Geodetic GPS Research Articles

Airborne GPS radio occultation refractivity profiles observed in tropical storm environments

AbstractAirborne GPS radio occultation (ARO) data have been collected during the 2010 PRE‐Depression Investigation of Cloud systems in the Tropics (PREDICT) experiment. GPS signals received by the airborne Global Navigation Satellite System Instrument System for Multistatic and Occultation Sensing (GISMOS) are used to retrieve vertical profiles of refractivity in the neutral atmosphere. The system includes a conventional geodetic GPS receiver component for straightforward validation of the analysis method in the middle to upper troposphere, and a high‐sample rate (10 MHz) GPS recorder for postprocessing complex signals that probe the lower troposphere. The results from the geodetic receivers are presented here. The retrieved ARO profiles consistently agree within ~2% of refractivity profiles calculated from the European Center for Medium‐Range Weather Forecasting model Interim reanalyses as well as from nearby dropsondes and radiosondes. Changes in refractivity obtained from ARO data over the 5 days leading to the genesis of tropical storm Karl are consistent with moistening in the vicinity of the storm center. An open‐loop tracking method was implemented in a test case to analyze GPS signals from the GISMOS 10 MHz recording system for comparison with geodetic receiver data. The open‐loop mode successfully tracked ~2 km deeper into the troposphere than the conventional receiver and can also track rising occultations, illustrating the benefit from the high‐rate recording system. Accurate refractivity retrievals are an important first step toward the future goal of assimilating moisture profiles to improve forecasting of developing storms using this new GPS occultation technique.

Temperature-Dependent Newtonian Rheology in Advection-Convection Geodynamical Model for Plate Spreading in Eastern Volcanic Zone, Iceland

Geodynamic process as advection-convection of the Mid-Atlantic Ocean Ridge (MAR), that is exposed on land in Iceland is investigated. Advection is considered for the plate spreading velocity. Geodetic GPS data during 2000-2010 is used to estimate plate spreading velocity along a profile in the Eastern Volcanic Zone (EVZ), Iceland striking N102。E, approximately parallel to the NUVEL-1A spreading direction between the Eurasian and North American plates. To predict subsurface mass flow patterns, temperature-dependent Newtonian rheology is considered in the finite-element models (FEM). All models are considered 2-D with steady-state, incompressible rheology whose viscosity depends on the subsurface temperature distribution. The thickness of lithosphere along the profile in the EVZ is identified by 700。C isotherm and 1022 Pa s iso-viscosity, those reach 50 ± 3 km depth at distance of 100 km from rift axis. Geodetic observation and model prediction results show the ~90% of spreading is accommodated within ~45 km of the rift axis in each direction. Model predicts ~8.5 mm.yr-1 subsidence at the surface of rift center when magmatic plumbing is inactive. The rift center (the highest magmatic influx is ~11 mm.yr-1) in model shifts ~10 - 20 km west to generate observed style surface deformation. The spreading velocity, isotherm and depth of isotherm are the driving forces resulting in the surface deformation. These three parameters have more or less equal weight. However, as the center of deformation in the EVZ shifts and most of the subsidence takes place in the volcanic system that is currently the active which is the located of plate axis.

Sea State Determination from Ship-Based Geodetic GPS

AbstractOcean waves have a profound impact on navigation, offshore operations, recreation, safety, and the economic vitality of a nation’s maritime and coastal communities. This study demonstrates that ships equipped with geodetic GPS and a radar gauge can provide accurate estimates of sea state. The Research Vessel (R/V) Kilo Moana recorded 1-Hz data for the entire period of a 10-day cruise around the Hawaiian Islands. Solving for precise kinematic positions for the ship and combining these solutions with the ranges from the ship to the sea surface provided by the radar gauge, it was possible to retrieve 1-Hz estimates of the sea surface elevation along the cruise track. Converting these into estimates of significant wave height, strong agreement was found with wave buoy measurements and hindcast wave data. Comparison with buoy data indicates the estimates have errors on the order of 0.22 m, or less than 11% of the wave height. Using wave model predictions of the dominant directions, the data were processed further to correct for the Doppler shift and to estimate the dominant wave period. Although relatively noisy in locations where the predicted wave directions are expected to be poor, in general these estimates also show a good agreement with the wave buoy observations and hindcast wave estimates. A segment of the cruise that formed a circuit allowed for testing the consistency of the ship-based estimates and for determining a dominant wave direction, which was found to agree closely with model predictions.

Inverse Modeling of GPS Multipath for Snow Depth Estimation—Part II: Application and Validation

GPS multipath reflectometry (GPS-MR) is a technique that uses geodetic quality GPS receivers to estimate snow depth. The accuracy and precision of GPS-MR retrievals are evaluated at three different sites: grasslands, alpine, and forested. The assessment yields a correlation of 0.98 and an rms error of 6–8 cm for observed snow depths of up to 2.5 m. GPS-MR underestimates in situ snow depth by 10%–15% at these three sites, although the validation methods do not measure the same footprint as GPS-MR.

GPS 시각 전송에서의 대류층 천정지연 모델과 매핑 함수에 따른 시각오프셋 비교

This paper shows effects of tropospheric delay models and mapping functions among delay features occurred when GPS code signal is transferred for GPS Time Transfer. GPS time transfer uses CGGTTS as the international standard format. For geodetic GPS receiver, ROB has provided r2cggtts software which generates CGGTTS data from RINEX data and all laboratories participated in TAI link use this software and send the CGGTTS results periodically. Though Saastamoinen zenith path model and Niell mapping function are commonly used in space geodesy, r2cggtts software applied NATO zenith path model and CHAO mapping function to the tropospheric delay model. Hence, this paper shows effects of two tropospheric delay models by implementing Saastamoinen model and Niell mapping function for the time offset.

The Alto Tiberina Near Fault Observatory (northern Apennines, Italy)

<p>The availability of multidisciplinary and high-resolution data is a fundamental requirement to understand the physics of earthquakes and faulting. We present the Alto Tiberina Near Fault Observatory (TABOO), a research infrastructure devoted to studying preparatory processes, slow and fast deformation along a fault system located in the upper Tiber Valley (northern Apennines), dominated by a 60 km long low-angle normal fault (Alto Tiberina, ATF) active since the Quaternary. TABOO consists of 50 permanent seismic stations covering an area of 120 × 120 km<span><sup>2</sup></span>. The surface seismic stations are equipped with 3-components seismometers, one third of them hosting accelerometers. We instrumented three shallow (250 m) boreholes with seismometers, creating a 3-dimensional antenna for studying micro-earthquakes sources (detection threshold is M<span><sub>L</sub></span> 0.5) and detecting transient signals. 24 of these sites are equipped with continuous geodetic GPS, forming two transects across the fault system. Geochemical and electromagnetic stations have been also deployed in the study area. In 36 months TABOO recorded 19,422 events with M<span><sub>L</sub></span> ≤ 3.8 corresponding to 23.36e-04 events per day per squared kilometres; one of the highest seismicity rate value observed in Italy. Seismicity distribution images the geometry of the ATF and its antithetic/synthetic structures located in the hanging-wall. TABOO can allow us to understand the seismogenic potential of the ATF and therefore contribute to the seismic hazard assessment of the area. The collected information on the geometry and deformation style of the fault will be used to elaborate ground shaking scenarios adopting diverse slip distributions and rupture directivity models.</p>

Using geodetic GPS receivers to measure vegetation water content

A GPS-based method is presented to measure vegetation water content. Commercially available geodetic-quality GPS receivers and antennas are used. The method is tested using GPS measurements collected over three field seasons. The GPS data are compared with in situ data for three plant types: desert grass, wheat, and alfalfa. The GPS retrievals of vegetation water content are based on the GPS signal-to-noise ratio (SNR) data. Instrumental issues that affect the SNR data are discussed, particularly satellite transmit power variations, footprint variations, and temperature effects. The amplitudes of the SNR data show a nearly linear relationship to the water content in grasses (0---0.5 kg/m2) and wheat crops (0---0.9 kg/m2). As predicted by theory, this simple linear relationship breaks down in vegetation with heavy water content, such as alfalfa. The field results are consistent with forward model predictions, whose effect restricts the use of this simple linear model for vegetation to water content of less than ~1 kg/m2.

Normalized Microwave Reflection Index: Validation of Vegetation Water Content Estimates From Montana Grasslands

The Normalized Microwave Reflection Index (NMRI) measures the intensity of GPS reflections, which is affected by vegetation within $\sim{\bf 100}\nbsp\hbox{\bf m}$ of GPS antennas. In a companion paper, the theoretical basis for NMRI and how it is derived from data archived at geodetic GPS installations are described. NMRI is calculated by normalizing the standard GPS metric ${\bf MP}_{\bf 1}{\bf rms}$ on a site-by-site basis to minimize terrain effects. Here, we validate NMRI as a metric for estimating vegetation water content (VWC) and evaluate the normalization procedure. In situ measurements of plant height, biomass, and VWC were taken on a biweekly basis during 2012 at four grassland sites in Montana. These measurements were compared to time series of ${\bf MP}_{\bf 1}{\bf rms}$ , NMRI, and Normalized Difference Vegetation Index (NDVI) from each site. At each site, a significant linear relationship exists between ${\bf MP}_{\bf 1}{\bf rms}$ and VWC. However, this relationship is not consistent across sites. Once normalized, a linear relationship exists between NMRI and VWC ( ${\bf r}^{\bf 2} = {\bf 0.71}$ ) that is consistent across the four sites. This suggests that VWC could be predicted from NMRI at sites without in situ observations, as long as vegetation and climate are similar. There is no clear relationship between NMRI and either vegetation height or biomass. The importance of normalization is shown using data from eight additional sites. After normalization, a strong positive correlation is apparent between NMRI and NDVI across all grassland GPS sites in Montana.

COMPARISON ANALYSIS OF INTERPOLATION TECHNIQUES FOR DEM GENERATION USING CARTOSAT-1 STEREO DATA

Digital Elevation Model (DEM) can be generated using several techniques such as photogrammetric technique, interferometry, Lidar, etc. In photogrammetric technique, a DEM generation using stereo images, accuracy of generated DEM is also dependent on interpolation techniques. The process of interpolation is conducted to generate DEM as a continuous data from the point map that contained height information as a discrete data. In this research, point map was extracted from Cartosat-1 stereo image and from geodetic single frequency GPS in differential mode. Different interpolation techniques were applied on these data sets with different combination within these data sets. In this study, analysis of DEM interpolation was conducted with deterministic interpolators such as inverse distance weighted (IDW), global polynomial, local polynomial, and radial basis functions (RBF); and probabilistic interpolators such as simple kriging, ordinary kriging, universal kriging, indicator kriging, probabilistic kriging, disjunctive kriging, and cokriging. The accuracy of generated DEMs through different interpolation techniques were evaluated with ground point data collected from geodetic single frequency GPS in differential mode. Based on the analysis, the range error of DEMs generated was between 1.29 m to 2.96 m. Interpolation method with the least error was ordinary kriging using point map data and GPS points, while the highest error was obtained from global polynomial method.

Mapping landslide displacements using Structure from Motion (SfM) and image correlation of multi-temporal UAV photography

In this study, we present a flexible, cost-effective, and accurate method to monitor landslides using a small unmanned aerial vehicle (UAV) to collect aerial photography. In the first part, we apply a Structure from Motion (SfM) workflow to derive a 3D model of a landslide in southeast Tasmania from multi-view UAV photography. The geometric accuracy of the 3D model and resulting DEMs and orthophoto mosaics was tested with ground control points coordinated with geodetic GPS receivers. A horizontal accuracy of 7 cm and vertical accuracy of 6 cm was achieved. In the second part, two DEMs and orthophoto mosaics acquired on 16 July 2011 and 10 November 2011 were compared to study landslide dynamics. The COSI-Corr image correlation technique was evaluated to quantify and map terrain displacements. The magnitude and direction of the displacement vectors derived from correlating two hillshaded DEM layers corresponded to a visual interpretation of landslide change. Results show that the algorithm can accurately map displacements of the toes, chunks of soil, and vegetation patches on top of the landslide, but is not capable of mapping the retreat of the main scarp. The conclusion is that UAV-based imagery in combination with 3D scene reconstruction and image correlation algorithms provide flexible and effective tools to map and monitor landslide dynamics.

The Accidental Tide Gauge: A GPS Reflection Case Study From Kachemak Bay, Alaska

For the last decade, it has been known that reflected GPS signals observed with specialized instruments could be used to measure sea level. In this letter, data from an existing geodetic-quality GPS site near Kachemak Bay, Alaska, are analyzed for a one-year time period. Daily sea-level variations are more than 7 m. Tidal coefficients have been estimated and compared with coefficients estimated from records from a traditional tide gauge at Seldovia Harbor, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim$</tex></formula> 30 km away. The GPS and Seldovia estimates of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$M_{2}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$S_{2}$</tex> </formula> coefficients agree to better than 2%; much of this residual can be attributed to true differences in the tide over 30 km as it propagates up Kachemak Bay. For daily mean sea levels the agreement is 2.3 cm. Because a standard geodetic GPS receiver/antenna is used, this GPS instrument can measure long-term sea-level changes in a stable terrestrial reference frame.

Testing GPS generated 1PPS against a rubidium standard

GPS receivers are often used in time-keeping applications, due to their availability, low price and high accuracy. We tested the capability of the GPS receiver to deliver a time-keeping accuracy needed for the time-critical applications, such as astrometry measurements, when a microsecond or better level of accuracy is needed in real-time. We tested a geodetic class GPS receiver against a rubidium standard, over a 24 hours period. In the overall view, the accuracy corresponds to the nominal values. However, we experienced outliers with the certain regularity that we could not explain with cycle-slips or the experiment set-up.

GPS phase difference variation statistics: A comparison between phase scintillation index and proxy indices

Scintillated GPS phase observations are traditionally characterized by the phase scintillation index, derived from specialized GPS receivers usually tracking at 50Hz. Geodetic quality GPS receivers, on the other hand, are normally tracking at frequencies up to 1Hz. However, availability of continuously operating geodetic receivers both in time and geographical location are superior to scintillation receiver’s coverage in many parts of the world. This motivates scintillation studies using regional and global geodetic GPS networks. Previous studies have shown the usefulness of GPS estimated total electron content variations for detecting ionospheric irregularities. In this paper, collocated geodetic and scintillation receivers are employed to compare proxy indices derived from geodetic receivers with the phase scintillation index during quiet and moderately disturbed ionospheric conditions. Sensitivity of the phase scintillation indices at high latitude stations to geomagnetic activity is discussed. Global mapping of ionospheric disturbances using proxy indices from real-time 1Hz GPS stations are also presented.

The Tohoku–Oki Earthquake: A Summary of Scientific Outcomes From Remote Sensing

On March 11, 2011, at 05:46:23 UTC, a megaearthquake (magnitude 9.0) occurred near the NE coast of Honshu Island (Japan), which originated near the subduction plate boundary between the Pacific and North American plates. The epicenter has been located at about 130 km east of Sendai City at a depth of about 32 km. A foreshock sequence took place days preceding the mainshock with magnitude 7.3 in March 9. The earthquake has been followed by a devastating tsunami. The location, the geometric parameters, and the focal mechanism are in agreement with the occurrence of the earthquake along the subduction plate boundary. The focal region from seismological analysis indicated that an area of about 500 km × 200 km moved with a maximum displacement of about 24 m near the hypocenter. The Earth Observation Community has made a strong effort to investigate the surface effects by exploiting the satellite data and the geodetic GPS measurements available. The Group on Earth Observation Geohazard Supersites established the Tohoku-Oki Event Supersite. This initiative provided a huge amount of satellite data to the scientific community. The main outcomes concern the measurement of the surface displacement pattern, the detection of surface changes due to damage, and the investigation of coastal changes due to inundation.

Pre- and post-seismic displacements associated with the Aswan 2011 earthquake (M = 4.2) derived from recent geodetic GPS data

Due to the importance of Aswan area, Egypt, it has been selected for the present study. Since 1982, after the main shock of November 14, 1981 with M = 5.6, several study programmes were initiated for monitoring seismicity and crustal movements at Aswan area (Tealeb 1996). Starting from 1997, GPS geodetic networks were established and observed many times in different campaigns. The observations are repeated twice a year. At the beginning of 2012, a moderate earthquake has occurred in the Aswan region with magnitude of 4.2 located at the southwest of the High Dam along Kalabsha fault. Before and after this event, these local geodetic networks were measured using GPS. Regional, Kalabsha-Sayal and Kalabsha networks were used in the present work. GPS data were processed using Bernese 5.0. The collected data before and after this event have shown that the area southwest of the High Dam and Kalabsha area suffered from stress and strain. The behaviour of the areas has changed after earthquake for pre-earthquake.

The Accuracy of GIS Tools for Transforming Assumed Total Station Surveys to Real World Coordinates

Most surveying works for mapping or GIS applications are performed with total station. Due to the remote nature of many of the sites surveyed, the surveys are often done in unprojected, local, assumed coordinate systems. However, without the survey data projected in real world coordinates, the range of possible analyses is limited and the value of existing imagery, elevation models, and hydrologic layers cannot be exploited. This requires a transformation from the local assumed to the real world coordinate systems. There are various built-in and add-in tools to perform transformations through GIS programs. This paper studies the effect of using Georeferencing tool, Spatial Adjustment tool (Affine and similarity) and CHaMP tool on the precision and relative accuracy of total station survey. This transformation requires real-world coordinates of at least two control points, which can be collected from different sources. This paper also studies the effect of using geodetic GPS, hand-held GPS, Google Earth (GE) and Bing Basemaps as sources for control points on the precision and relative accuracy of total station survey. These effects have been tested by using 111 points covered area of 60,000 m2and the results have shown that the CHaMP tool is the best for preserving the relative accuracy of the transformed points. The Georeferencing and spatial adjustment (similarity) tools give the same results and their accuracy are between 1/1000 and 1/300 depending on the source of control points. The results have also shown that the cornerstone to preserve the precision and relative accuracy of the transformed coordinates is the relative position of the control points despite their source.

Development of Virtual Reference Station in Kinematic Schemes of Geodetic GPS Network Using the Method of Maximum Informative Zone

The factual data on error of positioning in VRS GPS networks have been analyzed, where the mobile receiver is provided with VRS. The method of highly informative zone is suggested for removal of initial vagueness in selection of reference stations for purposes of development of VRS on the basis of minimum GPS network, composed of three reference stations. The recommendations on use of suggested method are given.

Evaluation of GPS Precise Point Positioning for Geoinformatics Community

Precise Geodetic GPS applications has evolved during the last two decades as the GPS processing and analysis techniques become more robotics and complicated. These progresses cover both the traditional relative positioning methods where more than one receiver are used simultaneously and the single positioning technique. In this study, GPS Precise Point Positioning (PPP) method, that does not require data from any other GPS receiver, was tested and evaluated through various computations environments. The un-differenced dual-frequency pseudo range and carrier phase observations were downloaded from one permanent international GNSS Service (IGS) station and one permanent CORS station. The GPS data were processed using various IGS orbit data in a post-processing manner using the ESA/UPC GNSS-Lab (gLAB) software. The results for various observations scenarios of the PPP solutions were investigated and analyzed. The conclusion and suggested future works are outlined in the research.

DORIS and GPS monitoring of the Gavdos calibration site in Crete

Due to its specific geographical location as well as its geodetic equipment (DORIS, GNSS, microwave transponder and tide gauges), the Gavdos station in Crete, Greece is one of the very few sites around the world used for satellite altimetry calibration. To investigate the quality of the Gavdos geodetic coordinates and velocities, we analyzed and compared here DORIS and GPS-derived results obtained during several years of observations. The DORIS solution is the latest ignwd11 solution at IGN, expressed in ITRF2008, while the GPS solution was obtained using the GAMIT software package. Current results show that 1–2mm/yr agreement can be obtained for 3-D velocity, showing a good agreement with current geophysical models. In particular, the agreement obtained for the vertical velocity is around 0.3–0.4mm/yr, depending on the terrestrial reference frame. As a by-product of these geodetic GPS and DORIS results, Zenith Tropospheric Delays (ZTDs) estimations were also compared in 2010 between these two techniques, and compared to ECMWF values, showing a 6.6mm agreement in dispersion without any significant difference between GPS and DORIS (with a 97.6% correlation), but with a 13–14mm agreement in dispersion when comparing to ECMWF model (with only about 90% correlation for both techniques). These tropospheric delay estimations could also provide an external calibration of the tropospheric correction used for the geophysical data of satellite altimetry missions.

Equivalency of geologic and geodetic rates in contractional orogens: New insights from the Pamir Frontal Thrust

Across contractional orogens, the equivalency between decadal convergence rates from geodetic GPS data and geologic shortening rates at time scales of thousands or millions of years has rarely been documented. Here, we present an example from the northern margin of Chinese Pamir, where the Main Pamir Thrust is tectonically quiescent, and recent deformation is concentrated on the Pamir Frontal Thrust (PFT). Based on dated and faulted fluvial terraces, magnetostratigraphy, and mapping, the horizontal shortening rate of the PFT is ∼6–7 mm/a at time scales of both ∼18.4 ka and ∼0.35 Ma, comparable to the geodetic rate of ∼6–9 mm/a across the same zone, implying that modern geodetic rates are a reasonable proxy for geologic rates since ∼0.35 Ma. Comparing this example with studies in other contractional orogens, we conjecture that a match or mismatch of geologic‐geodetic rates typically depends on the time scale of observation, fault geometry, and fault mechanics.