Zongsheng Huang

Attitude-correlated frames approach for a star sensor to improve attitude accuracy under highly dynamic conditions

The attitude accuracy of a star sensor decreases rapidly when star images become motion-blurred under dynamic conditions. Existing techniques concentrate on a single frame of star images to solve this problem and improvements are obtained to a certain extent. An attitude-correlated frames (ACF) approach, which concentrates on the features of the attitude transforms of the adjacent star image frames, is proposed to improve upon the existing techniques. The attitude transforms between different star image frames are measured by the strap-down gyro unit precisely. With the ACF method, a much larger star image frame is obtained through the combination of adjacent frames. As a result, the degradation of attitude accuracy caused by motion-blurring are compensated for. The improvement of the attitude accuracy is approximately proportional to the square root of the number of correlated star image frames. Simulations and experimental results indicate that the ACF approach is effective in removing random noises and improving the attitude determination accuracy of the star sensor under highly dynamic conditions.

An Improved Method for Dynamic Measurement of Deflections of the Vertical Based on the Maintenance of Attitude Reference

A new method for dynamic measurement of deflections of the vertical (DOV) is proposed in this paper. The integration of an inertial navigation system (INS) and global navigation satellite system (GNSS) is constructed to measure the bodys attitude with respect to the astronomical coordinates. Simultaneously, the attitude with respect to the geodetic coordinates is initially measured by a star sensor under quasi-static condition and then maintained by the laser gyroscope unit (LGU), which is composed of three gyroscopes in the INS, when the vehicle travels along survey lines. Deflections of the vertical are calculated by using the difference between the attitudes with respect to the geodetic coordinates and astronomical coordinates. Moreover, an algorithm for removing the trend error of the vertical deflections is developed with the aid of Earth Gravitational Model 2008 (EGM2008). In comparison with traditional methods, the new method required less accurate GNSS, because the dynamic acceleration calculation is avoided. The errors of inertial sensors are well resolved in the INS/GNSS integration, which is implemented by a Rauch–Tung–Striebel (RTS) smoother. In addition, a single-axis indexed INS is adopted to improve the observability of the system errors and to restrain the inertial sensor errors. The proposed method is validated by Monte Carlo simulations. The results show that deflections of the vertical can achieve a precision of better than 1″ for a single survey line. The proposed method can be applied to a gravimetry system based on a ground vehicle or ship with a speed lower than 25 m/s.

Region-confined restoration method for motion-blurred star image of the star sensor under dynamic conditions.

Under dynamic conditions, the centroiding accuracy of the motion-blurred star image decreases and the number of identified stars reduces, which leads to the degradation of the attitude accuracy of the star sensor. To improve the attitude accuracy, a region-confined restoration method, which concentrates on the noise removal and signal to noise ratio (SNR) improvement of the motion-blurred star images, is proposed for the star sensor under dynamic conditions. A multi-seed-region growing technique with the kinematic recursive model for star image motion is given to find the star image regions and to remove the noise. Subsequently, a restoration strategy is employed in the extracted regions, taking the time consumption and SNR improvement into consideration simultaneously. Simulation results indicate that the region-confined restoration method is effective in removing noise and improving the centroiding accuracy. The identification rate and the average number of identified stars in the experiments verify the advantages of the region-confined restoration method.

High-precision rolling angle measurement for a three-dimensional collimator.

We propose a precise rolling angle measurement for a collimator to extend its application in 3D angular deformation measurement, with performance significantly superior to that of the traditional 2D technique. The rolling angle measurement is realized by taking full advantage of the point array image, which is projected in terms of the collimated beam. The measurement error is estimated according to the proposed algorithm. The characteristics of the point array are analyzed to optimize the point array for precise measurement, including the point distribution, the point array resolution, and the point array area. Both simulations and experiments demonstrate that subarcsecond precision rolling angle measurement is achieved by our method, which is superior to those attained by other proposed targets.

A new integrated Gaussian-Markov process model for precision shipboard transfer alignment

In shipboard transfer alignment (TA), Kalman filter used to estimate the misalignment angle requires accurate ship dynamic flexure model. Traditionally, the ship dynamic flexure is modelled as a second-order Gaussian-Markov process according to experience. However, this model has not been validated in real applications, when the current model is differ with this used in Kalman filter design will result in a large measurement error. To solve this problem, an integrated Gaussian-Markov process model is proposed in this contribution which is based on the hydroelastic analysis and statistic from previous measured data. Specifically, theoretical analysis shows ship dynamic flexure is the response of elastic ship hull to sea wave loads, while the sea wave spectrum is of double-peaked in frequency filed for the swell and wind sea waves occurring simultaneously, and therefore, the dynamic flexure is also with double-peaked spectrum distribution. Furthermore, the frequency analysis based on our previous measured ship dynamic flexure data also demonstrates the power spectrum density (PSD) of actual dynamic flexure angle is of double-peaked distribution, which can be modelled more accurately by combined using two independent second-order Gaussian-Markov process models. Experimental results show that Kalman filter utilizing the proposed integrated Gaussian-Markov process model provides more accurate measurement of the misalignment angle compared with using traditional second-order Gaussian-Markov process model in shipboard TA.

A new time discrimination circuit for the 3D imaging Lidar

In order to enhance the time discrimination precision in the 3D imaging lidar, we propose a new time discrimination circuit, which improves both the delayer and the attenuator in the previous CFD (Constant Fraction Discriminator) circuit. The proposed circuit mainly includes a delayer, a low-pass filter, and a comparator. The delayer is implemented with a series of inductors and capacitors, which has some advantages: low signal distortion, small volume, easy adjustment, etc. The low-pass filter attenuates the signal amplitude and broadens the signal width, as well as reduces the noise by decreasing the equivalent noise bandwidth, and increases the signal slope at the discrimination time. Therefore, the time discrimination error is reduced significantly. This paper introduces the proposed circuit in detail, carries out a theoretical analysis for the noise and time discrimination error in the proposed circuit and compares them with the previous CFD circuit. The comparison results show that the proposed circuit can reduce the time discrimination error by about 50% under the same noise level. In addition, some experiments have been carried out to test the performances of the circuit. The experiments show that the time delay of the circuit is about 14ns, the time discrimination error is less than 150 ps when the voltage SNR ranges from 18.2 to 81.8, and the time discrimination error is less than 100 ps when the signal amplitude ranges from 0.2 V to 1.86 V. The tested time discrimination error is well in accordance with the theoretical calculation.

Study on optical 3D angular deformations measurement

3D angular deformations will be inevitable when ships are sailing, due to the changes of the environmental temperature and external stresses. The measurement of 3D angular deformations is one of the most critical and difficult issues in navy and shipbuilding industry around the world. In this paper, we propose an optical method to measure 3D ship angular deformations and discuss the measurement errors in detail. Theoretical analysis shows that the measured errors of the pitching and yawing deformations are induced by the installation errors of the image aperture, and the measured error of the rolling deformation depends on the subpixel location algorithm in image processing. It indicates that the measured errors of the optical measurement proposed in this paper are at the magnitude of angular seconds, when the elaborated installation and precise image processing technology are both performed.

A new 3D imaging lidar based on the high-speed 2D laser scanner

In order to enhance the imaging speed of the 3D imaging lidar (light detection and ranging) and implement high-speed 3D imaging under static conditions, we propose a new 3D imaging lidar based on a laser diode and a high-speed 2D laser scanner. The proposed 3D imaging lidar is mainly composed of a transmitter, a laser scanner, a receiver and a processor. This paper introduces the components and principle of the proposed 3D imaging lidar first. And then some experiments have been carried out to evaluate the performance of the 3D imaging lidar, in terms of scanning field, measuring precision, scanning speed, image resolution and etc. The results show that the scanning field of the 3D imaging lidar is about 26°×12°, the measuring precision is better than 5 cm (4 m distance), the scanning speed is greater than 30 fps (frame per second) and the image resolution can reach 16×101. In addition, the 3D imaging lidar can obtain both the 3D image and intensity image for the given target at the same time.

Mathematical Morphology Operations Applied in Star Image Processing for Star Trackers

We investigate the application of the mathematical morphology operations in star image processing for star trackers. Results verify its effectiveness in noise removal, star segmentation and broken star image reconstruction for the motion-blurred images.

An improved method for gravity disturbances compensation in INS/GPS integrated navigation

Though the gravity-induced velocity and position errors can be easily dumped in GPS/INS integrated navigation, the attitude estimation errors are more affected by the gravity disturbance errors. In this paper, an improved gravity disturbance compensation method is performed to reduce the gravity-induced attitude measurement error in inertial navigation system (INS) and global positioning system (GPS) integrated navigation. Specifically, the long and medium wavelength components of the gravity disturbance are compensated by using the EGM2008 global gravity model, and then a derivative second-order Gauss-Markov process is adopted as a statistical model of the short-wavelength components, which are estimated and removed online by a Kalman filter. Simulations results show that the proposed statistical model can decouple the short-wavelength components of gravity disturbances from the attitude errors. Therefore, the attitude errors are reduced effectively when the improved compensation method is applied.