Precise near-earth navigation with GPS: A survey of techniques

The tracking accuracy of the low earth orbiters (below about 3000 km altitude) can be brought below 10 cm with a variety of differential techniques that exploit the Global Positioning System (GPS). All of these techniques require a precisely known global network of GPS ground receivers and a receiver aboard the user satellite, and all simultaneously estimate the user and GPS satellite orbits. Three basic approaches are the geometric, dynamic, and nondynamic strategies. The last combines dynamic GPS solutions with a geometric user solution. Two powerful extensions of the nondynamic strategy show considerable promise. The first uses an optimized synthesis of dynamics and geometry in the user solution, while the second uses a novel gravity-adjustment method to exploit data from repeat ground tracks. These techniques will offer sub-decimeter accuracy for dynamically unpredictable satellites down to the lowesst possible altitudes

Precise tracking of the Magellan and Pioneer Venusorbiters by same-beam interferometry. Part 1: Dataaccuracy analysis

Simultaneous tracking of two spacecraft in orbit about a distant planet by two widely separated Earth-based radio antennas provides more-accurate positioning information than can be obtained by tracking each spacecraft separately. A demonstration of this tracking technique, referred to as same-beam interferometry (SBI), is currently being done using the Magellan and Pioneer 12 orbiters at Venus. Signals from both spacecraft fall within the same beamwidth of the Deep Space Station antennas. The plane-of-sky position difference between spacecraft is precisely determined by doubly differenced phase measurements. This radio metric measurement naturally complements line-of-sight Doppler. Data was first collected from Magellan and Pioneer 12 on August 11-12, 1990, shortly after Magellan was inserted into Venus orbit. Data were subsequently acquired in February and April 1991, providing a total of 34 hours of same-beam radio metric observables. Same-beam radio metric residuals have been analyzed and compared with model measurement error predictions. The predicted error is dominated by solar plasma fluctuations. The rms of the residuals is less than predicted by about 25 percent for 5-min averages. The shape of the spectrum computed from residuals is consistent with that derived from a model of solar plasma fluctuations. This data type can greatly aid navigation of a second spacecraft when the first is well-known in its orbit

센티미터 급 광역 보강항법 시스템의 반송파 위상 기반 보정정보 생성 알고리즘에 관한 연구

학위논문(박사)--서울대학교 대학원 :공과대학 기계항공공학부,2020. 2. 기창돈.Recently, the demand for high-precision navigation systems for centimeter-level service has been growing rapidly for various Global Navigation Satellite System (GNSS) applications. The network Real-Time Kinematic (RTK) is one of the candidate solution to provide high-accuracy position to user in real-time. However, the network RTK requires a lot of reference stations for nationwide service. Furthermore, it requires high-speed data-link for broadcasting their scalar-type corrections. This dissertation proposed a new concept of satellite augmentation system called Compact Wide-Area RTK, which provides centimeter-level positioning service on national or continental scales to overcoming the limitation of the legacy network RTK methods. Using the wide-area network of multiple reference stations whose distance is 200~1,000 km, the proposed system generates three types of carrier-phase-based corrections: satellite orbit corrections, satellite code/phase clock (CPC) corrections, tropospheric corrections. Through the strategy of separating the scalar-type corrections of network RTK into vector forms of each error component, it is enable to expand network RTK coverage to continental scale using a similar number of reference stations as legacy meter-level Satellite-Based Augmentation System (SBAS). Furthermore, it is possible to broadcast their corrections over a wide-area using geosynchronous (GEO) satellite with extremely low-speed datalink of 250 bps likewise of legacy SBAS. To sum up, the proposed system can improve position accuracy by centimeter-level while maintaining the hardware infrastructure of the meter-level legacy SBAS. This study mainly discussed on the overall system architecture and core algorithms for generating satellite CPC corrections and tropospheric corrections. This study proposed a new Three-Carrier Ambiguity Resolution (TCAR) algorithm using ionosphere-free combinations to correctly solve the integer ambiguity in wide-area without any ionospheric corrections. The satellite CPC corrections are calculated based on multiple stations for superior and robust performance under communication delay and outage. The proposed algorithm dramatically reduced the latency compensation errors and message amounts with compare to conventional RTK protocols. The tropospheric corrections of the compact wide-area RTK system are computed using GPS-estimated precise tropospheric delay and weather data based model together. The proposed algorithm adopts spherical harmonics function to significantly reduce the message amounts and required number of GPS reference stations than the network RTK and Precise Point Positioning-RTK (PPP-RTK), while accurately modeling the spatial characteristic of tropospheric delay with weather data together. In order to evaluate the user domain performance of the compact wide-area RTK system, this study conducted the feasibility test on mid-west and south USA using actual GPS measurements. As a result, the 95% horizontal position error is about 1.9 cm and the 95% vertical position error is 7.0 cm after the integer ambiguity is correctly fixed using GPS-only signals. The user ambiguity resolution takes about 2 minutes, and success-fix rate is about 100 % when stable tropospheric condition. In conclusion, the compact wide-area RTK system can provide centimeter-level positioning service to wide-area coverage with extremely low-speed data link via GEO satellite. We hope that this new system will consider as candidate solution for nationwide centimeter-level service such as satellite augmentation system of the Korea Positioning System (KPS).최근 자율주행자동차, 무인 드론 배송, 충돌 회피, 무인트랙터를 이용한 스마트 무인 경작 등 위성항법시스템(GNSS, Global Navigation Satellite System)을 사용하는 다양한 응용분야에서 수 cm 수준의 정밀 위치 정보에 대한 요구가 급격히 증가하고 있다. 본 학위논문에서는 1 m 급의 정확하고 신뢰성 높은 위치 서비스를 제공하는 기존의 정지궤도위성 기반 광역 보강항법 시스템(SBAS, Satellite-Based Augmentation System)의 기준국 인프라를 유지하면서 항법 성능을 수 cm 수준으로 향상시키기 위해 반송파 위상 기반의 초정밀 보정정보 생성 알고리즘에 관한 연구를 수행하였다. 실시간 정밀 측위(RTK, Real-Time Kinematic)는 반송파 위상 측정치에 포함된 미지정수를 정확하게 결정하여 수 cm 수준의 정밀 항법 서비스를 가능하게 하는 대표적인 기법이다. 그 중에서도 약 50~70 km 간격으로 분포된 다수의 기준국 정보를 활용하는 Network RTK 기법은 동적 사용자의 빠르고 정확한 위치 결정이 가능한 인프라로서 주목받고 있다. 하지만 스칼라 형태로 구성된 Network RTK 보정정보는 각 기준국 별로 관측된 위성 수에 따라 생성이 되기 때문에 보정 데이터 량이 상당히 방대하다. 메시지 전송에 필요한 데이터 량이 많을수록 고속의 통신 환경을 필요로 하며, 메시지 시간 지연이나 통신 단절에 매우 취약한 문제를 가지고 있다. 또한 스칼라 형태의 보정정보는 사용자와 기준국 간의 거리가 멀어질수록 보정 오차가 크게 발생하기 때문에 대륙 혹은 나라 규모의 광역에서 서비스하기 위해서는 수십~수백 개 이상의 기준국 인프라 구축이 필수적이다. 예를 들어, SBAS가 한반도 지역 서비스를 위해 5~7개의 기준국이 필요한 반면 Network RTK는 90~100개의 기준국이 필요하다. 즉 Network RTK는 시스템 구축 및 유지 비용이 SBAS 대비 약 15배 정도 많이 들게 된다. 본 논문에서는 기존 Network RTK의 문제점을 해결하기 위한 방법으로 대륙 급 광범위한 영역에서 실시간으로 cm급 초정밀 위치결정 서비스 제공이 가능한 Compact Wide-Area RTK 라는 새로운 개념의 광역보강항법시스템 아키텍처를 제안하였다. Compact Wide-Area RTK는 약 200~1,000 km 간격으로 넓게 분포된 기준국 네트워크를 활용하여 반송파 위상 기반의 정밀한 위성 궤도 보정정보, 위성 Code/Phase 시계 보정정보, 대류층 보정정보를 생성하는 시스템이다. 기존 스칼라 형태의 Network RTK 보정정보 대신 오차 요소 별 벡터 형태의 정밀 보정정보를 생성함으로써 데이터 량을 획기적으로 절감하고 서비스 영역을 확장할 수 있다. 최종적으로 SBAS와 마찬가지로 250 bps의 저속 통신 링크를 가진 정지궤도위성을 통해 광역으로 보정정보 방송이 가능하다. 본 논문에서는 3가지 보정정보 중 위성 Code/Phase 시계 보정정보와 대류층 보정정보 생성을 위한 핵심 알고리즘에 대해 중점적으로 연구하였다. 반송파 위상 기반의 정밀 보정정보 생성을 위해서는 먼저 미지정수를 정확하게 결정해야 한다. 본 논문에서는 삼중 주파수 반송파 위상 측정치의 무-전리층 조합을 활용하여 전리층 보정정보 없이도 정확하게 미지정수 결정 가능한 새로운 방법을 제안하였다. 위성 Code/Phase 시계 보정정보는 통신 지연 및 고장 시 우수하고 강건한 성능을 위해 다중 기준국의 모든 측정치를 활용하여 추정된다. 이 때 각 기준국 별 서로 다른 미지정수 때문에 발생하는 문제는 앞서 정확하게 결정된 기준국 간 이중차분 된 미지정수를 활용하여 수준을 조정하는 과정을 통해 해결이 가능하다. 그 결과 생성된 위성 Code/Phase 보정정보 메시지의 크기, 변화율, 잡음 수준이 크게 개선되었고, 통신 지연 시 오차 보상 성능이 기존 RTK 프로토콜 보다 99% 향상 됨을 확인하였다. 대류층 보정정보는 적은 수의 기준국 만을 활용하여 정확하게 대류층을 모델링하기 위해 자동 기상관측시스템으로부터 수집한 기상 정보를 추가로 활용하여 생성된다. 본 논문에서는 GNSS 기준국 네트워크로부터 정밀하게 추정된 반송파 위상 기반 수직 대류층 지연과 기상정보 기반으로 모델링 된 수직 대류층 지연을 함께 활용할 수 있는 새로운 알고리즘을 제안하였다. 구면조화함수를 사용하여 Network RTK 및 PPP-RTK 보다 필요한 메시지 양과 기준국 수를 크게 감소시키면서도 RMS 2 cm 수준으로 정확한 보정정보 생성이 가능함을 확인하였다. 본 논문에서 제안한 Compact Wide-Area RTK 시스템의 항법 성능을 검증하기 위해 미국 동부 지역 6개 기준국의 실측 GPS 데이터를 활용하여 테스트를 수행하였다. 그 결과 제안한 시스템은 미지정수 결정 이후 사용자의 95% 수평 위치 오차 1.9 cm, 95% 수직 위치 오차 7.0 cm 로 위치를 정확하게 결정하였다. 사용자 미지정수 결정 성능은 대류층 안정 상태에서 약 2분 내로 100% 의 성공률을 가진다. 본 논문에서 제안한 시스템이 향후 한국형 위성항법 시스템(KPS, Korean Positioning System)의 전국 단위 센티미터 급 서비스를 위한 알고리즘으로 활용되기를 기대한다.CHAPTER 1. Introduction 1 1.1 Motivation and Purpose 1 1.2 Former Research 4 1.3 Outline of the Dissertation 7 1.4 Contributions 8 CHAPTER 2. Overview of GNSS Augmentation System 11 2.1 GNSS Measurements 11 2.2 GNSS Error Sources 14 2.2.1 Traditional GNSS Error Sources 14 2.2.2 Special GNSS Error Sources 21 2.2.3 Summary 28 2.3 GNSS Augmentation System 29 2.3.1 Satellite-Based Augmentation System (SBAS) 29 2.3.2 Real-Time Kinematic (RTK) 32 2.3.3 Precise Point Positioning (PPP) 36 2.3.4 Summary 40 CHAPTER 3. Compact Wide-Area RTK System Architecture 43 3.1 Compact Wide-Area RTK Architecture 43 3.1.1 WARTK Reference Station (WRS) 48 3.1.2 WARTK Processing Facility (WPF) 51 3.1.3 WARTK User 58 3.2 Ambiguity Resolution and Validation Algorithms of Compact Wide-Area RTK System 59 3.2.1 Basic Theory of Ambiguity Resolution and Validation 60 3.2.2 A New Ambiguity Resolution Algorithms for Multi-Frequency Signals 65 3.2.3 Extra-Wide-Lane (EWL) Ambiguity Resolution 69 3.2.4 Wide-Lane (WL) Ambiguity Resolution 71 3.2.5 Narrow-Lane (NL) Ambiguity Resolution 78 3.3 Compact Wide-Area RTK Corrections 83 3.3.1 Satellite Orbit Corrections 86 3.3.2 Satellite Code/Phase Clock (CPC) Corrections 88 3.3.3 Tropospheric Corrections 89 3.3.4 Message Design for GEO Broadcasting 90 CHAPTER 4. Code/Phase Clock (CPC) Correction Generation Algorithm 93 4.1 Former Research of RTK Correction Protocol 93 4.1.1 Observation Based RTK Data Protocol 93 4.1.2 Correction Based RTK Data Protocol 95 4.1.3 Compact RTK Protocol 96 4.2 Satellite CPC Correction Generation Algorithm 100 4.2.1 Temporal Decorrelation Error Reduced Methods 102 4.2.2 Ambiguity Level Adjustment 105 4.2.3 Receiver Clock Synchronization 107 4.2.4 Averaging Filter of Satellite CPC Correction 108 4.2.5 Ambiguity Re-Initialization and Message Generation 109 4.3 Correction Performance Analysis Results 111 4.3.1 Feasibility Test Environments 111 4.3.2 Comparison of RTK Correction Protocol 113 4.3.3 Latency Compensation Performance Analysis 116 4.3.4 Message Data Bandwidth Analysis 119 CHAPTER 5. Tropospheric Correction Generation Algorithm 123 5.1 Former Research of Tropospheric Correction 123 5.1.1 Tropospheric Corrections for SBAS 124 5.1.2 Tropospheric Corrections of Network RTK 126 5.1.3 Tropospheric Corrections of PPP-RTK 130 5.2 Tropospheric Correction Generation Algorithm 136 5.2.1 ZWD Estimation Using Carrier-Phase Observations 138 5.2.2 ZWD Measurements Using Weather Data 142 5.2.3 Correction Generation Using Spherical Harmonics 149 5.2.4 Correction Applying Method for User 157 5.3 Correction Performance Analysis Results 159 5.3.1 Feasibility Test Environments 159 5.3.2 Zenith Correction Domain Analysis 161 5.3.3 Message Data Bandwidth Analysis 168 CHAPTER 6. Compact Wide-Area RTK User Test Results 169 6.1 Compact Wide-Area RTK User Process 169 6.2 User Performance Test Results 173 6.2.1 Feasibility Test Environments 173 6.2.2 User Range Domain Analysis 176 6.2.3 User Ambiguity Domain Analysis 182 6.2.4 User Position Domain Analysis 184 CHAPTER 7. Conclusions 189 Bibliography 193 초 록 207Docto

Ionospheric path delay models for spaceborne GPS receivers flying in formation with large baselines

GPS relative navigation filters could benefit notably from an accurate modeling of the ionospheric delays, especially over large baselines (>100 km) where double difference delays can be higher than several carrier wavelengths. This paper analyzes the capability of ionospheric path delay models proposed for spaceborne GPS receivers in predicting both zero-difference and double difference ionospheric delays. We specifically refer to relatively simple ionospheric models, which are suitable for real-time filtering schemes. Specifically, two ionospheric delay models are evaluated, one assuming an isotropic electron density and the other considering the effect on the electron density of the Sun aspect angle. The prediction capability of these models is investigated by comparing predicted ionospheric delays with measured ones on real flight data from the Gravity Recovery and Climate Experiment mission, in which two satellites fly separated of more than 200 km. Results demonstrate that both models exhibit a correlation in the excess of 80% between predicted and measured double-difference ionospheric delays. Despite its higher simplicity, the isotropic model performs better than the model including the Sun effect, being able to predict double differenced delays with accuracy smaller than the carrier wavelength in most cases. The model is thus fit for supporting integer ambiguity fixing in real-time filters for relative navigation over large baselines. Concerning zero-difference ionospheric delays, results demonstrate that delays predicted by the isotropic model are highly correlated (around 90%) with those estimated using GPS measurements. However, the difference between predicted and measured delays has a root mean square error in the excess of 30 cm. Thus, the zero-difference ionospheric delays model is not likely to be an alternative to methods exploiting carrier-phase observables for cancelling out the ionosphere contribution in single-frequency absolute navigation filters

Flight-Test Evaluation of Kinematic Precise Point Positioning of Small UAVs

An experimental analysis of Global Positioning System (GPS) flight data collected onboard a Small Unmanned Aerial Vehicle (SUAV) is conducted in order to demonstrate that postprocessed kinematic Precise Point Positioning (PPP) solutions with precisions approximately 6 cm 3D Residual Sum of Squares (RSOS) can be obtained on SUAVs that have short duration flights with limited observational periods (i.e., only ∼≤5 minutes of data). This is a significant result for the UAV flight testing community because an important and relevant benefit of the PPP technique over traditional Differential GPS (DGPS) techniques, such as Real-Time Kinematic (RTK), is that there is no requirement for maintaining a short baseline separation to a differential GNSS reference station. Because SUAVs are an attractive platform for applications such as aerial surveying, precision agriculture, and remote sensing, this paper offers an experimental evaluation of kinematic PPP estimation strategies using SUAV platform data. In particular, an analysis is presented in which the position solutions that are obtained from postprocessing recorded UAV flight data with various PPP software and strategies are compared to solutions that were obtained using traditional double-differenced ambiguity fixed carrier-phase Differential GPS (CP-DGPS). This offers valuable insight to assist designers of SUAV navigation systems whose applications require precise positionin

Characterizing the Impact of Precision Time and Range Measurements from Two-Way Time Transfer Systems on Network Differential GPS Position Solutions

Precise positioning plays an important role for both military and civilian users, from cell phones and OnStar to precision munitions and swarms of UAVs. Many applications require precise relative positioning of a network of vehicles (such as aircraft, tanks, troops, etc). Currently, the primary means for performing precise positioning is by using the Global Positioning System (GPS), and although GPS has become commonplace in today’s society, there are still limitations affecting the system. Recent advances in dynamic Two-Way Time Transfer (TWTT) have potentially provided a means to improve precise relative positioning accuracy over differential GPS (DGPS)-only approaches. TWTT is a technique in which signals are simultaneously exchanged between users. This research investigates the impact of using Two-Way Time Transfer (TWTT) time measurements to augment differential GPS systems to improve the relative positioning solutions of vehicle networks. Incorporating the TWTT time measurement into the DGPS solution improves the 3-D relative positioning accuracy by up to 44% with pseudorange measurements and 35% with carrier-phase measurements. Normally, the TWTT measurements are used in a manner that cancels out the impact of the vehicle position in order to obtain a precise relative time measurement. The research also implements an innovative approach to using TWTT measurements to actually obtain a precise measurement of the vehicle position in addition to the time measurement. The results show that 3-D relative positioning solutions can be improved by up to 48% when using pseudorange measurements augmented with TWTT time and range measurements, and up to 40% when using carrier-phase measurements augmented with TWTT time and range measurements

PRECISE KINEMATIC APPLICATIONS OF GPS: PROSPECTS AND CHALLENGES

GPS kinematic positioning in the post-processed or in the real-time mode is now increasingly used for many surveying and navigation applications on land, at sea and in the air. Techniques range from the robust pseudo-range-based differential GPS (DGPS) techniques capable of delivering accuracies at the few metre level, to sophisticated carrier phase-based centimetre accuracy techniques. The distance from the mobile receiver to the nearest reference receiver may range from a few kilometres to hundreds of kilometres. As the receiver separation increases, the problems of accounting for distance-dependent biases grows. For carrier phasebased techniques reliable ambiguity resolution becomes an even greater challenge. In the case of DGPS, more appropriate implementations such as Wide Area DGPS become necessary. In this paper, the challenges, progress and outlook for high precision GPS kinematic positioning for the short-range, medium-range and long-range cases, in both the post-processing and real-time modes will be discussed. Although the focus will be on carrier phase-based systems, some comments will also be made with regards to DGPS systems. Several applications of kinematic GPS positioning will be considered, so as to demonstrate the engineering challenges in addition to GPS, that have to be addressed

The impact of new signals on precise marine navigation - initial results from an experiment in Harwich Harbour

The General Lighthouse Authorities of the United Kingdom and Ireland (GLAs) are supporting a project at University College London (UCL) to study whether it is possible to meet the International Maritime Organisation’s (IMO) future requirements for port and harbour approach using future GNSS constellations, as detailed in IMO resolution A.915. This paper presents the results of a trial focusing on the accuracy, integrity, availability and continuity of port navigation, port approach, and docking. Abstract The required accuracy for docking is 0.1 m (95\%), which currently necessitates the use of Real Time Kinematic (RTK) processing. We consider the single-epoch geometry-based approach, which is robust against loss of lock and will fully benefit from the additional satellites. The trial was held at the beginning of May 2008 and saw THV Alert navigate into Harwich Harbour while satellite observation data were recorded from the vessel and from shore-based reference stations. Additional data were obtained from nearby Ordnance Survey reference stations, and two total stations were used to track the vessel’s passage to provide a truth model. Several modernised GPS satellites were tracked. The data were processed under different scenarios, using software developed at UCL, and the positioning performance analysed. Abstract Providing integrity for single-epoch RTK is particularly difficult. The identification of phase observation outliers is not possible before the integer ambiguities are resolved, but an undetected outlier could prevent successful ambiguity resolution. However, it will not always be necessary to fix every ambiguity to achieve the required precision, particularly with a multi-GNSS constellation. This paper introduces a new algorithm for partial ambiguity resolution in the presence of measurement bias that has been developed and tested at UCL.  This algorithm results in an improved ambiguity resolution success rate at the expense of computation time