As a new global navigation system, BeiDou has formally been in operation in the Asian region since December 27, 2012. To evaluate its positioning performance, several days’ observation data were collected using a global positioning system (GPS)/BeiDou receiver in Shanghai. In this paper, the constellation and signals of BeiDou are first reviewed. Carrier-to-noise density ratio (C/N0) and multipath combinations are analyzed, then multipath errors of GPS satellites and BeiDou satellites were compared in terms of elevation angle. A positioning performance comparison of three scenarios was conducted using three indicators including number of visible satellites (NVS), position dilution of precision (PDOP), and accuracy of positioning. All positioning results were solved using a single-point positioning mode. After six months of adjustment, comparison of the positioning performance before BeiDou began providing regional services, there were some significant improvements in performance. The accuracy requirements published before formal operation, in which positioning accuracy is less than 10 m, was met. The results show that the integration system has a large NVS, low PDOP value, and a high level of accuracy and precision. The positioning performance can be improved after using BeiDou, especially under the conditions of urban environment.
In the present study, experiments were conducted to investigate the interaction of supersonic underexpanded jet delivered from a convergent nozzle with a coannular tube on both the flowfield and noise emission. The effects of varying the tube length, L, and diameter, D, were investigated. The most pronounced noise reduction was observed at D/Dj = 2.2 at a jet nozzle pressure ratio (NPR) of 5, where the noise emission was reduced by approximately 30 dB compared with the baseline jet. Considerable sound pressure level (SPL) reduction over the measured frequency range was observed and the screech tone was completely suppressed. The schlieren photographs revealed that the shock-cell structure of the baseline jet disappeared at D/Dj = 1.9 as well as for higher values of D/Dj outside the tube. Detailed experimental investigations were carried out to explain the reduction in the noise radiated at D/Dj = 2.2. From these results, it was made clear that the Mach disk for the case of D/Dj = 2.2 extends up to the tube inner surface and the entire flowfield downstream of the Mach disk becomes subsonic. This was not observed for other cases of D/Dj where the time average flowfield is characterized by a repeated shock-cell structure inside or outside the tube. The effect of varying the jet nozzle pressure ratio (NPR), tube clearance and tube wall porosity on noise emission, flowfield and the associated thrust loss are reported.
The largest source of positioning error for single-frequency users of the Global Navigation Satellite System (GNSS) is typically the radio delay caused by the ionosphere. Although several analytic function-based models for correcting ionospheric errors are available, grid-based models are preferred because of their high estimation accuracy and low complexity. We propose an adaptive ionospheric pierce point cut-off radius control algorithm for the high-resolution, grid-based correction of ionospheric errors. The proposed scheme adjusts the IPP cut-off radii by comparing the target IPP densities with the current effective densities. Simulation results show that our method gives the required effective IPP density using this cut-off radius control, thus improving the accuracy and availability of the correction service. It is expected that the proposed scheme will make high-resolution, grid-based ionospheric correction available for Korean satellite navigation systems with a local distribution of ground reference stations. System development costs will also be reduced since it will require fewer reference stations.
This paper discusses a trajectory design for imaging both Mars and Deimos, which meets the requirements of the next Chinese mission to Mars and Deimos. Compared to Viking-1, being weak in systematic schedules in its original and extended missions, which resulted in the use of more fuel and reducing encounter opportunities, a multipurpose design is addressed in this paper for imaging both Mars and Deimos at Pre-Phase A, not after launch. A frozen and repeating orbit is employed to provide as many as 280 periodic encounters within 100 km of Deimos, in contrast to the fly-around mode used by Phobos-1/2 and Phobos-Grunt, requiring much fuel to guide the spacecraft to the vicinity of Phobos from the highly elliptical captured orbit. To enable encounters with Deimos under all of the perturbations and orbital control and determination errors, the station-keeping strategies for the arguments of periareon and latitude are implemented by some corrections to the semi-major axis and inclination, respectively. A numerical simulation is used to verify the encounter opportunities with the help of the station-keeping strategies. Detailed investigations on imaging, lighting and access conditions show that a working orbit is beneficial for imaging both the Martian surface and Deimos. Therefore, it is concluded that designing the trajectory of the piggyback spacecraft to be carried by the main orbiter in the next Chinese Mars mission during Pre-Phase A is practical from the engineering perspective.
In this paper, we describe a filtering algorithm for removing the error of wind velocity arises in airborne Doppler lidar measurements. The algorithm is based on the Kalman filter with a simplified Kalman gain, which assumes zero variance for correct wind velocity and infinite variance for incorrect wind velocity. The algorithm is applied to 17,487 seconds of airborne Doppler lidar measurements, where a sequence of measurements along the lidar's measurement range is obtained every one second. The reduction of incorrect wind velocity is evaluated at the distance where correct wind velocity exists at least 20–30% of the time out of all measurements. The average standard deviation of filtered wind velocity at the above mentioned distance results in 17.2% of the original value, which is similar magnitude to correct measurements.
The instability of the boundary layer on two-dimensional (2-D) surface corrugation was examined experimentally. The corrugation amplitude Aw was one order of magnitude smaller than the displacement thickness of the boundary layer. Sinusoidal-geometry corrugations with various wavelengths were considered to see how the growth of Tollmien-Schlichting (T-S) waves was affected by the scale of corrugation. The corrugation wavelength λw ranged from about one-tenth to the same order as the T-S wavelength. The destabilizing effect of the 2-D corrugation was observed for a wide range of corrugation wavelengths and was not strongly dependent on the wavelength. Even when a separation bubble was formed in each furrow for the smallest wavelength, the amplification of T-S waves was almost the same as in the case of longer wavelengths.
This paper documents a new guidance law for tactical missiles to intercept maneuvering targets. By incorporating linear and fractional power nonlinear terms, a faster convergent and chatter-free acceleration command is derived. With the aid of Lyapunov stability criteria, we prove that the line-of-sight angular rate can converge to a small region around the origin in finite time. Since the target maneuver cannot be obtained or measured in real applications, a nonlinear finite time convergence disturbance observer for target maneuver estimation is introduced and a composite guidance law with finite time convergence is presented to improve the overall performance. Theoretical analysis and numerical simulations demonstrate the effectiveness of the proposed method.
For many years, flight software (FSW) has been developed and verified using real hardware-based software test beds (STBs). Even though the STB presents the best real-time behavior of the FSW, it has a number of drawbacks such as very long delivery time for the target hardware, frequent changes in hardware configurations, and limited concurrent access to the STB. Therefore, software-based satellite simulators have been developed from the start of a project to overcome the restrictions and limitations imposed by the STB. They enable the development of FSW to progress in parallel with developing the target hardware from the initial phase of the project. In this approach, the processor emulator is the essential component of the satellite simulator, but its interpretation-based approach cannot meet the real-time requirements as the clock speed of the emulation target increases. In this paper, we discuss the design and development of a high-performance and cycle-accurate space processor emulator based on a just-in-time (JIT) dynamic binary translation (DBT) scheme. We also present a solution for self-modifying code emulation which is essential for satellite software but has been a limiting factor in the DBT scheme.
A novel flight dynamics-based model for a nonlinear aircraft tracking filter is proposed. The point mass flight dynamics (PMFD) model proposed in a previous study is elaborated. This study details the case for which the estimation of the state variables is essentially needed but the estimation of the target-dependent control variables is not. The present model, the regulated PMFD (RPMFD), adopts the target-independent control variables instead of the target-dependent variables. In other words, angle of attack and normalized thrust, which are treated as states with random processes in the original PMFD model, are replaced with normal and longitudinal load factors, respectively. The replacement of the target-dependent variables with the target-independent ones enables the unknown aircraft parameters to be removed from the dynamic model. The simulation indicates that RPMFD, which has an advantage of not requiring the unknown parameters, results in the same tracking performance as PMFD. This is because both are basically the same when viewed through implementation of the attitude effects on translational acceleration.