4044478
Comparison of Three Methods for Estimating GPS Multipath Repeat Time
Sidereal filtering is an effective method for mitigating multipath error in static GPS positioning. Using accurate estimates of multipath repeat time (MRT) in sidereal filtering can further improve the performance of the filter. There are three commonly used methods for estimating the MRT: Orbit Repeat Time Method (ORTM), Aspect Repeat Time Adjustment (ARTA), and Residual Correlation Method (RCM). This study utilizes advanced sidereal filtering (ASF) adopting the MRT estimates derived by the three methods to mitigate the multipath in observation domain, then evaluates the three methods in term of multipath reduction in both coordinate and observation domain. Normally, the differences between the MRT estimates from the three methods are less than 1.2 s on average. The three methods are basically identical in multipath reduction, with RCM being slightly better than the other two methods, whereas for a satellite affected by orbit maneuver (satellite number 13 in this study), the MRT estimated by the three methods differ by up to tens of seconds, and the RCM- and ARTA-derived MRT estimates are better than ORTM-derived ones for ASF multipath reduction. The RCM shows a slight advantage in multipath mitigation, while ORTM is the one of lowest computation and ARTA is the optimal one for real-time ASF. Thus, the best MRT estimation method for practical applications depends on which criterion overweighs the others.
120151756
Site-specific multipath characteristics of global IGS and CORS GPS sites
Abstract.The site-specific multipath characteristics of 217 Global Positioning System (GPS) sites worldwide were analyzed using the variability of the post-fit phase residuals. Because the GPS satellite constellation returns to the same configuration in a sidereal day (23 h 56 min 4 s), the multipath repeats on that period. However, daily GPS position estimates are usually based on the solar day. When several days of GPS data are processed, this steady change in the orientation of the satellite constellation with respect to the station manifests itself in the form of patterns in the post-fit phase residuals which shift by 3 min 56 s per day. It was found that the mean root mean square of the time-shifted post-fit phase residuals is highly dependent on the GPS antenna type. The conclusions derived from the analysis of the time-shifted post-fit residuals were verified by performing a cross-correlation of the post-fit residuals across many days for selected sites.
129807124
Rapid resolution of crustal motion at short ranges with the global positioning system
On the basis of the analysis of Global Positioning System (GPS) data collected at the U.S. Geological Survey Kennedy Ranch House alinement array near Parkfield, California, in November 1990 and February 1991, we assess the temporal resolution of crustal deformation for GPS receiver systems operating as strain meters and provide guidelines for achieving millimeter-level precision with short-range, kinematic-type GPS measurements. Site occupation of about one-half hour is sufficient for day-to-day repeatability of submillimeter precision in relative position during good satellite geometry. Signal multipath effects which dominate dispersive baseline errors are essentially averaged out and need not be modeled. However, as occupation times decrease, baseline solutions are increasingly sensitive to multipath effects which together with nondispersive receiver measurement noise, are of millimeter to centimeter magnitude in epoch-by-epoch solutions. On repeatedly monitored baselines we can account for both of these noise sources separately. The mostly random receiver noise is effectively suppressed by low-pass filters with corner frequencies of about 20 mHz. The multipath noise is filtered using the fact that it essentially repeats from day to day. Residual errors in filtered 1-s epoch-by-epoch solutions are below 2 mm during periods of good satellite geometry. Hence for a continuously monitored baseline, actual deformation in excess of this threshold can be detected with a time lag comparable to the group delay of the low-pass filter, i.e., typically within a minute. In terms of strain along a typical short baseline of 1 km sampled once per second we can detect 1–2 μs in l min with multipath modeling. Without multipath modeling, it takes 10 min to resolve the same magnitude of deformation. In 30 min we can detect 0.1 μs independent of multipath modeling. These results have important implications for kinematic-type GPS surveys. For millimeter-level precision without impractical multipath modeling it is necessary to occupy a site during good satellite geometry for a period longer than that of the multipath signature. Our experience in both benign and difficult multipath environments indicates that approximately 10 min are sufficient. The measurements at the Kennedy Ranch House alinement array suggest that this segment of the San Andreas fault appears to be virtually locked at the surface during the 3-month period between the two surveys.
54982878
A NEW APPROACH FOR FIELD CALIBRATION OF ABSOLUTE GPS ANTENNA PHASE CENTER VARIATIONS
This paper introduces a new approach for determining azimuth- and elevation-dependent phase center biases through a field measurement in an absolute sense. It gives particular attention to multipath effects. The model, the conditions for the field procedure, and preliminary analysis of the results are presented. The absolute antenna phase center calibration procedure is implemented in the GPS processing package GEONAP.
27215293
Modified sidereal filtering: Implications for high‐rate GPS positioning
[1] High-rate (1-Hz) Global Positioning System (GPS) data are beginning to be used for a variety of geophysical monitoring purposes, including seismology. Improving the precision of high-rate GPS position estimates will increase the value of these 1-Hz GPS monitoring systems. One technique that has been used to improve high-rate GPS positioning takes advantage of the ground track repeat period of the satellites. This study investigates the GPS orbital repeat period and determines that it varies for each satellite and differs significantly from the generally assumed sidereal period. Orbit repeat periods are calculated and used to filter 1-Hz GPS position estimates. Using the calculated orbit repeat period significantly reduces low frequency (0.001–0.04 Hz) errors in 1-Hz GPS position estimates.
123691477
Modified Sidereal Filter and Its Effect on High-rate GPS Positioning
The normal sidereal filter is modified,and a way to calculate the mean satellite orbit period by dividing broadcast ephemeris is provided,which is checked through comparison with examples.From the results we can see that,compared to the pre-revision sidereal filter,the modified one can improve the positioning accuracy more efficiently,the accuracy of NEU directions are increased 4.5%,5.2% and 6.2% respectively,and it has better performance between frequencies of 0.001-0.08 Hz.
17123828
GPS sidereal filtering: coordinate- and carrier-phase-level strategies
Multipath error is considered one of the major errors affecting GPS observations. One can benefit from the repetition of satellite geometry approximately every sidereal day, and apply filtering to help minimize this error. For GPS data at 1 s interval processed using a double-difference strategy, using the day-to-day coordinate or carrier-phase residual autocorrelation determined with a 10-h window leads to the steadiest estimates of the error-repeat lag, although a window as short as 2 h can produce an acceptable value with > 97% of the optimal lag’s correlation. We conclude that although the lag may vary with time, such variation is marginal and there is little advantage in using a satellite-specific or other time-varying lag in double-difference processing. We filter the GPS data either by stacking a number of days of processed coordinate residuals using the optimum “sidereal” lag (23 h 55 m 54 s), and removing these stacked residuals from the day in question (coordinate space), or by a similar method using double-difference carrier-phase residuals (observational space). Either method results in more consistent and homogeneous set of coordinates throughout the dataset compared with unfiltered processing. Coordinate stacking reduces geometry-related repeating errors (mainly multipath) better than carrier-phase residual stacking, although the latter takes less processing time to achieve final filtered coordinates. Thus, the optimal stacking method will depend on whether coordinate precision or computational time is the over-riding criterion.
119987986
Sidereal filtering based on single differences for mitigating GPS multipath effects on short baselines
Carrier-phase multipath effects are one of the most significant error sources in precise Global Positioning System (GPS) positioning applications. A new sidereal filtering algorithm based on single differences is developed to mitigate multipath effects for short-baseline high-rate GPS applications such as structural deformation monitoring. This method differs from traditional sidereal filtering in that our method operates on the single differences rather than the coordinates or double differences. A multipath model for the single differences on the reference day is established for each satellite and is used to remove multipath errors from observations of subsequent days by taking advantage of the sidereal repeatability of multipath signals. Using both simulated and real GPS observations, we demonstrate that this method is insensitive to different weighting strategies used in computing single differences from double differences. Applying the proposed method can reduce the root mean square (RMS) of positioning noises by 82% on average. Compared to sidereal filtering (in either coordinate or double differences domain) and aspect repeat time adjustment, this method can further reduce the RMS values by 13 and 7%, respectively. Wavelet spectra have shown that the proposed method is more effective in mitigating multipath errors of both long and short periods. This method is also more advantageous in that it is applicable when different GPS satellites are observed on different days.
119057106
Effectiveness of observation-domain sidereal filtering for GPS precise point positioning
AbstractSidereal filtering is a technique used to reduce errors caused by multipath in the positioning of static receivers via the Global Positioning System (GPS). It relies upon the receiver and its surrounding environment remaining static from one day to the next and takes advantage of the approximately sidereal repeat time of the GPS constellation geometry. The repeating multipath error can thus be identified, usually in the position domain, and largely removed from the following day. We describe an observation-domain sidereal filter algorithm that operates on undifferenced ionospheric-free GPS carrier phase measurements to reduce errors caused by multipath. It is applied in the context of high-rate (1 Hz) precise point positioning of a static receiver. An observation-domain sidereal filter (ODSF) is able to account for the slightly different repeat times of each GPS satellite, unlike a position-domain sidereal filter (PDSF), and can hence be more effective at reducing high-frequency multipath error. Using 8-h long datasets of GPS measurements from two different receivers with different antenna types and contrasting environments, the ODSF algorithm is shown overall to yield a position time series 5–10 % more stable, in terms of Allan deviation, than a PDSF over nearly all time intervals below about 200 s in length. This may be particularly useful for earthquake and tsunami early warning systems where the accurate measurement of small displacements of the ground over the period of just a few minutes is crucial. However, the sidereal filters are also applied to a third dataset during which two short episodes of particularly high-frequency multipath error were identified. These two periods are analyzed in detail and illustrate the limitations of using sidereal filters with important implications for other methods of correcting for multipath at the observation level.

122286550
Filtering GPS time-series using a Vondrak filter and cross-validation
Multipath disturbance is one of the most important error sources in high-accuracy global positioning system (GPS) positioning and navigation. A new data filtering method, based on the Vondrak filter and the technique of cross-validation, is developed for separating signals from noise in data series, and applied to mitigate GPS multipath effects in applications such as deformation monitoring. Both simulated data series and real GPS observations are used to test the proposed method. It is shown that the method can be used to successfully separate signals from noise at different noise levels, and for varying signal frequencies as long as the noise level is lower than the magnitude of the signals. A multipath model can be derived, based on the current-day GPS observations, with the proposed method and used to remove multipath errors in subsequent days of GPS observations when taking advantage of the sidereal day-to-day repeating characteristics of GPS multipath signals. Tests have shown that the reduction in the root mean square (RMS) values of the GPS errors ranges from 20% to 40% when the method is applied.
131214144
The Improved Sidereal Filtering of Time Series Considering Segments’Similarity in Coseismic Displacement
Based on the traditional sidereal filtering method,an improved sidereal filtering method is put forward,accounting for segments’similarity,namely before building filtering operator.Firstly determine the segments’ similarity of the coordinates time series between earthquake day and before that day or after that day,and then based on the similarity measure index to identify weight of each day for filtering.Different similarity measure indicators are analyzed.After that the universal image quality index(UIQI)method is recommended as the similarity measure for the improved sidereal filtering.It’s more scientific and reasonable than other similarity measure which considers the correlation coefficient between two series,own mean value and variance as well.The results show that the improved sidereal filtering can effectively improve the single epoch positioning accuracy and reliability,completely overcome the defects of traditional filtering method,is beneficial to batch processing without manual intervention.
59397640
Finding the repeat times of the GPS constellation
Single-epoch estimates of position using GPS are improved by removing multipath signals, which repeat when the GPS constellation does. We present two programs for finding this repeat time, one using the orbital period and the other the topocentric positions of the satellites. Both methods show that the repeat time is variable across the constellation, at the few-second level for most satellites, but with a few showing much different values. The repeat time for topocentric positions, which we term the aspect repeat time, averages 247 s less than a day, with fluctuations through the day that may be as much as 2.5 s at high latitudes.
110484339
Mitigation of multipath effect in GNSS short baseline positioning by the multipath hemispherical map
Multipath is one major error source in high-accuracy GNSS positioning. Various hardware and software approaches are developed to mitigate the multipath effect. Among them the MHM (multipath hemispherical map) and sidereal filtering (SF)/advanced SF (ASF) approaches utilize the spatiotemporal repeatability of multipath effect under static environment, hence they can be implemented to generate multipath correction model for real-time GNSS data processing. We focus on the spatial–temporal repeatability-based MHM and SF/ASF approaches and compare their performances for multipath reduction. Comparisons indicate that both MHM and ASF approaches perform well with residual variance reduction (50 %) for short span (next 5 days) and maintains roughly 45 % reduction level for longer span (next 6–25 days). The ASF model is more suitable for high frequency multipath reduction, such as high-rate GNSS applications. The MHM model is easier to implement for real-time multipath mitigation when the overall multipath regime is medium to low frequency.
130259571
Multi-antenna synchronized global navigation satellite system receiver and its advantages in high-precision positioning applications
The multi-antenna synchronized global navigation satellite system receiver is a high precision, low cost, and widely used emerging receiver. Using this type of receiver, the satellite and receiver clock errors can be eliminated simultaneously by forming between antenna single-differences, which is equivalent to the conventional double-difference model. However, current multi-antenna synchronized global navigation satellite system receiver products have not fully realized their potential to achieve better accuracy, efficiency, and broader applications. This paper introduces the conceptual design and derivable products of multi-antenna synchronized global navigation satellite system receivers involving the aspects of attitude determination, multipath effect mitigation, phase center variation correction, and ground-based carrier phase windup calibration. Through case studies, the advantages of multi-antenna synchronized global navigation satellite system receivers in high-precision positioning applications are demonstrated.
5889174
Reduction of Kinematic Short Baseline Multipath Effects Based on Multipath Hemispherical Map
Multipath hemispherical map (MHM) is a kind of multipath mitigation approach that takes advantage of the spatial repeatability of the multipath effect under an unchanged environment. This approach is not only suitable for static environments, but also for some kinematic platforms, such as a moving ship and airplane, where the dominant multipath effects come from the platform itself and the multipath effects from the surrounding environment are considered minor or negligible. Previous studies have verified the feasibility of the MHM approach in static environments. In this study, we expanded the MHM approach to a kinematic shipborne environment. Both static and kinematic tests were carried out to demonstrate the feasibility of the MHM approach. The results indicate that, after MHM multipath mitigation, the root mean square (RMS) of baseline length deviations are reduced by 10.47% and 10.57%, and the RMS of residual values are reduced by 39.89% and 21.91% for the static and kinematic tests, respectively. Power spectrum analysis has shown that the MHM approach is more effective in mitigating multipath in low-frequency bands; the high-frequency multipath effects still exist, and are indistinguishable from observation noise. Taking the observation noise into account, the residual reductions increase to 41.68% and 24.51% in static and kinematic tests, respectively. To further improve the performance of MHM for kinematic platforms, we also analyzed the influence of spatial coverage and resolution on residual reduction.
14959513
Formal Uncertainty and Dispersion of Single and Double Difference Models for GNSS-Based Attitude Determination
With multi-antenna synchronized global navigation satellite system (GNSS) receivers, the single difference (SD) between two antennas is able to eliminate both satellite and receiver clock error, thus it becomes necessary to reconsider the equivalency problem between the SD and double difference (DD) models. In this paper, we quantitatively compared the formal uncertainties and dispersions between multiple SD models and the DD model, and also carried out static and kinematic short baseline experiments. The theoretical and experimental results show that under a non-common clock scheme the SD and DD model are equivalent. Under a common clock scheme, if we estimate stochastic uncalibrated phase delay (UPD) parameters every epoch, this SD model is still equivalent to the DD model, but if we estimate only one UPD parameter for all epochs or take it as a known constant, the SD (here called SD2) and DD models are no longer equivalent. For the vertical component of baseline solutions, the formal uncertainties of the SD2 model are two times smaller than those of the DD model, and the dispersions of the SD2 model are even more than twice smaller than those of the DD model. In addition, to obtain baseline solutions, the SD2 model requires a minimum of three satellites, while the DD model requires a minimum of four satellites, which makes the SD2 more advantageous in attitude determination under sheltered environments.
122887839
Carrier phase multipath mitigation for BeiDou navigation satellite system
As is true of GPS, multipath disturbance is a limiting factor for further improvements in BDS high precision positioning applications. We investigate the characteristics of multipath errors and multipath mitigation for BDS geostationary earth orbit, inclined geosynchronous orbit and medium earth orbit satellites according to their orbital repeat periods. We argue that multipath errors can be mitigated using models extracted from a previous period since there is a strong agreement correlation in the multipath errors between subsequent periods. A multipath mitigation method, which is based on observation domain filtering and considers the repeat period of every satellite independently, was applied. The application of the proposed method shows that BDS positioning precision was improved by about 56, 48, and 48 % to 1.90, 1.28, and 4.37 mm in the north, east, and up components. After resolving BDS carrier phase multipath error, the positioning precision of the combined GPS&BDS with multipath correction reached to 0.95, 0.92, and 2.31 mm in the north, east, and up components that were more precise than either the GPS or BDS system alone. The more accurate and reliable precision obtained from the combined system after applying multipath mitigation method, would greatly contribute to high-performance precision deformation monitoring and seismological studies.