# Combined GNSS/SLAM-Based High Accuracy Indoor Dynamic Target Localization Method

Authors: Xiaona Zhang; Shufang Zhang; Qiaosong Li; Xin Wang; Shujing Sun
Journal: Wireless Communications and Mobile Computing
 (2021)

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## Abstract

As the indoor dynamic target localization does not detect and repair the circumferential jump value in time, which leads to large position and attitude errors and low-velocity stability, a combined Global Navigation Satellite System/Simultaneous Localization and Mapping- (GNSS/SLAM-) based high-precision indoor dynamic target localization method is proposed. The method uses Empirical Mode Decomposition (EMD) to decompose the noisy signal, obtains the noise energy as the dominant mode from the decomposed components, extracts the useful signal energy as the main dividing point, removes the high-frequency signal, constructs the low-frequency component to realize low-pass filtering and denoising, selects a suitable threshold processing function to make the high-frequency signal component retain the detailed signal effectively to realize high-frequency component denoising, detects and fixes the circumferential jump of the observed data, and detects and fixes the circumferential jump of each frequency. The indoor dynamic target positioning method is established by combining GNSS/SLAM to achieve high accuracy target positioning. The experimental results show that the position and attitude errors are small, and the velocity is stable, which indicates that the position information is closer to the dynamic target, i.e., the target positioning is more accurate. To address the problems of scale drift and frequent initialization due to environmental factors in monocular vision SLAM, an Ultra Wideband (UWB)/vision fusion localization model that takes into account the scale factor is proposed, which can give full play to the complementary characteristics between UWB and vision; solve the problems of visual initialization, scale ambiguity, and absolute spatial reference; and improve UWB localization accuracy and localization frequency as well as reducing the number of base stations. The model can reliably achieve 0.2 m accuracy in indoor environments with sparse texture or frequent light changes.


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## Body

## 1. Introduction

In recent years, with the growing market for public-facing location services, it has become extremely important to research an efficient, accurate quotient, and low consumption indoor mapping technology, so SLAM technology is gradually becoming a hot topic of research and development in the field of robotics. SLAM problem refers to placing a robot in an unknown environment, where the robot incrementally creates a continuous map of the unknown environment while determining its position in the map. It has been studied in the field of robotics for decades [1]. The cross-fertilization of mapping, navigation guidance and control, and remote sensing science has given rise to a combination of different sensors for positioning and mapping instead of a single navigation system. Multisensor combination navigation systems complement each other’s strengths and advantages and can obtain higher navigation performance than any single navigation system. In outdoor environments, the most common technique is to use a combination of Global Navigation Satellite System (GNSS) and Inertial Navigation System (IGS) for positioning, which can provide centimeter-level positioning accuracy when GNSS signal conditions are good. In general, as an important research direction in the field of navigation, seamless indoor-outdoor positioning technology has received extensive attention and research from governments and scholars [2]. At this stage, GNSS-based outdoor high-precision positioning technology is developing rapidly, and indoor positioning technologies such as UWB and INS have also made great progress, which can meet the positioning needs in most scenarios [3]. However, at this stage, it is still impossible to solve the problem of seamless indoor and outdoor positioning by a single sensor, and the method based on multisensor fusion has become the mainstream technical route [4]. Especially in the face of urban fires, earthquakes, and other emergency rescue, there are fires, smoke, noisy sound, power interruption, house collapse, road obstruction, rescue under the rubble, and other complex situations, intensifying the difficulty and challenge of rapid and high-precision positioning of indoor and outdoor rescue personnel and vehicles [5].However, in indoor environments, GNSS signals are highly attenuated to the extent that they cannot meet the needs of indoor navigation and positioning. In indoor SLAM, the sensors used are generally divided into two categories: environment-aware sensors and motion-aware sensors. Environment-aware sensors generally include laser range finders, vision sensors, sonar, and optical flow sensors, which are mainly used to achieve localization based on the characteristics of the surrounding environment [6]. Motion-aware sensors include INS and odometer, which can achieve autonomous navigation without relying on the external environment. LiDAR is widely used in indoor SLAM for its advantages such as high localization accuracy and high positioning accuracy, unlike vision sensors which are affected by ambient light and other factors and are more stable, but because LiDAR matching is highly dependent on the features of the surrounding environment, it performs poorly in environments with low features, such as promenades. The advantage of GPS positioning is convenience and low cost because it is done through cell phones. Theoretically, the location of the cell phone can be determined by calculating the signal difference of three base stations, and the terminal held by the user only needs a cell phone. Therefore, if the user’s cell phone has a signal, location positioning can be performed at any time without being affected by weather, tall buildings, location, etc. While inertial navigation can achieve autonomous navigation without relying on external environmental information, the sensor errors contained in it will gradually accumulate with the long working time, and the errors will fail after accumulating to a certain degree [7]. Therefore, the effective combination of the two sensors to complement each other’s shortcomings and to create highly accurate indoor maps with high efficiency has become an important research direction for scholars at home and abroad.To solve the existing problems, the GNSS/SLAM combined high-precision indoor dynamic target positioning method is proposed. By preprocessing the indoor dynamic observation data, detecting the weekly jump values of different frequencies, repairing both large and small weekly jumps, and obtaining high-quality data and accurate positioning, GNSS/SLAM combined navigation has the advantages of high positioning accuracy and stability. The combined GNSS/SLAM navigation has the advantages of high positioning accuracy and stability and has a broad application prospect for the development of indoor positioning and wireless communication technology. The original postprocessing algorithm is improved, and a near real-time improvement algorithm is proposed, which greatly reduces the output delay of the navigation results without reducing the mapping accuracy, and paves the way for transplanting the algorithm to the embedded platform, which can make real-time mapping of the drawn map in the computer while collecting information in the surrounding environment while walking on the mobile machine platform in real-time. As can be seen, sensor map-based positioning methods require preestablished maps, so in addition to the study of positioning technology, map-building technology is also an important research direction. The methods of map building can be generally divided into two categories: mapping and map building and simultaneous positioning and map building, in which the former process of map building requires other high-precision positioning systems to provide positioning information, while the latter estimates the location and the map of the surrounding environment simultaneously through the information provided by the sensors themselves. The mapping approach is more costly and difficult to use in GNSS signal occlusion areas, while the SLAM mapping approach is more flexible but less accurate in comparison.


## 2. Current Status of Research

The visual SLAM process consists of two parts: the front-end and the back-end. In the front-end part, the feature point method or the direct method is used to obtain data associations of visual information from between consecutive or interrupted frames [8]. In contrast, the direct method does not extract feature descriptors and directly uses pixel grayscale values to obtain image associations. Compared with the feature point method, the direct method can estimate denser maps and is more robust to scenes with insignificant textures, but the direct method is more sensitive to illumination changes and has difficulty handling closed-loop detection [9]. Among them, the feature point method is the more commonly used method, while the direct method has only been proposed in recent years [10]. In the back-end part, SLAM methods usually transform the whole problem into a maximizing a posteriori probability estimation (MAP) problem, using the data association provided by the front-end to estimate both the positional and surrounding environment maps [11]. The back-end estimation methods are generally divided into two types: filtering and optimization. The filtering approach is mainly based on the extended Kalman filter (EKF), while the optimization approach iteratively solves the problem by constructing a nonlinear least-squares problem using the Gauss-Newton method or the Levenberg-Marquardt method. The filtering method is more time-efficient, but its accuracy is poor, and it is difficult to deal with closed-loop problems, while the optimization method is more accurate and can deal with closed-loop detection and optimization problems, with the disadvantage that it is less time-efficient and in rare cases may fall into the local minimum problem and affect the robustness of the estimation [12]. Due to the improvement of hardware computing power and research on matrix sparsity in recent years, optimization-based back-end methods can now run in real-time on common computing platforms, so optimization methods are now more frequently used in visual SLAM research.Soatti et al. uses SURF as a feature descriptor to solve the closed-loop detection problem by training a bag-of-words model (BOM) of the feature descriptor, and this method is called FAB-MAP [13]. Where closed-loop detection specifically refers to determining whether the location of a frame appears in an established map, thus establishing a link between this frame and the map, and is used to optimize the map that improves the drift problem of the positional estimation. Later, Wen et al. used binary descriptors such as Brief or ORB to build bag-of-words models so that closed-loop detection can be done in a shorter time to meet the needs of real-time applications, and this method is called DBoW [14]. Hoang et al. proposed a visual SLAM method based on the direct method and the LSD-SLAM, which replaces the minimized reprojection error commonly used in the feature point method with minimized photometric error, proves that the direct method can also obtain the same performance as the feature point method and can build denser maps, and is a representative method of direct method visual SLAM [15]. Singh et al. introduce a tightly coupled EKF-based VIO method, the MSCKF, which uses a filtering approach to fuse visual and inertial guidance information, extending the back-end estimation to multiple frames of information in a sliding window instead of the then common optimization of two adjacent frames [16]. Based on this, MSCKF proposes a linearization point in the fixed Jacobi matrix computation to avoid the observable direction anomaly problem and introduces an online estimation method for the external parameters of the camera-IMU system [17].Aiming at the problem of missing absolute localization reference for visual SLAM, visual odometry, and other visual indoor localization technologies, we carry out the research of a single-image spatial-visual localization model based on coded graphics assistance. Firstly, the algorithm model and technical process of the coded image, automatic detection and recognition of coded image, and automatic extraction of coded image and object coordinates are studied; secondly, the coded image assisted single image space rear rendezvous positioning model is constructed, and Tukey antidifference weighting factor is introduced to weaken or suppress the influence of the residuals of coded image coordinate observation on the positioning results and achieve high accuracy positioning.


## 3. GNSS/SLAM Combined High-Precision Indoor Dynamic Target Positioning Analysis

### 3.1. GNSS/SLAM Combination Design

GNSS dynamic positioning in urban environments is limited by the observation environment, with high signal interference noise, serious multipath effects, and frequent signal interruptions by occlusion, which seriously affects positioning accuracy and continuity [18]. Currently, in GNSS positioning, the GNSS carrier-phase double-difference positioning technique can be used to remove highly correlated error sources in space and time, such as ephemeris errors, to improve the positioning accuracy. Usually, the carrier-phase double-difference observation model can be described as:
(1)∇Δp=∇Δp0−∇ΔT−∇ΔI−∇ΔM+∇Δεp,λ∇Δϕ=∇Δϕ0−∇ΔT−∇ΔI−1+λ∇ΔN+∇Δm+∇Δεϕ,where the∇Δ symbol refers to double differentiation. p0 and ϕ0 denote pseudo-range and carrier phase observations, the geometric distance between the receiver and the satellite is p0, T denotes tropospheric delay, I denotes ionospheric delay, λ denotes carrier phase wavelength, M denotes pseudo-range multipath error, m denotes carrier phase multipath error, and εp and εϕ denote pseudo-range noise and carrier phase noise. For GNSS RTK with GPS and BDS data, the equation for estimating the unknown parameters can be written as:
(2)HGAGHCACεp∇ΔNC=HGεp+AG∇ΔNCHCεp+AC∇ΔNC=∇ΔLG∇ΔLC,where the superscriptsG and C denote GPS and BDS, respectively; H is a geometric matrix containing satellite geometry information; εp denotes the baseline increment vector; A is the ambiguity coefficient matrix, and ∇ΔL is the observation minus the computation vector. Inertial devices mainly include three-axis accelerometers and three-axis gyroscopes. Limited by technical methods and process levels, inertial devices inevitably contain a variety of systematic and random errors, and at the same time, the influence of environmental factors makes these errors more complicated. Usually, inertial devices have been factory calibrated in a series of laboratory procedures; most of the system errors are compensated; the residual part needs to be estimated online through modeling; in addition, inertial devices also contain a variety of power-law spectrum of random noise, whether system errors or random errors will eventually be reflected by the accelerometer and gyroscope observations; this subsection focuses on the vehicle tactical grade. This subsection focuses on the analysis of SPAN-FSAS, an inertial guide, and discusses the effects of the selection of the systematic error term and the setting of the random error parameter on the combined GNSS/SINS results.Before introducing the method of visual map building, it is necessary to introduce the form of the composition of the visual map used in this paper. Whether the map is built with high precision GNSS or SLAM, the form of the map is the same, which ensures the consistency of the positioning map. When positioning, there is no need to consider what the building method is, and the same information for positioning is obtained [19]. The map data in this paper is organized in keyframes, which are some of the frames selected with certain rules among all the image frames in the construction map. The main rule is that there is a certain distance interval between adjacent keyframes, to avoid redundancy of data. And each keyframe contains the information extracted from the sensor information, which is enough for positioning, and the complete map is composed of this keyframe, as shown in Figure 1.Figure 1
Combined GNSS/SLAM framework.It should be noted that the vision sensor in this paper is a monocular camera, and as analyzed in the previous paper, the visual feature map for localization needs the 3D position information of the features, so the focus of the high-precision GNSS-based visual map building method is the accurate estimation of the 3D position of the features. The simplest 3D position estimation method is to use the matching information of the features and the localization information for triangulation. Suppose that for the same feature pointP, the normalized coordinates on the two frames obtained by feature detection and matching are x1 and x2, and the two frames obtained by the positioning information are R1,p1 and R2,p2. The depths of the high-precision pose GNSS are s1 and s2, which are also the depths of the feature point P in the two frames. There is the following constraint relation.
(3)s1x1=s2Rx2+ts2Rx2−t.In determining whether the current frame is a keyframe and added to the map, the most important judgment criterion is the average parallax between the feature matching pairs of the current frame and the previous keyframe, i.e., the distance between the positions of the matching point pairs under the image coordinates. This is because when the parallax is relatively small, it can lead to a large error in the depth calculated by triangulation due to the camera’s measurement unit of one pixel, which has a considerable impact on the matching localization. In addition, the matching accuracy of different feature point pairs may be different, and some of them are even mismatched, and the depths obtained by direct triangulation will become unreliable.For these reasons, it is not reasonable to simply perform triangulation, so in this paper, after triangulation of the feature points, a sliding window optimization is added to estimate the position of each feature point when building a map based on high accuracy GNSS. In this optimization problem, the only state quantity to be estimated is the depth of all feature points within the window, as in equation (4).
(4)X=λ0,λ1,⋯,λm.And the optimization problem takes the form of(5)maxliml,k⟶∞∑l,k∈CerAzlck,Xplck2.Since the window includes multiple keyframes with a certain distance interval, the features within the window generally have enough parallax, and the estimation of feature depth is more reliable after optimization. After the optimization of the feature depth, the reprojection error of each feature is also recalculated, and if the error is larger than a present threshold, the feature can be considered as a wrong match, and the depth is invalid. When the keyframe with the earliest timestamp in the window is removed, that is, when the feature depth information on the image is no longer changed by the optimization, it is ready to be joined to the map. The keyframe information described in the previous subsection needs to be computed before joining the map; that is, the feature descriptor is computed based on the position of the feature point on the original image, and then, the bag-of-words vector for this keyframe is computed based on the feature descriptor. After completing the computation of this information, this keyframe is added to the map incrementally.Closed-loop detection is a common concept in SLAM, which is essentially a method of information retrieval. The main purpose is to determine whether the current location has already appeared on the map, and when the information of the current location is found to be like some of the information in the map, it is considered that a closed-loop is found, and the map within the closed-loop can be optimized by establishing constraints on the matching relationship between the closed-loop frames. And in the localization process, the first step of global localization can be considered as the closed-loop detection process, which is to find similar locations in the map. Specifically, for the visual feature map in this paper, it is to find keyframes that are like the scene of the current image frame in the map composed of keyframes, and these keyframes will be used as candidate frames for feature matching with the current frame to get the global localization information. This is equivalent to the coarse localization process before the fine localization, which can greatly reduce the time complexity of global localization [20]. This is because the current frame does not need to feature match with every frame in the map but only needs to match with the map frames that are determined to be similar. The main solution for closed-loop detection in visual SLAM is to use the bag-of-words model. Bag-of-words (BOW) model is originally a document representation method in the field of information retrieval. In determining the similarity of two documents, the order of occurrence of words in a document is not considered, but only the frequency of occurrence of different words, i.e., two documents are considered similar when the frequency of occurrence of each word is similar. And the analogy of this concept to image features is to divide image features into a certain number of categories and count the frequency of occurrence of different categories in each image.As shown in Figure2, for an image, the histogram statistics of categories can be performed, and the images with similar histogram distribution have higher similarity, which is the information of bag-of-words vectors in keyframes mentioned before, and the distance between the bag-of-words vectors of different images can be considered as the similarity of images.Figure 2
Schematic diagram of the bag-of-words model.Our algorithm uses multiple fusion algorithms, which results in more accurate results and more efficient localization. And the classes of these features are called dictionaries, which are used to query which class the features belong to base on the descriptors of the features. The dictionary of feature descriptors in visual SLAM is obtained by clustering the features on many images, which has the advantage that it can cope with a wide variety of scenes and can perform closed-loop detection in unknown scenes. In the localization scenario of this paper, on the other hand, the map is well established, so the scene is known, and a dictionary specifically for this map can be generated by clustering all the feature points appearing on the map. Feature matching refers to determining the correspondence between feature points of different images and determining which points are the same in the 3D space. The similarity between feature points is generally measured by feature descriptors, which are usually expressed in terms of Euclidean distance for floating-point descriptors like SIFT and Hamming distance for binary descriptors like BRIEF. The Hamming distance refers to the number of values that differ between two binary numbers with the same number of bits, and the larger the Hamming distance, the smaller the similarity.(6)dx,y=Sup∑i=1nxi⊗yi.With the similarity measure, what needs to be done is to find which feature points are like each other. One of the simplest ideas is Brute-Force Matcher, which is the idea of calculating the distance between each feature point and all its possible matches and choosing the closest one as the matching point. This method is very simple, but it works quite well when the number of features is small. The number of features in a single keyframe in the map is generally small between 100 and 200, so using violent matching is a more efficient approach. Based on the sliding window state described above, the form of the optimization problem during fusion localization is somewhat altered from that of local positional estimation, specifically by adding reprojection error term for the features observed in the map frames.(7)maxliml,k⟶∞e∑l,k∈CrAzlck,Xplck2+liml,β⟶∞e∑l,β∈CrAzlcβ,Xplck2,X=x0,x1,⋯,xm,λ0,λ1,⋯,λm,xk=pbkw,vbkw,qbkw,bk,bg,xcb=pbc,qbc.The objective of this optimization problem is to simultaneously optimize the poses of the image frames in the window and the relative poses derived from PnP in the previous subsection. The final optimized relative poses will be based on the poses of the map frames to obtain the poses of the current frame in the map coordinate system, i.e., the global positioning result. When no closed-loop detection occurs, i.e., when no map frame is observed, the whole problem turns back to the relative pose estimation problem and continues to be derived based on the previous global positioning result. This is the fusion localization method of tightly coupled optimization, which simultaneously performs the estimation of relative positional and global localization, and finally fuses the two, improving the disadvantages of poor robustness and low frequency of global localization information based on feature point matching, giving global localization results with higher accuracy and frequency.


### 3.2. Indoor Dynamic Target Localization Analysis

SLAM is a synchronous positioning and mapping technology, and GNSS is a superior navigation technology, which is commonly used in various fields [21–23]. In some scenarios, GNSS signals cannot be received due to the complex propagation environment and interference in the propagation channel, and its navigation function fails, so GNSS/SLAM combination can obtain high-precision environmental features and provide accurate positioning information. In the GNSS SLAM combination algorithm, for the attitude angle of the LIDAR coordinate system in the reference coordinate system, the position vector of the sensor and all the observed landmark points are the system state vector at k moments [24–26]. Location recognition is a key element of mobile robotics. In practical usage scenarios, we usually need to estimate the current robot’s location information without any a priori information. This operation is also referred to as global localization. In this paper, the data in a 3D spatial coordinate system is first divided into two spheres: an outer sphere and an inner sphere. In this case, the radius of the outer sphere and the radius of the inner sphere can be measured. Based on the principle of quadrant distribution of the 3D spatial coordinate system, it can be divided into 8 subspaces. Eventually, 16 subspaces can be obtained. The most essential difference between the tight and loose combination is whether SINS provides position a priori information for the GNSS solution. The position information is the link between SINS and GNSS, and the two sensors share the position to achieve the fusion of observation information. Based on this argument, we design a combination method with a semicompact structure.SINS as the main filter distributes the position a priori information for all sensors, and each subfilter decides by itself whether to use the position a priori information through the judgment of the pretesters. This distributed architecture is essentially a semitight combination, where all sensor information is handshake confirmed and fused in the position domain, while each sensor’s internal solution remains independent, leaving only the receiving and sending interface for position information. The advantage of the semicompact combination is that it can robustly fuse data from multiple sensors to obtain a better result; here, only GNSS and SINS sensor fusion is used as an example. The SPAN-FSAS device from Novatel was used to collect a single GPS vehicle data in a suburban area with a duration of about 2.8 hours. To analyze the effect of satellite signal interruption on the semitight combination, we simulated three 60 s segments of partial satellite information loss lock starting from 0.75 hours, and only the first 4 satellites were retained in the first segment according to the satellite serial number size, and only the first 3 satellites were retained in the second segment. According to the size of satellite serial number, only the first 4 satellites are retained in the first segment, only the first 3 satellites are retained in the second segment, and only the first 2 satellites are retained in the third segment, as shown in Figure3 in the STC results labeled with the three segments of loss of lock.Figure 3
PDOP values and number of available satellites.From the error analysis of the positioning experiment, we can see that the lateral and longitudinal errors in most areas are within 20 cm, and the root mean square error of heading angle estimation is 0.420°, which can meet the needs of intelligent vehicle applications in most cases. The maximum lateral error of 84 cm was found in the open intersection area, where the road features on both sides are less constrained, so the lateral error was larger, while the maximum longitudinal error was found in the straight-line area due to the high repetition of the feature points on the straight-line road, which may cause some error in the estimation of the longitudinal positioning. Overall, the localization results are reasonable considering that most of the maps used were created by the VI SLAM approach. When using the maps created by the hybrid approach for localization, it not only shows the performance of the positioning system but also is affected by the accuracy of the maps. Therefore, to further compare and verify the effectiveness of the localization method and the map building method, additional experiments were conducted to localize the maps entirely using high precision GNSS building, as shown in Figure4.Figure 4
Flow chart of visual map creation.Local map and map alignment and optimization and the detailed methods for these two parts are described below. It should be noted that due to the lack of robust and reliable criteria for judging whether the satellite signal is obscured, the judgment here is performed by hand, which is easy to implement and reasonable since the map building process is an offline process. After having a more accurate estimate of the positional pose and feature depth, all that must be done is the creation of the local map [27, 28]. It is easy to find that this part is the same as the building steps in the method based on high-precision GNSS mapping; that is, the keyframe information that can be used for localization is calculated from the positional pose and feature position, including the calculation of feature descriptors and the calculation of bag-of-words vectors, which will not be described repeatedly here.In terms of system implementation, the map building method in this paper is an extension of local bit pose estimation, so the underlying programming platform is the same as the open-source tool, and the time synchronization of GNSS information with other sensor information is achieved through software synchronization, and the software timestamp of GNSS information is accurate enough in this system due to the small amount of data, low transmission delay, and high frequency, and the additional visual image closed-loop used in the experiment detection is implemented using the open-source library DBOW, and the map data is saved using the serialization function of the Boost library. Although the map building process is an offline process after data acquisition, the methods used in this paper are real-time, so all experiments are run in real-time on an ordinary laptop CPU.


## 3.1. GNSS/SLAM Combination Design

GNSS dynamic positioning in urban environments is limited by the observation environment, with high signal interference noise, serious multipath effects, and frequent signal interruptions by occlusion, which seriously affects positioning accuracy and continuity [18]. Currently, in GNSS positioning, the GNSS carrier-phase double-difference positioning technique can be used to remove highly correlated error sources in space and time, such as ephemeris errors, to improve the positioning accuracy. Usually, the carrier-phase double-difference observation model can be described as:
(1)∇Δp=∇Δp0−∇ΔT−∇ΔI−∇ΔM+∇Δεp,λ∇Δϕ=∇Δϕ0−∇ΔT−∇ΔI−1+λ∇ΔN+∇Δm+∇Δεϕ,where the∇Δ symbol refers to double differentiation. p0 and ϕ0 denote pseudo-range and carrier phase observations, the geometric distance between the receiver and the satellite is p0, T denotes tropospheric delay, I denotes ionospheric delay, λ denotes carrier phase wavelength, M denotes pseudo-range multipath error, m denotes carrier phase multipath error, and εp and εϕ denote pseudo-range noise and carrier phase noise. For GNSS RTK with GPS and BDS data, the equation for estimating the unknown parameters can be written as:
(2)HGAGHCACεp∇ΔNC=HGεp+AG∇ΔNCHCεp+AC∇ΔNC=∇ΔLG∇ΔLC,where the superscriptsG and C denote GPS and BDS, respectively; H is a geometric matrix containing satellite geometry information; εp denotes the baseline increment vector; A is the ambiguity coefficient matrix, and ∇ΔL is the observation minus the computation vector. Inertial devices mainly include three-axis accelerometers and three-axis gyroscopes. Limited by technical methods and process levels, inertial devices inevitably contain a variety of systematic and random errors, and at the same time, the influence of environmental factors makes these errors more complicated. Usually, inertial devices have been factory calibrated in a series of laboratory procedures; most of the system errors are compensated; the residual part needs to be estimated online through modeling; in addition, inertial devices also contain a variety of power-law spectrum of random noise, whether system errors or random errors will eventually be reflected by the accelerometer and gyroscope observations; this subsection focuses on the vehicle tactical grade. This subsection focuses on the analysis of SPAN-FSAS, an inertial guide, and discusses the effects of the selection of the systematic error term and the setting of the random error parameter on the combined GNSS/SINS results.Before introducing the method of visual map building, it is necessary to introduce the form of the composition of the visual map used in this paper. Whether the map is built with high precision GNSS or SLAM, the form of the map is the same, which ensures the consistency of the positioning map. When positioning, there is no need to consider what the building method is, and the same information for positioning is obtained [19]. The map data in this paper is organized in keyframes, which are some of the frames selected with certain rules among all the image frames in the construction map. The main rule is that there is a certain distance interval between adjacent keyframes, to avoid redundancy of data. And each keyframe contains the information extracted from the sensor information, which is enough for positioning, and the complete map is composed of this keyframe, as shown in Figure 1.Figure 1
Combined GNSS/SLAM framework.It should be noted that the vision sensor in this paper is a monocular camera, and as analyzed in the previous paper, the visual feature map for localization needs the 3D position information of the features, so the focus of the high-precision GNSS-based visual map building method is the accurate estimation of the 3D position of the features. The simplest 3D position estimation method is to use the matching information of the features and the localization information for triangulation. Suppose that for the same feature pointP, the normalized coordinates on the two frames obtained by feature detection and matching are x1 and x2, and the two frames obtained by the positioning information are R1,p1 and R2,p2. The depths of the high-precision pose GNSS are s1 and s2, which are also the depths of the feature point P in the two frames. There is the following constraint relation.
(3)s1x1=s2Rx2+ts2Rx2−t.In determining whether the current frame is a keyframe and added to the map, the most important judgment criterion is the average parallax between the feature matching pairs of the current frame and the previous keyframe, i.e., the distance between the positions of the matching point pairs under the image coordinates. This is because when the parallax is relatively small, it can lead to a large error in the depth calculated by triangulation due to the camera’s measurement unit of one pixel, which has a considerable impact on the matching localization. In addition, the matching accuracy of different feature point pairs may be different, and some of them are even mismatched, and the depths obtained by direct triangulation will become unreliable.For these reasons, it is not reasonable to simply perform triangulation, so in this paper, after triangulation of the feature points, a sliding window optimization is added to estimate the position of each feature point when building a map based on high accuracy GNSS. In this optimization problem, the only state quantity to be estimated is the depth of all feature points within the window, as in equation (4).
(4)X=λ0,λ1,⋯,λm.And the optimization problem takes the form of(5)maxliml,k⟶∞∑l,k∈CerAzlck,Xplck2.Since the window includes multiple keyframes with a certain distance interval, the features within the window generally have enough parallax, and the estimation of feature depth is more reliable after optimization. After the optimization of the feature depth, the reprojection error of each feature is also recalculated, and if the error is larger than a present threshold, the feature can be considered as a wrong match, and the depth is invalid. When the keyframe with the earliest timestamp in the window is removed, that is, when the feature depth information on the image is no longer changed by the optimization, it is ready to be joined to the map. The keyframe information described in the previous subsection needs to be computed before joining the map; that is, the feature descriptor is computed based on the position of the feature point on the original image, and then, the bag-of-words vector for this keyframe is computed based on the feature descriptor. After completing the computation of this information, this keyframe is added to the map incrementally.Closed-loop detection is a common concept in SLAM, which is essentially a method of information retrieval. The main purpose is to determine whether the current location has already appeared on the map, and when the information of the current location is found to be like some of the information in the map, it is considered that a closed-loop is found, and the map within the closed-loop can be optimized by establishing constraints on the matching relationship between the closed-loop frames. And in the localization process, the first step of global localization can be considered as the closed-loop detection process, which is to find similar locations in the map. Specifically, for the visual feature map in this paper, it is to find keyframes that are like the scene of the current image frame in the map composed of keyframes, and these keyframes will be used as candidate frames for feature matching with the current frame to get the global localization information. This is equivalent to the coarse localization process before the fine localization, which can greatly reduce the time complexity of global localization [20]. This is because the current frame does not need to feature match with every frame in the map but only needs to match with the map frames that are determined to be similar. The main solution for closed-loop detection in visual SLAM is to use the bag-of-words model. Bag-of-words (BOW) model is originally a document representation method in the field of information retrieval. In determining the similarity of two documents, the order of occurrence of words in a document is not considered, but only the frequency of occurrence of different words, i.e., two documents are considered similar when the frequency of occurrence of each word is similar. And the analogy of this concept to image features is to divide image features into a certain number of categories and count the frequency of occurrence of different categories in each image.As shown in Figure2, for an image, the histogram statistics of categories can be performed, and the images with similar histogram distribution have higher similarity, which is the information of bag-of-words vectors in keyframes mentioned before, and the distance between the bag-of-words vectors of different images can be considered as the similarity of images.Figure 2
Schematic diagram of the bag-of-words model.Our algorithm uses multiple fusion algorithms, which results in more accurate results and more efficient localization. And the classes of these features are called dictionaries, which are used to query which class the features belong to base on the descriptors of the features. The dictionary of feature descriptors in visual SLAM is obtained by clustering the features on many images, which has the advantage that it can cope with a wide variety of scenes and can perform closed-loop detection in unknown scenes. In the localization scenario of this paper, on the other hand, the map is well established, so the scene is known, and a dictionary specifically for this map can be generated by clustering all the feature points appearing on the map. Feature matching refers to determining the correspondence between feature points of different images and determining which points are the same in the 3D space. The similarity between feature points is generally measured by feature descriptors, which are usually expressed in terms of Euclidean distance for floating-point descriptors like SIFT and Hamming distance for binary descriptors like BRIEF. The Hamming distance refers to the number of values that differ between two binary numbers with the same number of bits, and the larger the Hamming distance, the smaller the similarity.(6)dx,y=Sup∑i=1nxi⊗yi.With the similarity measure, what needs to be done is to find which feature points are like each other. One of the simplest ideas is Brute-Force Matcher, which is the idea of calculating the distance between each feature point and all its possible matches and choosing the closest one as the matching point. This method is very simple, but it works quite well when the number of features is small. The number of features in a single keyframe in the map is generally small between 100 and 200, so using violent matching is a more efficient approach. Based on the sliding window state described above, the form of the optimization problem during fusion localization is somewhat altered from that of local positional estimation, specifically by adding reprojection error term for the features observed in the map frames.(7)maxliml,k⟶∞e∑l,k∈CrAzlck,Xplck2+liml,β⟶∞e∑l,β∈CrAzlcβ,Xplck2,X=x0,x1,⋯,xm,λ0,λ1,⋯,λm,xk=pbkw,vbkw,qbkw,bk,bg,xcb=pbc,qbc.The objective of this optimization problem is to simultaneously optimize the poses of the image frames in the window and the relative poses derived from PnP in the previous subsection. The final optimized relative poses will be based on the poses of the map frames to obtain the poses of the current frame in the map coordinate system, i.e., the global positioning result. When no closed-loop detection occurs, i.e., when no map frame is observed, the whole problem turns back to the relative pose estimation problem and continues to be derived based on the previous global positioning result. This is the fusion localization method of tightly coupled optimization, which simultaneously performs the estimation of relative positional and global localization, and finally fuses the two, improving the disadvantages of poor robustness and low frequency of global localization information based on feature point matching, giving global localization results with higher accuracy and frequency.


## 3.2. Indoor Dynamic Target Localization Analysis

SLAM is a synchronous positioning and mapping technology, and GNSS is a superior navigation technology, which is commonly used in various fields [21–23]. In some scenarios, GNSS signals cannot be received due to the complex propagation environment and interference in the propagation channel, and its navigation function fails, so GNSS/SLAM combination can obtain high-precision environmental features and provide accurate positioning information. In the GNSS SLAM combination algorithm, for the attitude angle of the LIDAR coordinate system in the reference coordinate system, the position vector of the sensor and all the observed landmark points are the system state vector at k moments [24–26]. Location recognition is a key element of mobile robotics. In practical usage scenarios, we usually need to estimate the current robot’s location information without any a priori information. This operation is also referred to as global localization. In this paper, the data in a 3D spatial coordinate system is first divided into two spheres: an outer sphere and an inner sphere. In this case, the radius of the outer sphere and the radius of the inner sphere can be measured. Based on the principle of quadrant distribution of the 3D spatial coordinate system, it can be divided into 8 subspaces. Eventually, 16 subspaces can be obtained. The most essential difference between the tight and loose combination is whether SINS provides position a priori information for the GNSS solution. The position information is the link between SINS and GNSS, and the two sensors share the position to achieve the fusion of observation information. Based on this argument, we design a combination method with a semicompact structure.SINS as the main filter distributes the position a priori information for all sensors, and each subfilter decides by itself whether to use the position a priori information through the judgment of the pretesters. This distributed architecture is essentially a semitight combination, where all sensor information is handshake confirmed and fused in the position domain, while each sensor’s internal solution remains independent, leaving only the receiving and sending interface for position information. The advantage of the semicompact combination is that it can robustly fuse data from multiple sensors to obtain a better result; here, only GNSS and SINS sensor fusion is used as an example. The SPAN-FSAS device from Novatel was used to collect a single GPS vehicle data in a suburban area with a duration of about 2.8 hours. To analyze the effect of satellite signal interruption on the semitight combination, we simulated three 60 s segments of partial satellite information loss lock starting from 0.75 hours, and only the first 4 satellites were retained in the first segment according to the satellite serial number size, and only the first 3 satellites were retained in the second segment. According to the size of satellite serial number, only the first 4 satellites are retained in the first segment, only the first 3 satellites are retained in the second segment, and only the first 2 satellites are retained in the third segment, as shown in Figure3 in the STC results labeled with the three segments of loss of lock.Figure 3
PDOP values and number of available satellites.From the error analysis of the positioning experiment, we can see that the lateral and longitudinal errors in most areas are within 20 cm, and the root mean square error of heading angle estimation is 0.420°, which can meet the needs of intelligent vehicle applications in most cases. The maximum lateral error of 84 cm was found in the open intersection area, where the road features on both sides are less constrained, so the lateral error was larger, while the maximum longitudinal error was found in the straight-line area due to the high repetition of the feature points on the straight-line road, which may cause some error in the estimation of the longitudinal positioning. Overall, the localization results are reasonable considering that most of the maps used were created by the VI SLAM approach. When using the maps created by the hybrid approach for localization, it not only shows the performance of the positioning system but also is affected by the accuracy of the maps. Therefore, to further compare and verify the effectiveness of the localization method and the map building method, additional experiments were conducted to localize the maps entirely using high precision GNSS building, as shown in Figure4.Figure 4
Flow chart of visual map creation.Local map and map alignment and optimization and the detailed methods for these two parts are described below. It should be noted that due to the lack of robust and reliable criteria for judging whether the satellite signal is obscured, the judgment here is performed by hand, which is easy to implement and reasonable since the map building process is an offline process. After having a more accurate estimate of the positional pose and feature depth, all that must be done is the creation of the local map [27, 28]. It is easy to find that this part is the same as the building steps in the method based on high-precision GNSS mapping; that is, the keyframe information that can be used for localization is calculated from the positional pose and feature position, including the calculation of feature descriptors and the calculation of bag-of-words vectors, which will not be described repeatedly here.In terms of system implementation, the map building method in this paper is an extension of local bit pose estimation, so the underlying programming platform is the same as the open-source tool, and the time synchronization of GNSS information with other sensor information is achieved through software synchronization, and the software timestamp of GNSS information is accurate enough in this system due to the small amount of data, low transmission delay, and high frequency, and the additional visual image closed-loop used in the experiment detection is implemented using the open-source library DBOW, and the map data is saved using the serialization function of the Boost library. Although the map building process is an offline process after data acquisition, the methods used in this paper are real-time, so all experiments are run in real-time on an ordinary laptop CPU.


## 4. Analysis of Results

### 4.1. System Performance Analysis

Figure5 shows that the UWB ranging data in the rectangular line is partially obscured by the columns and appears discontinuous. Its corresponding Δri is shown in Figure 5. The bars of different colors indicate the ranging residual values of the corresponding base stations, respectively. As can be seen from Figure 5, the range values are relatively unstable at several peaks, which is because the range error of the UWB increases relatively as the distance increases. This is because the range error of UWB increases as the distance increases. The corresponding range residuals also increase in Figure 5. The weight of the range residuals is analyzed to dynamically determine the weight of the covariance of the observations at different base stations. From Figure 5, the average range residuals of all four base stations are less than 0.1 m, which indicates that the overall quality of the range values of path 1 is high. The smallest residual value of 0 indicates that the ranging value is obscured at a certain moment, and the residual calculation is not done at this time. Continuous wide coverage scenario is the most basic coverage method for mobile communication, aiming at ensuring users’ mobility and service continuity and providing users with seamless high-speed service experience.Figure 5
Distance measurement data.Another advantage of the mapping method in this paper is that it does not require route closure in the actual sense to optimize the map. Visual closure is done only once in the whole route, so the accuracy of mapping using visual closure is experimentally lower than that of GNSS closed-loop mapping. Theoretically, both visual and GNSS closed-loop provide the same information, and if the number of closed-loops is the same, the result should be similar. The purpose of this paper is to deal with the visual mapping of the occluded and unoccluded areas, and the use of visual closed-loop is quite limited. The comparison experiments here illustrate the flexibility of this paper and the effectiveness of the framework.The experimental process around the figure of the conference table walking in a circle for this camera in the direction of different will automatically adjust the brightness, resulting in the video stream of light and dark changes; serious areas will be too black and too white, basically cannot see the textural information in the figure. Figure6 is a sparse texture of the experimental environment; you can see that the corridor walls for the brochure, the ground, and the top plate are relatively single tone. In addition, during the experiment, the camera is facing the direction of the corridor, and the wall tilt intersection increasing the texture is not clear. The experimental equipment used is a calibrated camera, as well as a UWB base station and tagging equipment, as shown in Figure 6. The UWB equipment details are consistent with Section 4 of this paper. The camera is a CCD (charge-coupled device) imaging device, supporting up to 1080P resolution video stream acquisition, which can be directly inputted to the computer through USB, and the development of the corresponding interface software to achieve real-time extraction of video streams and keyframes.Figure 6
Performance parameters.During the experiment, to see a larger range of objects, the camera depth of precision is set to infinity. In addition, because the camera is an industrial camera, the camera will automatically adjust the brightness after sensing light. Since a fixed light-sensitive parameter cannot be set, it results in a process of obvious light, and dark changes in the acquired video stream when the light changes. As a streaming technology, the vision sensor not only can obtain rich texture information but also can transmit it in real-time. If it is applied to localization methods, it is ideal for indoor localization as it can not only obtain location information but also understand the surrounding real-world scenery. However, the current vision-based indoor localization methods fail in localization due to sparse textures, too bright or too dark light, and scale ambiguity in monocular ORB-SLAM. To address these problems, this paper proposes a combined UWB/vision-based EKF indoor localization model, which achieves indoor localization accuracy of 0.2 m and solves the problems of visual scale ambiguity and localization failure caused by texture sparsity and light brightness variation.


### 4.2. Dynamic Target Localization Results

The actual road conditions are highly variable, and only five types of feature matching under typical conditions are given above. It can be seen that the SIFT operator extracts a lot of feature points, which are distributed in the corner points on the figure and the areas with obvious changes, and the actual number of feature points that can be matched is relatively much less, but they can also be distributed in different areas of the figure so that the geometric configuration strength of the rear rendezvous can be ensured, and the geometric configuration strength through the RANSAC algorithm and antidifference optimization estimation excludes still more error matching points, mainly some road sign points with high feature similarity. The target detection and semantic segmentation are not used when processing the image in real-time here, mainly considering that the difference between vehicle and pedestrian feature descriptors and environmental feature descriptors is relatively large and does not easily cause mutual interference, while the vehicle and pedestrian have been removed when building the map, so the matched feature points in the scene do not fall on the vehicle and pedestrian, but basically on the road, house, and tree. It is better to match the correct point distribution to get a good bit pose solving result.Figure7 gives the matching situation and localization accuracy under five road conditions, and we can see that the feature matching rate is only about 10.37%, and most of the extracted feature points cannot find the corresponding road marker points in the visual point cloud map, although the percentage is relatively small, the number of its matching feature points is sufficient, and the ratio of final matching error to correct matching is about 1 : 4.5. Using localization accuracy can also be around cm, and C and D are still slightly better than the optimized localization accuracy, but the optimized results are more stable and have higher accuracy, as shown in Figure 8. It should be noted that the vertical coordinates of the positional results and the positional results of antidifference optimization in the figure are different between the two. In addition, the left and front components of the position are the planes directly under the system, and the lower component is the elevation direction, where the positional positioned under the camera coordinate system, so the heading angle is the roll angle under the ENU system, and the roll angle is the heading angle, and the pitch angle remains the same.Figure 7
Matching cases and positioning accuracy in five cases.Figure 8
Position accuracy comparison.The above conclusion is only for data, and the same set of data is used for map building and positioning. In practice, map building and localization are often separated; for example, if the map built last month is used for localization at this moment, the point cloud map may not be able to accurately represent the actual road information at this moment due to the change of environmental factors. Whether the feature descriptors can still be matched needs to be studied in one step. With the promotion of crowdsourcing technology in the future, we can potentially achieve incremental real-time updates of the visual point cloud map and add more robust multidimensional feature information and semantic information, which can improve the success rate of localization in real scenarios. However, the average time required is 158 ms and 146 ms, and the maximum time required is 693 ms and 541 ms, accounting for the major part of the time consumed. On this basis, the position loosening combination with SINS can achieve continuous high sampling rate localization. Our positioning results are about 20% more efficient and a bit more accurate compared to others.


## 4.1. System Performance Analysis

Figure5 shows that the UWB ranging data in the rectangular line is partially obscured by the columns and appears discontinuous. Its corresponding Δri is shown in Figure 5. The bars of different colors indicate the ranging residual values of the corresponding base stations, respectively. As can be seen from Figure 5, the range values are relatively unstable at several peaks, which is because the range error of the UWB increases relatively as the distance increases. This is because the range error of UWB increases as the distance increases. The corresponding range residuals also increase in Figure 5. The weight of the range residuals is analyzed to dynamically determine the weight of the covariance of the observations at different base stations. From Figure 5, the average range residuals of all four base stations are less than 0.1 m, which indicates that the overall quality of the range values of path 1 is high. The smallest residual value of 0 indicates that the ranging value is obscured at a certain moment, and the residual calculation is not done at this time. Continuous wide coverage scenario is the most basic coverage method for mobile communication, aiming at ensuring users’ mobility and service continuity and providing users with seamless high-speed service experience.Figure 5
Distance measurement data.Another advantage of the mapping method in this paper is that it does not require route closure in the actual sense to optimize the map. Visual closure is done only once in the whole route, so the accuracy of mapping using visual closure is experimentally lower than that of GNSS closed-loop mapping. Theoretically, both visual and GNSS closed-loop provide the same information, and if the number of closed-loops is the same, the result should be similar. The purpose of this paper is to deal with the visual mapping of the occluded and unoccluded areas, and the use of visual closed-loop is quite limited. The comparison experiments here illustrate the flexibility of this paper and the effectiveness of the framework.The experimental process around the figure of the conference table walking in a circle for this camera in the direction of different will automatically adjust the brightness, resulting in the video stream of light and dark changes; serious areas will be too black and too white, basically cannot see the textural information in the figure. Figure6 is a sparse texture of the experimental environment; you can see that the corridor walls for the brochure, the ground, and the top plate are relatively single tone. In addition, during the experiment, the camera is facing the direction of the corridor, and the wall tilt intersection increasing the texture is not clear. The experimental equipment used is a calibrated camera, as well as a UWB base station and tagging equipment, as shown in Figure 6. The UWB equipment details are consistent with Section 4 of this paper. The camera is a CCD (charge-coupled device) imaging device, supporting up to 1080P resolution video stream acquisition, which can be directly inputted to the computer through USB, and the development of the corresponding interface software to achieve real-time extraction of video streams and keyframes.Figure 6
Performance parameters.During the experiment, to see a larger range of objects, the camera depth of precision is set to infinity. In addition, because the camera is an industrial camera, the camera will automatically adjust the brightness after sensing light. Since a fixed light-sensitive parameter cannot be set, it results in a process of obvious light, and dark changes in the acquired video stream when the light changes. As a streaming technology, the vision sensor not only can obtain rich texture information but also can transmit it in real-time. If it is applied to localization methods, it is ideal for indoor localization as it can not only obtain location information but also understand the surrounding real-world scenery. However, the current vision-based indoor localization methods fail in localization due to sparse textures, too bright or too dark light, and scale ambiguity in monocular ORB-SLAM. To address these problems, this paper proposes a combined UWB/vision-based EKF indoor localization model, which achieves indoor localization accuracy of 0.2 m and solves the problems of visual scale ambiguity and localization failure caused by texture sparsity and light brightness variation.


## 4.2. Dynamic Target Localization Results

The actual road conditions are highly variable, and only five types of feature matching under typical conditions are given above. It can be seen that the SIFT operator extracts a lot of feature points, which are distributed in the corner points on the figure and the areas with obvious changes, and the actual number of feature points that can be matched is relatively much less, but they can also be distributed in different areas of the figure so that the geometric configuration strength of the rear rendezvous can be ensured, and the geometric configuration strength through the RANSAC algorithm and antidifference optimization estimation excludes still more error matching points, mainly some road sign points with high feature similarity. The target detection and semantic segmentation are not used when processing the image in real-time here, mainly considering that the difference between vehicle and pedestrian feature descriptors and environmental feature descriptors is relatively large and does not easily cause mutual interference, while the vehicle and pedestrian have been removed when building the map, so the matched feature points in the scene do not fall on the vehicle and pedestrian, but basically on the road, house, and tree. It is better to match the correct point distribution to get a good bit pose solving result.Figure7 gives the matching situation and localization accuracy under five road conditions, and we can see that the feature matching rate is only about 10.37%, and most of the extracted feature points cannot find the corresponding road marker points in the visual point cloud map, although the percentage is relatively small, the number of its matching feature points is sufficient, and the ratio of final matching error to correct matching is about 1 : 4.5. Using localization accuracy can also be around cm, and C and D are still slightly better than the optimized localization accuracy, but the optimized results are more stable and have higher accuracy, as shown in Figure 8. It should be noted that the vertical coordinates of the positional results and the positional results of antidifference optimization in the figure are different between the two. In addition, the left and front components of the position are the planes directly under the system, and the lower component is the elevation direction, where the positional positioned under the camera coordinate system, so the heading angle is the roll angle under the ENU system, and the roll angle is the heading angle, and the pitch angle remains the same.Figure 7
Matching cases and positioning accuracy in five cases.Figure 8
Position accuracy comparison.The above conclusion is only for data, and the same set of data is used for map building and positioning. In practice, map building and localization are often separated; for example, if the map built last month is used for localization at this moment, the point cloud map may not be able to accurately represent the actual road information at this moment due to the change of environmental factors. Whether the feature descriptors can still be matched needs to be studied in one step. With the promotion of crowdsourcing technology in the future, we can potentially achieve incremental real-time updates of the visual point cloud map and add more robust multidimensional feature information and semantic information, which can improve the success rate of localization in real scenarios. However, the average time required is 158 ms and 146 ms, and the maximum time required is 693 ms and 541 ms, accounting for the major part of the time consumed. On this basis, the position loosening combination with SINS can achieve continuous high sampling rate localization. Our positioning results are about 20% more efficient and a bit more accurate compared to others.


## 5. Conclusion

Motion target detection and localization in the dynamic background, the current method cannot be repaired with high precision for different frequency regions in circumferential jump detection and repair, and it is difficult to obtain high-quality data, resulting in large position and attitude errors and unstable speed. That is, the accuracy of indoor dynamic target positioning is low. In this problem, a GNSS/SLAM combination-based high-precision indoor dynamic target positioning method is proposed to preprocess the indoor dynamic observation data, detect the circumferential jump occurring on each frequency, repair it with high precision, obtain high-quality data, and accurately position the indoor dynamic target, which effectively solves the problems existing in the current method. This method effectively solves the problems in current methods and provides the basis for the realization of high precision indoor dynamic target positioning. In the position error test, the error of method 1 is closer to 0, which proves that the positioning of this method is closer to the dynamic target, and the position is more accurate. The position error of method 2 and method 3 is larger due to the large fluctuation of latitude error, longitude error, and altitude error, which means that the positioning accuracy is lower. Because method 1 detects and fixes the observed data for circumferential hops, regardless of whether it is for large or small circumferential hops, it can detect the circumferential hops occurring at each frequency and can fix them with certain accuracy to obtain high-quality data, which makes the position error closer to 0, i.e., it accurately achieves indoor target positioning.



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*Source: 8380869-2021-06-28.xml*