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Healy, SB, Haase J, Lesne O.  2002.  Abel transform inversion of radio occultation measurements made with a receiver inside the Earth's atmosphere. Annales Geophysicae. 20:1253-1256. AbstractWebsite

Radio occultation measurements made with a receiver inside the Earth's atmosphere can be inverted, assuming local spherical symmetry, with an Abel transform to provide an estimate of the atmospheric refractive index profile. The measurement geometry is closely related to problems encountered when inverting seismic time-travel data and solar occultation measurements, where the Abel solution is well known. The method requires measuring both rays that originate from above and below the local horizon of the receiver. The Abel transform operates on a profile of "partial bending angles" found by subtracting the positive elevation measurement from the negative elevation value with the same impact parameter. In principle, the refractive index profile can be derived from measurements with a single frequency GPS receiver because the ionospheric bending is removed when the partial bending angle is evaluated.

Haase, JS, Ge MR, Vedel H, Calais E.  2003.  Accuracy and variability of GPS tropospheric delay measurements of water vapor in the western Mediterranean. Journal of Applied Meteorology. 42:1547-1568. AbstractWebsite

As a preliminary step for assessing the impact of global positioning system (GPS) refractive delay data in numerical weather prediction (NWP) models, the GPS zenith tropospheric delays (ZTDs) are analyzed from 51 permanent GPS sites in the western Mediterranean. The objectives are to estimate the error statistics necessary for future assimilation of GPS ZTD data in numerical models and to investigate the variability of the data in this area. The time series, which were derived continuously from November 1998 to June 2001, are compared with independent equivalent values derived from radiosonde profiles and the High-Resolution Limited-Area Model (HIRLAM) NWP model. Based on over two years of data, the difference between radiosonde and GPS ZTD has a standard deviation of 12 mm of delay and a bias of 7 mm of delay. Some sites have biases as high as 14 mm of delay. The bimodal distribution of residuals, with a higher bias for daytime launches, indicates these biases may be due to radiosonde day-night measurement biases. The biases between the GPS ZTD and HIRLAM estimates are smaller, but the 18-mm ZTD standard deviation is significantly greater. The standard deviation of the residuals depends strongly on the amount of humidity, which produces an annual signal because of the much higher variability of water vapor in the summer months. The better agreement with radiosonde data than HIRLAM estimates indicates that the NWP models will benefit from the additional information provided by GPS. The long-term differences between the observational data sources require further study before GPS-derived data become useful for climate studies.

Murphy, BJ, Haase JS, Muradyan P, Garrison JL, Wang KN.  2015.  Airborne GPS radio occultation refractivity profiles observed in tropical storm environments. Journal of Geophysical Research-Atmospheres. 120:1690-1709.   10.1002/2014jd022931   AbstractWebsite

Airborne 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 similar to 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 5days 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 similar to 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.

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.

Walpersdorf, A, Calais E, Haase J, Eymard L, Desbois M, Vedel H.  2001.  Atmospheric gradients estimated by GPS compared to a high resolution numerical weather prediction (NWP) model. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:147-152. AbstractWebsite

The estimation of horizontal atmospheric gradients, in addition to zenith delays, is a strategy now commonly used in geodetic Global Positioning System (GPS) positioning. This strategy compensates for inhomogeneities in the atmospheric water vapor distribution above GPS sites, and has shown to increase the positioning precision, e.g. in geodynamic networks. While the zenith delay has been successfully related to the pressure at the GPS site and the water vapor above the site, the relation of the GPS estimated horizontal gradients to atmospheric quantities remains unclear. To get a better understanding of the nature of these gradients inferred by GPS, this study compares GPS tropospheric observations from the MAGIC permanent network on the NW side of the Mediterranean Sea with simulations based on the high resolution NWP model ALADIN (Meteo France). To verify the model performance, we use meteorological measurements from the FETCH ship campaign in the Gulf of Lyon in March-April 1998. For this study, five stations of the MAGIC network close to the Golf of Lyon have been selected. Results from two periods, representing two different weather situations occuring within the FETCH observation campaign, are presented. (C) 2001 Elsevier Science Ltd. All rights reserved.

Saunders, JK, Haase JS.  2018.  Augmenting onshore GNSS displacements with offshore observations to improve slip characterization for Cascadia Subduction Zone earthquakes. Geophysical Research Letters. 45:6008-6017.   10.1029/2018gl078233   AbstractWebsite

For the Cascadia subduction zone, M-w similar to 8 megathrust earthquake hazard is of particular interest because uncertainties in the predicted tsunami size affect evacuation alerts. To reduce these uncertainties, we examine how augmenting the current Global Navigation Satellite Systems (GNSS) network in Cascadia with offshore stations improves static slip inversions for M-w similar to 8 megathrust earthquakes at different rupture depths. We test two offshore coseismic data types: vertical-only bottom pressure sensors and pressure sensors combined with GNSS-Acoustic aided horizontal positions. We find that amphibious networks best constrain slip for a shallow earthquake compared to onshore-only networks when offshore stations are located above the rupture. However, inversions using vertical-only offshore data underestimate shallow slip and tsunami impact. Including offshore horizontal observations improves slip estimates, particularly maximum slip. This suggests that while real-time GNSS-Acoustic sensors may have a long development timeline, they will have more impact for static inversion-based tsunami early warning systems than bottom pressure sensors. Plain Language Summary The Cascadia subduction zone is the region of highest tsunami hazard within the contiguous United States. This region has experienced many tsunamis over the last 10,000years that were generated by earthquakes of magnitude 8 to 9. Magnitude 8 earthquakes in the subduction zone can be tricky for tsunami early warning systems because it is difficult to determine the depth of the earthquake rupture, which strongly affects the anticipated tsunami size. This can make the difference between an evacuation order being issued or not. This study tests how estimating total slip on the earthquake fault during rupture and the resulting tsunami wave height for magnitude 8 earthquakes can be improved when combining the current land-based Global Navigation Satellite Systems monitoring network in the Pacific Northwest with offshore seafloor networks. We test hypothetical arrangements of offshore stations that measure the vertical seafloor motion with ocean bottom pressure sensors. We also test networks that measure motion in all three directions by including Global Navigation Satellite Systems measurements at the sea surface linked by acoustic communication to measurement points on the seafloor. This work can help plan where best to put new offshore instruments as they are developed for future tsunami early warning systems.

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.

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.
Rabier, F, Bouchard A, Brun E, Doerenbecher A, Guedj S, Guidard V, Karbou F, Peuch V-H, El Amraoui L, Puech D, Genthon C, Picard G, Town M, Hertzog A, Vial F, Cocquerez P, Cohn SA, Hock T, Fox J, Cole H, Parsons D, Powers J, Romberg K, VanAndel J, Deshler T, Mercer J, Haase J, Avallone L, Kalnajs L, Mechoso RC, Tangborn A, Pellegrini A, Frenot Y, Thépaut J-N, McNally A, Balsamo G, Steinle P.  2010.  The Concordiasi project in Antarctica. Bulletin of the American Meteorological Society. 91:69-86.   DOI: 10.1175/2009BAMS2764.1   Abstract

Within the framework of the International Polar Year, the Concordiasi project will makeinnovative observations of the atmosphere above Antarctica. The most important goals ofConcordiasi are:* To enhance the accuracy of weather prediction and climate records in Antarctica throughthe assimilation of in-situ and satellite data, with an emphasis on data provided byhyperspectral infra-red sounders. The focus will be on precipitation and the mass budget ofthe ice sheets. The improvements in dynamical model analyses and forecasts will be utilizedin chemical-transport models that describe the links between the polar vortex dynamics andozone depletion, and to advance the understanding of the Earth system by examining theinteractions between Antarctica and lower latitudes.* To improve our understanding of microphysical and dynamical processes controlling theozone content of the polar air masses, by providing the first quasi-Lagrangian observations ofozone and particle content of air masses, in addition to an improved characterization of the 3Dpolar vortex dynamics. Techniques for assimilating these Lagrangian observations will bedeveloped.A major Concordiasi component is a field experiment during the Austral springs of 2008 and2009. The field activities are based on a constellation of up to eighteen long durationstratospheric balloons deployed from the McMurdo station. Six of these balloons will carryGPS receivers and in-situ instruments measuring temperature, pressure, ozone, and particles.4All the balloons are capable of releasing dropsondes on demand for measuring atmosphericparameters. Finally, radiosounding measurements are collected at various sites, including theConcordia station.

Haase, JS, Shearer PM, Aster RC.  1995.  Constraints on temporal variations in velocity near Anza, California from analysis of similar event pairs. Bulletin of the Seismological Society of America. 85:194-206. AbstractWebsite

Similar earthquake pairs recorded by the Anza Seismic Network in southern California are used as repeatable sources to place an upper limit on temporal changes in seismic velocity which occurred in the vicinity of the Anza seismic gap in the last 9 yr. Relative arrival times for each pair of events are found using a cross-correlation method and relative locations are calculated to verify that the pairs have nearly identical hypocenters. The time separation between events in these pairs varies from less than a day to almost 7 yr. The longterm changes in seismic travel times, as measured from the pairs with the longest time separation, are not significantly greater than the noise level estimated from the short-time-separation event pairs. Almost all P-wave paths show less than 0.06% (0.007 sec) change in travel time and all S-wave paths have less than 0.03% (0.004 sec) change. Sensitivity tests place an upper bound on traveltime changes that could be compensated by hypocenter mislocation at 0.2%. There is no evidence that localized stress accumulation causes measurable changes in seismic velocity in the Anza region.

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.

Symithe, SJ, Calais E, Haase JS, Freed AM, Douilly R.  2013.  Coseismic slip distribution of the 2010 m 7.0 Haiti earthquake and resulting stress changes on regional faults. Bulletin of the Seismological Society of America. 103:2326-2343.   10.1785/0120120306   AbstractWebsite

The 12 January 2010 M-w 7.0 Haiti earthquake ruptured the previously unmapped Leogane fault, a secondary transpressional structure located close to the Enriquillo fault, the major fault system assumed to be the primary source of seismic hazard for southern Haiti. In the absence of a precise aftershock catalog, previous estimations of coseismic slip had to infer the rupture geometry from geodetic and/or seismological data. Here we use a catalog of precisely relocated aftershocks beginning one month after the event and covering the following 5 months to constrain the rupture geometry, estimate a slip distribution from an inversion of Global Positional Systems (GPS), Interferometric Synthetic Aperture Radar (InSAR) and coastal uplift data, and calculate the resulting changes of Coulomb failure stress on neighboring faults. The relocated aftershocks confirm a north-dipping structure consistent with the Leogane fault, as inferred from previous slip inversions, but with two subfaults, each corresponding to a major slip patch. The rupture increased Coulomb stresses on the shallow Enriquillo fault parallel to the Leogane rupture surface and to the west (Miragoane area) and east (Port-au-Prince). Results show that the cluster of reverse faulting earthquakes observed further to the west, coincident with the offshore Trois Baies fault, are triggered by an increase in Coulomb stress. Other major regional faults did not experience a significant change in stress. The increase of stress on faults such as the Enriquillo are a concern, as this could advance the timing of future events on this fault, still capable of magnitude 7 or greater earthquakes.

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.

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.

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.

Haase, JS, Nowack RL.  2011.  Earthquake scenario ground motions for the urban area of Evansville, Indiana. Seismological Research Letters. 82:176-185.
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.

Haase, JS, Murphy BJ, Muradyan P, Nievinski FG, Larson KM, Garrison JL, Wang K-N.  2014.  First results from an airborne GPS radio occultation system for atmospheric profiling. Geophysical Research Letters. :n/a–n/a.   10.1002/2013GL058681   AbstractWebsite

Global Positioning System (GPS) radio occultation (RO) from low Earth-orbiting satellites has increased the quantity of high-vertical resolution atmospheric profiles, especially over oceans, and has significantly improved global weather forecasting. A new system, the Global Navigation Satellite Systems Instrument System for Multistatic and Occultation Sensing (GISMOS), has been developed for RO sounding from aircraft. GISMOS also provides high-vertical resolution profiles that are insensitive to clouds and precipitation, and in addition, provides greater control on the sampling location, useful for targeted regional studies. The feasibility of the system is demonstrated with a flight carried out during development of an Atlantic tropical storm. The data have been evaluated through a comparison with dropsonde data. The new airborne RO system will effectively increase by more than 50% the number of profiles available for studying the evolution of tropical storms during this campaign and could potentially be deployed on commercial aircraft in the future.

Nicolas, J, Pierron F, Kasser M, Exertier P, Bonnefond P, Barlier F, Haase J.  2000.  French Transportable Laser Ranging Station: Scientific objectives, technical features, and performance. Applied Optics: Lasers, Photonics, and Environmental Optics. 39:402-410.
Haase, JS, Hauksson E, Kanamori H, Mori J.  1995.  Global Positioning System re-survey of Southern California Seismic Network Stations. Bulletin of the Seismological Society of America. 85:361-374. AbstractWebsite

Systematic errors in travel-time data from local earthquakes can sometimes be traced to inaccuracies in the published seismic station coordinates. This prompted a resurvey of the stations of the Caltech/USGS Southern California Seismic Network (SCSN) using the Global Positioning System (GPS). We surveyed 241 stations of the SCSN using Trimble and Ashtech dual-frequency GPS receivers and calculated positions accurate to 3 m using differential positioning from carrier phase measurements, Twelve percent of the stations that were surveyed were found to be mislocated by more than 500 m. Stations of the TERRAscope and USC networks were also surveyed, as well as a network of portable seismic stations deployed shortly after the 1992 Joshua Tree and Landers earthquakes. The new coordinates and the offsets from the old coordinates are given below. The new coordinates are being used in SCSN locations as of 1 January 1994.

Vey, S, Calais E, Llubes M, Florsch N, Woppelmann G, Hinderer J, Amalviet M, Lalancette MF, Simon B, Duquenne F, Haase JS.  2002.  GPS measurements of ocean loading and its impact on zenith tropospheric delay estimates: a case study in Brittany, France. Journal of Geodesy. 76:419-427.   10.1007/s00190-002-0272-7   AbstractWebsite

The results from a global positioning system (GPS) experiment carried out in Brittany, France, in October 1999, aimed at measuring crustal displacements caused by ocean loading and quantifying their effects on GPS-derived tropospheric delay estimates, are presented. The loading effect in the vertical and horizontal position time series is identified, however with significant disagreement in amplitude compared to ocean loading model predictions. It is shown that these amplitude misfits result from spatial tropospheric heterogeneities not accounted for in the data processing. The effect of ocean loading on GPS-derived zenith total delay (ZTD) estimates is investigated and a scaling factor of 4.4 between ZTD and station height for a 10degrees elevation cutoff angle is found (i.e. a 4.4-cm station height error would map into a 1-cm ZTD error). Consequently. unmodeled ocean loading effects map into significant errors in ZTD estimates and ocean loading modeling must be properly implemented when estimating ZTD parameters from GRS data for meteorological applications. Ocean loading effects Must be known with an accuracy of better than 3 cm in order to meet the accuracy requirements of meteorological and climatological applications of GPS-derived precipitable water vapor.

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.

Muradyan, P, Haase JS, Xie F, Garrison JL, Lulich T, Voo J.  2010.  GPS/INS navigation precision and its effect on airborne radio occultation retrieval accuracy. GPS Solutions. 10.1007/s10291-010-0183-7
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.