Pa Cross

Interferometric synthetic aperture radar (InSAR) atmospheric correction: GPS, Moderate Resolution Imaging Spectroradiometer (MODIS), and InSAR integration

Atmospheric effects represent one of the major limitations of repeat-pass interferometric synthetic aperture radar (InSAR). In this paper, GPS, and Moderate Resolution Imaging Spectroradiometer (MODIS) data were integrated to provide regional water vapor fields with a spatial resolution of 1 km × 1 km, and a water vapor correction model based on the resultant water vapor fields was successfully incorporated into the Jet Propulsion Laboratory/California Institute of Technology ROI_PAC software. The advantage of this integration approach is that only one continuous GPS station is required within a 2030 km × 1354 km MODIS scene. Application to ERS-2 repeat-pass data over the Los Angeles Southern California Integrated GPS Network (SCIGN) area shows that this integration approach not only helps discriminate geophysical signals from atmospheric artifacts but also reduces water vapor effects significantly, which is of great interest to a wide community of geophysicists.

Comparison of precipitable water vapor derived from radiosonde, GPS, and Moderate-Resolution Imaging Spectroradiometer measurements

Atmospheric water vapor is highly variable in both space and time across the Earth, and knowledge of the distribution of water vapor is essential in understanding weather and global climate. In addition, knowledge of the amount of atmospheric water vapor is required for high-precision interferometric synthetic aperture radar (InSAR) applications due to its significant impact on microwave signals, which is the principal motivation for this study. In order to assess the performance of different instruments, i.e., radiosondes (RS), Global Positioning System (GPS), and the Moderate-Resolution Imaging Spectroradiometer (MODIS) and for measuring precipitable water vapor (PWV), coincident observations collected at the Atmospheric Radiation Measurement Southern Great Plains site and at the Herstmonceux site over a 8-11 month period are used for time series intercomparisons. In this study, the Terra MODIS near-infrared water vapor products (Collection 3) were examined. In addition, a first spatial comparison of MODIS PWV and GPS PWV was performed using data covering all of Germany and kindly supplied by the GeoForschungsZentrum Potsdam. Time series comparisons of PWV between radiosondes and GPS show that the scale factors of PWV from radiosondes and GPS agreed to 4% with correlation coefficients higher than 0.98 and standard deviations about 1 mm. A significant day-night difference was found for Vaisala RS90 radiosondes in comparison with GPS PWV, with nighttime launches having a scale factor 4% larger, but agreeing overall better. It is also shown that GPS PWV and RS PWV agreed better with each other than with MODIS PWV, and the differences of MODIS PWV relative to GPS or RS were larger than those between GPS PWV and RS PWV. MODIS PWV appeared to overestimate PWV against RS, with scale factors from 1.14 to 1.20 and standard deviations from 1.6 to 2.2 mm. MODIS PWV appeared to overestimate PWV against GPS, with scale factors from 1.07 to 1.14 and standard deviations varying from 0.8 to 1.4 mm in time series. The larger differences relative to MODIS PWV are likely to be caused by uncertainties in the spectroscopic database for the MODIS retrievals, calibration uncertainties in the radiances measured by MODIS, operational differences of the three systems, and different mapping functions adopted in GPS and MODIS PWV retrievals. We derived a linear fit model to calibrate MODIS PWV, and better agreements between calibrated MODIS PWV and GPS PWV in space have been achieved. This indicates that MODIS PWV products should be updated or calibrated using a linear fit model before being applied to correct InSAR measurements. Also, the potential accuracy of standard resolution (resampled) radiosonde data from the UK Met Office and the University of Wyoming has been assessed. It is demonstrated that some caution needs to be exercised when using standard resolution data.

The strain rate field in the eastern Mediterranean region, estimated by repeated GPS measurements

We use the combined GPS velocity field of the eastern Mediterranean for the period 1988 to 1996 to determine crustal deformation strain rates in a region comprising the Hellenic arc, the Aegean Sea, and western Anatolia. We interpret the velocity field and determine the strain rate tensor by the spatial derivatives of the collocated motion vectors. The region following the line Marmara Sea, North Aegean Trough, northern central Greece, and the central Ionian islands is associated with strong right-lateral shear motion, with maximum shear strain rates of 180 nano-strain/a (180×10−9/a). In the central Aegean Sea, N–S-oriented extensional processes prevail, reaching 100 nano-strain/a. The southern Aegean is characterized by relatively small strain rates. Maximum extensional components of the strain rate tensor, reaching 150 nano-strain/a in a N–S direction, are found in central Greece. The Hellenic arc is associated with moderate arc-parallel extension and strong compression perpendicular to it. Projections of the strain rates parallel to the major fault zones reveal that the northern Aegean is governed by the westward continuation of the North Anatolian Fault Zone which is associated with strong dextral shearing (maximum 220 nano-strain/a), accompanied by numerous large earthquakes in this century.

Geodetic estimate of seismic hazard in the Gulf of Korinthos

The recent 15 June 1995, M0 = 6.0 × 1018 N m, Aigion earthquake in the western Gulf of Korinthos has focussed attention on the seismic hazard of the region. Although there have been few large earthquakes in the region during this century, the historical record suggests that there may have been many large earthquakes there in the interval 1750–1900. We present geodetic data that give estimates of the rate of extension of the Gulf of Korinthos during this century and which suggest that less than half of the elastic strain in the central and western Gulf of Korinthos has been released by earthquakes during this century. In contrast, the seismic and geodetic strains in the eastern Gulf of Korinthos are in agreement with each other. If the discrepancy between seismic and geodetic strains in the western Gulf of Korinthos that has accumulated during this century is removed in earthquakes, the moment release will be equivalent to several Ms > 6.5 earthquakes.

Assessment of the potential of MERIS near‐infrared water vapour products to correct ASAR interferometric measurements

Atmospheric water vapour is a major limitation for high precision Interferometric Synthetic Aperture Radar (InSAR) applications due to its significant impact on microwave signals. We propose a statistical criterion to test whether an independent water vapour product can reduce water vapour effects on InSAR interferograms, and assess the potential of the Medium Resolution Imaging Spectrometer (MERIS) near‐infrared water vapour products for correcting Advanced SAR (ASAR) data. Spatio‐temporal comparisons show c. 1.1 mm agreement between MERIS and GPS/radiosonde water vapour products in terms of standard deviations. One major limitation with the use of MERIS water vapour products is the frequency of cloud free conditions. Our analysis indicates that in spite of the low global cloud free conditions (∼25%), the frequency can be much higher for certain areas such as Eastern Tibet (∼38%) and Southern California (∼48%). This suggests that MERIS water vapour products show potential for correcting ASAR interferometric measurements in certain regions.

Integration of InSAR Time-Series Analysis and Water-Vapor Correction for Mapping Postseismic Motion After the 2003 Bam (Iran) Earthquake

Atmospheric water-vapor effects represent a major limitation of interferometric synthetic aperture radar (InSAR) techniques, including InSAR time-series (TS) approaches (e.g., persistent or permanent scatterers and small-baseline subset). For the first time, this paper demonstrates the use of InSAR TS with precipitable water-vapor (InSAR TS + PWV) correction model for deformation mapping. We use MEdium Resolution Imaging Spectrometer (MERIS) near-infrafred (NIR) water-vapor data for InSAR atmospheric correction when they are available. For the dates when the NIR data are blocked by clouds, an atmospheric phase screen (APS) model has been developed to estimate atmospheric effects using partially water-vapor-corrected interferograms. Cross validation reveals that the estimated APS agreed with MERIS-derived line-of-sight path delays with a small standard deviation (0.3-0.5 cm) and a high correlation coefficient (0.84-0.98). This paper shows that a better TS of postseismic motion after the 2003 Bam (Iran) earthquake is achievable after reduction of water-vapor effects using the InSAR TS + PWV technique with coincident MERIS NIR water-vapor data.

Code multipath modelling in the urban environment using large virtual reality city models: Determining the local environment

Positioning in the urban environment using GNSS is hampered by poor satellite availability due to signal obstruction created by both man-made and natural features of the urban environment. In addition, range measurement to satellites for positioning and for navigation is severely degraded by the multipath effect. The arrival and continuous enhancement of computerised geometric city models makes it possible to tackle these problems through modelling. In this paper description is given of a method for determining the local multipath environment, defined by the surfaces within a city model that will cause disruptive signal reflections to be presented to a receiver. An example simulation is performed, and graphical and numerical results are produced.

OSGM02: A NEW MODEL FOR CONVERTING GPS-DERIVED HEIGHTS TO LOCAL HEIGHT DATUMS IN GREAT BRITAIN AND IRELAND

Abstract The background to the recent computation of a new vertical datum model for the British Isles (OSGM02) is described After giving a brief description of the computational techniques and the data sets used for the derivation of the gravimetric geoid, the paper focuses on the fitting of this surface to the GPS and levelling networks in the various regions of the British Isles in such a way that it can be used in conjunction with GPS to form a replacement for the existing system of bench marks. The error sources induced in this procedure are discussed, and the theoretical basis given for the fitting procedure. Results for each major region (Great Britain, Ireland, and Northern Ireland) as well as the various independent island datums are described The problems to be expected when working between datums are discussed.

Variance component estimation applied to satellite laser ranging

The variance component estimation (VCE) method as developed by Helmert has been applied to the global SLR data set for the year 1987. In the first part of this study the observations have been divided into two groups: those from ruby and YAG laser systems, and their weights estimated over several months. It was found that the weights of both sets of stations altered slightly from month to month, but that, not surprisingly, the YAG systems consistently outperformed those based on ruby lasers. The major part of this paper then considers the estimation of the variance components (i.e. weights) at each SLR station from month to month. These were tested using the F-statistic and, although it indicated that most stations had significant temporal variations, they were generally small compared with the differences between the stations themselves, i.e. the method has been shown to be capable of discriminating between the precision with which the various laser stations are operating. The station coordinates and baseline lengths computed using both a priori, and estimated, weights where also compared and this showed that changes in the weights can have significant effects on the estimation of the station positions, particularly in the z component, and on the baseline lengths - so proving the importance of proper stochastic modelling when processing SLR data.

An Assessment of the RAIM Performance of a Combined Galileo/GPS Navigation System Using the Marginally Detectable Errors (MDE) Algorithm

Integrity relates to the trust that can be placed in the correctness of information supplied by a navigation system. It includes the ability of the navigation system to provide timely warning to users when the system fails to meet its stated accuracy. Specifically, a navigation system is required to deliver a warning (alarm) when the error in the derived user position solution exceeds an allowable level (alarm limit). This warning must be issued to the user within a given period of time (time-to-alarm) and with a given probability (integrity risk). The two main approaches to monitoring the integrity of satellite navigation systems are Receiver Autonomous Integrity Monitoring (RAIM), and monitoring based on an independent network of integrity monitoring stations and a dedicated Ground Integrity Channel (GIC). More recently Satellite Autonomous Integrity Monitoring (SAIM) methods have also been investigated.This article presents the results of a study to assess the RAIM capability of the Galileo system when used alone and when combined with the Global Positioning System (GPS). The assessment was based on the Marginally Detectable Error (MDE) algorithm. The results show a significant improvement in the capability to perform RAIM using a combined Galileo/GPS system compared to the performance using the Galileo system alone. This study was supported by Alcatel Space and was a contribution to the Galileo definition studies carried out for the European Community under the GALA project.