## 1 Introduction  
Lasing and imaging systems are increasingly required by new applications in the commercial, defense, and communication worlds. With these new applications, in-obit vibration throughout a wide bandwidth (DC-1K Hz) can be induced by external disturbance and various motions of the loads on the mobile carrier [1]. The unwanted line-of-sight (LOS) jitter can seriously affect the performance of the systems [2]. In order to achieve the mission objectives, their LOS system must provide precise pointing and tracking capabilities with suppression of jitter to levels of 1-3 micro-radians (rms) or less [3], especially in deep space laser communications [4] and directed energy weapons [5]. As a result, the requirements for the extremely accurate measurement of the LOS jitter are often specified in micro-radians or even nano-radians in wide bandwidth (DC-1K Hz) [6]. The magnetohydrodynamic (MHD) angular rate sensor (ARS) is developed for its unique advantage of extremely low noise level in ultra-wide bandwidth [7] and demonstrated in many systems, such as the measurement of angular acceleration in an impact environment [8], the Relay Mirror Experiment [9], the Advanced Land Observing Satellite [10], and the Mars Laser Communication Demonstration program [11]. However, the MHD ARS does not possess the capability of measurement at low frequency (<1 Hz), especially the constant inertial angular rate [12], which is essential for precision LOS pointing and wide-bandwidth LOS jitter suppression [13; 14]. [PERSON] et al. [15] investigated whether the differential of MHD ARS could be used to measure head angular acceleration in impact testing. The results showed that the differential of angular velocity deduced by MHD ARS are consistent with the vibratory response of the pendulum for various magnitudes of angular acceleration, but inconsistent due to its poor performance at low frequency. [PERSON] et al. [16] showed that the noise level of MHD ARS was better than two other rate gyroscopes while the MHD ARS's discrepancy in the experiment from the benchmark was larger due to its drift. [PERSON] et al. [17] discussed the application of MHD ARS in the rejection of angular disturbances in the flight laser transceiver and stated that the sensor's poor low-frequency noise performance could depress the combination measurement results. Thus, the limitation of MHD ARS at low frequency implies a large drift rate and has a negative impact on the sensor's tracking accuracy and application range. Substantial explorations have been carried out to expand the MHD ARS bandwidth at low frequency, especially focusing on combining it with the conventional gyroscope. [PERSON] et al. [18] proposed the design of a blending filter to combine the high-frequency MHD ARS sensor measurement with the low frequency rate estimate by a conventional gyroscope. The method was based on the extremely accurate estimation of two sensors' spectral characteristics and the disturbances that cause LOS jitter [19]. [PERSON] et al. [20] provided a detailed analysis of the predicted angular position error as a function of the measurement duration to compensate the sensor's drift. [PERSON] et al. [21] described an implementation of a Wiener-filter-based complementary filtering and a Kalman filtering, using data from a star tracker, gyroscopes, and MHD ARS. However, the procedure was hard to guarantee in real-time and adapt to complex application environments. [PERSON] et al. [22] designed a blended tracking concept by optical tracking sensors combing with MHD ARS to provide a composite pointing reference. The frequency and noise characteristics of the blended zone were not ideal in the dynamic experiment. Reviewing these existing combination measuring methods, we can find that the low-frequency compensation methods make strict demands on the real time and accuracy of the combinatorial technique and increase the complexity of the system.