Absolute Positional Accuracy Research Articles

Constraining a black hole companion for M87* through imaging by the Event Horizon Telescope

ABSTRACT The Event Horizon Telescope, a global very long baseline interferometric array observing at a wavelength of 1.3 mm, detected the first image of the M87 supermassive black hole (SMBH). M87 is a giant elliptical galaxy at the centre of the Virgo cluster, which is expected to have formed through merging of cluster galaxies. Consequently M87* hosted mergers of black holes through dynamical friction and could have one or multiple binary companions with a low mass ratio at large separations. We show that a long-term monitoring of the M87 SMBH image over ∼1 yr with absolute positional accuracy of 1 ≈ $\mu$as could detect such binary companions and exclude a large parameter space in semimajor axis (a0) and mass ratio (q), which is currently not constrained. Moreover, the presence of the accretion disc around M87* excludes a binary companion with a0 ≈ of the order of a milliparsec, as otherwise the accretion disc would have been tidally disrupted.

Improvement and Assessment of the Absolute Positioning Accuracy of Chinese High-Resolution SAR Satellites

In recent years, China has launched YaoGan-13 and GaoFen-3, high-resolution synthetic aperture radar (SAR) satellites that can acquire global high-resolution images. The absolute positioning accuracy of such satellites is important for mapping areas without ground reference points and for automated processing. However, satellites without geometric calibration have poor absolute positioning accuracy, greatly restricting their application (e.g., land resource surveys). Therefore, they cannot meet national demands for high-resolution SAR images with good geometric accuracy. Here, we propose a series of methods to improve the absolute positioning accuracy of YaoGan-13 and GaoFen-3, such as the multiple-image combined calibration strategy and geometric calibration model for a real continuously moving configuration, including consideration of atmospheric propagation delay. Using high-accuracy ground control data collected from different areas, the 2-D and 3-D absolute positioning accuracies of YaoGan-13 and GaoFen-3 were assessed after implementation of the improvement measures. Experimental results showed that, after calibration, the 2-D absolute positioning accuracy of YaoGan-13 and GaoFen-3 are improved from 43.86 m to 2.57 m and from 30.34 m to 4.29 m, respectively. In addition, the 3-D absolute positioning accuracies of YaoGan-13 in plane and elevation are 3.21 m and 2.22 m, respectively. Improving the absolute positioning accuracy of these satellites could broaden the scope of their potential applications in the future.

Microarcsecond VLBI Pulsar Astrometry with PSRπ II. Parallax Distances for 57 Pulsars

Abstract We present the results of PSRπ, a large astrometric project targeting radio pulsars using the Very Long Baseline Array (VLBA). From our astrometric database of 60 pulsars, we have obtained parallax-based distance measurements for all but 3, with a parallax precision that is typically ∼45 μas and approaches 10 μas in the best cases. Our full sample doubles the number of radio pulsars with a reliable (≳5σ) model-independent distance constraint. Importantly, many of the newly measured pulsars are well outside the solar neighborhood, and so PSRπ brings a near-tenfold increase in the number of pulsars with a reliable model-independent distance at d > 2 kpc. Our results show that both widely used Galactic electron density distribution models contain significant shortcomings, particularly at high Galactic latitudes. When comparing our results to pulsar timing, two of the four millisecond pulsars in our sample exhibit significant discrepancies in their proper motion estimates. With additional VLBI observations that extend our sample and improve the absolute positional accuracy of our reference sources, we will be able to additionally compare pulsar absolute reference positions between VLBI and timing, which will provide a much more sensitive test of the correctness of the solar system ephemerides used for pulsar timing. Finally, we use our large sample to estimate the typical accuracy attainable for differential VLBA astrometry of pulsars, showing that for sufficiently bright targets observed eight times over 18 months, a parallax uncertainty of 4 μas per arcminute of separation between the pulsar and calibrator can be expected.

Auditory localization accuracy and auditory spatial discrimination in children with auditory processing disorders

The present study investigated spatial hearing in children aged 6–12 years diagnosed with Auditory Processing Disorders (APD) and compared their results to those of a group of control children matched in age. Sound source localization accuracy was quantified using an absolute localization task and sound source discrimination by measuring the minimum audible angle. Low- and high-frequency noise bursts were presented from eight loudspeaker positions in the left and right hemifields (0°, 30, 60°, and 90° azimuth). Median absolute localization accuracy did not differ between children with APD and control children. However, the intra-individual variability of pointing behavior was higher for children with APD. In contrast, children with APD had significantly higher minimum audible angle thresholds than control children. These findings show that APD impairs sound source discrimination, but does not affect the median relationship between actual and judged sound source locations.

A Hall-Sensor-Based Localization Method With Six Degrees of Freedom Using Unscented Kalman Filter

Magnetic sensors are widely used in automotive and industrial applications to measure linear or angular movements. Although the measurement of the magnetic field vector offers the opportunity to estimate all mechanical degrees of freedom, previous approaches could not be applied in these environments. This paper presents an improved method for sensing a magnetic source’s position and orientation with six degrees of freedom. To solve the underlying inverse magnetostatic problem, an Unscented Kalman Filter in combination with an analytical model of the magnetic source’s field is used. The performance of the method is demonstrated on the basis of a novel multi-axis input device. It comprises a cuboid magnet and a CMOS (Complementary metal-oxide-semiconductor) Hall-Sensor array with up to 36 elements. The localization algorithm and the measurement model are implemented in an efficient way to achieve real-time capability and sampling rates up to 80Hz on embedded hardware. This outperforms known methods significantly and allows for a wide application of multi-degree of freedom sensors. Moreover, an absolute position and orientation accuracy of 71 $\mu \text{m}$ and 1.4° are achieved. The work describes the basis for advanced input devices but can also be transferred to other kinds of magnetic localization problems.

Multi-GNSS Induced Performance Enhancements in Constrained Environments

Nowadays, three global navigation satellite systems (GNSS), namely GPS, GLONASS and China’s BeiDou System (BDS), are fully-operational in the Asia-Pacific region. Furthermore, the European Galileo system and the Japanese Quasi Zenith Satellite System (QZSS), which is a regional navigation satellite system (RNSS), jointly provide 4 to 8 additional visible satellites in the region. Thus, it is expected that a combination of the above five systems will improve positioning performance as a result of enhanced satellite availability provided by multi-GNSS. In this research, we develop a method to combine GPS, GLONASS, BDS, Galileo, and QZSS pseudorange and carrier phase observations, and investigate positioning performance improvements brought by multi-GNSS. Experimental data were collected in Southern Taiwan to perform pseudorange-based, meter-level absolute (point) positioning as well as carrier phase-based, centimeter-level relative positioning. Test results indicate that (1) using multi-GNSS can effectively improve the accuracy of absolute (single point) and relative positioning, particularly in highly-masked, constrained environments, such as urban areas; (2) combining the five constellations can significantly shorten the Time-To-First-Fix (TTFF) for rapid ambiguity resolution required by Real-Time Kinematic (RTK) applications in constrained environments.

COMPARISON BETWEEN ABSOLUTE AND RELATIVE POSITIONAL ACCURACY ASSESSMENT - A CASE STUDY APPLIED TO DIGITAL ELEVATION MODELS

Abstract This paper presents a comparative study between the absolute and relative methods for altimetric positional accuracy of Digital Elevation Models (DEM). For the theoretical basis of this research, the definitions of accuracy (exactness) and precision, as well the concepts related to absolute and relative positional accuracy were explored. In the case study, the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the Shuttle Radar Topography Mission (SRTM) DEM were used. In the analysis of the absolute accuracy, 6,568 ground control points from GNSS orbital survey were used, collected through relative-static method. In the relative accuracy, it was used as reference DEM with spatial resolution of 5 meters generated by stereophotogrammetrical process for the Mapping Project of Bahia (Brazil). It was concluded that, once the accuracy of the reference DEM is better than the other two evaluated DEM, the results of the classification for the PEC-PCD for the relative evaluation are equal to or better than the absolute evaluation results, with the advantage to being able to verify the pixel population of the evaluated models, which makes it possible to identify outliers, distortions and displacements, including delimiting regions, which is much less likely with a limited set of control points.

Evaluation of a Low Cost Hand Held Unit with GNSS Raw Data Capability and Comparison with an Android Smartphone.

A newly available portable unit with GNSS raw data recording capability is assessed to determine static and kinematic position accuracy in various environments. This unit is the GPSMap 66, introduced by Garmin in early September. It is all-weather and robust for field use, and comes with a helix antenna. The high sensitivity chipset is capable of acquiring and tracking signals in highly attenuated environments. It can track single frequency GPS, GPS + GLONASS or GPS + Galileo and record code, Doppler and carrier phase data every second in the RINEX format. The evaluation presented herein focusses on GPS and Galileo. Static and kinematic test results obtained under a wide range of realistic field conditions are reported. Differential GNSS methods and Precise Point Positioning (PPP) are used to assess absolute position accuracy in ITRF coordinates, which is sufficiently close to the GPS and Galileo reference frame for the current purpose. Under low multipath conditions, measurements are found to be sufficiently accurate to provide single epoch, bias free position accuracy of a few metres. Accuracy is a function of signal attenuation and multipath conditions. The use of an external geodetic antenna significantly reduces measurement noise and multipath in high multipath environments. Carrier phase measurements, available more or less continuously under open sky conditions, significantly improve performance in differential mode. Accuracy in vehicular mode using code and carrier phase differential RTK solution is at the level of a few to several dm. Tests were conducted in parallel with a Huawei P10 Android 8.0 smartphone. The code measurement noise of this unit was found to be significantly higher than that of the GPSMap 66, a major reason being its lower performance PIFA antenna; carrier phase was only available for short time intervals, significantly degrading differential position accuracy performance.

ACCURATE VISUAL LOCALIZATION IN OUTDOOR AND INDOOR ENVIRONMENTS EXPLOITING 3D IMAGE SPACES AS SPATIAL REFERENCE

Abstract. In this paper, we present a method for visual localization and pose estimation based on 3D image spaces. The method works in indoor and outdoor environments and does not require the presence of control points or markers. The method is evaluated with different sensors in an outdoor and an indoor test field. The results of our research show the viability of single image localization with absolute position accuracies at the decimetre level for outdoor environments and 5 cm or better for indoor environments. However, the evaluation also revealed a number of limitations of single image visual localization in real-world environments. Some of them could be addressed by an alternative AR-based localization approach, which we also present and compare in this paper. We then discuss the strengths and weaknesses of the two approaches and show possibilities for combining them to obtain accurate and robust visual localization in an absolute coordinate frame.

A Compensation Method for Enhancing Aviation Drilling Robot Accuracy Based on Co-Kriging

The positional error of aviation drilling robot has a great influence on the strength and lives of aircrafts in the aircraft assembly. In order to improve the position accuracy of aviation drilling robot, an error compensation method based on co-kriging is proposed in this paper. The error similarity based on the kinematic of drilling robot is proposed firstly. Then, the positional errors of a set of points in the workspace are measured by using precision laser tracker. The measurement data are used to fit the cross-variogram of positional error. After the cross-variogram is obtained, the predicted positional errors of verification points can be estimated based on co-kriging. The positions after compensation are given to the drilling robot. The proposed method is carried out on an aviation drilling robot for practical compensation to verify the correctness and effectiveness of the error compensation method. The experimental results show that the average absolute positional error is reduced to 0.1150 mm from 0.7168 mm, and that the maximum absolute positional error is reduced to 0.2664 mm from 1.3073 mm. The experimental results also demonstrate that the proposed method can improve the absolute position accuracy of aviation robot and could meet the requirement of aircraft assembly.

MACS-Mar: a real-time remote sensing system for maritime security applications

The modular aerial camera system (MACS) is a development platform for optical remote sensing concepts, algorithms and special environments. For real-time services for maritime security (EMSec joint project), a new multi-sensor configuration MACS-Mar was realized. It consists of four co-aligned sensor heads in the visible RGB, near infrared (NIR, 700–950 nm), hyperspectral (HS, 450–900 nm) and thermal infrared (TIR, 7.5–14 µm) spectral range, a mid-cost navigation system, a processing unit and two data links. On-board image projection, cropping of redundant data and compression enable the instant generation of direct-georeferenced high-resolution image mosaics, automatic object detection, vectorization and annotation of floating objects on the water surface. The results were transmitted over a distance up to 50 km in real-time via narrow and broadband data links and were visualized in a maritime situation awareness system. For the automatic onboard detection of floating objects, a segmentation and classification workflow based on RGB, IR and TIR information was developed and tested. The completeness of the object detection in the experiment resulted in 95%, the correctness in 53%. Mostly, bright backwash of ships lead to an overestimation of the number of objects, further refinement using water homogeneity in the TIR, as implemented in the workflow, couldn’t be carried out due to problems with the TIR sensor, else distinctly better results could have been expected. The absolute positional accuracy of the projected real-time imagery resulted in 2 m without postprocessing of images or navigation data, the relative measurement accuracy of distances is in the range of the image resolution, which is about 12 cm for RGB imagery in the EMSec experiment.

Measurements on the Absolute 2-D and 3-D Localization Accuracy of TerraSAR-X

The German TerraSAR-X radar satellites TSX-1 and TDX-1 are well-regarded for their unprecedented geolocation accuracy. However, to access their full potential, Synthetic Aperture Radar (SAR)-based location measurements have to be carefully corrected for effects that are well-known in the area of geodesy but were previously often neglected in the area of SAR, such as wave propagation and Earth dynamics. Our measurements indicate that in this way, when SAR is handled as a geodetic measurement instrument, absolute localization accuracy at better than centimeter level with respect to a given geodetic reference frame is obtained in 2-D and, when using stereo SAR techniques, also in 3-D. The TerraSAR-X measurement results presented in this study are based on a network of three globally distributed geodetic observatories. Each is equipped with one or two trihedral corner reflectors with accurately (<5 mm) known reference coordinates, used as a reference for the verification of the SAR measured coordinates. Because these observatories are located in distant parts of the world, they give us evidence on the worldwide reproducibility of the obtained results. In this paper we report the achieved results of measurements performed over 6 1/2 years (from July 2011 to January 2018) and refer to some first new application areas for geodetic SAR.

A compensation method based on extreme learning machine to enhance absolute position accuracy for aviation drilling robot

To enhance the absolute position accuracy and solve complex modeling and computational complexity problems in traditional compensation methods for aviation drilling robots, a compensation method ba...

USE OF GEOSTATISTICS ON ABSOLUTE POSITIONAL ACCURACY ASSESMENT OF GEOSPATIAL DATA

Abstract: In the area of Geosciences it is intuitive to think of spatial correlation as a phenomenon under study, and Geostatistics has tools to identify and represent the behavior of such dependency. The spatial analysis of the results of an inspection of the quality of a cartographic product is generally not addressed in the standards, which are restricted to descriptive and tabular findings, based on the assumption of the Classical Statistics of independence of observed data. At the Brazilian National Infrastructure of Spatial Data (INDE), various cartographic products should be made available to society, along with their metadata. This paper proposes a methodology for quality inspection based on international standards and on the Cartographic Accuracy Standard (PEC), using geostatistical methods and spatial representation of this benchmarking, through positional quality maps. The method of evaluating the quality of data was applied to Brazil's Continual Cartographic Base, at the scale of 1:250000 - BC250, with a focus on absolute positional accuracy. The quality map generated presented regionalizations of the planimetric error confirmed by the producer team of the referred cartographic base of IBGE. Such information can help users and producers to understand the spatial behavior of cartographic product quality in study

Towards High Accuracy Robot-Assisted Surgery

In this article, we propose a new error modeling approach for robot manipulators in order to improve the absolute accuracy of a tools pose by using polynomial regression method. The core idea is based on a well-known fact: accuracy of repeatedly reaching a given position is much higher than the accuracy of absolute positioning (i.e. moving the manipulator to a given position). The underlying reason is, that positioning errors are dominated by systematic errors - while stochastic errors are significantly smaller. This fact is then exploited to apply some learning algorithm to derive an error-compensation model. Technically, this means that the robot is being calibrated a priori with some external sensor once. Afterwards it can operate with a much better quality. In detail, we propose to first perform a coordinate transformation using a least mean square approach (for registration). Then, to account for deviations of measured position in comparison to nominal (robot) position, the frame transformation model at each robots joint is extended by translational and rotational error parameters. This is then used to built an error compensation model with regression techniques. We evaluate the method on a data set obtained using a 7DOF robot manipulator and show that this approach brings positioning error to the order of repeatability errors for this manipulator.

Augmenting Ultra-Wideband Localization with Computer Vision for Accurate Flight

Ultra-wideband radio networks enable low-cost, low-computation robot localization in semi-structured environments; however, previous results have shown that these localization systems suffer from spatially-varying measurement biases, leading to a spatially-varying offset between the physical and the estimated position. In tasks where absolute positioning or high tracking accuracy is required, this offset can lead to failure of the task. This paper proposes augmenting ultra-wideband-based localization with visual localization to improve estimation accuracy for critical tasks. It also presents a control strategy that takes the camera measurement process into account, and allows the ultra-wideband system’s measurement biases to be learned and compensated over multiple executions of the task. This bias compensation can be used to improve the accuracy of the task in the case of visual impairment. The effectiveness of the proposed framework is demonstrated by accurately flying a quadrocopter to a landing platform using on-board estimation and control.

Robot calibration using a portable photogrammetry system

Abstract This work investigates the potential use of a commercially-available portable photogrammetry system (the MaxSHOT 3D) in industrial robot calibration. To demonstrate the effectiveness of this system, we take the approach of comparing the device with a laser tracker (the FARO laser tracker) by calibrating an industrial robot, with each device in turn, then comparing the obtained robot position accuracy after calibration. As the use of a portable photogrammetry system in robot calibration is uncommon, this paper presents how to proceed. It will cover the theory of robot calibration: the robot's forward and inverse kinematics, the elasto-geometrical model of the robot, the generation and ultimate selection of robot configurations to be measured, and the parameter identification. Furthermore, an experimental comparison of the laser tracker and the MaxSHOT 3D is described. The obtained results show that the FARO laser tracker ION performs slightly better: The absolute positional accuracy obtained with the laser tracker is 0.365 mm and 0.147 mm for the maximum and the mean position errors, respectively. Nevertheless, the results obtained by using the MaxSHOT 3D are almost as good as those obtained by using the laser tracker: 0.469 mm and 0.197 mm for the maximum and the mean position errors, respectively. Performances in distance accuracy, after calibration (i.e. maximum errors), are respectively 0.329 mm and 0.352 mm, for the laser tracker and the MaxSHOT 3D. However, as the validation measurements were acquired with the laser tracker, bias favors this device. Thus, we may conclude that the calibration performances of the two measurement devices are very similar.

Multi-optimization of a spherical mechanism for minimally invasive surgery

In order to obtain the remote center motion (RCM) mechanism with better performance indexes and avoid the collision of multi-manipulators in minimally invasive surgery (MIS), a novel multi-objective optimization model was presented. There were two optimization objectives: a global kinematic performance index and a comprehensive stiffness index. Other indexes to characterize the design requirements such as collision probability, workspace, mechanism parameter, mass, and wall thickness were considered as constraints. Angles between two adjacent joints and cross-section dimensions of links were chosen as the design variables. The non-dominated sorting genetic algorithm II (NSGA-II) was adopted to solve the complex multi-objective optimization problem. Then, a 3-degree of freedom (DoF) MIS robotic prototype based on optimization results has been built up. The experiments to test the spatial position change of the remote center point and to test the absolute position accuracy and repetitive position accuracy of the MIS robot were achieved, and the experimental results meet the requirements of MIS.

Comparative evaluation of IRNSS performance with special reference to positional accuracy

The Indian Regional Navigation Satellite System (IRNSS) is a seven-satellite constellation developed by Indian Space Research Organization (ISRO), India. IRNSS provides two services, with the standard positioning service open for civilian use, and for authorized users (including the military) with an assured absolute positional accuracy. IRNSS is designed to cover all the parts of India and also extending beyond the Indian borders. The coverage area of IRNSS with routes of satellites means and includes area covering entire Pakistan, Afghanistan, most of China and Middle East along with Indian Ocean. IRNSS downlink signals have two bands: S1 band and L5 Band. Presently Indian users are dependent on the civil GPS signals with a positional accuracy of 5 m. IRNSS system is intended to provide an absolute position accuracy of (i) better than 10 m throughout Indian landmass (ii) better than 20 m over the Indian Ocean and (iii) approximately in a region of 1500 km around India. There are 15 ground stations across the country responsible for operating and/or monitoring the IRNSS seven satellite constellation. A comparative analysis of IRNSS and GPS navigational satellites shows that the IRNSS consists of sufficient number of satellites for regional navigation system and it can also exhibit better performance. The main parameters selected for our study are orbital characteristics, position accuracy and positional error including Circular Error Probable (CEP), Altitude variation, Geometric Dilution of Precision (GDOP) with respect to Number of Satellites (NSAT), variations in Carrier to Noise (C/N0) ratio and scintillation effect of both GPS and IRNSS satellite systems. Our analysis showed that IRNSS systems shall be used as stand-alone systems.

MODEL FOR DETERMINATION OF THREE-DIMENSIONAL COORDINATES OF HIDDEN POINTS

One of the problems encountered during surveys is obstruction of points to be monitored, an example of this can be found in industrial environments where there are usually pipes and equipment that do not allow the establishment of straight lines;. An alternative to the application of topographic survey techniques is the use of plane mirrors to diversion of straight lines where the points of interest are observed indirectly. This study presents a new approach to the problem, whose analytical solution is based on the principles of Geometrical Optics and Surveying. The relative positional accuracy achieved in the preliminary tests is submillimetric order, and the absolute positional accuracy is millimetric order.