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Adhikari, L, Xie FQ, Haase JS.  2016.  Application of the full spectrum inversion algorithm to simulated airborne GPS radio occultation signals. Atmospheric Measurement Techniques. 9:5077-5087.   10.5194/amt-9-5077-2016   AbstractWebsite

With a GPS receiver on board an airplane, the airborne radio occultation (ARO) technique provides dense lower-tropospheric soundings over target regions. Large variations in water vapor in the troposphere cause strong signal multipath, which could lead to systematic errors in RO retrievals with the geometric optics (GO) method. The space-borne GPS RO community has successfully developed the full-spectrum inversion (FSI) technique to solve the multipath problem. This paper is the first to adapt the FSI technique to retrieve atmospheric properties (bending and refractivity) from ARO signals, where it is necessary to compensate for the receiver traveling on a non-circular trajectory inside the atmosphere, and its use is demonstrated using an end-to-end simulation system. The forward-simulated GPS L1 (1575.42 MHz) signal amplitude and phase are used to test the modified FSI algorithm. The ARO FSI method is capable of reconstructing the fine vertical structure of the moist lower troposphere in the presence of severe multipath, which otherwise leads to large retrieval errors in the GO retrieval. The sensitivity of the modified FSI-retrieved bending angle and refractivity to errors in signal amplitude and errors in the measured refractivity at the receiver is presented. Accurate bending angle retrievals can be obtained from the surface up to similar to 250m below the receiver at typical flight altitudes above the tropopause, above which the retrieved bending angle becomes highly sensitive to the phase measurement noise. Abrupt changes in the signal amplitude that are a challenge for receiver tracking and geometric optics bending angle retrieval techniques do not produce any systematic bias in the FSI retrievals when the SNR is high. For very low SNR, the FSI performs as expected from theoretical considerations. The 1% in situ refractivity measurement errors at the receiver height can introduce a maximum refractivity retrieval error of 0.5% (1 K) near the receiver, but the error decreases gradually to similar to 0.05% (0.1 K) near the surface. In summary, the ARO FSI successfully retrieves the fine vertical structure of the atmosphere in the presence of multipath in the lower troposphere.

and Aster, R.C., S*JS.  1993.  Comprehensive Characterization of Waveform Similarity in Large Microearthquake Data Sets. Bull. Seis. Soc. Am. Note. 83:1307-1314.
Bowling, T, Calais E, Haase JS.  2013.  Detection and modelling of the ionospheric perturbation caused by a Space Shuttle launch using a network of ground-based Global Positioning System stations. Geophysical Journal International. 192:1324-1331.   10.1093/gji/ggs101   AbstractWebsite

The exhaust plume of the Space Shuttle during its ascent triggers acoustic waves which propagate through the atmosphere and induce electron density changes at ionospheric heights which changes can be measured using ground-based Global Positioning System (GPS) phase data. Here, we use a network of GPS stations to study the acoustic wave generated by the STS-125 Space Shuttle launch on May 11, 2009. We detect the resulting changes in ionospheric electron density, with characteristics that are typical of acoustic waves triggered by explosions at or near the Earth's surface or in the atmosphere. We successfully reproduce the amplitude and timing of the observed signal using a ray-tracing model with a moving source whose amplitude is directly scaled by a physical model of the shuttle exhaust energy, acoustic propagation in a dispersive atmosphere and a simplified two-fluid model of collisions between neutral gas and free electrons in the ionosphere. The close match between observed and model waveforms validates the modelling approach. This raises the possibility of using ground-based GPS networks to estimate the acoustic energy release of explosive sources near the Earth's surface or in atmosphere, and to constrain some atmospheric acoustic parameters.

Calais, E, Haase JS, Minster JB.  2003.  Detection of ionospheric perturbations using a dense GPS array in Southern California. Geophysical Research Letters. 30 AbstractWebsite

[1] We present observations of high-frequency ionospheric perturbations detected using Global Positioning System (GPS) data from the Southern California Integrated GPS Network (SCIGN), a densely spaced GPS array of about 250 stations centered on the Los Angeles area. We show examples of perturbations with 3 - 10 minute periods that may result from coupling between the ionosphere and neutral gravity waves generated in the lower atmosphere. Although the signal-to-noise ratio of the perturbations is relatively small, we show how multi-station array processing techniques can take advantage of the high spatial density of the measurements and the coherence of the signal over a region the size of the SCIGN to considerably improve the detection capability.

Carvajal, M, Araya-Cornejo C, Sepulveda I, Melnick D, Haase JS.  2019.  Nearly instantaneous tsunamis following the Mw 7.5 2018 Palu earthquake. Geophysical Research Letters. 46:5117-5126.   10.1029/2019gl082578   AbstractWebsite

The tsunami observations produced by the 2018 magnitude 7.5 Palu strike-slip earthquake challenged the traditional basis underlying tsunami hazard assessments and early warning systems. We analyzed an extraordinary collection of 38 amateur and closed circuit television videos to show that the Palu tsunamis devastated widely separated coastal areas around Palu Bay within a few minutes after the mainshock and included wave periods shorter than 100 s missed by the local tide station. Although rupture models based on teleseismic and geodetic data predict up to 5-m tsunami runups, they cannot explain the higher surveyed runups nor the tsunami waveforms reconstructed from video footage, suggesting either these underestimate actual seafloor deformation and/or that non-tectonic sources were involved. Post-tsunami coastline surveys combined with video evidence and modeled tsunami travel times suggest that submarine landslides contributed to tsunami generation. The video-based observations have broad implications for tsunami hazard assessments, early warning systems, and risk-reduction planning. Plain Laguage Summary Tsunami hazard assessment is routinely based on assessing the impacts of long-period waves generated by vertical seafloor motions reaching the coast tens of minutes after the earthquake in typical subduction-zone environments. This view is inadequate for assessing hazard associated with strike-slip earthquakes such as the magnitude 7.5 2018 Palu earthquake, which resulted in tsunami effects much larger than would normally be associated with horizontal fault motion. From an extraordinary collection of 38 amateur and closed circuit television videos we estimated tsunami arrival times, amplitudes, and wave periods at different locations around Palu Bay, where the most damaging waves were reported. We found that the Palu tsunamis devastated widely separated coastal areas within a few minutes after the mainshock and included unusually short wave periods, which cannot be explained by the earthquake fault slip alone. Post-tsunami surveys show changes in the coastline, and this combined with video footage provides potential locations of submarine landslides as tsunami sources that would match the arrival times of the waves. Our results emphasize the importance of estimating tsunami hazards along coastlines bordering strike-slip fault systems and have broad implications for considering shorter-period nearly instantaneous tsunamis in hazard mitigation and tsunami early warning systems.

Chen, SH, Zhao Z, Haase JS, Chen AD, Vandenberghe F.  2008.  A study of the characteristics and assimilation of retrieved MODIS total precipitable water data in severe weather simulations. Monthly Weather Review. 136:3608-3628.   10.1175/2008mwr2384.1   AbstractWebsite

This study determined the accuracy and biases associated with retrieved Moderate Resolution Imaging Spectroradiometer (MODIS) total precipitable water (TPW) data, and it investigated the impact of these data on severe weather simulations using the Weather Research and Forecast (WRF) model. Comparisons of MODIS TPW with the global positioning system (GPS) TPW and radiosonde-derived TPW were carried out. The comparison with GPS TPW over the United States showed that the root-mean-square (RMS) differences between these two datasets were about 5.2 and 3.3 mm for infrared (IR) and near-infrared (nIR) TPW, respectively. MODIS IR TPW data were overestimated in a dry atmosphere but underestimated in a moist atmosphere, whereas the nIR values were slightly underestimated in a dry atmosphere but overestimated in a moist atmosphere. Two cases, a severe thunderstorm system (2004) over land and Hurricane Isidore (2002) over ocean, as well as conventional observations and Special Sensor Microwave Imager (SSM/I) retrievals were used to assess the impact of MODIS nIR TPW data on severe weather simulations. The assimilation of MODIS data has a slightly positive impact on the simulated rainfall over Oklahoma for the thunderstorm case, and it was able to enhance Isidore's intensity when the storm track was reasonably simulated. The use of original and bias-corrected MODIS nIR TPW did not show significant differences from both case studies. In addition, SSM/I data were found to have a positive impact on both severe weather simulations, and the impact was comparable to or slightly better than that of MODIS data.

Chen, XM, Chen SH, Haase JS, Murphy BJ, Wang KN, Garrison JL, Chen SY, Huang CY, Adhikari L, Xie F.  2018.  The impact of airborne radio occultation observations on the simulation of Hurricane Karl (2010). Monthly Weather Review. 146:329-350.   10.1175/mwr-d-17-0001.1   AbstractWebsite

This study evaluates, for the first time, the impact of airborne global positioning system radio occultation (ARO) observations on a hurricane forecast. A case study was conducted of Hurricane Karl during the PreDepression Investigation of Cloud-Systems in the Tropics (PREDICT) field campaign in 2010. The assimilation of ARO data was developed for the three-dimensional variational (3DVAR) analysis system of the Weather Research and Forecasting (WRF) Model version 3.2. The impact of ARO data on Karl forecasts was evaluated through data assimilation (DA) experiments of local refractivity and nonlocal excess phase (EPH), in which the latter accounts for the integrated horizontal sampling along the signal ray path. The tangent point positions (closest point of an RO ray path to Earth's surface) drift horizontally, and the drifting distance of ARO data is about 2 to 3 times that of spaceborne RO, which was taken into account in these simulations. Results indicate that in the absence of other satellite observations, the assimilation of ARO EPH resulted in a larger impact on the analysis than local refractivity did. In particular, the assimilation of ARO observations at the actual tangent point locations resulted in more accurate forecasts of the rapid intensification of the storm. Among all experiments, the best forecast was obtained by assimilating ARO data with the most accurate geometric representation, that is, the use of nonlocal EPH operators with tangent point drift, which reduced the error in the storm's predicted minimum sea level pressure (SLP) by 43% beyond that of the control experiment.

Crowell, BW, Melgar D, Bock Y, Haase JS, Geng JH.  2013.  Earthquake magnitude scaling using seismogeodetic data. Geophysical Research Letters. 40:6089-6094.   10.1002/2013gl058391   AbstractWebsite

The combination of GPS and strong-motion data to estimate seismogeodetic waveforms creates a data set that is sensitive to the entire spectrum of ground displacement and the full extent of coseismic slip. In this study we derive earthquake magnitude scaling relationships using seismogeodetic observations of either P wave amplitude or peak ground displacements from five earthquakes in Japan and California ranging in magnitude from 5.3 to 9.0. The addition of the low-frequency component allows rapid distinction of earthquake size for large magnitude events with high precision, unlike accelerometer data that saturate for earthquakes greater than M 7 to 8, and is available well before the coseismic displacements are emplaced. These results, though based on a limited seismogeodetic data set, support earlier studies that propose it may be possible to estimate the final magnitude of an earthquake well before the rupture is complete.

Dautermann, T, Calais E, Haase J, Garrison J.  2007.  Investigation of ionospheric electron content variations before earthquakes in southern California, 2003-2004. Journal of Geophysical Research-Solid Earth. 112   B0210610.1029/2006jb004447   AbstractWebsite

[1] It has been proposed that earthquakes are preceded by electromagnetic signals detectable from ground- and space-based measurements. Ionospheric anomalies, such as variations in the electron density a few days before earthquakes, are one of the precursory signals proposed. Since Global Positioning System (GPS) data can be used to measure the ionospheric total electron content (TEC), the technique has received attention as a potential tool to detect ionospheric perturbations related to earthquakes. Here, we analyze 2 years ( 2003 - 2004) of data from the Southern California Integrated GPS Network (SCIGN), a dense network of 265 continuous GPS stations centered on the Los Angeles basin, for possible precursors. This time period encompasses the December 2003, M6.6, San Simeon and September 2004, M6.0, Parkfield earthquakes. We produce TEC time series at all SCIGN sites and apply three different statistical tests to detect anomalous TEC signals preceding earthquakes. We find anomalous TEC signals but no statistically significant correlation, in time or in space, between these TEC anomalies and the occurrence of earthquakes in southern California for the 2003 - 2004 period. This result does not disprove the possibility of precursory phenomena but show the signal-to-noise ratio of a hypothetical TEC precursor signature is too low to be detected by the analysis techniques employed here. Precursors may still be revealed for future large earthquakes in well instrumented areas such as California and Japan, if the tests can be developed into techniques that can better separate external influences from the actual TEC signal.

Douilly, R, Haase JS, Ellsworth WL, Bouin M-P, Calais E, Symithe S, Armbruster JG, Mercier De Lepinay BF, Deschamps A, Saint-Louis M, Meremonte ME, Hough SE.  2012.  Improving the resolution of the 2010 Haiti earthquake fault geometry using temporary seismometer deployments. Bull. Seis. Soc. Am. in review Abstract

Haiti has several active faults that are capable of producing large earthquakes such as the 2010 Mw 7.0 Haiti earthquake. This earthquake was not unexpected, given geodetic measurements showing strain accumulation on the Enriquillo Plantain Garden Fault Zone, the major fault system in southern Haiti (Manaker et al. 2008). GPS and INSAR data (Calais et al., 2010) show, however, that this rupture occurred on the previously unmapped Léogâne fault, a 60° north dipping oblique blind thrust located immediately north of the Enriquillo Fault. Following the earthquake, several groups installed temporary seismic stations to record aftershocks. Natural Resources Canada installed three broadband seismic stations, Géoazur installed 21 ocean bottom seismometers, L’Institut de Physique du Globe de Paris installed 5 broadband seismometers, and the United States Geological Survey deployed 17 short period and strong motion seismometers in and around Port-au-Prince. We use data from this complete set of stations, along with data from permanent regional stations, to relocate all of the events from March 17 to June 24, to determine the regional one-dimensional crustal structure and determine focal mechanisms. The aftershock locations from the combined data set clearly delineate the Léogâne fault. The strike and dip closely agrees with that of the global centroid moment tensor solution, but appears to be more steeply dipping than the finite fault inversions. The aftershocks also delineate a flat structure on the west side of the rupture zone and may indicate triggered seismicity on the Trois Baies fault, although the depths of these events are not as well constrained. There is no clear evidence for aftershocks on the other rupture segments inferred in the Hayes et al. (2010) mainshock rupture model. There is a cluster of aftershocks in the hanging wall near the western patch of high slip identified by Calais et al. (2010) and Meng et al. (2011), or central patch in the Hayes et al. (2010) model. We use first-motion focal mechanism solutions to clarify the relationship of the fault geometry to the mechanisms of the larger events.

Douilly, R, Haase JS, Ellsworth WL, Bouin MP, Calais E, Symithe SJ, Armbruster JG, de Lepinay BM, Deschamps A, Mildor SL, Meremonte ME, Hough SE.  2013.  Crustal structure and fault geometry of the 2010 Haiti earthquake from temporary seismometer deployments. Bulletin of the Seismological Society of America. 103:2305-2325.   10.1785/0120120303   AbstractWebsite

Haiti has been the locus of a number of large and damaging historical earthquakes. The recent 12 January 2010 M-w 7.0 earthquake affected cities that were largely unprepared, which resulted in tremendous losses. It was initially assumed that the earthquake ruptured the Enriquillo Plantain Garden fault (EPGF), a major active structure in southern Haiti, known from geodetic measurements and its geomorphic expression to be capable of producing M 7 or larger earthquakes. Global Positioning Systems (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data, however, showed that the event ruptured a previously unmapped fault, the Leogane fault, a north-dipping oblique transpressional fault located immediately north of the EPGF. Following the earthquake, several groups installed temporary seismic stations to record aftershocks, including ocean-bottom seismometers on either side of the EPGF. We use data from the complete set of stations deployed after the event, on land and offshore, to relocate all aftershocks from 10 February to 24 June 2010, determine a 1D regional crustal velocity model, and calculate focal mechanisms. The aftershock locations from the combined dataset clearly delineate the Leogane fault, with a geometry close to that inferred from geodetic data. Its strike and dip closely agree with the global centroid moment tensor solution of the mainshock but with a steeper dip than inferred from previous finite fault inversions. The aftershocks also delineate a structure with shallower southward dip offshore and to the west of the rupture zone, which could indicate triggered seismicity on the offshore Trois Baies reverse fault. We use first-motion focal mechanisms to clarify the relationship of the fault geometry to the triggered aftershocks.

Emore, GL, Haase JS, Choi K, Larson KA, Yamagiwa A.  2007.  Recovering seismic displacements through combined use of 1-Hz GPS and strong-motion accelerometers. Bulletin of the Seismological Society of America. 97:357-378.   10.1785/0120060153   AbstractWebsite

Retrieving displacement from seismic acceleration records is often difficult because unknown small baseline offsets in the acceleration time series will contaminate the doubly integrated record with large quadratic errors. One-hertz Global Positioning System (GPS) position estimates and collocated seismic data are available from the 2003 M-W 8 Tokachi-Oki (Hokkaido) earthquake. After a process of correcting for possible misorientation of the seismic sensors, an inversion method is used to simultaneously solve for ground displacement with both data sets as input constraints. This inversion method takes into account the presence of unknown offsets in the acceleration record, and the relatively large uncertainties in the estimated 1-Hz GPS positions. In this study, 117 channels of seismic data were analyzed. Only 5% of the time does the static displacement retrieved from traditional baseline correction processing without GPS information agree with the absolute displacement measured with 1-Hz GPS to within the errors of the GPS data. In solving simultaneously for constrained displacements that agree with both the seismic and GPS data sets, an optimal solution was found that included only one- or two-step functions in the acceleration records. Potential explanations for the offsets are analyzed in terms of tilt of the sensor or electronic noise. For nine stations, clear misorientations of the seismic sensors of more than 20 deg from the reported orientation were found. For this size event, the 30-sec sampled GPS solutions were also a sufficient constraint for establishing the offset errors and recovering reliable displacements. The results significantly extend the frequency band over which accelerometer data are reliable for source inversion studies.

Evans, C, Archambault HM, Cordeira JM, Fritz C, Galarneau Jr. TJ, Gjorjievska S, Griffin KS, Johnson A, Komaromi WA, Monette S, Muradyan P, Murphy B, Riemer M, Sears J, Stern D, Tang B, Thompson S.  2012.  The Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) field campaign: Perspectives of early career scientists. Bulletin of the American Meteorological Society. 92:173-187.   10.1175/BAMS-D-11-00024.1  
Frankel, A, Harmsen S, Mueller C, Calais E, Haase J.  2011.  Seismic Hazard Maps for Haiti. Earthquake Spectra. 27:S23-S41.   10.1193/1.3631016   Abstract

We have produced probabilistic seismic hazard maps of Haiti for peak ground acceleration and response spectral accelerations that include the hazard from the major crustal faults, subduction zones, and background earthquakes. The hazard from the Enriquillo-Plantain Garden, Septentrional, and Matheux-Neiba fault zones was estimated using fault slip rates determined from GPS measurements. The hazard from the subduction zones along the northern and southeastern coasts of Hispaniola was calculated from slip rates derived from GPS data and the overall plate motion. Hazard maps were made for a firm-rock site condition and for a grid of shallow shear-wave velocities estimated from topographic slope. The maps show substantial hazard throughout Haiti, with the highest hazard in Haiti along the Enriquillo-Plantain Garden and Septentrional fault zones. The Matheux-Neiba Fault exhibits high hazard in the maps for 2% probability of exceedance in 50 years, although its slip rate is poorly constrained.

Fritts, DC, Abdu MA, Batista BR, Batista IS, Batista PP, Buriti R, Clemesha BR, Dautermann T, de Paula E, Fechine BJ, Fejer B, Gobbi D, Haase J, Kamalabadi F, Laughman B, Lima PP, Liu H-L, Medeiros A, Pautet D, Riggin DM, Sabbas SF, Sobral JHA, Stamus P, Takahashi H, Taylor MJ, Vadas SL, Wrasse C.  2009.  The Spread F Experiment (SpreadFEx): Program overview and first results. Earth Planets Space. 61:411-430.
Fritts, DC, Abdu MA, Batista BR, Batista IS, Batista PP, Buriti R, Clemesha BR, Dautermann T, de Paula E, Fechine BJ, Fejer B, Gobbi D, Haase J, Kamalabadi F, Laughman B, Lima PP, Liu H-L, Medeiros A, Pautet D, Riggin DM, Sabbas SF, Sobral JHA, Stamus P, Takahashi H, Taylor MJ, Vadas SL, Wrasse C.  2009.  Overview and Summary of the Spread F Experiment (SpreadFEx). Annales Geophysicae. 27:1-15.
Garrison, JL, Lee SCG, Haase JS, Calais E.  2007.  A method for detecting ionospheric disturbances and estimating their propagation speed and direction using a large GPS network. Radio Science. 42   Rs601110.1029/2007rs003657   AbstractWebsite

A technique is developed for detecting short period (3-10 min) ionospheric disturbances and estimating their propagation speed and direction using data from a large GPS network (a hundred or more receivers). This method increases the signal-to-noise ratio of small signals and could be applied, autonomously, to process a large set of data for the study of the potential signal sources and statistical distributions of these disturbances. The integral electron content (IEC) for every satellite-station pair in the network is extracted from dual frequency phase data. These IEC time series are then band-pass-filtered and cross-correlated with each other. The resulting correlation power is an indication of the presence of a common disturbance recorded at the two stations, and the delay to the maximum correlation is a measurement of the propagation time between the ionospheric pierce points of the respective stations. A threshold on correlation power is used to select a subset of these delay measurements. The velocity of the detected perturbation is then estimated by fitting a two-dimensional plane wave model to this subset of measurements. A technique is developed to remove the effects of time-varying satellite motion and to reconstruct the waveform that would have been observed at a fixed point within the ionosphere. Consistency of the resulting velocity estimates is checked using a stacking-alignment method and a time-distance mapping that accounts for the motion of the GPS satellites. The sensitivity of the velocity estimate to both the assumed height of a thin-layer ionosphere and the detection threshold value is studied. A simulation is used to demonstrate the IEC waveform distortion due to satellite motion, and an example is shown in which this distortion is able to shift the dominant frequencies of an actual disturbance outside of the passband of the filter, thereby preventing detection. Four weeks of data, in different seasons, collected using the Southern California Integrated GPS Network (SCIGN), were processed. Over the total of 28 days, 127 significant disturbances were detected, most with horizontal propagation speeds between 50-1000 m/s and westward directions of propagation. A few cases with exceptionally high speed (>2000 m/s) were observed. It is hypothesized that these are manifestations of disturbances that occur simultaneously throughout the ionosphere, rather than traveling waves. The rate of occurrence of disturbances in the 3-10 min band was found to be larger than expected. Observational biases of this method are discussed.

Garrison, JL, Voo J, Yueh SH, Grant MS, Fore AG, Haase JS.  2011.  The estimation of sea surface roughness effects in microwave radiometric measurements of salinity using reflected Global Navigation Satellite System signals. IEEE Geoscience and Remote Sensing Letters. PP:1-5.   10.1109/LGRS.2011.2159323   Abstract

In February–March 2009, an airborne field campaign was conducted using the Passive Active L- and S-band (PALS) microwave sensor and the Ku-band Polarimetric Scatterometer to collect measurements of brightness temperature and near-surface wind speeds. Flights were conducted over a region of expected high-speed winds in the Atlantic Ocean, for the purposes of algorithm development for sea surface salinity (SSS) retrievals. Wind speeds encountered during the March 2, 2009, flight ranged from 5 to 25 m/s. The Global Positioning System (GPS) delay mapping receiver from the National Aeronautics and Space Administration (NASA) Langley Research Center was also flown to collect GPS signals reflected from the ocean surface and generate postcorrelation power-versus-delay measurements. These data were used to estimate ocean surface roughness. These estimates were found to be strongly correlated with PALS-measured brightness temperature. Initial results suggest that reflected GPS measurements made using small low-power instruments can be used to correct the roughness effects in radiometer brightness temperature measurements to retrieve accurate SSS.

Ge, MR, Calais E, Haase J.  2000.  Reducing satellite orbit error effects in near real-time GPS zenith tropospheric delay estimation for meteorology. Geophysical Research Letters. 27:1915-1918. AbstractWebsite

We investigate the influence of using IGS predicted orbits for near real-time zenith tropospheric delay determination from GPS and implement a new processing strategy that allows the use of predicted orbits with minimal degradation of the ZTD estimates. Our strategy is based on the estimation of the three Keplerian parameters that represent the main error sources in predicted orbits (semi-major axis, inclination, and argument of perigee), and their associated variance directly from the real-time GPS data set. A comparison with the use of IGS final orbits shows a negligible bias and an rms less than 6 mm in the ZTD estimates at all the stations. The improvement compared to a strategy based only on the quality index provided with the predicted orbits is 20%. The level of accuracy and turn-around time shown here meet the current requirements for operational meteorology.

Ge, M, Calais E, Haase J.  2001.  Automatic orbit quality control for near real-time GPS zenith tropospheric delay estimation. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:177-181. AbstractWebsite

We implement an automatic orbit quality control procedure in order to reduce the effect of satellite orbit error for near real-time estimating of zenith total delay (ZTD). We estimate the three Keplerian parameters of the GPS orbits that represent the main error sources (semimajor axis, inclination, and argument of perigee). We start with an a priori constraint corresponding to the typical orbit accuracy for each parameter and adjust it iteratively according to its estimated value and variance. This data analysis procedure shows an improvement of 20% in ZTD rms compared to a strategy based only on the quality index provided with the IGS predicted orbits. A real-time test with IGS ultra-rapid orbits also shows significant improvement compared to fixing IGS ultra-rapid orbits. This strategy allows for a totally automated estimation of orbital parameters directly from the real-time GPS data, without altering the accuracy of the ZTD estimates. (C) 2001 Elsevier Science Ltd. All rights reserved.

Ge, MR, Calais E, Haase J.  2002.  Sensitivity of zenith total delay accuracy to GPS orbit errors and implications for near-real-time GPS meteorology. Journal of Geophysical Research-Atmospheres. 107   431510.1029/2001jd001095   AbstractWebsite

[1] Global Positioning System (GPS) measurements have been demonstrated to provide precipitable water vapor (PWV) estimates with a level of accuracy that is comparable to that of radiosondes and microwave radiometers. GPS measurements therefore have the potential to become a significant source of data for operational weather forecasting, provided that PWV (or the intermediate zenith total delay (ZTD)) can be made available in near real-time with a minimum accuracy degradation. Despite the recent decrease in the latency and increase in accuracy provided by the International GPS Service (IGS) ultrarapid predicted GPS orbit products, we show that the accuracy of these orbits continues to be a limiting factor for the accuracy of near real-time GPS-derived atmospheric estimates. In this work, a coefficient matrix is derived from the normal equations of the least squares adjustment model for the GPS observables that maps the orbital parameter errors into ZTD errors. This is used to analyze the sensitivity of GPS derived tropospheric errors to an extensive set of parameters, including their time dependence, in a computationally efficient manner. We show that ZTD errors are dominated by biases in the orbital semimajor axis, with minor contributions from the inclination and argument of perigee, and that this error increases significantly after the fourth to fifth hour of the prediction window. We implemented a GPS data processing strategy based on an iterative estimation of the three most critical orbital parameters (semimajor axis, inclination and argument of perigee) together with the ZTD parameters. We tested this strategy in a 3500 3500 km network of 15 GPS sites in western Europe providing hourly data files. We show that the standard deviation improvement compared to a strategy based only on the orbit quality index provided with the predicted orbit products is on the order of 20%. The analysis of one month of data in near-real-time shows a bias lower than 1 mm ZTD and a standard deviation lower than 6 mm ZTD compared to using the most precise IGS final orbits. We also show that this strategy is robust and capable of dealing with very large orbit errors appropriately. We demonstrate that the same quality is achievable with a 1500 1500 km network which has positive implications for decentralized processing strategies. The near real-time processing methodology described here meets the current timeliness requirements of operational meteorology (30 mn to 2 hours, depending on the application), while ensuring a level of accuracy similar to that provided in postprocessed mode with precise final IGS orbits (1 mm ZTD bias, 6 mm ZTD RMS). The method we propose can also be considered as an "on-the-fly'' orbit quality control for near real-time GPS applications.

Geng, JH, Bock Y, Melgar D, Crowell BW, Haase JS.  2013.  A new seismogeodetic approach applied to GPS and accelerometer observations of the 2012 Brawley seismic swarm: Implications for earthquake early warning. Geochemistry Geophysics Geosystems. 14:2124-2142.   10.1002/ggge.20144   AbstractWebsite

The 26 August 2012 Brawley seismic swarm of hundreds of events ranging from M1.4 to M5.5 in the Salton Trough, California provides a unique data set to investigate a new seismogeodetic approach that combines Global Positioning System (GPS) and accelerometer observations to estimate displacement and velocity waveforms. First in simulated real-time mode, we analyzed 1-5 Hz GPS data collected by 17 stations fully encircling the swarm zone at near-source distances up to about 40km using precise point positioning with ambiguity resolution (PPP-AR). We used a reference network of North American GPS stations well outside the region of deformation to estimate fractional-cycle biases and satellite clock parameters, which were then combined with ultrarapid orbits from the International GNSS Service to estimate positions during the Brawley seismic swarm. Next, we estimated seismogeodetic displacements and velocities from GPS phase and pseudorange observations and 100-200 Hz accelerations collected at three pairs of GPS and seismic stations in close proximity using a new tightly coupled Kalman filter approach as an extension of the PPP-AR process. We can clearly discern body waves in the velocity waveforms, including P-wave arrivals not detectable with the GPS-only approach for earthquake magnitudes as low as M-w 4.6 and significant static offsets for magnitudes as low as M-w 5.4. Our study shows that GPS networks upgraded with strong motion accelerometers can provide new information for improved understanding of the earthquake rupture process and be of critical value in creating a robust early warning system for any earthquake of societal significance.

Haase, J, Calais E, Talaya J, Rius A, Vespe F, Santangelo R, Huang XY, Davila JM, Ge M, Cucurull L, Flores A, Sciarretta C, Pacione R, Boccolari M, Pugnaghi S, Vedel H, Mogensen K, Yang X, Garate J.  2001.  The contributions of the MAGIC project to the COST 716 objectives of assessing the operational potential of ground-based GPS meteorology on an international scale. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:433-437. AbstractWebsite

MAGIC (Meteorological Applications of GPS Integrated Column Water Vapor Measurements in the Western Mediterranean) is a 3 year project financed in part by the European Commission for research on deriving and validating robust GPS integrated water vapor (IWV) and zenith tropospheric delay (ZTD) data sets and developing methods to assimilate the data into numerical weather prediction models (NWP) and test their impact. It was conceived independently from the COST 716 action, which seeks to coordinate research in the domain at an international scale, but addresses some of the same objectives. This has led to a productive cooperation between the two initiatives and their participants, and motivated the decision of MAGIC participants to provide research results as part of the COST demonstration system. Currently a database of 1.5 years of ZTD data are available on the MAGIC web site which has been validated through comparisons with radiosondes which gives differences with a standard deviation of 10 mm ZTD or the equivalent error in IWV of 1.6 kg/m(2). NWP assimilation tests will be carried out in the final year of the project. (C) 2001 Elsevier Science Ltd. All rights reserved.

Haase, JS, Vedel H, Ge M, Calais E.  2001.  GPS zenith tropospheric delay (ZTD) variability in the Mediterranean. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:439-443. AbstractWebsite

In the context of the MAGIC project (Meteorological Applications of GPS Integrated Column Water Vapor Measurements in the Western Mediterranean), zenith tropospheric delays (ZTD) are computed from Global Positioning System (GPS) signals at permanent sites in the western Mediterranean area continuously since November 1998. These time series are compared to the equivalent values estimated by integrating the atmospheric column derived from radiosonde profiles and the HIRLAM numerical weather prediction model in order to determine the error statistics of the data. From a year and a half of data, we show that the difference between radiosonde and GPS ZTD has a standard deviation of 12 mm and a bias of less than 5 mm for most stations. The biases between the GPS ZTD and HIRLAM estimates are smaller, but the standard deviation is greater, usually on the order of 17 mm. The standard deviation of the residuals from both comparisons depends greatly on the humidity which produces an annual signal because of the much higher humidity variability in the summer months. (C) 2001 Elsevier Science Ltd. All rights reserved.