Chen Yongqi

Tight integration of digital map and in-vehicle positioning unit for car navigation in urban areas

Now GPS has been widely used for land, sea and air navigation. However, due to signal blockage and severe multipath environments in urban areas, such as in Hong Kong, GPS alone can not satisfy most land vehicle navigation requirements. Dead Reckoning (DR) systems have been widely used to bridge the gaps of GPS and to smooth GPS position errors. However, the DR drift errors increase with time rapidly and frequent calibration is required. Under the normal situation, GPS is sufficient to provide the calibration to the DR unit. However, GPS may not be available in urban areas for more than 20 min, and the DR position errors can reach hundreds of meters during the period. As land vehicles have to be on roads, digital map can be used to constrain the locations of vehicles, known as map-matching. One of the main problems for map-matching techniques is mis-matching, that may be caused by the positioning sensor errors and the complexity of city road network. In this paper, a newly developed model to tightly integrate digital map and in-vehicle positioning unit for car navigation is introduced. With this method, it improves the position accuracy by constraining the vehicle location on the roads. Moreover it provides the close-loop controls for the DR drift errors by feeding back the coordinates of the feature points of the road network and road bearings to the DR unit and therefore the navigation system can be used for longer period when GPS is not available. Extensive tests have been carried out in Hong Kong. It demonstrates that this close-loop approach is much better on the reliability of map-matching, as the positioning sensor errors are constantly calibrated by the digital map.

Determination of weighted mean tropospheric temperature using ground meteorological measurements

The weighted mean tropospheric temperature is a critical parameter in the conversion of wet zenith delay to precipitable water vapor in GPS Meteorology. This parameter can not be calculated from the radiosonde data in real time through the conventional methods. In this study, we first discuss the admissible error of weighted mean temperature to enable the accuracy of the conversion better than 1 mm, then summarize the performance of some of the existing methods. An empirical formula is established that satisfies the real-time requirement in GPS meteorology using Sequential Regression Analysis method. It is shown that this real-time formula as compared with other empirical methods is more accurate for local applications.

Development and Field Testing of a Multi- Antenna GPS System for Deformation Monitoring

GPS has become an important technology for monitoring deformations of structures and the crust of the Earth. A limiting factor for large-scale use of GPS in such applications is however its high hardware cost. For automatic monitoring of deformations, each point to be monitored needs equipped with a set of GPS instruments. This makes many applications such as routine monitoring of landslides too expensive in most cases. A multi-antenna GPS system has been developed and tested aiming at significantly reducing the cost of GPS when used for monitoring deformations of objects such as slopes. The system uses special hardware and software to allow one GPS receiver to be used with a number of GPS antennas. One set of such equipment can therefore be used to monitor a number of points. The system normally reduces the cost of GPS hardware by a number of folds. Besides, such a system design also eases the tasks of data communication, management and system control. This paper describes the design and system configurations of the multi-antenna GPS system that has been developed by the research team. The system consists of integrated hardware and software components for data acquisition, transmission, processing, analysis and visualization. Integration of the GPS system with conventional slope monitoring systems will also be discussed in brief.

Wet Refractivity Tomography with an Improved Kalman-Filter Method

An improved retrieval method, which uses the solution with a Gaussian constraint as the initial state variables for the Kalman Filtering (KF) method, was developed to retrieve the wet refractivity profiles from slant wet delays (SWD) extracted by the double-differenced (DD) GPS method. The accuracy of the GPS-derived SWDs is also tested in this study against the measurements of a water vapor radiometer (WVR) and a weather model. It is concluded that the GPS-derived SWDs have similar accuracy to those measured with WVR and are much higher in quality than those derived from the weather model used. The developed method is used to retrieve the 3D wet refractivity distribution in the Hong Kong region. The retrieved profiles agree well with the radiosonde observations, with a difference of about 4 mm km−1 in the low levels. The accurate profiles obtained with this method are applicable in a number of meteorological applications.

Seafloor sediment classification based on multibeam sonar data

The multibeam sonars can provide hydrographic quality depth data as well as hold the potential to provide calibrated measurements of the seafloor acoustic backscattering strength. There has been much interest in utilizing backscatters and images from multibeam sonar for seabed type identification and most results are obtained. This paper has presented a focused review of several main methods and recent developments of seafloor classification utilizing multibeam sonar data or/and images. These are including the power spectral analysis methods, the texture analysis, traditional Bayesian classification theory and the most active neural network approaches.

Schreiber approach for GPS carrier phase ambiguity resolution

Differenced GPS carrier phase observations are usually used in GPS positioning to eliminate the various common GPS errors. A different approach is proposed in this paper where no differencing of the observations is required. In this method, the common GPS errors are modelled explicidy and eliminated by introducing the Schreiber method, which is used in traditional geodesy to eliminate the azimuth unknowns in observation equations. The paper first introduces the Schreiber method, and then discusses its application to the various GPS common-mode error models. It is shown that if only the real ambiguity solution is concerned, the estimate of the position parameters from the Schreiber approach is the same as that from the differencing approach. However, the variance of the ambiguity estimate from the Schreiber approach is smaller than that from the differencing approach. Numerical examples are given to show that the results are reliable.

The inverse problem of optimal regulators and its application

This paper presents a new solution to the inverse problem of linear optimal regulators to minimize a cost function and meet the requirements of relative stability in the presence of a constant but unknown disturbance. A state feedback matrix is developed using Lyapunov’s second method. Moreover, the relationships between the state feedback matrix and the cost function are obtained, and a formula to solve the weighting matrices is suggested. The developed method is applied successfully to design the horizontal loops in the inertial navigation system.

Remote sensing of water vapor content using ground-based GPS data

Spatial and temporal resolution of water vapor content is useful in improving the accuracy of short-term weather prediction. Dense and continuously tracking regional GPS arrays will play an important role in remote sensing atmospheric water vapor content. In this study, a piecewise linear solution method was proposed to estimate the precipitable water vapor (PWV) content from ground-based GPS observations in Hong Kong. To evaluate the solution accuracy of the water vapor content sensed by GPS, the upper air sounding data (radiosonde) that are collected locally was used to calculate the precipitable water vapor during the same period. One-month results of PWV from both ground-based GPS sensing technique and radiosonde method are in agreement within 1–2 mm. This encouraging result will motivate the GPS meteorology application based on the establishment of a dense GPS array in Hong Kong.

GPS Rapid Static and Kinematic Positioning Based on GPS Active Network

This paper presents a data processing strategy for GPS kinematic positioning by using a GPS active network to model the GPS errors in double difference observable. Firstly, the double difference residuals are estimated between the reference stations in the active network. Then the errors at a user station are predicted as the network corrections to user measurements, based on the location of the user. Finally conventional kinematic positioning algorithms can be applied to determine the position of the user station. As an example, continuous 24-hour GPS data in March 2001 has been processed by this method. It clearly demonstrates that, after applying these corrections to a user within the network, both the success rate for ambiguity resolution and the positioning accuracy have been significantly improved.

Monitoring the dynamic characteristics of tall buildings by GPS technique

Dynamic characteristics of large structures, such as tall buildings, longspan suspension, cable-stayed bridges and tall chimneys, are key to assess their drift and stress conditions. The dynamic characteristics of large structures are difficult to measure directly under the condition of earthquakes or strong winds using traditional techniques such as laser collimator, total station and accelerometers. Therefore there is a great need for developing new method or technique for this purpose. Recent advances in Global Positioning System (GPS) technology provide a great opportunity to monitor long-period changes of structures reliably. GPS receivers capable to gauge the motion at the centimeter or sub-centimeter level with sampling frequency 10Hz or even 20 Hz are now available from several manufacturers. To the authors’ knowledge, the capability of identifying dynamic characteristics from GPS observations has not been widely verified. For the feasibility study on using kinematic GPS technology to identify the dynamic characteristics of tall buildings, some experiments were conducted in a simulative environment. This paper discusses in detail the experiment device, and the ways through them GPS data are recorded, processed and analyzed. With post-processing version of NovAtel’s Softsurv software and auto-regressive (AR) spectral analysis method, relative displacements and corresponding vibrating frequencies have been derived from GPS observations. The results indicate that the dynamic characteristics can be identified accurately by kinematic GPS technology.