pulse signal of a SF waveform in the same scenario. It is possible to achieve coherent accumulation only using the echo information, and a channel calibration method using a novel accumulation method of calibration satellite echoes is proposed in this paper. The method accumulates multi-period echoes coherently based on the pulse compression result including the peak position and corresponding phase, estimates the channel transfer function by accumulated signals, and calculates the accurate calibration coefficient of bistatic ISAR imaging systems. The remainder of this paper is organized as follows. In Section 2, the bistatic ISAR echo signal model with a non-ideal channel is introduced.