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See detailEffect of unmodelled tidal displacements in GPS and GLONASS coordinate time series
Abraha, Kibrom Ebuy UL; Teferle, Felix Norman UL; Hunegnaw, Addisu UL et al

in Geophysical Journal International (2018), 214(3), 2195-2206

This study demonstrates the different effects of unmodelled (sub-)daily tidal displacement in Global Positioning System (GPS) and GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) coordinate time ... [more ▼]

This study demonstrates the different effects of unmodelled (sub-)daily tidal displacement in Global Positioning System (GPS) and GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) coordinate time-series. The results show that more than two propagated periodic signals appear in GPS and GLONASS Precise Point Positioning (PPP) coordinate time-series in the presence of an unmodelled M2 /O1 tidal displacements as a result of a non-overlapping 24-hr data sampling. To summarize the propagated periodic signals at the fortnightly period, an unmodelled M2 tidal displacement propagates predominately into two long-period signals at 13.6x (x is a positive integer) and 14.76 d for GPS, while only one significant propagated periodic signal at 14.76 d is discernible for GLONASS. Similarly, significant propagated periodic signals at 13.6x and 14.19 d for GPS and only at 14.19 d for GLONASS are evident as a result of an unmodelled O1 tidal displacement. However, an unmodelled M f (long- period) signal results in a strong power of similar magnitude at 13.6x d (∼13.66 d) for both GPS and GLONASS solutions. The appearance of different periodic signals as a result of the same unmodelled tidal displacement is attributed to the different ground repeat periods of the constellations. The latter is likely to explain the reason why the 13.6x-d fortnightly signal is present only in GPS solutions. Comparing the powers of the M2 propagated periodic signals at 13.6x and 14.76 d on average from 32 globally distributed stations, the amplitude of the former is larger than for the latter by an order of magnitude. The results of this study demonstrate that the 13.6x-d periodic signal in GPS/GNSS (Global Navigation Satellite System) derived products is a joint contribution of the propagation of unmodelled (sub-)daily tidal displacements and errors at longer periods with the former appearing to contribute more. Significant reduction of the propagated periodic signals is achieved from combined-system solutions where including Galileo (the European GNSS) to the combined solution already shows benefits by reducing the effect even before the system has reached its full constellation. Combined GNSS solutions will benefit the applications of GNSS time-series for retrieving tidal harmonic signals such as Mf as they reduce constellation specific propagation effects. [less ▲]

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See detailOn the Impact of Multi-GNSS Solutions on Satellite Products and Positioning
Abraha, Kibrom Ebuy UL; Teferle, Felix Norman UL; Hunegnaw, Addisu UL et al

Poster (2016, December 12)

In Global Navigation Satellite System (GNSS) coordinate time series unrecognised errors and un-modelled (periodic) effects may bias non-linear motions induced by geophysical signals. Those spurious ... [more ▼]

In Global Navigation Satellite System (GNSS) coordinate time series unrecognised errors and un-modelled (periodic) effects may bias non-linear motions induced by geophysical signals. Those spurious signals can be caused either due to un-modelled long periodic signals or propagation of sub-daily signals into the time series. Understanding and mitigating these errors is vital to reduce biases and on revealing subtle geophysical signals. Mostly, the spurious signals are caused by unmodelled errors which occur due to the draconitic years, satellite ground repeats and absorption into resonant GNSS orbits. Accordingly, different features can be observed in GNSS-derived products from different single-GNSS or combined-GNSS solutions. To assess the nature of periodic signals on station coordinate time series Precise Point Positioning (PPP) solutions are generated using the Bernese GNSS Software V5.2. The solutions consider only GPS, only GLONASS or combined GPS+GLONASS (GNSS) observations. We assess the periodic signals of station coordinates computed using the combined International GNSS Service (IGS) and four of its Analysis Centers (ACs) products. [less ▲]

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See detailGNSS related periodic signals in coordinate time-series from Precise Point Positioning
Abraha, Kibrom Ebuy UL; Teferle, Felix Norman UL; Hunegnaw, Addisu UL et al

in Geophysical Journal International (2016)

In Global Navigation Satellite System (GNSS) coordinate time series unrecognised errors and un-modelled (periodic) effects may bias non-linear motions induced by geophysical signals. Hence, understanding ... [more ▼]

In Global Navigation Satellite System (GNSS) coordinate time series unrecognised errors and un-modelled (periodic) effects may bias non-linear motions induced by geophysical signals. Hence, understanding and mitigating these errors is vital to reducing biases and on revealing subtle geophysical signals. To assess the nature of periodic signals in coordinate time series Precise Point Positioning (PPP) solutions for the period 2008 to 2015 are generated. The solu- tions consider Global Positioning System (GPS), GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) or combined GPS+GLONASS (GNSS) observations. We assess the pe- riodic signals of station coordinates computed using the combined International GNSS Service (IGS) and four of its Analysis Centers (ACs) products. Furthermore, we make use of different filtering methods to investigate the sources of the periodic signals. A faint fortnightly signal in our PPP solution based on Jet Propulsion Laboratory (JPL) products and the existence of an 8-day period for those ACs generating combined GPS+GLONASS products are the main features in the GPS-only solutions. The existence of the 8-day period in the GPS-only solution indicates that GPS orbits computed in a combined GNSS solution contain GLONASS-specific signals. The GLONASS-only solution shows highly elevated powers at the 3rd draconitic harmonic ( ~ 120-day period), at the 8-day period and its harmonics (4 days, 2.67 days) besides the well-known annual, semi-annual and other draconitic harmonics. We show that the GLONASS constellation gaps before December 2011 contribute to the power at some of the frequencies. However, the well known fortnightly signal in GPS-only solutions is not discernible in the GLONASS-only solution. The combined GNSS solution contains periodic signals from both systems, with most of the powers being reduced when compared to the single-GNSS solutions. A 52% reduction for the horizontal components and a 36% reduction for the vertical compo- nent are achieved for the fortnightly signal from the GNSS solution compared to the GPS-only solution. Comparing the results of the employed filtering methods reveals that the source of most of the powers of draconitic and fortnightly signals are satellite-induced with a non-zero contribution of site-specific errors. [less ▲]

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See detailMulti-GNSS Benefits to Real-Time and Long-Term Monitoring Applications
Teferle, Felix Norman UL; Ding, Wenwu; Abraha, Kibrom Ebuy UL et al

Scientific Conference (2016, July 30)

The processing of observations from multiple Global Navigation Satellite Systems (GNSSs) has been shown to benefit high-precision applications on time scales from real-time (RT) to long-term monitoring ... [more ▼]

The processing of observations from multiple Global Navigation Satellite Systems (GNSSs) has been shown to benefit high-precision applications on time scales from real-time (RT) to long-term monitoring. While the improvements for RT applications have been widely documented and stem largely from the availability of additional observations, often with better satellite geometry, especially in obstructed environments, the improvements to long-term monitoring applications are less well understood. In this evaluation two distinct examples from recent studies carried out at the University of Luxembourg will be presented. Firstly, we will discuss RT estimates of Zenith Tropospheric Delay (ZTD) obtained using integer ambiguity fixed Precise Point Positioning (PPP) solutions based on GPS, GLONASS, Galileo and BDS observations. This study revealed that the largest improvement in the ZTD estimates stemmed from the additional GNSS observations to those of GPS. The fixing of integer ambiguities (GPS only) had less of an effect. Secondly, we will discuss long-term PPP solutions using GPS and GLONASS observations in combination with various satellite orbit and clock products from the International GNSS Service and its analysis centres. Here of particular interest are the constellation specific draconitic signals and the impact of signal obstructions on the long-term position time series. [less ▲]

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See detailImpact of Limited Satellite Visibility on Estimates of Vertical Land Movements
Abraha, Kibrom Ebuy UL; Teferle, Felix Norman UL; Hunegnaw, Addisu UL et al

in International Association of Geodesy Symposia (2016)

The number of Global Navigation Satellite System (GNSS) satellites and their geometry directly affect the quality of positioning and derived satellite products. Accordingly, the International GNSS Service ... [more ▼]

The number of Global Navigation Satellite System (GNSS) satellites and their geometry directly affect the quality of positioning and derived satellite products. Accordingly, the International GNSS Service (IGS) recommends GNSS antennas to be installed away from natural and man-made surfaces and structures, which may affect the incoming signals through severe multipath or obstructions. Following these recommendations, continuous GNSS (cGNSS) stations are generally located in low multipath environments with minimal signal obstructions. However, some applications require GNSS antennas to be installed at specific locations in order to measure local processes. In support of sea level studies, cGNSS stations are established at or close to tide gauges in order to accurately monitor the local vertical land movements experienced by the sea level sensors. However, the environment at the tide gauge might not be optimal for GNSS observations due to the aforementioned station-specific effects, which may degrade the quality of coordinate solutions. This study investigates the impact of severe signal obstructions on long-term position time series for some selected stations. A masking profile from an actually obstructed site is extracted, simulated and applied to unobstructed IGS sites. To investigate these effects, we imple- mented a new feature called azimuth-dependent elevation masking in the Bernese GNSS Software version 5.2. We present our preliminary results on the use of this new feature to study the impact of different obstruction scenarios on long-term GNSS position time series and vertical land movement estimates. The results show that a certain obstruction, with the effect being highly dependent on its severity and azimuthal direction, affects all coordinate components with the effect being more significant for the Up component. Moreover, it causes changes in the rate estimates and increases the rate uncertainty with the effect being site-specific. [less ▲]

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See detailSignal Obstructions at GNSS Stations: Benefits From Multi-GNSS Observations
Abraha, Kibrom Ebuy UL; Teferle, Felix Norman UL; Hunegnaw, Addisu UL et al

Poster (2015, October 27)

The current accuracy of IGS products, few centimeter level, requires amongst other things that the location for GNSS antennas are nearly optimal for GNSS observations. This includes a low multipath ... [more ▼]

The current accuracy of IGS products, few centimeter level, requires amongst other things that the location for GNSS antennas are nearly optimal for GNSS observations. This includes a low multipath environment and little to no signal obstructions. However, this is not guaranteed for every station especially in urban areas and mountainous regions. As some applications such as GNSS for sea level studies or to monitor landslides require GNSS antennas to be installed at a specific site, it is clear that the environment might not be favourable for GNSS observations. In this study, we investigate the effect of signal obstructions on station positions, specifically the height component, based on simulated obstruction scenarios using a modified Bernese GNSS Software version 5.2 (BSW52). The behaviours of different obstruction scenarios and the impact of multi-GNSS (GPS+GLONASS for now) observations for both clear and obstructed stations are discussed. [less ▲]

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See detailImpact of Limited Multi-GNSS Visibility on Vertical Land Movement Estimates
Abraha, Kibrom Ebuy UL; Teferle, Felix Norman UL; Hunegnaw, Addisu UL et al

Poster (2015, June 27)

The number of GNSS satellites and their geometry directly affect the quality of positioning and derived satellite products. Accordingly, the International GNSS Service (IGS) recommends GNSS antennas to be ... [more ▼]

The number of GNSS satellites and their geometry directly affect the quality of positioning and derived satellite products. Accordingly, the International GNSS Service (IGS) recommends GNSS antennas to be installed away from natural and man-made surfaces and structures, which may affect the incoming signals through severe multipath or obstructions. Following these recommendations, continuous GNSS (cGNSS) stations are generally located in low multipath environments with minimal signal obstructions. However, some applications require GNSS antennas to be installed at specific locations in order to measure local processes. Hence, in support of sea level studies, cGNSS stations must be installed close to or at tide gauges in order to accurately monitor the local vertical land movements experienced by the sea level sensors. However, the environment at the tide gauge might not be optimal for GNSS observations due to the aforementioned station-specific effects, which degrade the quality of coordinate solutions.This first study investigates the impact of severe signal obstructions on long-term monitoring results by use of simulated and real observations for selected cGNSS stations, and evaluates if the use of multi-GNSS (GPS+GLONASS) constellations will benefit derived results. To investigate these effects, we implemented azimuth and elevation dependent masking in the Bernese GNSS Software version 5.2. We present our preliminary results on the impact of different obstruction scenarios and combined GPS and GLONASS solutions on coordinate and vertical land movement estimates. [less ▲]

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See detailDetecting offsets in GPS time series: First results from the detection of offsets in GPS experiment
Gazeaux, Julien; Williams, Simon; King, Matt et al

in Journal of Geophysical Research. Solid Earth (2013), 118

The accuracy of Global Positioning System (GPS) time series is degraded by the presence of offsets. If these are not detected and adjusted correctly they bias velocities, and hence geophysical estimates ... [more ▼]

The accuracy of Global Positioning System (GPS) time series is degraded by the presence of offsets. If these are not detected and adjusted correctly they bias velocities, and hence geophysical estimates, and degrade the terrestrial reference frame. They also alter apparent time series noise characteristics as undetected offsets resemble a random walk process. As such, offsets are a substantial problem. A number of offset detection methods have been developed across a range of fields, and some of these are now being tested in geodetic time series. The DOGEx (Detection of Offsets in GPS Experiment) project aims to test the effectiveness of automated and manual offset detection approaches and the subsequent effect on GPS-derived velocities. To do this, simulated time series were first generated that mimicked realistic GPS data consisting of a velocity component, offsets, white and flicker noises (1/f spectrum noises) composed in an additive model. We focus on offset detection and together with velocity biases induced by incorrect offset detection. We show that, at present, manual methods (where offsets are hand -picked by GPS time series experts) almost always give better results than automated or semi-automated methods (two automated methods give quite similar velocity bias as the best manual solutions). For instance, the 5th percentile ranges (5% to 95%) in velocity bias for automated approaches is equal to 4.2mm/year,whereas it is equal to 1.8mm/yr for the manual solutions. However the True Positive detection rate of automated solutions is significantly higher than those for the manual solutions, being around 37% for the best automated, and 42% for the best manual solution. The amplitude of offsets detectable by automated solutions is greater than for hand picked solutions, with the smallest detectable offset for the two best manual solutions equal to 5mm and 7mm and to 8mm and 10mm for the two best automated solutions. The best manual solutions yielded velocity biases from the truth commonly in the range ±0.2mm/yr, whereas the best automated solutions produced biases no better than double this range. Assuming the simulated time series noise levels continue to be representative of real GPS time series, robust geophysical interpretation of individual site velocities lower than these levels is therefore not robust. Further work is required before we can routinely interpret sub-mm/yr velocities for single GPS stations. [less ▲]

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