Shuanggen Jin

Seasonal variability of GPS-derived zenith tropospheric delay (1994-2006) and climate implications

[1] The total zenith tropospheric delay (ZTD) is an important parameter of the atmosphere and directly or indirectly reflects the weather and climate processes and variations. In this paper the ZTD time series with a 2-hour resolution are derived from globally distributed 150 International GPS Service (IGS) stations ( 1994 - 2006), which are used to investigate the secular trend and seasonal variation of ZTD as well as its implications in climate. The mean secular ZTD variation trend is about 1.5 +/- 0.001 mm/yr at all IGS stations. The secular variations are systematically increasing in most parts of the Northern Hemisphere and decreasing in most parts of the Southern Hemisphere. Furthermore, the ZTD trends are almost symmetrically decreasing with increasing altitude, while the summation of upward and downward trends at globally distributed GPS sites is almost zero, possibly reflecting that the secular ZTD variation is in balance at a global scale. Significant annual variations of ZTD are found over all GPS stations with the amplitude from 25 to 75 mm. The annual variation amplitudes of ZTD near oceanic coasts are generally larger than in the continental inland. Larger amplitudes of annual ZTD variation are mostly found at middle latitudes (near 20 degrees S and 40 degrees N) and smaller amplitudes of annual ZTD variation are located at higher latitudes ( e. g., Antarctic) and the equator areas. The phase of annual ZTD variation is about 60 degrees in the Southern Hemisphere ( about February, summer) and about 240 degrees in the Northern Hemisphere ( about August, summer). The mean amplitude of semiannual ZTD variations is about 10 mm, much smaller than annual variations. The semiannual amplitudes are larger in the Northern Hemisphere than in the Southern Hemisphere, indicating that the semiannual variation amplitudes of ZTD in the Southern Hemisphere are not significant. In addition, the higher-frequency variability (RMS of ZTD residuals) ranges from 15 to 65 mm of delay, depending on altitude of the station. Inland stations tend to have lower variability and sites at ocean and coasts have higher variability. These seasonal ZTD cycles are due mainly to the wet component variations (ZWD).

GNSS Reflectometry and Remote Sensing: New Objectives and Results

Abstract The Global Navigation Satellite System (GNSS) has been a very powerful and important contributor to all scientific questions related to precise positioning on Earth’s surface, particularly as a mature technique in geodesy and geosciences. With the development of GNSS as a satellite microwave (L-band) technique, more and wider applications and new potentials are explored and utilized. The versatile and available GNSS signals can image the Earth’s surface environments as a new, highly precise, continuous, all-weather and near-real-time remote sensing tool. The refracted signals from GNSS radio occultation satellites together with ground GNSS observations can provide the high-resolution tropospheric water vapor, temperature and pressure, tropopause parameters and ionospheric total electron content (TEC) and electron density profile as well. The GNSS reflected signals from the ocean and land surface could determine the ocean height, wind speed and wind direction of ocean surface, soil moisture, ice and snow thickness. In this paper, GNSS remote sensing applications in the atmosphere, oceans, land and hydrology are presented as well as new objectives and results discussed.

Hydrological and oceanic effects on polar motion from GRACE and models

[1] Terrestrial water storage (TWS) and ocean bottom pressure (OBP) are major contributors to the observed polar motion excitations, second only to atmospheric mass movement. However, quantitative assessment of the hydrological and oceanic effects on polar motion remains unclear because of the lack of global observations. In this paper, hydrological and oceanic mass excitations to polar motion are investigated using monthly TWS and OBP derived from the Gravity Recovery and Climate Experiment (GRACE) for January 2003 until December 2008. The results from this analysis are compared with hydrological model excitations from the European Center for Medium-Range Weather Forecasts (ECMWF) and oceanic model excitations obtained from the Jet Propulsion Laboratory (JPL) using Estimating the Circulation and Climate of the Ocean (ECCO). Results show that the GRACE-derived OBP and TWS better explain the geodetic residual polar motion excitations for the Px component at the annual period, while the GRACE OBP and ECMWF hydrological angular momentum agree better with the geodetic residuals for the annual Py excitation. GRACE ocean and hydrology excitations better explain the geodetic residuals for the semiannual Py excitation. However, the JPL ECCO and ECMWF models better explain the intraseasonal geodetic residual of polar motion excitation in the Px and Py components. The GRACE data demonstrate much higher intraseasonal variability than either the models or the geodetic observations.

Pattern and evolution of seismo‐ionospheric disturbances following the 2011 Tohoku earthquakes from GPS observations

Global Positioning System (GPS) has been widely used to sense crustal deformation and ionospheric anomalies, particularly seismic ionospheric disturbances. In March 2011, the earthquakes with magnitude of up to Mw = 9 occurred in Tohoku near the east coast of Honshu, Japan. The GPS Earth Observation Network (GEONET) in Japan with more than 1200 continuously operating stations provides a unique opportunity to study the detailed seismic ionospheric disturbances. In this paper, the pattern and evolution of seismic ionospheric disturbances following the Tohoku earthquakes are investigated by dense GEONET data, including amplitude, propagation pattern, direction, speed, and evolution. Maximal coseismic ionospheric disturbances are found with up to more than 4 TECU, and the disturbance period is around 10–20 min. The seismic ionospheric effects following the aftershocks are attenuated with the increase of the time and distance between the ionospheric pierce point and the epicenter of the main event, which last more than 2 h. Seismic ionospheric disturbance detected by GPS measurement is not only related to the main shock but also the giant aftershocks. Propagation velocities of the total electron content (TEC) disturbance show a decrease when it spreads 400–600 km away from the epicenter in the north-western direction, where it is just near the west coast of Japan. Furthermore, the TEC disturbance also has obvious directional features. In the first half hour, the TEC disturbance in the southeast direction has the biggest amplitude, while the dominant direction is changed to northwest tens of minutes later. In addition, signals with higher frequencies are existed in seismic TEC variation at the epicenter region but do not appear in the far field.

Variability and Climatology of PWV From Global 13-Year GPS Observations

Water vapor plays the key role in the global hydrologic cycle and climate change. However, the distribution and variability of water vapor in the troposphere is not understood well in the globe, particularly the high-resolution variation. In this paper, 13-year 2-h precipitable water vapors (PWV) are derived from globally distributed 155 Global Positioning System sites observations and global three-hourly surface weather data and six-hourly National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis products, which are the first used to investigate multiscale water-vapor variability on a global scale. It has been found that the distinct seasonal cycles are in summer with a maximum water vapor and in winter with a minimum water vapor. The higher amplitudes of annual PWV variations are located in midlatitudes with about 10-20 plusmn 0.5 mm, and the lower amplitudes are found in high latitudes and equatorial areas with about 5 plusmn 0.5 mm. The larger differences of mean PWV between in summer and winter are located in midlatitudes with about 10-30 mm, particularly in the Northern Hemisphere. The semiannual variation amplitudes are relatively weaker with about 0.5 plusmn 0.2 mm. In addition, significant diurnal variations of PWV are found over most International Global Navigation Satellite Systems Service stations. The diurnal (24 h) cycle has amplitude of 0.2-1.2 plusmn 0.1 mm, and the peak time is from the noon to midnight. The semidiurnal (12 h) cycle is weaker, with amplitude of less than 0.3 mm.

TEC response to the 2008 Wenchuan Earthquake in comparison with other strong earthquakes

We registered near-field global positioning system (GPS) total electron content (TEC) response to the Wenchuan Earthquake on 12 May 2008. The Wenchuan Earthquake (magnitude 8.0) occurred at 06:28 UT as the result of motion on a northeast striking reverse fault (thrust fault) on the northwestern margin of the Sichuan Basin. The earthquake reflects tectonic stresses resulting from the convergence of crustal material slowly moving from the high Tibetan Plateau, to the west, against a strong crust underlying the Sichuan Basin and southeastern China. We found that intensive N-shaped shock-acoustic waves with a plane waveform and with a half-period of about 200 s propagated south-eastwards with a velocity 580 m s − 1 for a distance of about 1000 km from the epicentre. The wavefront of N-shaped disturbance was parallel with the earthquake rupture direction (from southwest to northeast). The main directional lobe of shock-acoustic wave emitter is directed southeastwards, i.e. transversely to the rupture. We speculate that the above properties of TEC response are determined by the geodynamics of the Wenchuan Earthquake. No noticeable TEC response to that earthquake was found in far-field regions in South Korea and Japan. We compared TEC response to the 2008 Wenchuan earthquake with other strong earthquakes.

Co-seismic ionospheric and deformation signals on the 2008 magnitude 8.0 Wenchuan Earthquake from GPS observations

The 8.0 magnitude Wenchuan Earthquake occurred at the Longmenshan Fault along the eastern boundary between the Tibetan Plateau and the western Sichuan Basin in southwestern China on 12 May 2008, killing tens of thousands of people in several cities along the western Sichuan Basin. In this paper, co-seismic ionospheric and deformation signals from the mainshock of this event are extracted from national global positioning system (GPS) network observations, which provide unique insights into this event. The co-seismic deformation moves towards the earthquake epicentre, and the largest magnitude reaches 2.3 m in Beichuan. The total moment of the co-seismic rupture is 2.4 × 1021 nm, equivalent to a magnitude of 8.1 and nearly identical to the seismological estimate. Furthermore, co-seismic ionospheric disturbances indicate a shock-acoustic wave propagation at a mean velocity of about 600 m s−1 towards the rupture direction.

An improvement of GPS height estimations: stochastic modeling

The results of GPS positioning depend on both functional and stochastic models. In most of the current GPS processing programs, however, the stochastic model that describes the statistical properties of GPS observations is usually assumed that all GPS measurements have the same accuracy and are statistically independent. Such assumptions are unrealistic. Although there were only a few studies modeling the effects on the GPS relative positioning, they are restricted to short baselines and short session lengths. In this paper, the stochastic modeling for IGS long-baseline positioning (with 24-hour session) is analyzed in the GAMIT software by modified stochastic models. Results show that any mis-specifications of stochastic model result in unreliable GPS baseline results, and the deviation of baseline estimations reaches as much as 2 cm in the height component. Using the stochastic model of satellite elevation angle-based cosine function, the precision of GPS baseline estimations can be improved, and the GPS baseline component is closest to the reference values, especially GPS height.

GNSS Remote Sensing

This book presents the theory and methods of GNSS remote sensing as well as its applications in the atmosphere, oceans, land and hydrology. It contains detailed theory and study cases to help the reader put the material into practice.

GPS ionospheric tomography: A comparison with the IRI-2001 model over South Korea

The International Reference Ionosphere model 2001 (IRI-2001) is one of the most comprehensive empirical models of the ionosphere and has been widely used to estimate the electron density profiles in the altitude ranging from about 60 to 2000 km and the total electron content (TEC) at any given location, time and date, which reflect smooth-average global ionospheric behaviors. However, whether it provides normal actual estimations in the ionosphere over some regions should be tested with real observation data. In this paper, the three-dimensional ionospheric electron density profiles over South Korea in 2003 are obtained using the ionospheric tomography reconstruction technique with the permanent Korean GPS Network (KGN) data, and its validity is further verified by another independent ionosonde data. The GPS ionospheric reconstruction results are used to compare then results obtained with the IRI-2001 model in South Korea in terms of NmF2 and TEC. The monthly averaged diurnal values of these key parameters in January, April, July and October 2003 are considered to represent the winter, spring, summer and autumn seasons, respectively. Compared with the GPS reconstruction results, averaged monthly NmF2 medians from the IRI-2001 are overestimated in daytime and underestimated in nighttime for all seasons, but the deviation magnitudes in autumn and winter are smaller than in spring and summer. In addition, averaged monthly TEC medians from the IRI-2001 are overestimated in daytime in winter, but almost always underestimated in other seasons.