Differential Carrier Phase Measurements Research Articles

Galileo-Based Doppler Shifts and Time Difference Carrier Phase: A Static Case Demonstration.

The European Commission is designing and implementing new regulations for vehicle navigation in different sectors. Commission Delegated Regulation 2017/79 defines the compatibility and performance of the 112-based eCall in-vehicle systems. The regulation has a large impact on road transportation because it requires that all cars and light duty vehicles must be equipped with eCall devices. For heavy duty vehicles, a set of new regulations has been developed, starting from EU Regulation No 165/2014, in which the concept of smart tachographs was introduced to enforce the EU legislation on professional drivers' driving and resting times. In addition, intelligent speed assistance (ISA) devices increase the safety of road users. These new devices fully exploit the Global Navigation Satellite System (GNSS) to compute position velocity and time (PVT) information. In all these systems, the velocity of the vehicle plays a fundamental role; hence, a reliable and accurate velocity estimate is of utmost importance. In this work, two methods for velocity estimation using Galileo are presented and compared. The first exploits Doppler shift measurements, while the second uses time difference carrier phase (TDCP) measurements. The Doppler-based technique for velocity estimation is widely adopted in current devices, while the TDCP technique is emerging due to its promising high accuracy. The two methods are compared considering all the Galileo signals including E1, E5a, E5b, E5 Alt BOC and E6. The methods are compared in terms of velocity errors for both horizontal and vertical components using real static data. From the tests performed, it emerged that the TDCP has increased performance with respect to the Doppler-based solution. Among the Doppler-based solutions, the most accurate solution is the one obtained with the E5 Alt BOC signal.

Polarization Diversity-Enabled LOS/NLOS Identification via Carrier Phase Measurements

The provision of accurate localization is an increasingly important feature of wireless networks. To this end, a reliable distinction between line-of-sight (LOS) and non-LOS (NLOS) radio links is necessary to avoid degenerative localization estimation biases. Interestingly, LOS and NLOS transmissions affect the polarization of the received signals differently. In this work, we leverage this phenomenon to propose a threshold-based LOS/NLOS classifier exploiting weighted differential carrier phase measurements over a single link with different polarization configurations. Operation in either full or limited polarization diversity systems is possible. We develop a framework for assessing the performance of the proposed classifier, and show through simulations the performance impact of the reflecting materials in NLOS scenarios. For instance, the classifier is far more efficient in NLOS scenarios with wooden reflectors than in those with metallic reflectors. Numerical results evince the potential performance gains from exploiting full polarization diversity, properly weighting the differential carrier phase measurements, and using multi-carrier/tone transmissions. Finally, we show that the optimum decision threshold is inversely proportional to the path power gain in dB, while it does not depend significantly on the material of potential NLOS reflectors.

Global Navigation Satellite System Real-Time Kinematic Positioning Framework for Precise Operation of a Swarm of Moving Vehicles.

The global navigation satellite system (GNSS) real-time kinematic (RTK) technique is used to achieve relative positioning centimeter levels among multiple agents on the move. A typical GNSS RTK estimates the relative positions of multiple rover receivers with respect to a single-base receiver. In a fleet of rover GNSS receivers, this approach is inefficient because each rover receiver only uses GNSS measurements of its own and those sent from a single-base receiver. In this study, we propose a novel GNSS RTK framework that facilitates the precise positioning of a swarm of moving vehicles through the GNSS measurements of multiple receivers and broadcasts fixed-integer ambiguities of GNSS carrier phases. The proposed framework not only provides efficient RTK positioning but also reliable performance with a limited number of GNSS satellites in view. Our experimental flight tests with six GNSS receivers showed that the systematic procedure of the proposed framework could maintain lower than 6 cm of 3D RMS positioning errors, whereas the conventional RTK failed to resolve the correct integer ambiguities of double difference carrier phase measurements more than 13% in five out of nine total baselines.

GNSS real‐time instantaneous velocimetry based on moving‐window polynomial modelling

The instantaneous kinematic velocity obtained by using a stand-alone global navigation satellite system (GNSS) receiver has attracted increasing attention, and has significant for numerous applications. Using the time difference carrier phase (TDCP) measurements, or the displacements they produced, we propose two real-time instantaneous velocity determination methods based on the moving-window polynomial model. The two methods are called the observation-domain and the coordinate-domain, correspondingly. In the calculation process, the Cholesky update method is utilized to improve numerical efficiency. The proposed methods are a finite impulse in nature, and hence are robust against modeling uncertainties, such as temporal outlying measurements, inaccurate information of noise statistics. Two vehicle-mounted experiments were performed to verify the performance of the coordinate-domain and observation-domain methods. The reference velocities used to evaluate the accuracy of velocity estimates are chosen as GNSS/INS smoothing solution and high-rate RTK solution in these two experiments, respectively. The results show that the coordinate-domain and observation-domain methods were comparable in performance, both better than that of the Nonlinear Tracking Differentiator (NTD) method. Specifically, the improvement in terms of root mean square of the proposed methods, compared with the NTD, can be 29.3–40.3% and 6.25–16.4% in the two experiments, respectively.

Detecting Hazardous Spatial Gradients at Satellite Acquisition in GBAS

In this article, we develop a ground monitor capable of detecting anomalous signal-in-space spatial gradients for rising, newly acquired, and reacquired satellites in the ground-based augmentation system using either single or dual frequency. These gradients can be caused by satellite orbit ephemeris faults and ionospheric fronts. The monitor utilizes differential code and carrier phase measurements across multiple reference receiver antennas as the basis for detection. We show that the new monitor significantly improves performance over existing detection algorithms and is capable of meeting Category III precision approach and landing requirements.

Dual-frequency carrier smoothed code filtering with dynamical ionospheric delay modeling

In GNSS applications, carrier-smoothed-code is a widely used technique to combine code pseudo-range and carrier phase measurements. A dynamical ionospheric delay modeling method is proposed based on Kalman filter and least-squares theory. The level of the process noise is adaptively tuned along with the real-time KF state estimation, based on the online variance component estimation method. Meanwhile, the correlations of the time differenced carrier phase measurements are considered. This approach avoids overly optimistically evaluating the estimate and improves the transient accuracy of the estimates. A real GPS dataset is employed to check the performance of the proposed method under different conditions. The results show that the new algorithm can model the ionospheric delay variation well with different sampling intervals or even in ionospheric abnormal environment. The positioning accuracy can be confirmed, about 21%, 35% and 16% better are obtained in the N, E, and U direction than raw dataset.

Estimating maneuvers for precise relative orbit determination using GPS

Precise relative orbit determination is an essential element for the generation of science products from distributed instrumentation of formation flying satellites in low Earth orbit. According to the mission profile, the required formation is typically maintained and/or controlled by executing maneuvers. In order to generate consistent and precise orbit products, a strategy for maneuver handling is mandatory in order to avoid discontinuities or precision degradation before, after and during maneuver execution. Precise orbit determination offers the possibility of maneuver estimation in an adjustment of single-satellite trajectories using GPS measurements. However, a consistent formulation of a precise relative orbit determination scheme requires the implementation of a maneuver estimation strategy which can be used, in addition, to improve the precision of maneuver estimates by drawing upon the use of differential GPS measurements. The present study introduces a method for precise relative orbit determination based on a reduced-dynamic batch processing of differential GPS pseudorange and carrier phase measurements, which includes maneuver estimation as part of the relative orbit adjustment. The proposed method has been validated using flight data from space missions with different rates of maneuvering activity, including the GRACE, TanDEM-X and PRISMA missions. The results show the feasibility of obtaining precise relative orbits without degradation in the vicinity of maneuvers as well as improved maneuver estimates that can be used for better maneuver planning in flight dynamics operations.

Improving the Performance of INS/GNSS System Using the Approach of Carrier Phase Measurements

This paper discusses the techniques of attitude, velocity ad position estimation from GNSS carrier phase measurements, and investigates the performance of the lower precision MEMS based INS/GNSS system based on carrier phase measurements. Generally, a GPS receiver estimates the position and velocity from code phase and Doppler measurements. Double differenced carrier phase measurements provide more accurate velocity and position estimation compared to code and Doppler measurements. However, for position measurement, the integer ambiguity is required to be removed. Multiples antennae approach is used to derive the attitude information from carrier phase measurements in order to control the large initial misalignment angles for initialization of the integration process or to utilize during benign dynamics. Lever arm effect is considered to compensate for the separation of GNSS antenna and IMU location. The derived three GNSS observables are used to correct the INS through optimal Kalman filtering in a closed loop. Simulation results indicates the effectiveness of the integrated system for airborne as well as for land navigation vehicles.

Optimal antenna topologies for spatial gradient detection in differential GNSS

AbstractThis paper describes new methods to determine optimal reference antenna topologies for detection of spatial gradients in differential Global Navigation Satellite Systems (GNSS). Such gradients can be caused by ionospheric fronts and orbit ephemeris faults, and if undetected, represent major threats to aircraft navigation integrity. Differential carrier phase measurements between ground antennas are highly sensitive to spatial gradients. Therefore, monitors using spatially separated ground antennas have recently attracted great interest. However, they cannot detect gradients of all sizes and directions due to the presence of integer ambiguities. These ambiguities cannot be resolved because the gradient magnitude is unknown a priori. Furthermore, the performance of such monitors is highly dependent on the spatial distribution of reference antennas. In this work, we introduce new methods to find optimized antenna topologies for spatial gradient detection.

High-Accuracy GPS-Based Aircraft Navigation for Landing using Pseudolites and Double-Difference Carrier Phase Measurements

The GPS code phase measurements provide accuracy about 5-20 m. This accuracy level is not sufficient for some applications such as aircraft landing. Precise positioning can be achieved by using differential carrier phase measurements. But these measurements contain unknown integers. The Carrier phase integer ambiguity can be resolved with the aid of ranging information from two integrity beacons, placed one on each side of the approach path to a runway. The clock bias present in the single difference carrier phase measurements is cancelled by the double-differencing. However, the double-difference measurements are correlated. The decorrelation is achieved by using LAMBDA method. This paper presents the integer ambiguity resolution algorithm and position estimation with single-difference and double-difference carrier phase measurements.

Carrier Phase Ionospheric Gradient Ground Monitor for GBAS with Experimental Validation

This paper describes a Ground Based Augmentation System (GBAS) ground-based monitor capable of instantly detecting anomalous ionospheric gradients at the time of satellite acquisition. The monitor utilizes differential carrier phase measurements across multiple reference station baselines as the basis for detection. Performance analysis shows that the monitor is highly sensitive to the quality of the carrier phase measurements. Therefore, data collected from a GBAS prototype ground facility is used to quantify the measurement quality and validate the concept monitor.

Comparison of GPS-based autonomous vehicle following using global and relative positioning

Differential GPS methods are developed for use in automated vehicle convoy positioning. The GPS pseudo-range and carrier phase measurements are used to compute relative position vectors between two vehicles with sub-meter errors. The carrier phase measurement makes this level of accuracy attainable, but carrier phase ambiguity must be resolved prior to the relative position estimation. An algorithm, referred to as the Dynamic base Real Time Kinematic (DRTK) algorithm, is described to estimate carrier phase ambiguity and the relative position vector between two GPS receivers. A comparative study of the performance of the algorithm using either single or dual frequency measurements is presented. The use of relative positioning to autonomously follow a human-driven lead vehicle is presented. Time Difference Carrier Phase (TDCP) measurements are used to estimate the change in the position of the following vehicle between measurement epochs. The TDCP algorithm is combined with the DRTK algorithm to estimate the position of the following vehicle relative to a virtual lead vehicle position. Analysis of the accuracy of the TDCP algorithm at individual measurement epochs and over varying time intervals is presented. The DRTK/TDCP following method (using relative positioning information) is compared to a GPS waypoint following method (using global positioning information) using an all-terrain vehicle which is automated to follow a human-driven lead vehicle.

The Analysis and Validation of Two-dimensional Attitude Measurement with GPS

Global Positioning System (GPS) is more and more being used in the attitude measurement, the error accumulates over time and the accuracy of the system can’t be ensured after a long time, the GPS has the performance of effective real-time and stability in accuracy through the application of the GPS measurement of attitude in resolving. After determining the attitude of carriers using measured carrier phase of GPS,solving the carrier phase ambiguitie problem, Real-time attitude determination is derived from extremely precise differential carrier phase measurements among GPS antennas mounted on the vehicle.A simple and efficient way to determine the attitude,use the GPS OEM boards,carrier phase difference method,QR decomposition algorithm.The feasibility and applicability of the design are validated through experiments.The results show that:the method can get the attitude in a short time and is of higher accurate.

Noise analysis and suppression method in attitude determination using the global positioning system (GPS)

A noise suppression method is developed for attitude determination using the global positioning system. The influence of noise on attitude determination application is analyzed to determine the relationship of errors. In order to suppress the noise, the total least squares method is utilized for double difference carrier phase measurements and unit vectors between satellites and antennas. Experimental results indicate that compared to the traditional least squares method without noise suppression, the accuracy of the measurement of attitude angles is increased by about 30–50%. The increased computation time of this method does not significantly influence the real time performance for land vehicle application.

Carrier Phase Relative RAIM Algorithms and Protection Level Derivation

The concept of Relative Receiver Autonomous Integrity Monitoring (RRAIM) using time differential carrier phase measurements is investigated in this paper. The precision of carrier phase measurements allows for mitigation of integrity hazards by implementing RRAIM monitors with tight thresholds without significantly affecting continuity. In order to avoid the need for cycle ambiguity resolution, time differences in carrier phase measurements are used as the basis for detection. In this work, we examine RRAIM within the context of the GNSS Evolutionary Architecture Study (GEAS), which explores potential architectures for aircraft navigation utilizing the satellite signals available in the mid-term future with GPS III. The objectives of the GEAS are focused on system implementations providing worldwide coverage to satisfy LPV-200 operations, and potentially beyond. In this work, we study two different GEAS implementations of RRAIM. General formulas are derived for positioning, fault detection, and protection level generation to meet a given set of integrity and continuity requirements.

Antenna phase center calibration for precise positioning of LEO satellites

Phase center variations of the receiver and transmitter antenna constitute a remaining uncertainty in the high precision orbit determination (POD) of low Earth orbit (LEO) satellites using GPS measurements. Triggered by the adoption of absolute phase patterns in the IGS processing standards, a calibration of the Sensor Systems S67-1575-14 antenna with GFZ choke ring has been conducted that serves as POD antenna on various geodetic satellites such as CHAMP, GRACE and TerraSAR-X. Nominal phase patterns have been obtained with a robotic measurement system in a field campaign and the results were used to assess the impact of receiver antenna phase patterns on the achievable positioning accuracy. Along with this, phase center distortions in the actual spacecraft environment were characterized based on POD carrier phase residuals for the GRACE and TerraSAR-X missions. It is shown that the combined ground and in-flight calibration can improve the carrier phase modeling accuracy to a level of 4 mm which is close to the pure receiver noise. A 3.5 cm (3D rms) consistency of kinematic and reduced dynamic orbit determination solutions is achieved for TerraSAR-X, which presumably reflects the limitations of presently available GPS ephemeris products. The reduced dynamic solutions themselves match the observations of high grade satellite laser ranging stations to 1.5 cm but are potentially affected by cross-track biases at the cm-level. With respect to the GPS based relative navigation of TerraSAR-X/TanDEM-X formation, the in-flight calibration of the antenna phase patterns is considered essential for an accurate modeling of differential carrier phase measurements and a mm level baseline reconstruction.

Smoothing GPS carrier phase double differences using inertial measurements for high performance applications

This paper will describe an enhancement to the GPS double difference carrier phase measurements on a single airborne platform by smoothing them with inertial measurements while preserving the dynamic bandwidth. This enhancement will reduce the impact of carrier phase multipath and carrier phase noise on baseline determination between multiple antennas on an aircraft when in flight. This type of measurement system has numerous applications where platform pointing and relative body motion must be determined at the mm-level for applications such as sensor stabilization, Synthetic Aperture Radar, long range RADAR (i.e. angle-of-arrival measurements). Lower noise levels (mm-level and below) enable more performance to the stabilized system such as increased aperture for longer range, operation at higher frequencies, and more image resolution. The focus of this paper will be on a technique to provide this enhanced performance for these various applications using the available navigation systems. Additionally, this type of smoothing can effectively remove the additional noise induced by carrier phase measurement differencing. The noise level of a double or triple difference can be reduced below that of the raw measurement. A complimentary synthesized double difference quantity with ultra-low-noise characteristics will be used to smooth the GPS carrier phase double difference measurements without losing dynamic bandwidth since it follows the airborne dynamics. Flight test data will be presented to demonstrate the performance improvement in the midst of aircraft dynamics. Results will show that the noise reduction follows the theoretical prediction.

10 Hz or 10 s?

Data sampling frequencies in many kinematic GNSS applications are often in the range of 2–10 Hz or even higher. In contrast, the sampling frequency of standard reference stations is usually not higher than 1 Hz, and many stations even deliver data at sampling intervals as large as 30 s. An easily implemented algorithm for data interpolation will be presented and it will be demonstrated that interpolation of pseudorange and carrier phase reference station data is possible at a high level of accuracy. This technique—which has not received proper attention so far—is helpful to reduce data storage capacity for postprocessing applications, but and can also be beneficial for real-time applications suffering from slow data links. Results of test trials indicate that a standard deviation better than 2 mm can be reached for interpolated carrier phases collected at reference sites sampling data with 5-s intervals in double difference mode. These interpolated double difference data obviously still follow a Gaussian distribution. A trend function for the expected standard deviation of interpolated double difference carrier phase measurements will be presented. From this function, a recommendation for an optimal sampling rate of reference station data can be derived which is close to 10 s.

GPS를 이용한 저궤도 위성 자세 결정의 미지정수 결정 성공확률 향상

GPS 반송파 측정값을 이용하게 되면 수 cm 이하의 정밀도로 위치 결정을 수행 할 수 있으며, 두 개 이상의 안테나를 움직이는 물체에 장착할 경우 정밀한 자세 결정을 수행할 수 있다. GPS 반송파 측정값을 사용하는 경우 항상 미지정수 결정의 어려움이 발생하게 되며, 미지정수 결정 과정에서 올바른 결정을 할 수 있는 척도로 성공확률을 확인할 필요가 있다. 본 논문에서 저궤도 위성의 자세 결정을 위하여 이중차분된 측정값을 사용하게 되며, 이때 만들어진 측정행렬과 측정값의 공분산을 이용하여 성공확률을 계산하게 된다. 성공확률을 향상시키기 위하여 저궤도 위성의 위치벡터와 자세벡터가 수직이라는 제한 조건을 사용하여 측정값을 증가시키는 방법을 제안하였다. 적용된 저궤도 위성의 궤도는 우리별 3호의 궤도 정보를 이용하여 만들었으며 시뮬레이션을 통하여 그 결과를 확인 하였다. To determine precise position GPS carrier phase measurements are used. In addition, the multi-antenna system consisting of 2 or more GPS antennas can make attitude determination effectively. When GPS carrier phase measurements are used the integer ambiguity must be fixed. The success rate is used to validate the integer ambiguity. For LEO satellite attitude determination the double difference carrier phase measurements are used, the success rate is calculated using the covariance matrix and the measurement matrix. The constraint that LEO satellite position vector and attitude vector is orthogonal is suggested for improving the success rate. The LEO satellite orbit model is KITSAT3. The results of the simulation are shown and analyzed.

Tightly coupled GPS/INS integration for missile applications

An approach to enhance the performance of tightly coupled GPS/INS systems is described. First, the advantages of tightly coupled systems compared to loosely coupled systems are clarified. Then, it is shown in hardware-in-the-loop tests and in a test drive that processing time differenced carrier phase measurements instead of delta-range measurements results in an increased velocity and attitude accuracy for the tightly coupled system, which is of great importance in the beginning of a time interval with purely inertial navigation, e.g. when GPS is lost due to jamming. A measurement equation is derived allowing to process the time differenced carrier phase measurements in the navigation Kalman filter. Finally, the proposed method is applied to missile navigation systems, where significant vibrations enter the inertial sensor data.