remote sensing Article High-Accuracy Positioning in Urban Environments Using Single-Frequency Multi-GNSS RTK/MEMSIMU Integration Tuan Li 1 , Hongping Zhang 1, *, Zhouzheng Gao 2,3 , Qijin Chen 1 and Xiaoji Niu 1 1 2 3 * GNSS Research Center, Wuhan University, 129 Luoyu Road, Wuhan 430079, China; tuanli@whu.edu.cn (T.L.); chenqijin@whu.edu.cn (Q.C.); xjniu@whu.edu.cn (X.N.) School of Land Science and Technology, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, China; zhouzhenggao@whu.edu.cn German Research Centre for Geosciences (GFZ), Telegrafenberg, 14473 Postsdam, Germany Correspondence: hpzhang@whu.edu.cn Received: 18 December 2017; Accepted: 26 January 2018; Published: 30 January 2018 Abstract: The integration of Global Positioning System (GPS) real-time kinematics (RTK) and an inertial navigation system (INS) has been widely used in many applications, such as mobile mapping and autonomous vehicle control. Such applications require high-accuracy position information. However, continuous and reliable high-accuracy positioning is still challenging for GPS/INS integration in urban environments because of the limited satellite visibility, increasing multipath, and frequent signal blockages. Recently, with the rapid deployment of multi-constellation Global Navigation Satellite System (multi-GNSS) and the great advances in low-cost micro-electro-mechanical-system (MEMS) inertial measurement units (IMUs), it is expected that the positioning performance could be improved significantly. In this contribution, the tightly-coupled single-frequency multi-GNSS RTK/MEMS-IMU integration is developed to provide precise and continuous positioning solutions in urban environments. The innovation-based outlier-resistant ambiguity resolution (AR) and Kalman filtering strategy are proposed specifically for the integrated system to resist the measurement outliers or poor-quality observations. A field vehicular experiment was conducted in Wuhan City to evaluate the performance of the proposed algorithm. Results indicate that it is feasible for the proposed algorithm to obtain high-accuracy positioning solutions in the presence of measurement outliers. Moreover, the tightly-coupled single-frequency multi-GNSS RTK/MEMS-IMU integration even outperforms the dual-frequency multi-GNSS RTK in terms of AR and positioning performance for short baselines in urban environments. Keywords: multi-GNSS (GPS/BeiDou/GLONASS); real-time kinematic (RTK); MEMS-IMU; tightly-coupled integration; urban environments; fault detection and exclusion 1. Introduction High-accuracy positioning is an issue for many applications, such as machine control, unmanned aerial vehicles, mobile mapping, etc. For outdoor environments, Global Positioning System (GPS) real-time kinematics (RTK) has been proven to be a reliable and efficient tool because it can provide centimeter-level positioning after correctly resolving the carrier-phase integer ambiguities. Research has shown that rapid ambiguity resolution (AR) can be achieved in open sky conditions with dual-frequency GPS receivers [1], but the high-cost of dual-frequency receivers definitely makes many potential applications impossible. Generally, the single-frequency GPS RTK has a low AR success rate and reliability [2,3]. With the rapid development of multi-GNSS [4], the performance of single-frequency RTK can be improved Remote Sens. 2018, 10, 205; doi:10.3390/rs10020205 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 205 2 of 21 significantly due to the increasing satellite visibility and better spatial geometry, as it was studied in GPS/BDS [1,5,6], GPS/GLONASS [3], GPS/BDS/GLONASS [7], and BDS/GALILEO/QZSS/GPS [8]. In this research, the combination of three main satellite navigation systems, namely the U.S. GPS, Russia’s GLObal NAvigation Satellite System (GLONASS), and the Chinese BeiDou navigation satellite system (BDS), will be considered to enhance the positioning capabilities of single-frequency RTK in urban environments. Currently, both GPS and GLONASS have full constellations of thirty-two and twenty-four medium earth orbit (MEO) satellites, respectively. The Chinese BDS has begun to provide position, velocity, and timing service in the Asia-Pacific region since the end of 2012, with the constellation of five geostationary earth orbit (GEO) satellites, five inclined geo-synchronous orbit (IGSO) satellites, and four MEO satellites [4]. Different from the code division multiple access (CDMA) modulation that is adopted by GPS and BDS, GLONASS uses the frequency division multiple access (FDMA) modulation, i.e., different frequencies for different satellites [2,9]. This FDMA modulation makes GLONASS AR difficult due to the carrier phase inter-frequency bias (IFB) caused by the analog hardware delay and the digital signal processing [10]. The IFB cannot be eliminated in the double-differenced (DD) process like GPS or BDS, thus preventing the integer ambiguity resolution. Recent research has shown that the IFB can be pre-calibrated or estimated in real-time, which makes GLONASS AR possible [11,12]. In recent years, there has been increasing demand for high-accuracy positioning in urban environments. However, the RTK performance degrades in such environments due to the frequent signal blockages and multipath, even with multiple constellations. In order to provide continuous high-accuracy positioning solutions, GNSS is often integrated with INS due to their complementary characteristics [13–16]. Actually, the GPS/INS integrated system has been used in many applications that require high-accuracy position, velocity, and attitude information [14,17]. In the last few years, some researchers have investigated the positioning performance of GPS/INS integration in urban environments [18–20]. These studies were mainly based on pseudo-range and Doppler observations and only meter-level positioning accuracy could be obtained. With the rapid advances in MEMS inertial sensor technology, the low-cost GNSS/INS integration becomes increasingly attractive and suitable for many applications [21,22]. Recently, MEMS-IMUs have been integrated with low-cost GPS receivers to provide an accurate and reliable navigation solution in urban environments [23–25]. Since the GNSS measurements are easily contaminated with large multipath errors, especially for code measurements, the fault detection and exclusion (FDE) algorithm is necessary for integrated GNSS/INS system working in urban environments. Some FDE algorithms have been developed for the GPS/INS system in the literature. For example, quality control for an integrated navigation system using innovations and recursive filtering is proposed in [26]. An innovation-based detection, identification, and adaptation (DIA) procedure in an integrated navigation system is presented in [27]. In [28], a snapshot innovation method is used for improved positioning in foliage environments. Hewitson et al. extended the GNSS receiver autonomous integrity monitoring (RAIM) algorithm to the integrated GNSS/INS systems [29]. Even though these methods are effective to detect and isolate faulty measurements, a highly-reliable FDE algorithm is still needed, especially for carrier-phase-based high-accuracy positioning in GNSS-challenged environments. Although the positioning performance in urban environments has been investigated in some previous studies, they mainly focused on the integration of GPS-only and INS, and few investigated the centimeter-level positioning capabilities in urban environments by combining the multi-GNSS data and low-cost MEMS-IMU data. In this contribution, we investigate the feasibility of high-accuracy positioning in urban environments by using the tightly-coupled integration of single-frequency GPS/BDS/GLONASS RTK and INS. The outlier-resistant AR and Kalman filtering strategy are proposed specifically for the tightly-coupled integration to resist the measurement outliers. A field vehicular experiment was conducted in Wuhan City to evaluate the AR and positioning performance of the proposed algorithm. Comparisons will be conducted with respect to the single- and dual-frequency Remote Sens. 2018, 10, 205 3 of 21 multi-GNSS RTK. In addition, the positioning drifts during real GNSS signal outages instead of simulated outages will be evaluated. The paper is organized as follows: Section 2 presents the tightly-coupled multi-GNSS RTK/INS integration models including the INS dynamic model, measurement model, and single-epoch ambiguity resolution with inertial aiding. The innovation-based outlier-resistant AR and Kalman filtering strategy is also given in this section. Next, the field experiment and data processing strategies are described in Section 3. The corresponding results and discussion are presented in Section 4, followed by a summary of the work and conclusions in Section 5. 2. Methods 2.1. Tightly-Coupled GPS/BDS/GLONASS RTK/INS Integration Model In this research, the Extended Kalman Filter (EKF) is used to implement the tightly-coupled multi-GNSS RTK/INS integration. The EKF directly fuses the multi-GNSS data and IMU data to obtain optimal estimates of the integrated system state. In order to show the tightly-coupled integration algorithm, the INS dynamic model, the measurement model and the single-epoch AR with inertial aiding will be introduced in this section. 2.1.1. INS Dynamic Model In this research, the INS dynamic model is constructed as the ψ-angle error model [30]. In this model, the error analysis is done with respect to the computer (c) frame that is locally levelled at the computed position. The ψ-angle error model can be described as: .c c × δrc + δvc δr = −ωec c + ωc × δvc + δgc + c p δfb δv = fc × ψ − 2ωie ec b . c + ωc × ψ − c p δωb ψ = − ωie ec ib b . . .c (1) . where δr, δv and ψ denote the time derivative of position, velocity, and attitude error vectors, c is the angular rate of e-frame (i.e., Earth-centered, Earth-fixed (ECEF)) with respect respectively; ωie c is the angular rate of c-frame with to the inertial (i) frame, projected to the computer (c) frame; ωec c respect to e-frame, projected to c-frame; δg is the gravity error vector projected in the c-frame; b denote the inertial sensor errors; C p is the rotation matrix from the body (b) frame δfb and δωib b (i.e., forward-right-down (FRD)) to the platform (p) frame. In order to improve the navigation performance of low-cost MEMS inertial sensors, the IMU errors including the bias and scale factor of both the gyroscope and accelerometer are augmented into the filter state and estimated on-line. In the implementation of GNSS/INS integration algorithms, they are generally modeled as first-order Gauss-Markov process [31]. Therefore, the complete error state vector can be described as: h X = (δrc ) T (δvc )T ψ T bTg bTa sTg sTa iT (2) where δrc and δvc are position and velocity errors in the navigation frame (i.e., north-east-down (NED)), respectively; ψ is the attitude error; bg denotes the gyro bias error; ba denotes the accelerometer bias error; sg and sa are the scale factor errors of the gyro and accelerometer, respectively. 2.1.2. Measurement Model In the single-frequency GPS/BDS/GLONASS RTK positioning, the code and carrier phase observations on f 1 frequency (herein GPS L1: 1575.42 MHz; BDS B1: 1562.098 MHz; GLONASS L1: 1602.006 + k × 0.5625 MHz, where k is the corresponding satellite frequency number) will be used together. Since there is no frequency overlap in the three GNSS systems, the double-differencing Remote Sens. 2018, 10, 205 4 of 21 (DD) formulation should be applied within the individual GNSS system, i.e., one reference satellite per system [5]. This is a loosely-coupled way to combine observations from different GNSS systems. The double-differenced code and carrier-phase observation equations of a single GNSS system are given as follows in units of range, and the time stamps are omitted for brevity: ∇∆PR = ∇∆ρ + ∇∆T + ∇∆I + ∇∆ε ρ (3) λ∇∆ϕ = ∇∆ρ + ∇∆T − ∇∆I + λ∇∆N + ∇∆ε ϕ (4) where ∇∆(·) denotes the DD operator; PR and ϕ are code and carrier phase observations, respectively; ρ is the geometric distance in units of meters between the receiver and satellite; T and I denote the tropospheric and ionospheric delay, respectively; λ and N are carrier phase wavelength and integer ambiguity, respectively; ε ρ and ε ϕ are the measurement noise and un-modeled residual error (multipath, etc.) of code and carrier phase observations, respectively. Specifically, for GLONASS carrier phase observations, λ∇∆ϕ and λ∇∆N could be written as follows: λ∇∆ϕ = λk ∆ϕk − λr ∆ϕr (5) λ∇∆N = λk ∆N k − λr ∆N r = λk ∇∆N kr + (λk − λr )∆N r (6) where the superscripts k and r denote the non-reference and reference satellite, respectively; ∆(·) represents the single-differenced (SD) operator. In Equation (6), the SD ambiguity can be estimated with the SD pseudo-range observations [32]. As the observation of low-elevation satellite is generally noisier, the elevation-dependent weight model is adopted to determine the a priori variance for GNSS observations [33]: ( σ02 , ele ≥ π/6 2 σ = (7) (σ0 / sin(ele))2 , else where σ0 is the un-differenced observation standard deviation (STD) at zenith, and ele is the elevation angle. For RTK positioning of short baseline, the tropospheric term T and ionospheric term I in Equations (3) and (4) can be neglected. The remaining unknown parameters that need to be estimated are the baseline increment vector and integer ambiguities. For single-frequency GPS/BDS/GLONASS RTK, the linearized DD observation with unknown parameters can be described in matrix form as follows: " # " #" # " # ερ H 0n × n δpr ∇∆PR − ∇∆r0 = − (8) εϕ H Λ ∇∆N λ∇∆ϕ − ∇∆r0 with iT H G HC H R Λ = diag ΛG ΛC Λ R H= N= h h NG NC NR iT (9) (10) (11) where n is the total number of DD ambiguities; δpr is the baseline increment vector; ∇∆r0 denotes the computed DD ranges with approximate rover coordinates and satellite positions; the superscripts ‘G’, ‘C’, ‘R’ represent GPS, BDS and GLONASS, respectively; H is the design matrix containing the relative receiver-satellite geometry; Λ is the diagonal matrix that contains the wavelengths of f 1 frequencies from the GPS, BDS and GLONASS satellites. For the tightly-coupled integration, the measurement model in the discrete-time form can be expressed as Zk = Hk Xk + Vk (12) Remote Sens. 2018, 10, 205 5 of 21 where Hk is the design matrix, and Zk is the measurement vector with the following formula: " Zk = # ∇∆ρ̂ I NS − ∇∆PR,GNSS ∇∆ρ̂ I NS − λ∇∆ϕGNSS (13) where the subscripts “INS” and “GNSS” represent the INS-predicted ranges and GNSS observations, respectively. The INS-predicted ranges are calculated using the INS-updated position and satellite positions. Since the IMU measurement center and the GNSS antenna phase center cannot be installed at the same place, the lever-arm offset should be considered when fusing the two different kinds of data. The lever-arm correction can be described in the e-frame as: reGNSS = reI MU + Cen Cnb `bGNSS (14) where reGNSS and reI MU are position coordinates in the e-frame for the GNSS rover receiver and IMU center, respectively; Cen is the rotation matrix form the n-frame to the e-frame; Cnb is the rotation matrix form the b-frame to the n-frame; `bGNSS denotes the lever-arm offset vector in the b-frame. After the error perturbation analysis, the position error term between the IMU and GNSS antenna can be written as [34]: h i δreGNSS ≈ δreI MU + Cen Cnb `bGNSS × ψ (15) where × denotes the cross-product operator. Considering that the position error term in the system state vector is expressed in the n-frame, the final design matrix Hk in (12) can be derived from (3), (4), (9), (13), and (15) as: h h i i Hk = H·Cen 0n×3 H·Cen · Cnb `bGNSS × 0n×12 (16) 2.1.3. Single-Epoch Ambiguity Resolution with Inertial Aiding Differential carrier-phase based centimeter-level positioning relies on the correct integer ambiguity resolution, but there are still challenges for reliable ambiguity resolution in urban environments, especially for single-frequency RTK. For the tightly-coupled RTK/INS integration, a priori information from INS can be used to reduce the search space of integer ambiguities, and then improve the AR reliability [35]. In this research, the INS predicted position information is used as a virtual measurement. Assuming that the observation equation is linearized at the INS predicted position, and then the virtual observation equation from INS predicted position can be written as: ε I NS = h I3 × 3 03 × n i " δpr ∇∆N # − 03×1 (17) where I3×3 is the identity matrix. Combining the above equation and (8), the unknown parameters and their corresponding covariance can be obtained through the weighted least-squares estimation: T B WB " δpr ∇∆N # = BT WL (18) with: B= H H I3×3 0n × n λIn×n , W = diag Wρ 03×n Wϕ WINS ∇∆PR − ∇∆r0 , L = λ∇∆ϕ − ∇∆r0 03×1 Remote Sens. 2018, 10, 205 6 of 21 where Wρ , Wϕ, and WINS are the inverse of the covariance matrices of the code, carrier phase, and INS virtual measurements, respectively. Further, the coefficient matrix of the normal Equation (18) can be derived as follows: " # HT Wρ H + HT WϕH + WINS λHT Wϕ T B WB = (19) λWρ λ 2 Wρ Generally, the strength of the normal equation is weak if the code and carrier phase observations are used only because the code measurements are imprecise. With the term WINS in the top-left corner of the above equation, the strength will be enhanced and then the accuracy of the float solution can be improved. Since the precision of INS predicted position is high in the short-term period, this virtual measurement will definitely improve the AR performance. Once the real-valued ambiguities and their corresponding variance-covariance (VC) matrix are obtained, the Least-squares AMBiguity Decorrelation Adjustment (LAMBDA) method is performed to obtain the integer-valued ambiguities [36]. Then, the ambiguity validation test is performed to determine whether the searched ambiguities should be accepted or not. The validation test is very crucial, as incorrect ambiguity resolution will lead to unacceptable positioning result. In practice, the data-driven ratio-test and model-driven bootstrapped success rate are used together for ambiguity validation [37,38], which will be adopted in this research as well. 2.2. Innovation-Based Outlier-Resistant Ambiguity Resolution and Kalman Filtering Strategy In order to keep the optimality of the system state, faulty measurements should be excluded before the Kalman updates. In urban environments, the code and carrier phase observations are susceptible to multipath errors, especially for the code measurements. In the tightly-coupled RTK/INS integration, large code multipath errors will bias the estimates of the float ambiguities dramatically and then reduce the probability of correctly fixing the ambiguities. To reduce the effects of measurement outlier on the parameter estimation, the observation variance should be inflated properly for those with outliers. Under normal operating condition, the measurement innovation sequence of Kalman filtering should be normally distributed with zero mean. When discrepancies in innovation sequence are found, the faulty measurements are detected. In this research, the following innovation-based inflating factor γii is constructed to model the measurements [39]: νek,i ≤ k0 1, e ν | k,i | k1 −k0 γii = × , k0 < νek,i ≤ k1 k e 0 k − ν 1 | k,i | ∞, νek,i > k1 (20) where k0 and k1 are two constants, usually chosen as 2.0–3.0 and 4.5–8.5, respectively; νek,i is the i-th normalized measurement innovation at epoch k, which can be written as: νek,i = q Vk,i (Hk Pk,k−1 HkT + Rk )i,i (21) where Vk,i is the i-th measurement innovation at epoch k; Pk,k−1 is the time update of covariance matrix in the Kalman filtering; Rk is the measurement covariance matrix. In Equation (20), γii is equal to one if the measurement is normal; when the measurement is outlying, γii is infinite, i.e., this gross measurement will be eliminated; when the normalized value is between k0 and k1 , the effect of this measurement on the parameter estimation will be reduced. Remote Sens. 2018, 10, 205 7 of 21 Since the DD measurements are correlated mathematically, the inflated measurement covariance matrix should preserve the original correlation. Therefore, the following equivalent covariance matrix should be used in the measurement update: γ11 σ12 .. Rk = . γn1 σn1 · · · γ1n σ1n .. .. . . 2 · · · γnn σn (22) γii γ jj (23) with: γij = p where γij is the inflating factor of the covariance at i-th row and j-th column. In the ambiguity resolution process, the outlier-resistant scheme will be applied to model the code measurements so that the effects of the abrupt measurement errors on the float ambiguities can be eliminated. In the measurement update stage of Kalman filter, the outlier-resistant filtering will be activated again to model the code or ambiguity-fixed carrier phase observations in case of wrongly accepted ambiguities. With the outlier-resistant ambiguity estimation and filtering, the unbiased float ambiguities and system states can be achieved; otherwise, the system states may become biased or even make the integration filter diverged. Obviously, the two-step outlier-resistant scheme is easy to implement in the framework of RTK/INS integration and little extra computation is required. 2.3. Overview of Multi-GNSS RTK/INS Tightly-Coupled Integration with Innovation-Based FDE According to the description above, an overview of the proposed multi-GNSS RTK/INS integration with innovation-based FDE is shown in Figure 1. After the system initialization, the compensated raw IMU outputs are used in the INS mechanization to provide high data-rate position, velocity and attitude (PVA) information. In the process of INS mechanization, the second-order coning correction term, the rotational and sculling motion effect are considered to weaken their influences on the attitude and velocity update [40,41]. Once the multi-GNSS data from base and rover receivers are available, the double-differenced code and carrier phase observations will be formed within the individual GNSS system. Then, the outlier-resistant ambiguity resolution with INS aiding will be used to resist the code measurement outliers. The LAMBDA method is employed for ambiguity resolution, and a validation process will be used to confirm the correctness of the fixed ambiguities. If the searched ambiguities pass the validation test, the precise ambiguity-fixed carrier phase measurements will be used to update the tightly-coupled integration filter; otherwise, the code observations should be used at this epoch. The measurement inputs of the integration filter is the difference between INS-derived DD ranges and DD code or carrier-phase observations. In the EKF update phase, the outlier-resistant filtering is applied again to model the potential code measurement outlier or the ambiguity-fixed carrier phase measurement in case the incorrectly-fixed ambiguities are accepted. Since the output rate of IMU data is much higher than that of GNSS data, the standalone INS mode will process the IMU data epoch-by-epoch when the GNSS data is not available. Finally, the estimated IMU sensor errors including the biases and scale factors of the gyroscope and accelerometer are fed back to compensate the errors of raw IMU data. Meanwhile, the navigation solution provided by INS mechanization is updated with the estimated PVA errors from the integration filter. measurement in case the incorrectly-fixed ambiguities are accepted. Since the output rate of IMU data is much higher than that of GNSS data, the standalone INS mode will process the IMU data epochby-epoch when the GNSS data is not available. Finally, the estimated IMU sensor errors including the biases and scale factors of the gyroscope and accelerometer are fed back to compensate the errors of raw IMU data. Meanwhile, the navigation Remotesolution Sens. 2018,provided 10, 205 by INS mechanization is updated with the estimated PVA errors from the8 of 21 integration filter. n PINS ∇Δϕ ∇ΔP Figure 1. Implementation innovation-based outlier-resistant resolution and and Kalman Figure 1. Implementation of of thethe innovation-based outlier-resistantambiguity ambiguity resolution Kalman filtering for the tightly-coupled integration of multi-GNSS RTK and MEMS-IMU. filtering for the tightly-coupled integration of multi-GNSS RTK and MEMS-IMU. 3. Field Test Description and Data Processing Strategy 3. Field Test Description and Data Processing Strategy In order to evaluate the performance of the proposed robust single-frequency multi-GNSS In order to evaluate the performance of the proposed robusttest single-frequency RTK/MEMS-IMU integration in urban environments, a field vehicular was carried out inmulti-GNSS Wuhan City, China on integration 12 June 2015.in The trajectory, as shown in 2a, is about 7.0was km in the north-south RTK/MEMS-IMU urban environments, a Figure field vehicular test carried out in Wuhan 7.2 km in the east-west direction. A range of scenarios was incorporated in north-south the test City, direction China onand 12 June 2015. The trajectory, as shown in Figure 2a, is about 7.0 km in the route, including relatively open-sky condition, under trees and overpasses, and sub-dense direction and 7.2 km in the east-west direction. A range of scenarios was incorporated in theurban test route, including relatively open-sky condition, under trees and overpasses, and sub-dense urban canyon with high buildings (Figure 2b). Figure 2c shows the test platform and equipment used in this paper. The inertial data from MEMS grade IMU (POS1100), which consists of three MEMS gyroscopes and three quartz accelerometers, was used in this research. The tactical-grade IMU (POS310) with three fiber optic gyroscopes (FOGs) was used to provide the reference solutions. Both of the two IMUs are provided by Wuhan MaiPu Space Time Technology Company (Wuhan, China), and their output rate are 200 Hz. Their main performance specifications are shown in Table 1. A Trimble NetR9 multi-GNSS receiver (Sunnyvale, CA, USA), as the reference station, was located on the rooftop of the GNSS Research Center at Wuhan University to collect raw GNSS data. The rover receiver (Trimble BD982 OEM board) was fixed on the vehicle during the driving test. The lever arm offset between the phase center of the rover GNSS antenna and the IMU measuring center was accurately measured in advance. In the experiment, the sampling rate of multi-GNSS data in the base and rover station was set to 1 Hz. As shown in Figure 2d, the field test took about 45 min and the vehicle stayed static in the first several minutes. In the data processing, broadcast ephemeris are used to provide satellite clocks and orbits for GPS, BDS, and GLONASS. As the baseline separation is less than 7 km, the tropospheric and ionospheric delay are not considered. Empirically, a priori standard deviations for the un-differenced GPS, BDS, and GLONASS carrier phase observation at the zenith are set to 3 mm. The values for code observations of GPS, BeiDou MEO/IGSO, BeiDou GEO, and GLONASS satellites are 0.35 m, 0.35 m, 0.5 m, and 0.5 m, respectively. In terms of the ambiguity validation, the predefined success rate is set to 0.99, and the critical ratio value is 3.0 for the GPS-only and 2.0 for the combined GNSS system. The main reason why smaller ratio value is applied to the combined GNSS system is that it provides much better satellite geometry and model strength for reliable AR than the single GNSS system [42]. located on the rooftop of the GNSS Research Center at Wuhan University to collect raw GNSS data. The rover receiver (Trimble BD982 OEM board) was fixed on the vehicle during the driving test. The lever arm offset between the phase center of the rover GNSS antenna and the IMU measuring center was accurately measured in advance. In the experiment, the sampling rate of multi-GNSS data in the base station was set to 1 Hz. As shown in Figure 2d, the field test took about 45 min9 and Remoteand Sens.rover 2018, 10, 205 of 21 the vehicle stayed static in the first several minutes. (a) (b) VN VE VD Velocity (m/s) 10 5 0 -5 -10 -15 2500 3000 3500 4000 4500 5000 GPS Time - 43,5000 (sec) (c) 5500 (d) Figure 2. Test description of the land land vehicle vehicle experiment China. (a) Figure 2. Test description of the experiment on on 12 12 June June 2015 2015 in in Wuhan, Wuhan, China. (a) Test Test trajectory; (b) typical scenarios; (c) platform and equipment; and (d) vehicle speed in the field test. trajectory; (b) typical scenarios; (c) platform and equipment; and (d) vehicle speed in the field test. Table 1. Performance specifications of the IMU sensors in the experiment. Table 1. Performance specifications of the IMU sensors in the experiment. IMU Grade IMU Grade POS310 Tactical POS310 Tactical POS1100 MEMS POS1100 MEMS Dimensions (mm) Dimensions (mm) 151 × 120 × 101 151 × 120 81.8 × 68××101 70 81.8 × 68 × 70 Bias Random Walk Bias Random Walk Velocity Angular Gyro. Acce. Gyro. Acce. (°/ ) (mGal) Angular (°/√ ) Velocity ( √/ /√ ) √ (◦ /h) (mGal) (◦ / h) (m/s/ h) 0.5 300 0.05 0.10 0.510.0 300 0.05 0.10 1500 0.33 0.18 10.0 1500 0.33 0.18 In the data processing, broadcast ephemeris are used to provide satellite clocks and orbits for 4. Results GPS, BDS,and andDiscussion GLONASS. As the baseline separation is less than 7 km, the tropospheric and 4.1. Satellite Availability The satellite availability in sub-dense urban environments is investigated at first for the GPS (G), BDS (C) and GLONASS (R). As shown in Figure 3, the PRN numbers between 1 and 32 are for the GPS satellites, between 33 and 67 for the BDS, and between 68 and 91 for the GLONASS. Currently, the constellations of GPS and GLONASS consist of MEO satellites only, whereas the BDS constellation is comprised of GEO, IGSO, and MEO satellites. In this field test, there are four GEO satellites (C33, C35, C36, and C37) and five IGSO satellites (C38–C42) tracked, but no MEO satellites were tracked. It can be seen that some satellites’ tracking conditions are very poor due to the high buildings and overpasses in the urban environments wherein the longest partial GNSS outage lasted about 2 min with only 1–2 satellites tracked. Obviously, the multi-GNSS provides many more available satellites than the GPS only, which will definitely bring benefits for RTK positioning in urban environments. BDS (C) and GLONASS (R). As shown in Figure 3, the PRN numbers between 1 and 32 are for the GPS satellites, between 33 and 67 for the BDS, and between 68 and 91 for the GLONASS. Currently, the constellations of GPS and GLONASS consist of MEO satellites only, whereas the BDS constellation is comprised of GEO, IGSO, and MEO satellites. In this field test, there are four GEO satellites (C33, C35, C36, and C37) and five IGSO satellites (C38–C42) tracked, but no MEO satellites were tracked. It can be seen that some satellites’ tracking conditions are very poor due to the high Remote Sens. 2018, 10, 205 10 of 21 buildings and overpasses in the urban environments wherein the longest partial GNSS outage lasted about 2 min with only 1–2 satellites tracked. Obviously, the multi-GNSS provides many more available satellites than the GPS only, which will definitely bring benefits for RTK positioning in The number of satellites and the corresponding position dilution of precision (PDOP) of GPS, urban environments. ◦ cut-off(PDOP) number of satellites the corresponding of GPS, GPS + BDS (G The + C), and GPS + BDSand + GLONASS (G +position C + R) dilution with a of 15precision elevation angle in the GPS + BDS (G + C), and GPS + BDS + GLONASS (G + C + R) with a 15° cut-off elevation angle in the rover station are depicted in Figure 4a,b, respectively. rover station are depicted in Figure 4a,b, respectively. It can be seen 4 that thethe satellite and PDOP can be improved It can from be seenFigure from Figure 4 that satelliteavailability availability and PDOP can be improved visibly byvisibly by using the combined GPS/BDS and GPS/BDS/GLONASS systems. According to the from using the combined GPS/BDS and GPS/BDS/GLONASS systems. According to the statisticsstatistics from Figure 4, the average number of satellites of GPS, GPS + BDS, and GPS + BDS + GLONASS are 6.0, Figure 4, the average number of satellites of GPS, GPS + BDS, and GPS + BDS + GLONASS are 6.0, 12.3, 12.3, and 16.5, and the corresponding average PDOPs are 3.12, 2.05, and 1.71, respectively. Compared and 16.5, and the corresponding average PDOPs are 3.12, 2.05, and 1.71, respectively. Compared with with the GPS, the PDOP improvements of the multi-GNSS are about 34.3–45.2%. Obviously, the the GPS, the PDOP improvements the multi-GNSS are about are 34.3–45.2%. Obviously, the average average number of available of satellites in urban environments less than that in open-sky number ofconditions. available satellites in urban environments are less than that in open-sky conditions. 90 80 Satellite PRN number 70 GLONASS: [68, 91] BeiDou: [33, 67] GPS: [1, 32] 60 50 40 30 20 10 0 3000 3500 4000 4500 GPS Time - 43,5000 (sec) 5000 Remote Sens. 2018, 10, x FOR PEER REVIEW 10 of 22 Figure 3. Satellite availability of GPS (1 ≤ PRN ≤ 32), BeiDou (33 ≤ PRN ≤ 67), and GLONASS Figure 3. Satellite availability of GPS (1 ≤ PRN ≤ 32), BeiDou (33 ≤ PRN ≤ 67), and GLONASS (68 (68 ≤ PRN ≤ 91) with a 15◦ cut-off elevation angle. ≤ PRN ≤ 91) with a 15° cut-off elevation angle. GPS G+C GPS G+C+R 25 G+C G+C+R 30 20 PDOP Number of satellites 30 15 10 20 10 5 0 2500 3000 3500 4000 4500 GPS Time - 43,5000 (sec) 5000 3000 3500 4000 4500 GPS Time - 43,5000 (sec) (a) 5000 (b) Figure 4. The number of available satellites and PDOP of GPS (G), GPS/BDS (G + C), and Figure 4. The number of available satellites and PDOP of GPS (G), GPS/BDS (G + C), and GPS/BDS/ GPS/BDS/GLONASS (G + C + R) with a 15° cut-off elevation angle in the rover station. (a) Number of GLONASS (G + C + R) with a 15◦ cut-off elevation angle in the rover station. (a) Number of satellites; satellites; and (b) PDOP. and (b) PDOP. 4.2. Multi-GNSS RTK/INS Integration with Innovation-Based FDE 4.2. Multi-GNSS RTK/INS Integration with Innovation-Based FDE One of the key issues for GNSS-based high-accuracy positioning in urban environments is the fault detection exclusion because the GNSS observations are more likely to contain measurement One of the keyand issues for GNSS-based high-accuracy positioning in urban environments is the outliers. As the code measurements can have very large multipath error, they should be detected and fault detection and exclusion because the GNSS observations are more likely to contain measurement excluded in the data processing. As stated previously, the measurement innovations of Kalman outliers. As the code measurements can have very large multipath error, they should be detected filtering should be distributed with zero mean and there will be discrepancies for those and excluded in the data processing. As stated previously, the measurement innovations of Kalman measurements with outliers. Figure 5 depicts the code innovations for all available GPS + BDS + filtering should be distributed with zero mean and there will be discrepancies for those measurements GLONASS satellites with a 15° cut-off elevation angle. It can be seen that very large code with outliers. Figure 5 depicts the code innovations forare alldetected availableinGPS + BDS +filter’s GLONASS satellites measurement outliers (shown in three red circles) the Kalman innovation sequence of DD code observation. A zoom-in window is given to show the spread of those code innovations with small magnitudes. It shows that most of the code innovations are within ±2 m, but there are still small discrepancies. There is no doubt that these code measurements with abrupt outliers will affect ambiguity resolution and positioning accuracy if no effective measures are taken. 4 Figure 4. The number of available satellites and PDOP of GPS (G), GPS/BDS (G + C), and GPS/BDS/GLONASS (G + C + R) with a 15° cut-off elevation angle in the rover station. (a) Number of satellites; and (b) PDOP. 4.2. Multi-GNSS RTK/INS Integration with Innovation-Based FDE One of the key issues for GNSS-based high-accuracy positioning in urban environments is the Remote Sens. 2018, 10,detection 205 fault and exclusion because the GNSS observations are more likely to contain measurement 11 of 21 outliers. As the code measurements can have very large multipath error, they should be detected and excluded in the data processing. As stated previously, the measurement innovations of Kalman filteringelevation should be angle. distributed with there willcode be discrepancies for those with a 15◦ cut-off It can bezero seenmean thatand very large measurement outliers (shown measurements with outliers. Figure 5 depicts the code innovations for all available GPS + BDS + in three red circles) are detected in the Kalman filter’s innovation sequence of DD code observation. GLONASS satellites with a 15° cut-off elevation angle. It can be seen that very large code A zoom-in window is given show of are those codein innovations with small magnitudes. measurement outliers to (shown in the threespread red circles) detected the Kalman filter’s innovation of DD observation. A zoom-in window is show the spread of those code It shows thatsequence most of thecode code innovations are within ±given 2 m, tobut there are still small discrepancies. innovations with small magnitudes. It shows that most of the code innovations are within ±2 m, but There is no doubt that these code measurements with abrupt outliers will affect ambiguity resolution there are still small discrepancies. There is no doubt that these code measurements with abrupt and positioning accuracy no effective measures are taken. outliers will affectifambiguity resolution and positioning accuracy if no effective measures are taken. 4 2 0 -2 -4 20 Code Innovation (m) Zoom in 0 -20 -40 Large Outlier -60 -80 3000 3500 4000 4500 GPS Time - 43,5000 (sec) 5000 Figure 5. Kalman filter’s innovation sequence of double-differenced code observation for all available GPS + BDS + GLONASS satellites with a 15◦ cut-off elevation angle. Each color denotes a different satellite pair. The points in three red circles are very large code measurement outliers. Remote Sens. 2018, 10, x FOR PEER REVIEW 11 of 22 Figure 6 givesFigure an example about the effects code measurement outlier the estimation of float 5. Kalman filter’s innovation sequence of of double-differenced code observation for allon available GPS + BDS + GLONASS satellites with a 15° cut-off elevation angle. Each color denotes a different ambiguities. It shows the ambiguity biases with and without innovation-based FDE in the presence satellite pair. The points in three red circles are very large code measurement outliers. of one code measurement outlier on a GLONASS satellite whose code innovation is –19.8 m (given Figure 6 gives example about the effects of code as measurement outlier onbetween the estimation by the integration filter). The an ambiguity bias is defined the difference theofestimated float float ambiguities. It shows the ambiguity biases with and without innovation-based FDE in the ambiguity and presence its trueofinteger value. If the ambiguity biassatellite is closer zero, it is more one code measurement outlier on a GLONASS whoseto code innovation is –19.8likely to fix the m (given by the integration filter). The ambiguity bias is defined as the difference between the ambiguity correctly. It can be seen that the ambiguity biases are within 0.15 cycles, except the satellite estimated float ambiguity and its true integer value. If the ambiguity bias is closer to zero, it is more with the outlier likely (about 0.26 cycles), when the innovation-based FDE strategy is applied. By comparison, to fix the ambiguity correctly. It can be seen that the ambiguity biases are within 0.15 cycles, except theare satellite withlarger the outlier (about 0.26 cycles), when innovation-based FDE strategy is is even more the ambiguity biases much without FDE and thethemaximum ambiguity bias applied. By comparison, the ambiguity biases are much larger without FDE and the maximum than 0.6 cycles, ambiguity which will prevent correct ambiguity fixing. bias is even more than 0.6 cycles, which will prevent correct ambiguity fixing. 0.7 Ambiguity bias (cycle) 0.6 0.5 With FDE Without FDE Satellite with outlier 0.4 0.3 0.2 0.1 0 3 14 16 26 29 32 33 37 38 39 40 41 42 69 75 77 78 79 87 Satellite PRN number Figure 6. Ambiguity biases with and without innovation-based FDE in the presence of one code Figure 6. Ambiguity biases and without FDE in the of one code measurement outlierwith on a GLONASS satellite innovation-based at epoch 439,618 s (The PRN number is 69presence with an elevation and the DDsatellite code innovation given 439,618 by the integration is −19.8 m). is 69 with an elevation measurement outlier onofa21.4°, GLONASS at epoch s (Thefilter PRN number of 21.4◦ , and theInDD innovation given by the integration filter is presence −19.8 m). ordercode to verify the feasibility of high-accuracy positioning in the of measurement outliers, Figure 7 shows the root-mean-square (RMS) of code innovations of the tightly-coupled GPS/BDS/GLONASS/INS integration and the AR fixing differences with and without innovationbased FDE. The epochs whose AR fixing difference is equal to zero (shown in red) means that the fixing states of these epochs are the same, i.e., both fixed or float. The epochs that can be fixed only with FDE are depicted with green circles. It can be seen that the ambiguity-fixed solution can be obtained in some cases even though a large code measurement outlier is present. Clearly, the innovation-based FDE strategy is effective to resist the code measurement outliers and, thus, improve the ambiguity resolution performance. Remote Sens. 2018, 10, 205 12 of 21 In order to verify the feasibility of high-accuracy positioning in the presence of measurement outliers, Figure 7 shows the root-mean-square (RMS) of code innovations of the tightly-coupled GPS/BDS/GLONASS/INS integration and the AR fixing differences with and without innovation-based FDE. The epochs whose AR fixing difference is equal to zero (shown in red) means that the fixing states of these epochs are the same, i.e., both fixed or float. The epochs that can be fixed only with FDE are depicted with green circles. It can be seen that the ambiguity-fixed solution can be obtained in some cases even though a large code measurement outlier is present. Clearly, the innovation-based FDE strategy is effective to resist the code measurement outliers and, thus, improve the ambiguity resolution performance. Since the ambiguity is resolved in the single-epoch mode, the integrated filter will be insensitive to the frequent cycle slips in urban environments. In the filter’s measurement update stage, the outlier-resistant Kalman filtering strategy will be applied for the code or ambiguity-fixed carrier-phase measurements. For carrier-phase based high-accuracy positioning, the ambiguity-fixed Remote Sens. 2018, 10, x FOR PEER REVIEW 12 of 22 carrier phase residual can indicate the correctness of ambiguity resolution and positioning accuracy. Figures 8 and 9 show 18the DD carrier phase residual and the corresponding RMS of all the available satellites of the tightly-coupled GPS/BDS/GLONASS/INS integration, respectively. RMS of code innovation Remote Sens. 2018, 10, x FOR PEER REVIEW 12 of 22 Both fixed or float (with and without FDE) RMSFixed of codeonly innovation with FDE 18 14 16 12 RMS of code innovation (m) and AR fixing difference RMS of code innovation (m) and AR fixing difference 16 10 12 8 10 6 8 4 2 Both fixed or float (with and without FDE) Fixed only with FDE 14 6 4 2 0 0 3000 3000 3500 4000 3500 4000 45004500 GPSTime Time --43,5000 (sec) GPS 43,5000 (sec) 5000 5000 Figure 7. RMS of code innovations for the tightly-coupled GPS/BDS/GLONASS/INS integration and Figure 7. RMS of code innovations for the tightly-coupled GPS/BDS/GLONASS/INS integration and the AR fixing difference with without FDE. Figure 7. RMS of code innovations forandthe tightly-coupled GPS/BDS/GLONASS/INS integration and the AR fixing difference with and without FDE. the AR fixing difference with and without FDE. 0.06 L1 Carrier Phase Residual (m) L1 Carrier Phase Residual (m) 0.06 0.04 0.04 0.02 0.02 0 0 -0.02 -0.04 -0.02 -0.04 -0.06 -0.08 -0.06 3000 3500 4000 4500 GPS Time - 43,5000 (sec) 5000 Figure 8. DD carrier phase residual for the tightly-coupled GPS/BDS/GLONASS/INS integration. -0.08 Each color denotes a different satellite pair. (Measurement noise, multipath, and atmospheric delay are included). 3000 3500 4000 4500 5000 GPS - 43,5000 (sec) are below 10 cm for all the It can be seen from Figure 8 that the DDTime carrier phase residuals available satellites and most of them are within ±2 cm. The obvious discrepancies of some residuals Figure 8. be DD carrier for the tightly-coupled GPS/BDS/GLONASS/INS may mainly duephase to the residual un-modeled effects. The corresponding RMSs of carrierintegration. phase Figure 8. DD carrier phase residual for themultipath tightly-coupled GPS/BDS/GLONASS/INS integration. Each residuals color denotes a different satellite (Measurement multipath, and atmospheric are below 20 mm, except forpair. one BDS IGSO satellitenoise, and one GLONASS satellite (shown indelay Each colorare denotes a different satellite pair. (Measurement noise, multipath, and atmospheric delay Figure 9). The relatively large residuals of these two satellites may be caused by low-elevation included). are included). multipath. It can be seen from Figure 8 that the DD carrier phase residuals are below 10 cm for all the available satellites and most of them are within ±2 cm. The obvious discrepancies of some residuals may be mainly due to the un-modeled multipath effects. The corresponding RMSs of carrier phase residuals are below 20 mm, except for one BDS IGSO satellite and one GLONASS satellite (shown in Figure 9). The relatively large residuals of these two satellites may be caused by low-elevation multipath. Remote Sens. 2018, 10, 205 13 of 21 It can be seen from Figure 8 that the DD carrier phase residuals are below 10 cm for all the available satellites and most of them are within ±2 cm. The obvious discrepancies of some residuals may be mainly due to the un-modeled multipath effects. The corresponding RMSs of carrier phase residuals are below 20 mm, except for one BDS IGSO satellite and one GLONASS satellite (shown in Figure 9). The relatively large residuals of these two satellites may be caused by low-elevation multipath. Remote Sens. 2018, 10, x FOR PEER REVIEW 13 of 22 RMS of carrier phase residual (mm) 30 25 20 15 10 5 0 3 14 16 26 29 31 32 33 35 36 37 38 39 40 41 42 69 75 77 78 79 87 88 Satellite PRN number Figure 9. RMS of the DD carrier phase residual for the tightly-coupled GPS/BDS/GLONASS/INS integration. Figure 9. RMS of the DD carrier phase residual for the tightly-coupled GPS/BDS/GLONASS/ INS integration. 4.3. Single-Epoch Ambiguity Resolution and Positioning In order to assess the AR and positioning performance of the single-frequency multi-GNSS RTK/INS tightly-coupled integration algorithm in urban environments, the reference trajectory was 4.3. Single-Epoch Ambiguity Resolution and Positioning generated firstly by using the measurements of a tactical-grade POS310 IMU and dual-frequency GPS/BDS/GLONASS data in the tightly-coupled RTK/INS integration mode with Rauch-Tung- In order to assess the and[43]. positioning performance ofenvironments, the single-frequency multi-GNSS Striebel (RTS)AR smoothing Since frequent loss-of-lock occurs in urban we focus on the single-epoch ambiguity resolution and positioning performance in this research. This method has RTK/INS tightly-coupled integration algorithm in urban environments, the reference trajectory the advantage that the positioning results are insensitive to the cycle slips. The ambiguity resolution and positioning performance of singleand dual-frequency RTK and multi-GNSS) willPOS310 IMU and was generated firstly by using the measurements of(single a GPS tactical-grade also be investigated and their results will be compared with the solution of the tightly-coupled singledual-frequency GPS/BDS/GLONASS thethetightly-coupled RTK/INS integration mode with frequency RTK/INS integration.data In orderin to show difference of RTK with and without INS aiding, the positioning results of the tightly-coupled RTK/INS integration are excluded at those epochs when Rauch-Tung-Striebelthe (RTS) smoothing [43]. Since frequent loss-of-lock occurs in urban environments, GNSS-only solution is not available (e.g., the number of satellites is less than four for GPS). We first evaluate the AR and positioning performance GPS, GPS + BDS (G + C), and GPS we focus on the single-epoch ambiguity resolution andof the positioning performance in this research. + BDS + GLONASS (G + C + R) with a customary cut-off elevation angle of 15°. Figure 10 shows the This method has the advantage that the positioning results are insensitive to the cycle slips. time series of position difference in the north, east, and vertical directions for single-frequency RTK (GPS, G + C, G + C + R, top two rows), dual-frequency RTK (GPS, G + C, G + C + R, middle two rows), The ambiguity resolution and positioning performance of single- and dual-frequency RTK (single GPS and single-frequency RTK/INS integration (GPS, G + C, G + C + R, bottom two rows). The correctlyfixedalso solutions, solutions, float solutions are shown red, and grey,with the solution of and multi-GNSS) will be incorrectly-fixed investigated andandtheir results will inbegreen, compared respectively. The zoom-in windows are provided for both the horizontal (north, east) scatterplots and the tightly-coupled single-frequency RTK/INS integration. Insolutions. order The to correctness show the vertical (down) time series to show the details of the correctly-fixed of difference of RTK ambiguity fixing is checked according to the positioning difference between the fixed solution and with and without INS aiding, the positioning results of the tightly-coupled RTK/INS integration are the reference solution. If the position difference is larger than 0.1 m in the north or east components, or 0.15 m in the vertical direction, the integer ambiguitiesis willnot be considered as incorrectly excluded at those epochs when the GNSS-only solution available (e.g., fixed. the number of satellites is It can be seen from Figure 10 that the AR and positioning performance of single-frequency GPSless than four for GPS). only RTK is very poor, indicating that it is difficult to use single-frequency GPS RTK for highaccuracy kinematic positioning in urban environments. The addition of BDS increases the AR We first evaluate the ARdramatically, and positioning performance the GPS, + BDS (G + C), and GPS performance and the inclusion of GLONASSof together further GPS improves the withathecustomary single-frequency cut-off GPS RTK, elevation the dual-frequency GPS RTK + BDS + GLONASS performance. (G + C +Compared R) with angle of can 15◦ . Figure 10 shows obviously improve the positioning performance due to the doubled observations on another the time series of position difference inis worse the than north, east, andmulti-GNSS vertical directions for single-frequency frequency, but its performance the single-frequency RTK. Therefore, better satellite availability from multi-GNSS brings benefits for single-frequency RTK in urban RTK (GPS, G + C, Genvironments. + C + R,Significantly, top twotherows), dual-frequency RTK (GPS, G + C, AR and positioning performance of dual-frequency multi-GNSS G + C + R, middle RTK are further improved with a substantial decrease of ambiguity-float two rows), and single-frequency RTK/INS integration (GPS, G +solutions C, Gin+comparison C + R, bottom two rows). with the single-frequency multi-GNSS RTK. The correctly-fixed solutions, incorrectly-fixed solutions, and float solutions are shown in green, red, and grey, respectively. The zoom-in windows are provided for both the horizontal (north, east) scatterplots and vertical (down) time series to show the details of the correctly-fixed solutions. The correctness of ambiguity fixing is checked according to the positioning difference between the fixed solution and the reference solution. If the position difference is larger than 0.1 m in the north or east components, or 0.15 m in the vertical direction, the integer ambiguities will be considered as incorrectly fixed. It can be seen from Figure 10 that the AR and positioning performance of single-frequency GPS-only RTK is very poor, indicating that it is difficult to use single-frequency GPS RTK for high-accuracy kinematic positioning in urban environments. The addition of BDS increases the AR performance dramatically, and the inclusion of GLONASS together further improves the performance. Compared with the single-frequency GPS RTK, the dual-frequency GPS RTK can obviously improve the positioning performance due to the doubled observations on another frequency, but its performance is worse than the single-frequency multi-GNSS RTK. Therefore, better satellite availability from Remote Sens. 2018, 10, 205 14 of 21 multi-GNSS brings benefits for single-frequency RTK in urban environments. Significantly, the AR and positioning performance of dual-frequency multi-GNSS RTK are further improved with a substantial decrease of ambiguity-float solutions in comparison with the single-frequency multi-GNSS RTK. The results from Figure 10 also show that the ambiguity resolution and positioning performance of the single-frequency multi-GNSS RTK/INS integration can be significantly improved with fewer ambiguity-float solutions than the corresponding single- and dual-frequency RTK solutions. Compared with single-frequency GPS RTK, the corresponding tightly-coupled GPS RTK/INS integration has only small improvements in AR performance, but the accuracy and stability of the ambiguity-float position series improves significantly due to the short-term accuracy and strong constraints of the INS. For kinematic positioning in urban environments, this is one of the most important advantages of the integrated GNSS/INS system. In case of RTK positioning, the ambiguity-float position error can be large because it mainly depends on the quality and precision of the code measurements. In Table 2, we provide the corresponding statistical information in terms of RMS of position difference of the float and correctly-fixed single-epoch solution, positioning availability and fixing rate of GPS, GPS + BDS, and GPS + BDS + GLONASS. The position availability, i.e., the percentage of all epochs that the RTK positioning, is available to the total number of epochs. The AR fixing rate for full ambiguity resolution was computed by: PFR = number o f correctly f ixed epochs total number o f epochs (24) For the single-epoch-based kinematic positioning, this fixing rate directly reflects the high-accuracy positioning availability. It can be seen from Table 2 that the positioning availability is increased from 85.5% of the GPS to 91.8% of the GPS + BDS and 92.4% of the GPS + BDS + GLONASS, respectively. The fixing rate of the single-frequency GPS RTK is only 0.1%. By comparison, the fixing rate of single-frequency GPS + BDS and GPS + BDS + GLONASS RTK increases to 25.1% and 44.7%, respectively. Obviously, the dual-frequency multi-GNSS RTK greatly improves the AR performance with fixing rates of 75.8% and 76.7% for the GPS + BDS and GPS + BDS + GLONASS systems, respectively. The dual-frequency GPS-only RTK has a low fixing rate of 10.4% due to the poor satellite geometry (mean number of satellites is 6.0). The single-frequency multi-GNSS RTK/INS integration shows better AR performance with fixing rate of 86.1% for both the GPS + BDS and GPS + BDS + GLONASS system. This means that centimeter-level positioning accuracy is available over 86% of the time in the field test with the tightly-coupled single-frequency multi-GNSS RTK/INS integration. Similar to the dual-frequency GPS-only RTK, the fixing rate of single-frequency GPS RTK/INS integration is also very low (only 5.8%). Table 2. The positioning availability (PA, %), RMS of positioning differences (north, east, down) of the float and correctly-fixed single-epoch solution, and fixing rate PFR (%) for different system configurations with a 15◦ cut-off elevation angle. System G PA G+C 85.5 G+C+R 91.8 92.4 RMS (cm) N E D FR N E D FR N E D FR L1 RTK Fixed/Float 0.35 105.9 0.45 97.2 1.58 179.0 0.1 1.27 74.42 2.21 76.27 2.09 180.9 25.1 0.35 83.54 0.38 86.67 1.13 188.9 44.7 L1 + L2 RTK Fixed/Float 0.72 91.19 0.74 90.88 1.47 147.1 10.4 0.57 81.66 0.62 95.23 1.59 170.9 75.8 0.57 86.83 0.56 91.90 1.41 143.9 76.7 L1 + INS Fixed/Float 1.60 58.40 1.60 34.94 1.45 85.61 5.8 0.55 23.54 0.65 37.35 1.79 53.61 86.1 0.53 16.44 0.57 27.78 1.50 44.81 86.1 The results from Table 2 also show that the RMSs of the position differences of the correctly-fixed solutions for the GPS, GPS + BDS, and GPS + BDS + GLONASS are all within 3 cm in the north, Remote Sens. 2018, 10, 205 15 of 21 east, and vertical directions. Noticeably, the RMSs of the float solutions of the tightly-coupled single-frequency RTK/INS integration are much smaller than that of the single- and dual-frequency RTK. We also notice that some RMS values of the L1-RTK fixed solutions are smaller than that of L1/L2-RTK. The main reason for this is that the fixing rate of L1-RTK is much lower and the majority of these fixed solutions are obtained under relatively good observation conditions with small PDOP and multipath error. Therefore, the precision of these fixed solutions can be very high and the RMS value of them is smaller. Similarly, the reason why some RMS values of L1/L2-RTK float solutions Remote Sens. 2018, 10, x FOR PEER REVIEW 15 of 22 is greater than that of L1 RTK is that the percentage of L1/L2 float solutions is much lower and the multipath error. Therefore, the precision of these fixed solutions can be very high and the RMS value of them isof smaller. Similarly, the reason why some RMS valuespoor of L1/L2-RTK solutions is greatersatellites. Overall, observation condition these epochs is generally very withfloat fewer visible than that of L1 RTK is that the percentage of L1/L2 float solutions is much lower and the observation the L1 RTK can obtain fixed solutions in good condition while L1/L2 RTK can condition of these epochs is generally very poor with fewer visible satellites. Overall, the L1obtain RTK can fixed solutions in obtain fixed solutions in good condition while L1/L2 RTK can obtain fixed solutions in more more challenged cases. challenged cases. 0.05 0 0 -5 0 5 East(m) 0 -10 3000 4000 5000 3000 -0.05 -0.05 0 0.05 -5 -5 North(m) 2 3000 5000 0 -0.05 -0.05 0 0.05 2 3000 0 3000 4000 5000 GPS Time - 43,5000 (sec) 0 -2 -0.05 -0.05 0 0.05 -5 0 5 East(m) 0 -10 5000 3000 4000 5000 L1 G+C+R+INS(15°) 2 L1 G+C+INS(15°) 0 -0.05 -0.05 0 0.05 0 East(m) 5000 0 0 -5 0.05 -2 -2 4000 0.05 5 0.05 0 -0.05 10 4000 0 0.05 0 -0.05 2 -2 0 5 East(m) 0 -10 Down (m) 0.05 0 -0.05 2 3000 L1+L2 G+C+R(15°) -0.05 -0.05 0 0.05 2 0.05 0 East(m) 0 -5 L1 GPS+INS(15°) -2 -2 Down (m) 4000 0 0.05 0 0.05 0 -0.05 10 0 -10 5 -5 0 5 East(m) Down (m) Down (m) 0.05 0 -0.05 10 -10 5000 Down (m) 0 North(m) 0.05 0 4000 0 5 East(m) 0 L1+L2 G+C(15°) North(m) North(m) L1+L2 GPS(15°) 5 0 -0.05 -0.05 0 0.05 0.05 0 -0.05 10 0 -10 0.05 0 -5 0 5 East(m) Down (m) Down (m) 0.05 0 -0.05 10 North(m) -5 5 -0.05 -0.05 0 0.05 -5 -0.05 -0.05 0 0.05 -5 -5 0.05 0 -0.05 10 Down (m) 5 North(m) 0 0.05 -0.05 -0.05 0 0.05 0.05 0 -0.05 2 3000 4000 5000 GPS Time - 43,5000 (sec) 0 0 -2 -2 2 Down (m) 0.05 0 North(m) North(m) 5 L1 G+C+R(15°) North(m) L1 G+C(15°) L1 GPS(15°) 0 East(m) 2 0 -2 3000 4000 5000 GPS Time - 43,5000 (sec) Figure 10. Time series of position difference in the north, east, and vertical directions for GPS (1st column), GPS + BDS (G + C, 2nd column) and GPS + BDS + GLONASS (G + C + R, 3rd column) with a 15◦ cut-off elevation angle. The top two rows are single-frequency RTK (GPS, G + C, and G + C + R). The middle two rows are dual-frequency RTK (GPS, G + C, and G + C + R). The bottom two rows are the single-frequency RTK/INS integration (GPS, G + C, and G + C + R). The correctly-fixed solutions are shown in green, incorrectly-fixed solutions in red, and float solutions in grey. A zoom-in window is provided to show the details of the correctly fixed solutions in the horizontal (north, east) and vertical (down) time-series. Note the different scale for the tightly-coupled RTK/INS integration. Remote Sens. 2018, 10, 205 16 of 21 For the high-accuracy positioning in urban environments, the measurements are easily affected by multipath outliers, especially for those from low-elevation satellites. These measurement outliers will make reliable AR impossible and bias the final positioning result. With the greatly increased observations from multi-GNSS, the high-accuracy positioning with higher cut-off elevation angles becomes possible. Therefore, we also investigate the positioning capabilities of the combined GPS, BeiDou, and GLONASS systems with elevation cut-off angles of 25◦ , 30◦ , and 35◦ . Figure 11 shows the time series of the position differences in the north, east, and vertical directions for single-frequency RTK (1st column), dual-frequency RTK (2nd column), and single-frequency RTK/INS integration (3rd column), respectively. The first two rows are for a 25◦ cut-off elevation, while the middle two rows and last two rows are for elevation cut-off angles of 30◦ and 35◦ , respectively. The results from Figure 11 indicates that the AR and positioning performance of dual-frequency GPS/BDS/GLONASS RTK are much better than that of the corresponding single-frequency RTK, especially under the higher cut-off elevation angle. Meanwhile, the performance of the dual-frequency multi-GNSS RTK does not degrade obviously with the increasing cut-off elevation angles, while this is not the case for the single-frequency multi-GNSS RTK. In comparison with dual-frequency multi-GNSS RTK, the performance of the tightly-coupled single-frequency multi-GNSS RTK/INS integration is improved further with fewer ambiguity-float solutions and a smaller positioning error. Generally, the quality of the measurements from high-elevation satellites are better than that from low-elevation satellites. However, the position error of ambiguity-float RTK can also be very large with high elevation cut-off angles. Table 3 shows the corresponding AR and positioning performance of the combined GPS, BDS, and GLONASS system with elevation cut-off angles of 25◦ , 30◦ , and 35◦ . It includes the fixing rate and RMSs of positioning differences of the float and correctly-fixed single-epoch solution. It can be seen from Table 3 that the fixing rate of single-frequency GPS + BDS + GLONASS RTK is 36.1% with a 25◦ cut-off elevation angle, and it drops to 20.3% with a 30◦ cut-off elevation angle and to only 14.7% with a 35◦ cut-off elevation angle. By comparison, the dual-frequency GPS + BDS + GLONASS RTK achieves much higher AR fixing rates of 77.7%, 74.3%, and 71.5% for cut-off elevation angles of 25◦ , 30◦ , and 35◦ , respectively. For the tightly-coupled single-frequency GPS + BDS + GLONASS RTK/INS integration, further improvements of 8.5%, 10.3%, and 10.7% can be obtained with cut-off elevation angles of 25◦ , 30◦ , and 35◦ , respectively. The results in Table 3 also indicate that centimeter-level poisoning accuracy is available once the ambiguities are correctly fixed. The RMSs of position differences of the correctly-fixed solutions with three different cut-off elevation angles are all within 3 cm in the north-east-down components. The RMSs of the ambiguity-float solutions of the tightly-coupled RTK/INS integration are all within 1 m in the north, east, and vertical directions. By contrast, the RMSs of the corresponding ambiguity-float RTK solutions can reach more than 2 m in the vertical direction. Table 3. RMSs of positioning difference (north, east, down) of the float and correctly-fixed single-epoch solution and fixing rate PFR (%) of the GPS + BDS + GLONASS system with higher cut-off elevation angles. Cut-off (◦ ) 25 30 35 RMS (cm) N E D FR N E D FR N E D FR L1 RTK Fixed/Float 0.35 78.44 0.44 88.31 1.33 206.3 36.1 0.41 81.30 0.52 87.39 1.95 223.9 20.3 0.46 81.14 0.54 87.75 2.07 226.4 14.7 L1 + L2 RTK Fixed/Float 0.50 82.56 0.56 100.9 1.56 183.5 77.7 0.60 102.8 0.73 102.5 2.28 221.2 74.3 0.55 107.9 0.76 103.4 2.60 230.4 71.5 L1 + INS Fixed/Float 0.54 20.35 0.62 32.28 1.75 43.03 86.2 0.76 35.47 0.78 50.04 2.51 67.94 84.7 0.78 36.00 0.77 47.65 2.37 67.59 82.2 Remote Sens. 2018, 10, 205 17 of 21 Remote Sens. 2018, 10, x FOR PEER REVIEW 17 of 22 0 -5 -0.05 -0.05 0 0.05 -5 0 5 East(m) 0 -10 -5 0.05 0 -0.05 10 0 -10 5000 0 0 -0.05 -0.05 0 0.05 -5 -5 3000 4000 -0.05 -0.05 0 0.05 -5 Down (m) 0.05 0 -0.05 10 0 -10 0 5 East(m) 0 -0.05 -0.05 0 0.05 -5 0 5 East(m) 3000 4000 5 0.05 0 0 -5 0 5 East(m) -1 -1 3000 3000 4000 5000 GPS Time - 43,5000 (sec) 4000 5000 0 0 0.05 0 -0.05 2 -10 0 -0.05 -0.05 0 0.05 0 1 East(m) L1 G+C+R+INS(35°) 1 0.05 0.05 0 -0.05 10 3000 4000 5000 GPS Time - 43,5000 (sec) 5000 0 -1 -1 0 1 4000 0 -2 5000 -0.05 -0.05 0 0.05 -5 3000 0.05 0 -0.05 2 0 0 East(m) L1 G+C+R+INS(30°) 1 0.05 Down (m) -5 -2 5000 L1+L2 G+C+R(35°) North(m) 0 0 -0.05 -0.05 0 0.05 0 Down (m) North(m) 0.05 0 -10 5000 L1 G+C+R(35°) 5 0.05 0.05 0 -0.05 10 0 -10 5 -5 0 5 East(m) Down (m) Down (m) 0.05 0 -0.05 10 4000 North(m) 0.05 North(m) North(m) 5 3000 L1+L2 G+C+R(30°) L1 G+C+R(30°) 0 -1 -1 0.05 0 -0.05 2 0 5 East(m) Down (m) 4000 0.05 0 -0.05 -0.05 0 0.05 North(m) 3000 0 -5 Down (m) Down (m) 0.05 0 -0.05 10 0.05 0 North(m) 0 5 Down (m) 0.05 North(m) North(m) 5 L1 G+C+R+INS(25°) 1 L1+L2 G+C+R(25°) L1 G+C+R(25°) -0.05 -0.05 0 0.05 0 East(m) 1 0 -2 3000 4000 5000 GPS Time - 43,5000 (sec) Figure 11.11. Time series of position differences in the east, and directions for single-frequency Figure Time series of position differences innorth, the north, east,vertical and vertical directions for singleRTK (G + C RTK + R, (G 1st +column), RTK (G +RTK C + R, and single-frequency frequency C + R, 1stdual-frequency column), dual-frequency (G 2nd + C +column), R, 2nd column), and single◦ , 30◦ , and 35◦ ). RTK/INS integration (G + C + R,(G 3rd column) higher cut-off angles (25 frequency RTK/INS integration +C + R, 3rdwith column) with higherelevation cut-off elevation angles (25°, 30°, The correctly-fixed solutions are shown in green, incorrectly-fixed solutions in red, and float solutions and 35°). The correctly-fixed solutions are shown in green, incorrectly-fixed solutions in red, and float in solutions grey. A zoom-in is given istogiven show details of the solutions in grey. Awindow zoom-in window to the show the details of correctly-fixed the correctly-fixed solutionsininthe the horizontal east)vertical and vertical (down) time-series. the different scale fortightly-coupled the tightlyhorizontal (north,(north, east) and (down) time-series. Note Note the different scale for the coupled integration. RTK/INS integration. RTK/INS epoch solution and fixing rate elevation angles. Cut-off (°) 25 RMS (cm) N E D L1 RTK 0.35 0.44 1.33 Fixed/Float Remote Sens. 2018, 10, 205 78.44 88.31 206.3 L1 + L2 RTK 0.50 0.56 1.56 Fixed/Float 82.56 100.9 183.5 L1 + INS 0.54 0.62 1.75 Fixed/Float 32.28 43.03 4.4. Position Drift Error 20.35 for Multi-GNSS (%) of the GPS + BDS + GLONASS system with higher cut-off 30 N E D 0.41 0.52 1.95 36.1 81.30 87.39 223.9 0.60 0.73 2.28 77.7 102.8 102.5 221.2 0.76 0.78 2.51 86.2 35.47 50.04 67.94 RTK/INS Integration after FR FR N 0.46 20.3 81.14 0.55 74.3 107.9 0.78 84.7 36.00 the End of 35 E D FR 0.54 2.07 14.7 87.75 226.4 0.76 2.60 71.5 103.4 230.4 0.77 2.37 82.2 47.65 Outages 67.59 GNSS 18 of 21 4.4. Position Driftapplications, Error for Multi-GNSS Integration after the End of GNSS Outages For most dynamic it is RTK/INS inevitable for vehicles to go through the overpasses, trees and tunnels, etc. In these conditions, the satellite signals may lose lock frequently no matter For most dynamic applications, it is inevitable for vehicles to go through the overpasses, treesthe single and tunnels, etc. In these conditions, the satellite signals may lose lock frequently no matter the single GPS or the multi-GNSS is used. The frequent signal loss makes the GNSS data processing more GPSespecially or the multi-GNSS used. The frequent signal loss makes thepositioning. GNSS data processing complicated, for theiscarrier-phase based high-accuracy Figure more 12 shows the complicated, especially for the carrier-phase based high-accuracy positioning. Figure 12 shows the GNSS outage (it cannot provide RTK solutions) durations during the driving test for the combined GNSS outage (it cannot provide RTK solutions) durations during the driving test for the combined ◦ GPS, BDS, and system system with awith 15 acut-off elevation angle. windows GPS, GLONASS BDS, and GLONASS 15° cut-off elevation angle.Three Three zoom-in zoom-in windows are are given to show the of duration the outage duration the longest outage lasts morethan than 22 min and to show thegiven magnitude of magnitude the outage whereinwherein the longest outage lasts more min and some others are within 10 s. some others are within 10 s. Outage duration (seconds) 120 100 80 60 40 20 6 4 2 0 10 5 0 4050410041504200 3000 3100 3200 8 6 4 2 0 -2 4700 4800 4900 0 3000 3500 4000 4500 GPS Time - 43,5000 (sec) 5000 5500 Figure 12. GNSS outage durations during the driving test for the combined GPS, BDS, and GLONASS Figure 12. GNSS outage durations during the driving test for the combined GPS, BDS, and GLONASS system with a 15° cut-off elevation angle. system with a 15◦ cut-off elevation angle. Since the high-precision position information during the long outages is not available in the real experiments, the maximum position error is used to evaluate the positioning performance in the data Since the high-precision position information during the long outages is not available in the real analysis. Generally, the position errors reach the maximum at the end of outages and the accurate experiments, the maximum position error is used by to applying evaluatebackward the positioning in the data reference position at this epoch can be obtained smoothing.performance Table 4 shows all the outage durations and theerrors corresponding maximum positionat drift analysis. Generally, the position reach the maximum theerrors. end of outages and the accurate reference position at this epoch can be obtained by applying backward smoothing. Table 4 shows all Table 4. The GNSS outage durations in the field test and the corresponding maximum position drift the outage durations errors. and the corresponding maximum position drift errors. Outage Items #1 #2 0.4 6.2 7.1 9.4 #3 #3 #4 #5 #6 0.9 15.3 4.8 16.1 9 523.5 1739.3 #6 2817.3 3352.1 127 #7 #8 #9 #10 #11 #12 #13 6.8 6.7 1.5 0.3 14.7 4.2 #9 #10 8.7 1.4 1.4 11.0 2 16.2 2 4.7 #11 Table 4. The GNSS outage the corresponding position Duration (s) durations 4 3 in the 6 field 2 test 9 and127 4 6 2 maximum 2 7 2 4 drift errors. Outage Items MAX (cm)#1 Duration (s) MAX (cm) North East Down 3D 4 0.4 6.2 7.1 9.4 North East #2 Down 3D 3 1.2 0.4 1.6 2.0 1.2 0.4 1.6 2.0 6 5.6 9.3 #4 5.5 12.22 5.6 9.3 5.5 12.2 1.9 1.7 1.4 2.9 1.9 1.7 1.4 2.9 #5 0.9 15.3 4.8 16.1 523.5 1739.3 2817.3 3352.1 1.9 1.1 #7 6.9 7.24 1.9 1.1 6.9 7.2 5.8 14.6 1.1 15.7 7.1 1.5 2.3 67.6 #8 5.8 14.6 1.1 15.7 7.1 1.5 2.3 7.6 6.8 0.3 8.7 11.0 3.2 3.2 1.7 4.8 7 #12 #13 2 4 6.7 14.7 1.4 16.2 1.5 4.2 1.4 4.7 3.2 3.2 1.7 4.8 It can be seen from Table 4 that the maximum drift at the longest outage duration is up to 5.235, 17.393, and 33.521 m in the north, east, and vertical components. It is also clear that the position drift becomes larger when the outage duration increases. It is mainly due to the uncompensated IMU errors and these errors will lead to the rapid growth of position drift for MEMS inertial sensors. Generally, centimeter-level positioning accuracy can be obtained when the outage duration is within 4 s for the POS1100 MEMS grade IMU. This capability to provide high-accuracy navigation results within short-term outages is meaningful for some navigation applications. 5. Conclusions Carrier-phase-based high-accuracy positioning in urban environments is still a challenging task, even using high-cost dual-frequency receivers. In this contribution, the tightly-coupled integration of Remote Sens. 2018, 10, 205 19 of 21 the single-frequency multi-GNSS RTK and low-cost MEMS-IMU was developed to provide reliable and continuous high-accuracy positioning in GNSS-challenged environments. The outlier-resistant AR and Kalman filtering strategy was proposed specifically for the integration algorithm to resist the measurement outliers. A field vehicular test was carried out to investigate the high-accuracy positioning capabilities of the proposed algorithm in urban environments. Comparisons in terms of AR and positioning performance have been conducted with respect to the single- and dual-frequency multi-GNSS RTK. The following conclusions can be drawn based on the presented results and analysis in this research. In urban environments, the satellite tracking condition is poor due to the frequent loss-of-lock of the satellite signal, and the degraded satellite availability poses serious challenges to positioning, especially for the single GPS system. The GNSS observations are susceptible to outliers in such environments. Therefore, the fault detection and exclusion is definitely required for reliable positioning; otherwise, the reliable ambiguity resolution is impossible when measurement outliers are in the GNSS data. The proposed outlier-resistant ambiguity resolution and Kalman filtering strategy are suitable for the tightly-coupled integration algorithm and effective to improve the ambiguity resolution performance. By applying an outlier-resistant ambiguity resolution and filtering strategy, the ambiguity-fixed solutions can be obtained even though the measurements are contaminated with large outliers. The AR and positioning performance of GPS is very poor in urban environments, even using the dual-frequency RTK. As expected, the multi-GNSS can greatly improve the positioning performance due to the increased satellite availability and better spatial geometry structure. The fixing rate of single-frequency GPS RTK increases from 0.1% to 25.1% and 44.7% of the GPS/BDS and GPS/BDS/GLONASS RTK, respectively. The dual-frequency RTK can obtain much better results than the corresponding single-frequency RTK with fixing rates of 10.4%, 75.8%, and 76.7% for the GPS, GPS/BDS, and GPS/BDS/GLONASS, respectively. The performance can be further improved by the single-frequency multi-GNSS RTK/MEMS-IMU tightly-coupled integration with fewer ambiguity-float solutions and smaller positioning error than the corresponding dual-frequency multi-GNSS RTK. The corresponding fixing rate for the GPS, GPS/BDS, and GPS/BDS/GLONASS are 5.8%, 86.1%, and 86.1%, respectively. Additionally, the position errors of ambiguity-float RTK are large even with a higher cut-off elevation angle because it mainly relies on the relatively imprecise code measurements. By contrast, the ambiguity-float solution of the RTK/INS tightly-coupled integration is more stable and accurate due to the strong constraint from the INS in the short-term period. The results also show that high-accuracy positioning is feasible with limited performance loss for the single-frequency GPS/BDS/GLONASS/INS integration when higher cut-off elevation angles are applied in urban environments. Its AR fixing rate is 86.2%, 84.7%, and 82.2% when the cut-off elevation angles are set to 25◦ , 30◦ , and 35◦ , respectively. Compared with the high-cost dual-frequency RTK, the tightly-coupled integration of single-frequency multi-GNSS and low-cost MEMS-IMU are promising and preferred for some applications in urban environments. For long outage durations, it is difficult to obtain centimeter-level positioning accuracy during the GNSS outage. Therefore, other aiding sensors and methods will be considered in the future to further improve the high-accuracy positioning capabilities. In addition, suitable methods should be developed to model the multipath error on the carrier phase. Acknowledgments: We would like to thank the anonymous reviewers for their valuable comments and critical remarks, which greatly improved the quality of the manuscript. This work is supported in part by the National Key Research and Development Program of China (nos. 2016YFB0501800, 2016YFB0501803 and 2016YFB0501804), in part by the National High Technology Research and Develop Program of China (no. 2015AA124002) and, in part, by the National Natural Science Foundations of China ( no. 41404029). Author Contributions: Tuan Li and Hongping Zhang conceived the initial idea; Tuan Li designed the experiment and implemented the software for this contribution; Tuan Li analyzed the data and wrote the main manuscript; Zhouzheng Gao and Qijin Chen helped with the data analysis; and Hongping Zhang and Xiaoji Niu helped with the writing. All authors reviewed the manuscript. Remote Sens. 2018, 10, 205 20 of 21 Conflicts of Interest: The authors declare no conflict of interest. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. He, H.; Li, J.; Yang, Y.; Xu, J.; Guo, H.; Wang, A. 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