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Spatial and temporal characteristics of optimum process noise values of tropospheric parameters for kinematic analysis of Global Navigation Satellite System (GNSS) sites in Japan
© The Author(s) 2016
Received: 7 March 2016
Accepted: 26 November 2016
Published: 8 December 2016
The application of a Global Navigation Satellite System (GNSS) to the understanding of crustal deformation has significant advantages. Kinematic analysis of GNSS data is a key technique for understanding short-timescale crustal deformation with periods of less than 1 day and has been utilized in many studies of crustal deformation. In particular, high sampling rate (e.g., interval of 1 Hz or greater) kinematic GNSS analysis can be used to detect seismic waves caused by large earthquakes. Larson et al. (2003) found good agreement between surface displacements integrated from strong ground-motion records and long-baseline (several 100 km) 1-Hz GNSS position estimates for the 2002 Denali earthquake. Other studies have similarly succeeded in detecting seismic waves caused by large earthquakes (e.g., Ohta et al. 2006; Bilich et al. 2008). Furthermore, several studies deduced the rupture processes of large earthquakes from high sampling rate kinematic GNSS time series (e.g., Miyazaki et al. 2004; Yokota et al. 2009; Delouis et al. 2010). Recently, kinematic GNSS analysis has been used for real-time estimation of the magnitude and fault expansion of large earthquakes (e.g., Ohta et al. 2012, 2015; Melgar et al. 2013, 2015; Melgar and Bock 2013; Kawamoto et al. 2016).
These previous studies mainly focused on the coseismic time period. Miyazaki and Larson (2008) subsequently investigated early after slip following the 2003 Tokachi-oki earthquake, deduced from 30-s baseline kinematic GNSS analysis, in which they assumed a tightly constrained random walk stochastic process for coordinate estimation (Larson and Miyazaki 2008). Kinematic GNSS time series usually show large disturbances in the lower frequency band (e.g., Genrich and Bock 2006), due to the difficulty of strict separation between the coordinate parameters and other unknown parameters, such as the tropospheric parameters.
The tropospheric delay (TD) provides information on the amount of water vapor integrated over the path between a GNSS satellite and a receiver. TD is a function of zenith distance or satellite elevation (e.g., Davis et al. 1985) and is factorized into dry (hydrostatic) and wet components. In precise GNSS data processing, the zenith total delay (ZTD) is typically estimated as a function of time. Approximately 90% of the total TD caused by refraction is due to the hydrostatic tropospheric component. The hydrostatic component strongly depends on atmospheric pressure at the Earth’s surface; thus, it can be accurately modeled. The remaining 10% of the total TD comprises the wet tropospheric component, which is spatially and temporally dependent on the water vapor in the atmosphere and is therefore much more difficult to model precisely (e.g., Webb et al. 2016). Furthermore, the tropospheric delay gradient model (e.g., MacMillan 1995) expresses the tropospheric delay as a combination of the ZTD and an additional term to express azimuthal dependence, represented by the tropospheric delay gradient. Bar-Sever et al. (1998) investigated the impact of tropospheric delay gradient estimates in precise GPS analysis, using GIPSY-OASIS software (Lichten and Border 1987). GIPSY-OASIS adopts a Kalman filter-based (KF) approach, which is useful for estimating the unknown time-dependent parameters. The standard least squares method is typical and robust in estimating daily GNSS coordinates, but is unsuitable for more frequent coordinates and for simultaneously estimating a large number of unknown parameters. In KF-based processing, we must assume the stochastic process mode, such as white noise or random walk, for each unknown parameter. Furthermore, we also must assume process noise values for each time-dependent unknown parameter. These process noise values control the dynamics of the unknown parameters. However, setting these parameters, including each process noise value, is strongly dependent on the method of analysis. In a study by Bar-Sever et al. (1998), the optimum strategy comprised a low-elevation cutoff angle (7°), combined with a model of the zenith wet delay (ZWD) and tropospheric gradient as a relatively low random walk process noise value (5 × 10−8 km s−1/2 for ZWD and 5 × 10−9 km s−1/2 for tropospheric gradient, respectively). These optimum tropospheric parameters have now been adopted as one of the recommended values in GIPSY-OASIS for kinematic coordinate estimation in a slow-moving body. The process noise values for tropospheric parameters usually adopt units of “mm h−1/2.” In this study, however, we adopted units of “km s−1/2” because this is used in GIPSY-OASIS for setting the process noise of tropospheric parameters. Penna et al. (2015) showed that kinematic precise point positioning (PPP) with appropriately tuned process noise constraints is capable of recovering synthetic tidal displacements. They searched for optimum process noise for the ZWD and coordinate time series based on long-term GNSS data from each site and concluded that tuned coordinate and ZWD process noise values enable accurate 0- to 6-mm amplitude semidiurnal and diurnal periodic tidal ground displacements to be detected with accuracy better than 0.2 mm. Those results clearly indicate the importance of determining the optimum process noise of unknown parameters under a stochastic process approach for precise GNSS data analysis. In particular, the treatment of process noise is fundamentally important for kinematic analysis because the coordinate time series must be solved using limited data compared to static analysis.
Based on these background studies, we assess the spatial and temporal characteristics of the optimum process noise settings of unknown tropospheric parameters for kinematic PPP data analysis. In this paper, we initially focus on the spatial distribution of the optimum parameter settings between ZWD and tropospheric gradient across the Japanese nationwide GNSS network, which comprises more than 1300 stations. We also discuss the long-term stability of the optimum tropospheric parameters for specific sites. Finally, we discuss the effects of optimizing process noise for kinematic GNSS data analysis.
GNSS data and analysis
To assess the spatial characteristics of the optimum process noise, we analyzed 24-h data recorded across the entire GEONET network from March 10, July 4, and November 22, 2011. The data were processed using GIPSY-OASIS (version 6.3), which provides GNSS data analysis and simulation. We adopted a kinematic PPP strategy (Zumberge et al. 1997) for the coordinate estimation. The reference GPS satellite orbit and clock information were obtained using the Jet Propulsion Laboratory (JPL) final products (known as flinnR). For comparison with the recommended process noise values for the tropospheric parameters suggested by Bar-Sever et al. (1998), we defined an elevation cutoff angle of 7° during data processing. To correct for phase center variation of both the GEONET and GNSS satellites, we applied the absolute antenna phase center variation table provided by the International GNSS Service (IGS). Single-receiver carrier-phase ambiguities were resolved using uncalibrated phase delay (UPD) information provided by the JPL (Bertiger et al. 2010). We corrected the ocean tide loading effect based on the NAO.99b model (Matsumoto et al. 2000) . We estimated the site coordinates every 30 s, assuming a white noise stochastic model with a fixed process noise value (10−2 km). In addition, we estimated ZWD and the tropospheric gradient every 30 s using a random walk stochastic process. Furthermore, we applied a priori information for the zenith tropospheric delay based on the gridded Vienna Mapping Functions 1 (VMF1; Boehm and Schuh 2004) for all of the sites. We computed nominal hydrostatic and wet delays for each site using gridded VMF1 data, which comprises 6-h data with 2.5° × 2.0° spatial resolution. We then applied these calculated nominal tropospheric parameter values as a priori information during kinematic PPP processing.
The stochastic nature of tropospheric parameters such as ZWD and tropospheric gradient affects the changeability of the time series for those unknown parameters. Furthermore, it should also indirectly affect the site coordinate time series. Thus, we assumed that small disturbances in the site coordinate time series should be taken as an optimized result in this study. The three-dimensional root-mean-square (3D-RMS) value is a useful index for assessing the stability of the coordinate time series. However, if 3D-RMS values of the time series are adopted for assessing optimum process noise values, it is difficult to distinguish the effect of process noise value for each coordinate component. Thus, we use the three individual coordinate components for assessing appropriate process noise values. Based on these assumptions, we carried out a grid search for the optimum combination of process noise parameters for the ZWD and tropospheric gradient (hereafter termed TROP and GRAD, respectively), based on the stability of the coordinate time series. We estimated the kinematic site coordinate time series under different combinations of TROP and GRAD parameters for each site. We varied the process noise values for TROP (1 × 10−9 to 1 × 10−5 km s−1/2) and GRAD (1 × 10−11 to 1 × 10−7 km s−1/2). Each parameter space is divided into 10 combinations, so we estimated the kinematic coordinate time series in 100 combinations. Finally, we calculated the standard deviation (SD) of the time series for each combination of tropospheric process noise parameters for three coordinate components (east–west, north–south, and up–down).
To assess the long-term stability of the optimum tropospheric parameters for a specific site, we analyzed data recorded continuously at stations 0098 and 0032 throughout the year 2010. The locations of these sites are indicated in Fig. 1. We estimated the optimum combination of tropospheric process noise values for each day, using the same procedure as for the spatial characteristic assessment described above.
Characteristics of estimated optimum process noise at specific sites
The horizontal coordinate components (e.g., east–west, EW, and north–south, NS) would be expected to correlate with the gradient parameter process noise (e.g., Miyazaki et al. 2003). However, it is difficult to confirm the dependency of the different GRAD process noise values in Fig. 2. Weather conditions on March 10, 2011, were fine in and around station 0098; thus, the atmospheric gradient change should be small. If the atmospheric gradient amount is large, such as during the passage of a weather front, the GRAD process noise value will have a large effect.
Characteristics of the estimated optimum process noise combination
Frequency distribution of TROP and GRAD parameter combinations
Additional file 1: Figure S1 and Additional file 2: Figure S2 show the frequency distribution of the TROP and GRAD parameter combinations for each component for July 4 and November 22, 2011, respectively. Weather conditions on July 4 were rainy throughout Japan due to the passage of a cold front. In contrast, November 22 showed clear, fine weather due to high atmospheric pressure across the entire region of Japan. The results in Additional file 1: Figure S1 clearly show the concentration of optimum combinations in a specific region compared with the results from March 10, 2011 (Fig. 4). Furthermore, the maximum value of the GRAD frequency distribution (~1 × 10−8 km s−1/2) is larger than that for the March 10 results (Fig. 4, ~2 × 10−9 km s−1/2). This may reflect the differing weather conditions between the 2 days. This differs from the results for November 22 (Additional file 2: Figure S2), in which the frequency distribution shows slightly complex characteristics; it is clear that the horizontal components did not show a simple distribution, with a comparatively large concentration region in both of the horizontal components (indicated by the dashed red square for EW component and dashed black square for NS component in Additional file 2: Figure S2). The maximum frequency value for the NS component, however, did not locate within this region. In contrast, it is possible to recognize the clear concentration region of the frequency distribution in the vertical component.
Based on these results from the different days, we found that the tendency for concentration of frequency distribution in specific regions is basically a common characteristic despite the twin peaks observed in the horizontal components in the case of November 22 (Additional file 2: Figure S2). Furthermore, the maximum frequency combination differed for each day, which may reflect the differing weather conditions.
Spatial distribution characteristics of the optimum TROP and GRAD parameters
In this section, we show the spatial distribution of the optimum TROP and GRAD parameters for each calculated day. Firstly, we show the general characteristics, based on the results of the 3 days. Secondly, we show the results for sub-divided region based on the results for March 10, 2011.
The distributions of estimated optimum TROP for the vertical component on March 10 and November 22 (Fig. 5(a-3) and Additional file 4: Figure S4(a-3)) clearly show regional characteristics; however, the optimum TROP histogram (Fig. 6e; Additional file 6: Figure S6(e)) for the vertical component shows a steeper distribution than the GRAD parameter (Fig. 6f; Additional file 6: Figure S6(f)). The overall trend indicates that the optimum TROP value for the vertical component in the northern part of Japan is small compared to that in the south (Fig. 5(a-3); Additional file 4: Figure S4(a-3)). However, it is difficult to recognize this trend in the case of July 4, 2011 (Additional file 3: Figure S3(a-3)). The majority (67.7%) of the optimum TROP values were concentrated within a very narrow range around 6 × 10−8 km s−1/2 (Additional file 5: Figure S5(e)). Weather conditions on July 4 were rainy throughout Japan, which might affect this characteristic spatial pattern of the optimum TROP distribution.
In comparison with the TROP parameter, the spatial characteristics of the GRAD parameter are less well defined (Fig. 5; Additional file 3: Figure S3; Additional file 4: Figure S4). The frequency histograms, however, show clear characteristics in each day. For example, in the case of March 10, the frequency histograms of the optimum GRAD parameter for the horizontal components clearly show broader distributions compared to the TROP parameter for the vertical component (Fig. 6). In contrast, the frequency histogram for July 4 clearly shows the steep characteristic for the horizontal components (Additional file 5: Figure S5(b, d)). Weather conditions on July 4 were strongly influenced by the passage of a cold front, such that the obtained results should reflect these weather conditions.
For a more detailed understanding of the spatial characteristics, we show the results for sub-divided regions. In Additional file 7: Figure S7, frequency histograms show the optimum TROP and GRAD for sub-divided regions A–G for the case of March 10 (see also region sub-divisions indicated in Fig. 1) to assess the regional dependency of the TROP and GRAD parameters. Comparison of the calculated histograms for regions (A) and (E) shows that in region (A), the histogram is characterized by a broad distribution with highest frequency optimum TROP value of 1 × 10−8 km s−1/2 (Additional file 7: Figure S7). In contrast, the histogram for region (E) shows a much steeper distribution with optimum TROP value of 2 × 10−8 km s−1/2 (Additional file 7: Figure S7). These clear differences suggest the optimum process noise might be region-dependent.
The TROP parameter distribution shows a second important characteristic, namely the influence of recording station elevation. Additional file 8: Figure S8 shows the ratio of each optimum TROP parameter within each ellipsoidal GNSS station elevation range for the case of March 10. It is clear that the ratio of low TROP parameter values increased with site elevation. This is a reasonable result because higher elevation is associated with less integration of water vapor. These results suggest that the optimum ZWD process noise parameter might depend on each sub-divided region and the elevation of each site. At this time, the scale of the sub-divided region is several hundred kilometers. Thus, the parameter at least depends on such spatial expansion.
Temporal characteristics of the estimated optimum process noise combination
Interestingly, the optimum TROP time series for the vertical component indicates limited disturbance compared with the horizontal components. Furthermore, the moving average time series obtained at station 0098 shows stability throughout the year (Fig. 7) with small annual pattern. Similarly, at station 0032 the obtained time series is stable despite the minor long-term pattern that developed following day of year (DOY) 100 (Fig. 7). In contrast, the optimum GRAD parameters for the horizontal components show a different tendency. It is clear that the obtained time series did not stabilize during the year, and shows a clear annual pattern in the moving average time series. In the previous section, we suggested that the GRAD parameter might not have a significant spatial characteristic within the scale of the GEONET on that specific day. The obtained time series, however, suggests that the optimum GRAD parameter might vary following an annual pattern, despite the relatively large disturbance compared with the optimum TROP parameter for the vertical component.
Yoshida (2010) discussed the spatial annual pattern of tropospheric gradient in Japan deduced from the spatial distribution of the estimated ZWD value at each GEONET site. It was found that the spatial gradient of the ZWD is approximately 130 mm/1000 km and 100 mm/1000 km for summer and winter seasons, respectively. It was also found that the large spatial gradient of ZWD appeared in higher- and lower-latitude regions in the summer and winter seasons, respectively. These results suggest that the annual characteristic of the GRAD parameter at each site might relate to the spatial and temporal characteristics of the GRAD and TROP parameters, even though in the present study the GRAD parameter did not display significant spatial characteristic within 1 day. It is difficult to discuss the relationship between the spatial annual pattern of tropospheric gradient found by Yoshida (2010) and the annual pattern of the optimum GRAD parameter identified in the present study, because our analysis was restricted to only two sites due to limitations on computation time. To investigate the relationship between the two study findings, spatially and temporally dense analysis will be required.
As summarized in “Introduction” section, determining the appropriate process noise for unknown tropospheric parameters is important for precise analysis of kinematic data. In this section, we discuss the impact of process noise optimization on the kinematic GNSS time series.
Effectiveness of common optimum process noise values for a specific date
In Fig. 4, we presented the frequency distribution of the TROP and GRAD parameter combinations for each coordinate component, deduced from data recorded across the entire GEONET network on March 10, 2011. The obtained results showed that the optimum combinations for each component are concentrated within specific regions of the parameter space (indicated by dashed red squares in Fig. 4). However, the smallest SD combination is different for each component. Thus, we averaged the frequencies of the three components to extract the common optimum process noise combination. The parameter combination comprising a TROP value of 2 × 10−8 km s−1/2 and a GRAD value of 2 × 10−9 km s−1/2 produced the highest frequency value (~4.3%). Thus, we adopted these process noise values as the “common” optimum parameters for this specific date.
Practical application of optimum process noise values in kinematic analysis
In this study, we assessed the spatial and temporal characteristics of process noise values for unknown tropospheric parameters. The findings clearly indicate the importance of optimizing the process noise. However, using a daily grid-search approach to determine the optimum process noise combination for each site may be unfeasible, due to excessive computational demands. In the previous section, we described the effectiveness of using a “common” process noise value for each specific day across the GEONET network; however, this approach also requires considerable computational resources. Thus, for practical use, it is important to develop a method of determining these “optimum” values within realistic computation times, which is beyond the scope of the present study. These approaches should be useful for both GEONET and other GNSS networks. Once the optimum process noise parameters are experimentally and/or theoretically determined for each station or each network, the results will contribute significantly to the understanding of short-timescale crustal deformation with periods of less than 1 day, using kinematic GNSS analysis.
In this study, we assessed the spatial and temporal characteristics of process noise values for unknown tropospheric parameters in kinematic analysis of GNSS data from the GEONET network, Japan. We used a grid-search approach to extract optimum process noise values for each site on 3 days during 2011.
Based on the values determined for each site, we investigated the spatial and temporal characteristics of the process noise parameters. The spatial distribution of the optimum process noise value for the zenith wet tropospheric parameter with vertical site coordinate clearly shows regional characteristics and is also dependent on station elevation. In contrast, the optimum process noise for the tropospheric gradient might not have significant spatial distribution characteristics within the scale of the GEONET network on that specific day. The temporal characteristics of the optimum process noise parameters for each site coordinate component at specific sites showed a clear annual pattern in the tropospheric gradient parameter of the horizontal components.
Finally, we assessed the impact of process noise optimization for the kinematic GNSS site coordinate time series. For the calculation, we assumed the “common” optimum process noise values for a specific day (March 10, 2011) across the entire GEONET network. These gave improved site coordinate time series (i.e., smaller standard deviation) compared with the recommended values proposed by Bar-Sever et al. (1998), with the exception of some outlier sites. These results suggest that the use of appropriate process noise values is important for analyzing kinematic GNSS time series.
We are grateful to the GSI for providing the GEONET GNSS data and to the Jet Propulsion Laboratory for providing high-quality precise GPS ephemerides and clock information. We would like to thank three anonymous reviewers and the associate editors for their many constructive comments on an earlier draft of the manuscript. This study was partly supported by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program. This work was also supported by the Japan Society for the Promotion of Science (JSPS) under KAKENHI Grant Number 15H03713 and 15K13556. We would like to thank Editage (www.editage.jp) for English language editing.
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