Attitude errors in a strapdown inertial navigation system due to gravity disturbances and system noises can be relatively large, although they are bound within the Schuler and the Earth rotation period. The principal objective of the investigation is to determine to what extent accurate gravity data can improve the attitude accuracy. The way the gravity disturbances affect the attitude were analyzed and compared with system noises by the analytic solution and simulation. The gravity disturbances affect the attitude accuracy by introducing the initial attitude error and the equivalent accelerometer bias. With the development of the high precision inertial devices and the application of the rotation modulation technology, the gravity disturbance cannot be neglected anymore. The gravity compensation was performed using the EGM2008 and simulations with and without accurate gravity compensation under varying navigation conditions were carried out. The results show that the gravity compensation improves the horizontal components of attitude accuracy evidently while the yaw angle is badly affected by the uncompensated gyro bias in vertical channel.
A sonobuoy system based on a fiber optic vector hydrophone (FOVH) is demonstrated. Phase Generated Carrier– Arctangent (PGC-ATAN) demodulation algorithm was used to acquire real-time underwater acoustic signals. After the optimal design of the laser configuration, the background noise of the FOVH is -104.3dB re <i>rad</i> √ <i>Hz</i> at 1 kHz, with an acceleration sensitivity of 41.5dB re rad/g which allows the system detecting signals at DSS0. The theoretical derivation of FOVH directivity is proposed and the design criterion is discussed. The ratio of the minimum to the maximum amplitude of the FOVH directivity is -35dB by symmetrical structure design of the FOVH. A lake trial shows that the maximum detection range of the sonobuoy system is more than 15km for an acoustic signal of 210dB re μPa, and the bearing of a moving target can be estimated.
Former studies have proved that the attitude error in a single-axis rotation INS/GPS integrated system tracks the high frequency component of the deflections of the vertical (DOV) with a fixed delay and tracking error. This paper analyses the influence of the nominal process noise covariance matrix Q on the tracking error as well as the response delay, and proposed a Q-adjusting technique to obtain the attitude error which can track the DOV better. Simulation results show that different settings of Q lead to different response delay and tracking error; there exists optimal Q which leads to a minimum tracking error and a comparatively short response delay; for systems with different accuracy, different Q-adjusting strategy should be adopted. In this way, the DOV estimation accuracy of using the attitude error as the observation can be improved. According to the simulation results, the DOV estimation accuracy after using the Q-adjusting technique is improved by approximate 23% and 33% respectively compared to that of the Earth Model EGM2008 and the direct attitude difference method.
In asymmetrical suspension systems, triaxial signals' phase differences of fiber optic vector hydrophones are nonzero, which is a serious problem for direction of arrival (DOA) estimations of underwater acoustic signals. In this paper, an asymmetrical suspension system is described. Dynamics analysis of the suspension system is performed by using the analytic geometry method. Triaxial resonant frequencies of the suspension system are gotten, phase delays between the outer signals and the hydrophone's triaxial signals are derived, and influence of the suspension system on phase differences in low frequency zone is theoretically explained and simulated. Then frequency responses of the hydrophone in four suspension states are tested in a standing wave tube. The results indicate that both phase differences of xy axes and zy axes are large at resonant frequencies of the suspension system, which is approximately coincided with the theoretical analysis. Phase difference of zy axes at frequency higher than 500 Hz is obvious, which results from resonant responses of the fiber optic vector hydrophone. It proves that symmetry of the suspension system has great influence on phase differences of the fiber optic vector hydrophones.
Deflections of the vertical (DOV) are normally ignored in the gravity compensation procedure, which become one of the primary error sources in inertial navigation. In a single-axis rotation INS/GPS system, bias of the gyro and the accelerometer can be ignored, the attitude error is mainly affected by DOV. In this paper, the ideal system assumption is abandoned and the influence of DOV on the attitude is comprehensively discussed, which can be divided into two parts i.e. the direct influence and the indirect influence. The attitude error tracks the DOV along the trajectory belongs to the former. A relatively fixed delay between the attitude error and the DOV belongs to the latter. The delay is essentially induced by the weak observability of the system to the violent DOV. Factors which affect the delay are carefully analyzed. The simulation results show that the delay is mainly affected by accuracies of the inertial sensors and the GPS. It decreases with the GPS accuracy increasing, but increases with the inertial sensor accuracy increasing. The process noise covariance matrix Q plays an important role. With analysis of the characteristics of the delay, influence of the DOV on attitude is studied further, which is necessary for the attitude correction in future.