ITG: A New Global GNSS Tropospheric Correction Model
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Abstract Tropospheric correction models are receiving increasing attentions, as they play a crucial role in Global Navigation Satellite System (GNSS). Most commonly used models to date include the GPT2 series and the TropGrid2. In this study, we analyzed the advantages and disadvantages of existing models and developed a new model called the Improved Tropospheric Grid (ITG). ITG considers annual, semi-annual and diurnal variations and includes multiple tropospheric parameters. The amplitude and initial phase of diurnal variation are estimated as a periodic function. ITG provides temperature, pressure, the weighted mean temperature (Tm) and Zenith Wet Delay (ZWD). We conducted a performance comparison among the proposed ITG model and previous ones, in terms of meteorological measurements from 698 observation stations, Zenith Total Delay (ZTD) products from 280 International GNSS Service (IGS) station and Tm from Global Geodetic Observing System (GGOS) products. Results indicate that ITG offers the best performance on the whole.본 연구는 GNSS (Global Navigation Satellite System) 폐색지역에서 위치결정이 가능하도록 적합한 GNSS 측위 방안을 도출하고, 이를 통해 다양한 LBS의 개발 및 활용을 지원하기 위해 수행되었다. 이를 위해 국내의 GNSS 폐색지역 해소에 적합한 것으로 분석된 다중신호 의사 GNSS 기법을 적용한 의사위성(Pseudolite) 측위시스템을 구성하고, 실험에 적합한 환경의 GNSS 폐색지역을 선정하여 그 성능과 활용성을 검증하고자 하였다. 실험 지역으로 다중경로의 영향이 적으며, 완전 폐색지역과 부분 폐색지역으로 구분이 명확한 대형 교량 하부에 의사위성 장비를 설치하고, 측량용 GNSS 수신기를 이용한 단독측위 방식의 위치결정 실험을 수행하였다. 그 결과 국내의 GNSS 폐색지역 위치결정에 적합한 의사 GNSS 측위 모델을 정립할 수 있었으며, 본 연구에서 제안한 의사 GNSS 기술을 활용하는 경우 GNSS 폐색지역 내에서도 개활지와 유사한 수준의 GNSS 단독측위 정확도(수 m 이내)로 수평 위치결정이 가능하였다.
GNSS augmentation
Air navigation
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การผลิตแผนที่ภาพถ่ายทางอากาศด้วยอากาศยานไร้คนขับขนาดเล็กในปัจจุบัน ได้มีการนำเทคโนโลยีระบบนำทางด้วยดาวเทียม GNSS มาใช้ในการระบุตำแหน่งหมุดควบคุมภาคพื้นดิน รวมถึงการระบุตำแหน่งของอากาศยานฯนั้นด้วย ซึ่งความถูกต้องทางตำแหน่งของหมุดควบคุมฯ และตำแหน่งอากาศยานฯ ดังกล่าวนั้น เป็นส่วนสำคัญของความถูกต้องทางตำแหน่งของแผนที่ภาพถ่ายทางอากาศที่ได้จากงานสำรวจด้วยอากาศยานนี้ โดยปัจจัยที่จะทำให้ได้ตำแหน่งที่มีความถูกต้องสูง ได้แก่ ชนิดของเครื่องรับสัญญาณดาวเทียม GNSS ที่ใช้ในการรังวัด และเทคนิคการรังวัดดาวเทียม GNSS ที่นำมาประยุกต์ใช้ ซึ่งปัจจุบันนิยมใช้เครื่องรับสัญญาณดาวเทียม GNSS แบบรังวัด (Survey grade) ชนิดหลายความถี่ และเทคนิคการรังวัดดาวเทียมแบบจลน์ คือเทคนิค RTK (Real Time Kinematic) หรือ PPK (Post Processing Kinematic) ตามลำดับ อย่างไรก็ตามเครื่องรับสัญญาณดาวเทียม GNSS แบบรังวัดนั้น มีราคาค่อนข้างสูง รวมทั้งเทคนิคการรังวัดด้วยดาวเทียมแบบ RTK และ PPK นั้นยังคงมีข้อจำกัดในการนำมาประยุกต์ใช้ในงานการผลิตแผนที่ภาพถ่ายทางอากาศด้วยวิธีการนี้ กล่าวคือความถูกต้องทางตำแหน่งที่ได้รับขึ้นอยู่กับระยะห่างระหว่างสถานีฐานถึงตำแหน่งหมุดควบคุมฯ หรือตำแหน่งของอากาศยานนั้น แล้วแต่กรณี ซึ่งจะทำให้อากาศยานไร้คนขับถูกจำกัดระยะทางในการบิน ดังนั้นในงานวิจัยนี้จึงได้ทดสอบประสิทธิภาพของเครื่องรับและเสาอากาศรับสัญญาณดาวเทียม GNSS ราคาประหยัดแบบหลายความถี่ U-blox F9P ด้วยเทคนิคการหาตำแหน่งแบบจุดเดี่ยวความละเอียดสูง ซึ่งต้องการเครื่องรับสัญญาณดาวเทียมเพียงเครื่องเดียวและความถูกต้องทางตำแหน่งที่ได้รับไม่ขึ้นอยู่กับระยะห่างจากสถานีฐานใด ๆ�โดยทดสอบทั้งในการรังวัดแบบสถิตและแบบจลน์ ผลการวิจัยพบว่าความถูกต้องทางตำแหน่งจากการรังวัดแบบสถิตของเครื่องรับและเสาอากาศราคาประหยัดดังกล่าวให้ความถูกต้องทางตำแหน่งทางราบและทางดิ่งที่ดีกว่า 0.03�เมตร (ซึ่งอยู่ในระดับเดียวกับเครื่องรับสัญญาณดาวเทียมแบบรังวัดที่รังวัดด้วยเทคนิค RTK) ในการทดสอบรังวัดแบบจลน์ที่ความเร็ว 10 และ 20 เมตรต่อวินาที สามารถให้ความถูกต้องทางตำแหน่งทางราบดีกว่า 0.6�เมตร และทางดิ่งดีกว่า 1.2�เมตร
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Attempts have been made to study the different energy components of the troposphere prior to the occurrence of nor’westers in Bangladesh during the pre-monsoon season. The energy components of the troposphere on the dates of occurrence of nor’westers have also been compared with the energy components of non-occurrence day, weekly and monthly normal values. During the pre-monsoon season, the sensible heat (SH) and latent heat (LH) content of the troposphere increase as the season progresses due to the warming of the troposphere and availability of moisture respectively. The SH and LH of the troposphere on the dates of occurrence of nor’westers increases significantly from near the surface to about 300-200 hPa and about 900 hPa level to about 400 hPa or more respectively in most of the cases, although it has inter-layer variations. There are some exceptions especially in the lower troposphere where there exists decrease in LH of the troposphere at 0000 UTC. These exceptions may be due to the availability of less moisture in the lower atmosphere on some occasions early in the morning. It has also been found that the SH and LH of the troposphere on the dates of occurrence of nor’westers become significantly above weekly normal from near the surface to about 300-200 hPa and in the lower troposphere respectively in most of the cases. The SH is found to be below normal in the layer between 850 and 650 hPa levels and significantly above monthly normal in the upper troposphere, having maximum in the layer 400-300 hPa. The LH is significantly above the monthly normal in the lower troposphere, and it becomes below monthly normal in the mid-troposphere. During the pre-monsoon season, mean monthly potential energy (PE) of a parcel of air in the troposphere decreases in the lower troposphere upto about mid-troposphere and then increases in the upper troposphere as the season progresses. The PE has a decreasing tendency near the surface on the dates of occurrence of nor’westers in many cases and then it shows erratic behaviour in the vertical. The variation of PE on the dates of occurrence of nor’westers from the weekly normal is minimum in the lower atmosphere and does not show any definite trend onwards.
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Ubiquitous positioning using combined lowcost, Micro-electro-mechanical-systems (MEMS) Inertial Navigation Systems (INS) and Global Navigation Satellite System (GNSS) systems remains a challenge in difficult GNSS environments. This is due to the GNSS signals being partially or completely obstructed by building structures and consequently no information being available to calibrate the MEMS INS errors. In these environments although there may be insufficient signals for a two or three dimensional GNSS solution, some signals can still be received, particularly when utilizing high sensitivity GNSS receivers. These signals can be used to aid the INS/GNSS integrated system but can only be realized when it utilizes a tightly coupled (TC) as opposed to a loosely coupled (LC) integration architecture. This is particularly useful for MEMS based INS/GNSS integrated system as its performance can degrade rapidly over short period of GNSS outages. This paper aims to evaluate the capabilities of MEMS based INS/GNSS using TC integration architecture during extended period of GNSS outages. The results obtained show that TC integration architecture does help limit the error growth even when INS/GNSS integrated system experienced relatively long period of partial GNSS outages.
GNSS augmentation
Air navigation
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An increasing number of mobile applications and services require that devices are aware of their location. Global navigation satellite systems (GNSS) are the predominant enabling technology. But location information provided by commercial GNSS is not secure, unlike what is the usual assumption. There are only few exceptions in the literature that present GNSS vulnerabilities. In this paper, we contribute the first detailed quantitative analysis of attacks against GNSS-based localization. We show how replay attacks against GNSS can have a significant impact: even against cryptographically secured GNSS instantiations, an adversary can manipulate the location and time calculated by victim GNSS receivers. We explain in detail how such attacks can be mounted, measure their impact, and discuss the effectiveness of possible countermeasures.
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Replay attack
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In all GNSS satellite time scales are referenced to GNSS Time and GNSS Time is synchronized to Reference Time which is steered to UTC. GNSS Time accuracy parameters influence the accuracy of positioning and timing as well as the accuracy of calculating and predicting GNSS-GNSS Time Offsets for GNSS interoperability. GNSS time scales generation and maintenance have been analyzed and the results of their accuracy estimation are provided on the basis of available information.
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近年では,斜面防災に利用できる多様なモニタリングデータが獲得されている.衛星を用いた測位システムであるRTK-GNSSを活用し,ChangeFinderによる変化点検知手法が提案されているが,実用化のためには適用範囲の拡大が課題として残されている.本研究では,GNSSデータに含まれる測定誤差を処理する手法を新たに提案し,検知精度の向上を試みた.具体的には,マルチパス誤差を除去する恒星日周期補正法と偶然誤差を含むGNSSデータから真の位置を区別する偶然誤差処理モデルを提案し,粒子フィルタによる逐次推定を援用した変化点検知を行う.さらに,実際の高速道路において観測されたGNSSデータを用いて検証し,斜面の変位量が標本標準偏差の1.5倍を超える場合に変化点を検知できることを示した.
GNSS augmentation
Precise Point Positioning
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다양한 위성 항법의 등장과 전자 장비의 발달로 인해 GNSS 측량의 활용성은 많은 분야에서 입증되고 있다. 특히 기준점 측량 중 수평 위치 결정 분야에서는 GNSS 측량이 가능한 조건에서는 이를 이용하여 기준점 측량을 수행하고 있는 상황이다.BR 그러나, 수평위치와 달리 높이측량 분야에서는 레벨측량 방법으로 높이 결정을 수행하고 있으며 많은 작업시간과 오차를 포함할 수 있는 상황이다.BR 이에 본 연구에서는 높이 결정에 많은 영향을 미치는 GNSS 타원체고 정확도를 비교하고자 하였다. 비교를 위해 관측 시간별로 정확도를 분석하였으며, 상업용 프로그램과 학술용 프로그램을 사용하여 GNSS 높이 정확도를 분석하였다. 분석결과 타원체 고 정확도 향상을 위해서는 8시간의 관측시간이 필요하다는 것을 입증할 수 있었다.
Ellipsoid
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Development of Global Navigation Satellite System (GNSS) applications requires a methodology for GNSS positioning performance assessment in targeted GNSS positioning environment conditions, and its compliance with requirements of targeted GNSS application. We propose such a methodology that utilizes experimentally collected GNSS observations in the GNSS positioning environment conditions of concern, and open-source software. The proposed methodology include the GNSS observations collection with the consumer-grade smartphone GNSS receivers, and statistical analysis of the estimated components of the post-processed GNSS positioning error vector. Demonstrated in the real scenario of the GNSS positioning performance assessment based on experimental data, the proposed methodology was proven to become an indispensable tool for GNSS applications developers, operators and users, especially in the segment of Location-Based Services.
GNSS augmentation
Hybrid positioning system
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