Acta Geodaetica et Cartographica Sinica

Three multi-frequency and multi-system GNSS high-precision point positioning methods and their performance in complex urban environment

GENG Jianghui, CHANG Hua, GUO Jiang, LI Guangcai, WEI Na

2020, 49(1): 1-13. doi:10.11947/j.AGCS.2020.20190106

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High-precision applications, such as intelligent transportation and automated driving in urban environment, require a dynamic positioning accuracy at decimeter to centimeter level. However, GNSS performance (such as availability and positioning accuracy) in urban environments is strongly degraded by man-made and natural objects that obstruct satellite signals. Taking advantage of the available multi-frequency and multi-system GNSS (GPS/BDS/Galileo/QZSS) data, we adopt the new single-epoch PPP wide-lane ambiguity fixing method (PPP-WAR) for vehicle dynamic positioning. It is compared with the traditional PPP method and wide-area pseudorange enhanced precision positioning method in three typical urban environments with different complexity, including a road in metropolitan area, a narrow road in small town and an open road in industrial area. The integrity of triple-frequency GNSS data is currently up to 94% in urban area, which assures that single-epoch PPP-WAR method is applicable in complex urban area. It's shown that a positioning accuracy of 0.41 m in the horizontal direction is achieved using the single-epoch PPP-WAR method in the narrow road in town with the threshold of 3.0 m, which satisfies the accuracy requirement at decimeter level. The accuracy is largely improved by 53.9% and 21.2% compared with those of wide-area pseudorange enhanced precision positioning solutions and traditional PPP solutions, respectively. The availability of three methods in metropolitan area is higher than 70%, as well as more than 90% in small town and industrial area. The availability of PPP-WAR and traditional PPP can still reach ~70% with a strict threshold of 0.5 m. Four typical features (i.e. tunnels, crossovers, street trees and skyscrapers) in urban environment have the least impact on the performance of single-epoch PPP-WAR among the three methods. In sum, single-epoch PPP-WAR method has the best performance in complex urban environment with more obstructions, while traditional PPP method is the best in open urban environment with less obstructions.

Optimized estimation of foF2 using GNSS data ingestion to NeQuick model during magnetic storm

HAN Ling, WANG Jiexian, CHEN Yanling, LIU Jingbin, LI Haojun

2020, 49(1): 14-23. doi:10.11947/j.AGCS.2020.20180260

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The critical frequency of F2 is an important parameter for high frequency communications. Ionosonde measured foF2 is an effective way but it would be affected by geomagnetic storm which will even lead to the data missing. NeQuick model could estimate the TEC and foF2.But the accuracy would be degraded during geomagnetic storm. Using the dual frequency GNSS data from the CMONOC(Crustal Movement Observation Network of China)and applying the regional polynomial model, the accurate TEC could be retrieved. Based on the condition that the sum of the squared errors is the minimum, the effective ionized parameter Az could be estimated using the calculated TEC. Using the NeQuick model input by ionized parameter Az, the TEC and foF2 could be retrieved. Using the observations of five ionosonde stations located in different latitudes is in China, the result indicated that the TEC RMS improved 20%~40%, and the foF2 RMS improved 10%~25%. This method could improve the TEC and foF2 accuracy. It showed the benefit of low cost. So it could be considered as the alternative way to the ionosonde observations especially for the foF2 data missing period.

Post-maneuver orbit determination based on the predicted clocks

CHEN Qian, CHEN Junping, WU Shan, ZHANG Yize, YU Chao

2020, 49(1): 24-33. doi:10.11947/j.AGCS.2020.20190078

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The BDS system is designed as a GEO/IGSO/MEO mixed constellation. Satellite maintaining operation is performed every 7 to 10 days for GEO/IGSO satellites. With the maneuver satellite's status being unhealthy in the post-maneuver period, which lasts 5~6 hours, the BDS system usability is affected. In the long unhealthy periods, observation data together with satellite/station clocks are accumulated until the processed orbits meet precision requirement. To shorten the satellite's unhealthy time during post-maneuver, we propose an orbit determination method making use of the predicted satellite/station clocks. With the clocks being modeled with high precision, data of longer observation arc could be used, thus improve the orbit accuracy. In the modeling of satellite and station clocks, the reference inconsistency between the two-way satellite time frequency transfer(TWSTFT) and dynamic precise orbit determination(POD) techniques are considered. Three satellite maneuver events in November 2018 are analyzed. Results show that the satellite unavailability time could be shortened by at least 1 hour. And it improves the accuracy of orbit prediction with maximum of 84.82% for the first group of orbits in post-maneuver. For the POD from 3h to 8h after maneuver, the mean improvement of orbit prediction accuracy is of 26.06%(C01),31.58%(C04) and 9.95%(C03) by using the proposed method.

A method for determining the weight in weighted mixed estimation

SONG Yingchun, SONG Caiwei, ZUO Tingying

2020, 49(1): 34-41. doi:10.11947/j.AGCS.2020.20180359

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In this paper, multi-source observation models with various heteroscedastic and combined adjustment methods in geodesy are summarized. It shows that the mixed estimation method can be used for measurement data fusion, and can balance the influence of additional information and sample information on parameter estimation. By calculating the weights to minimize the trace of the covariance matrix of parameter estimation, an optimal selection method of weights is given. This paper extends the existing weighted mixing method so that the given method are not limited by the prior unit weight variance, and can be effectively applied to large measurement data processing.

Accuracy analysis of IRI2016 international reference ionosphere at altitude of 60~100 km

CHENG Huhua, ZHAN Caiju, ZHAO Liang, WANG Yibai, XIU Xingtao

2020, 49(1): 42-54. doi:10.11947/j.AGCS.2020.20180508

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The low-altitude ionosphere (60~100 km) has great influence on radio propagation and measurement. IRI2016 is a widely used international reference ionospheric model. Therefore, it is of great practical significance to analyze the reliability of its electron density at an altitude of 60~100 km. Based on electron density data (2014-2016) provided by medium-frequency (MF) radar in Langfang, the reliability of electron density of IRI2016 model at an altitude of 60~100 km in the mid-latitude was quantitatively analyzed in this study using deviation, absolute difference, correlation coefficient, relative deviation and Lomb-Scargle periodogram. At the altitude of 60~100 km in the mid-latitude, the results showed as follows:① The deviation, absolute difference and relative deviation of electron density was closely related to season. At the altitude of 86~100 km, they increased rapidly with the increase of altitude. In addition, correlation coefficient was also closely related to season, but presented an alternation of increasing and decreasing with the increase of altitude. ② The electron density accuracy of the IRI2016 model was related to solar activity and geomagnetic conditions, and had the highest accuracy under low solar activity and magnetic tranquility. ③ The electron density of MF radar and IRI2016 model at the altitude of 82~84 km both showed significant (verified by 90% significance test) quasi-diurnal tidal wave and quasi-8-hour gravity wave. Meanwhile, IRI2016 model presented significant quasi-semidiurnal tidal wave, which was not found in MF radar. At the altitude of 86~92 km, the electron density of both MF radar and IRI2016 model had significant quasi-diurnal tidal wave and quasi-semidiurnal tidal wave, but no quasi-8-hour gravity wave. ④ The electron density of both MF radar and IRI2016 model showed differences in significant wave period and power spectrum at the same altitude, and the significant wave period and power spectrum had differences among different altitudes. ⑤ Quasi-diurnal tidal wave played a main role in the change of the electron density of both MF radar and IRI2016 model at the altitude of 82~92 km. Quasi-8-hour gravity wave played a secondary role at the altitude of 82 km, and quasi-semidiurnal tidal wave played a secondary role at the altitude of 86~92 km.

Abnormal data detection and process by using BDS satellite offset semiparametric adjustment model

LI Wentao, YAN Xiong, XIA Lei, REN Qingyang, CHEN Jie, PAN Xiong

2020, 49(1): 55-64. doi:10.11947/j.AGCS.2020.20180384

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In view of the high frequency of abnormal data such as clock jump, gross error and so on in the BeiDou satellite clock offset data, this paper introduces the semiparametric adjustment model to the processing of the clock offset data of BDS. First of all, when considering the system error, we improved the common clock offset model. Secondly, when considering the abnormal data and the system error comprehensively, we use the compensation of principle of least squares and the extreme value solving method, the relation expression of estimated value of the parameters and non-parameters before and after the separation of the abnormal data and the corrected values of the corresponding observation values is derived in detail, and separation of the fixed value of the abnormal data, the parameter solution and the system error is realized. On this basis, the Cook distance is introduced, the Cook distance of parameter components and nonparametric components and the mixed Cook distance are presented to determine the location of the abnormal value. And the selection method of some parameters and the corresponding treatment measures are presented to achieve the location and fixed value of the anomaly data of the satellite clock offset and the separation of some system errors. Finally, the comparison is conducted by the method adopted in this paper and common method using the precision clock offset data of the BeiDou satellite, which has a sampling interval of 5 min from the GNSS center of Wuhan University. The experimental results show that the proposed method can effectively identify abnormal value in the satellite clock and deal with it, and effectively overcome the deficiency of the anomaly detection method based on the empirical threshold difference, and this method has a good detection effect for the abnormal clock offset data of smaller magnitude. To a certain extent, it has enriched the existing BDS clock offset data quality control method.

Land cover classification of polarimetric SAR with fully convolution network and conditional random field

ZHAO Quanhua, XIE Kailang, WANG Guanghui, LI Yu

2020, 49(1): 65-78. doi:10.11947/j.AGCS.2020.20190038

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Aiming at the problems of low classification accuracy and poor effect in the traditional fully convolution network (FCN), and insufficient consideration on the scattering characteristics of ground object features in the traditional polarimetric synthetic aperture radar (PolSAR) land cover classification methods. To overcome this limitation, this paper proposes a land cover classification algorithm of polarimetric SAR with improved FCN and conditional random field (CRF). First of all, the Freeman and Pauli decompositions are used to model the full-polarimetric SAR image to obtain the scattering features of scattering mechanisms, and Freeman decompositions are referenced to obtain the main scattering object corresponding to the main scattering component. Learning from the FCN-Vgg19-8s network with excellent performance in the field of image classification, and considering the large amount of high-level convolution parameters and the insufficient optimization of low-level convolution model parameters, Then an improved FCN, named FCN-MD-8s, is designed though constructing multi-scale convolution group and cost function in the upper and middle layers based on FCN-Vgg19-8s to guarante dimensionality reduction and optimization of overall model parameters. Additionally, FCN-MD-8s network is trained and tested for scattering mechanisms from Freeman decomposition by Cascade-migration-learning structure. Afterwards, according to the main scattering feature corresponding to the main scattering component, the main feature object is extracted from each component prediction image to obtain a component classification result. The result of each component classification is superimposed to gain a global rough classification. Finally, the fully-connected CRF with false color image, which is visualized by Pauli coherent decomposition, is used to transfer full image information over global rough classification for fine classification. The qualitative and quantitative analyses of classification results demonstrate that the proposed algorithm has effectiveness and feasibility.

Using DEM as full controls in block adjustment of satellite imagery

CAO Hui, TAO Pengjie, LI Haihong, ZHANG Zuxun

2020, 49(1): 79-91. doi:10.11947/j.AGCS.2020.20190226

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An algorithm is proposed in this study which directly uses digital elevation model (DEM) as full controls in block adjustment of satellite images, it is an application and extension of "cloud control" in photogrammetry. Different from the indirect geo-referencing method based on the registration of image-derived DSM or DEM with reference DEM, a DEM is considered as full geometric constraints and directly introduced into the block adjustment of satellite images based on the RFM model, by minimizing the squared sum of distances from tie points to the terrain surface described by DEM. By using DEM as exclusive control, the horizontal and vertical positioning accuracies of stereo satellite images can be upgraded without any ground control points (GCPs). A total of 330 stereo scenes of Chinese Mapping Satellite-1 images covering the whole area of Shandong Province are used for experimental tests, and three different open source DEM data of 1″ grid AW3D30, ASTER GDEM and 3″ grid SRTM GL3 are used as control data, respectively. The adjustment results are validated by using 100 well-distributed ground check points. Experiments show that the positioning accuracy is depended on the accuracy of DEM itself. While using AW3D30 as full controls, which is the most accurate one among the three global DEMs in experiments, the block adjustment accuracy of 4.9 m (about 1 pixel) in horizontal direction and 2.8 m in vertical direction can be achieved, respectively. Which is similar with the results of using 100 GCPs. It is proved that the effectiveness and feasibility of using DEM instead of ground control points as controls for block adjustment.

Fast visibility detection without specifying the user-defined biases in multi-view texture mapping

HUANG Xiangxiang, ZHU Quansheng, JIANG Wanshou

2020, 49(1): 92-107. doi:10.11947/j.AGCS.2020.20180468

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The Z-buffer and ray-tracing algorithms are the two popular approaches used in visibility handling for multi-view based texture mapping. However, the accuracies of the two algorithms are limited by the user-defined biases. We propose a solution in which no biases are specified. First, a shader-based rendering is designed according to the projection parameter of oblique photogrammetry to generate each view's initial visibility map (IVM). The fully occluded primitives will be excluded by the depth test of graphics pipeline.Second, projection coverage refinement (PCR) is given to the visible primitives in IVM based on vector rasterization criterion and pixel depth. Last, lazy projection (LP) and iterative vertex-edge sampling (IVES) are proposed to distinguish the partially visible and fully visible primitives. We use two datasets to prove our method's validity. The experimental results show that our method has a better performance than the mainstream algorithm.

Oblique image matching algorithm based on adaptive initial object patch

ZHANG Chunsen, ZHANG Qiyuan, GUO Bingxuan, XUE Wanchang

2020, 49(1): 108-116. doi:10.11947/j.AGCS.2020.20190050

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In order to solve the problem of low degree of image connection points and too few aero triangulation points with large depth discontinuity or difference when using the least square image matching method based on object patch to match oblique images, an algorithm of oblique image matching based on adaptive initial object patch is proposed. The algorithm uses the initial values of interior and exterior orientation elements of the oblique image and the object points generated in the matching process, and adaptively calculates the elevation of the object patch and the initial value of the normal vector direction angle by the method of multi-patch forward intersection and object variance partition. Then it solves the problem that the low accuracy initial value of the oblique image with the least squares image matching method of the object patch is not easily matched to corresponding points when matching the oblique image. Finally, two groups of oblique images are matched by the proposed method and the least square image matching method, in which the initial state of the object patch is horizontal object patch, respectively, to verify the effectiveness of the proposed method.

Comparison of LM and EM parameter optimization methods for airborne laser bathymetric full-waveform decomposition

GUO Kai, LIU Yanxiong, XU Wenxue, HE Xiufeng, ZHANG Ziyin, LIU Haitao

2020, 49(1): 117-131. doi:10.11947/j.AGCS.2020.20180242

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As operating environment and scanning conditions would affect the accuracy of the parameters of initial waveform decomposition, it is necessary to use some optimization method to adjust the initial parameters for making the results more precise. With the measured and simulated data, the strategies of Levenberg-Marquardt (LM) and expectation-maximization (EM) have been chosen typically in this paper. It is studied that the features of the waveform fitting precision, the scanning condition response and the depth of the deviation in the same initial parameters condition. The accuracy and stability of the data processing results has been discussed in detail, and the main technical features and the difference of the results of the two methods has been summarized in this paper.

Study on the influence of cloud adjacency effects on the aerosol optical depth retrieval and its reducing method

HE Junliang

2020, 49(1): 132-132. doi:10.11947/j.AGCS.2020.20190060

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