Hee-Sung Kim

Analytic equivalence of iterated TOA and TDOA techniques under structured measurement characteristics

This paper compares the iterated time-of-arrival and the time-difference-of arrival techniques analytically under the assumption of structured measurement error characteristics. The assumed measurement error characteristics consider different channel qualities and correlations. To compare the two popular localization techniques dealing with different number of states, information preserving matrices are introduced to transform original measurement vectors to modified measurement vectors. After the information preserving transformations, it is proved that the location estimates produced by both techniques are exactly the same under structured measurement error characteristics. A Monte-Carlo simulation result verifies the analysis result.

Elimination of Clock Jump Effects in Low-Quality Differential GPS Measurements

Most of single frequency GPS receivers utilize low-quality crystal oscillators. If a low- quality crystal oscillator is utilized as the time reference of a GPS receiver, the receivers clock bias grows very fast due to its inherent low precision and poor stability. To prevent the clock bias becoming too large, large clock jumps are intentionally injected to the clock bias and the time offset for clock steering purpose. The abrupt changes in the clock bias and the time offset, if not properly considered, induce serious accuracy degradation in relative differential positioning. To prevent the accuracy degradation, this paper proposes an efficient and systematic method to eliminate the undesirable clock jump effects. Experiment results based on real measurements verify the effectiveness of the propose method.

A geometric wireless location system utilizing downlink pilot strength measurement messages

In this paper, the performance of the wireless-signal map-matching method under a code division multiple access network environment is demonstrated. The wireless-signal map-matching method is the key step to implement the recently introduced wireless location technology referred to as the localization exploring network measurement occurrences. The wireless-signal map-matching method is applied to the downlink pilot strength measurement messages collected under typical urban environments. From the experiment results it is shown that an accuracy improvement is possible by revealing the spatial structure of non-line-of-sight errors without any GPS information.

A WLAN/GPS Hybrid Localization Algorithm for Indoor/Outdoor Transit Area

For improved localization around the indoor/outdoor transit area of buildings, this paper proposes an efficient algorithm combining the measurements from the WLAN (Wireless Local Area Network) and the GPS (Global Positioning System) for. The proposed hybrid localization algorithm considers both multipath errors and NLOS (Non-Line-of-Sight) errors, which occur in most wireless localization systems. To detect and isolate multipath errors occurring in GPS measurements, the propose algorithm utilizes conventional multipath test statistics. To convert WLAN signal strength measurements to range estimates in the presence of NLOS errors, a simple and effective calibration algorithm is designed to compute conversion parameters. By selecting and combining the reliable GPS and WLAN measurements, the proposed hybrid localization algorithm provides more accurate location estimates. An experiment result demonstrates the performance of the proposed algorithm.

A Location Technique Based On Calibrated Radio Frequency Propagation Model For Wireless Local Area Networks

This paper proposes an efficient location technique to find an indoor location under the IEEE 802.11 wireless local area networks. The proposed method is based on the range measurements obtained from a simple radio frequency propagation model. Thus, unlike the radio frequency fingerprint correlation method, it does not suffer from the computational burden during the real-time location service period and can quickly reply the location requests of many users at the same time. To increase the location accuracy in spite of the frequent non-line-of-sight error occurrences, the proposed method calibrates the distortion of the non-line-of-sight error by a simple measurement surveying procedure that does not require the surveyor`s manual interaction. Experimental results show the capability of the proposed method.

Design and Implementation of Receiver Network Elements for Real-Time Precise GPS/GNSS

Due to the deployment of various wireless networks originating from CDMA, GSM, and WLAN, it became very convenient to exchange information from one place to another. As compared with the traditional environments for one-way information distribution based on fixed radio frequency bands, the convenient wireless network environments will bring about many changes in positioning technologies based on global navigation satellites. Among the many changes to come, the reconfigurable receiver network is one of the most attractive concepts since it can be tailored to a specific application area among networked robots, formation flying, bridge monitoring, and traffic monitoring. As an initial study to develop a reconfigurable receiver network, this paper deals with the design and implementation of the key elements of the reconfigurable receiver netowork; server, broadcaster, and client. In the designed receiver network, a sever receives and decodes measurements from a reference receiver installed at a known location, a broadcaster processes and transfers the messages from servers to clients and manages connections with servers and clients, a client receives the messages from the broadcaster and performs differential positioning. A real-time experiment result is demonstrated to validate the functionalities of each network element.

Estimation of relative ionospheric delay by GNSS Algorithm for Accuracy and Safety

In this paper, a GNSS software package named GAFAS (GNSS Algorithm For Accuracy and Safety) is introduced. GAFAS is a general-purpose software package developed since 2005. It is written in C/C++ language being purposed to build diversified GNSS infrastructure and user applications. Currently, GAFAS is upgrading its capability in medium/long baseline integer ambiguity resolution for more accurate ionospheric delay estimation. To verify the validity of its concept, an experiment was performed with real measurements.