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Assimilation of GPS radio occultation data at DWD

Assimilation of GPS radio occultation data at DWD

resolution (of the order of 100 m). They provide information on temperature and humidity from the atmospheric refrac- tion of the GPS radio signals as measured by a satellite on a low-earth orbit. In contrast to radiance data GPS radio occultations are unaffected by clouds or precipitation and are bias-free (at least at the level of the raw observations). These properties make them interesting in particular for inter-instrument calibration studies, e.g. in climate research. Finally, the horizontal coverage of the radio occultation observations is close to uniform, which is beneficial for their use in data assimilation. Thus, GPSRO data have proven to be a valuable source of information in atmospheric research and weather prediction, and are now being used opera- tionally at many NWP centers, e.g. MetOffice, ECMWF, NCEP, M´et´eo-France, Environment Canada, and JMA (for an overview see the talks and proceedings of the GRAS SAF workshop (2008) and OPAC workshop (2010) and references cited therein).
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Ionospheric correction of GPS radio occultation data in the troposphere

Ionospheric correction of GPS radio occultation data in the troposphere

Abstract. For inversions of the GPS radio occultation (RO) data in the neutral atmosphere, this study investigates an op- timal transition height for replacing the standard ionospheric correction using the linear combination of the L1 and L2 bending angles with the correction of the L1 bending angle by the L1–L2 bending angle extrapolated from above. The optimal transition height depends on the RO mission (i.e., the receiver and firmware) and is different between rising and setting occultations and between L2P and L2C GPS signals. This height is within the range of approximately 10–20 km. One fixed transition height, which can be used for the pro- cessing of currently available GPS RO data, can be set to 20 km. Analysis of the L1CA and the L2C bending angles shows that in some occultations the errors of standard iono- spheric correction substantially increase around the strong inversion layers (such as the top of the boundary layer). This error increase is modeled and explained by the horizontal in- homogeneity of the ionosphere.
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Information content in reflected signals during GPS Radio Occultation observations

Information content in reflected signals during GPS Radio Occultation observations

Abstract. The possibility of extracting useful information about the state of the lower troposphere from the surface re- flections that are often detected during GPS radio occulta- tions (GPSRO) is explored. The clarity of the reflection is quantified, and can be related to properties of the surface and the low troposphere. The reflected signal is often clear enough to show good phase coherence, and can be tracked and processed as an extension of direct non-reflected GPSRO atmospheric profiles. A profile of bending angle vs. impact parameter can be obtained for these reflected signals, char- acterized by impact parameters that are below the apparent horizon, and that is a continuation at low altitude of the stan- dard non-reflected bending angle profile. If there were no re- flection, these would correspond to tangent altitudes below the local surface, and in particular below the local mean sea level. A forward operator is presented, for the evaluation of the bending angle of reflected GPSRO signals, given atmo- spheric properties as described by a numerical weather pre- diction system. The operator is an extension, at lower impact parameters, of standard bending angle operators, and repro- duces both the direct and reflected sections of the measured profile. It can be applied to the assimilation of the reflected section of the profile as supplementary data to the direct section. Although the principle is also applicable over land, this paper is focused on ocean cases, where the topographic height of the reflecting surface, the sea level, is better known a priori.
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The GPS Radio Occultation Concept: Theoretical Performance and Initial Results

The GPS Radio Occultation Concept: Theoretical Performance and Initial Results

THE GPS RADIO OCCULTATION CONCEPT THEORETICAL PERFORMANCE AND INITIAL RESULTS Thesis by E Robert Kursinski In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California[.]

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On the determination of gravity wave momentum flux from GPS radio occultation data

On the determination of gravity wave momentum flux from GPS radio occultation data

Abstract. Global Positioning System (GPS) radio occul- tation (RO) is a well-established technique for obtaining global gravity wave (GW) information. RO uses GPS sig- nals received by low Earth-orbiting satellites for atmospheric limb sounding. Temperature profiles are derived with high vertical resolution and provide a global coverage under any weather conditions, offering the possibility of global monitoring of the vertical temperature structure and at- mospheric wave parameters. The six-satellite constellation COSMIC/FORMOSAT-3 delivers approximately 2000 tem- perature profiles daily. In this study, we use a method to obtain global distributions of horizontal gravity wave wave- lengths, to be applied in the determination of the vertical flux of horizontal momentum transported by gravity waves. Here, a method for the determination of the real horizontal wave- length from three vertical profiles is applied to the COSMIC data. The horizontal and vertical wavelength, the specific po- tential energy (E p ), and the vertical flux of horizontal mo-
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Orographic and convective gravity waves above the Alps and Andes Mountains during GPS radio occultation events – a case study

Orographic and convective gravity waves above the Alps and Andes Mountains during GPS radio occultation events – a case study

Abstract. Gravity waves (GWs) and convective systems play a fundamental role in atmospheric circulation, weather, and climate. Two usual main sources of GWs are orographic ef- fects triggering mountain waves and convective activity. In addition, GW generation by fronts and geostrophic adjust- ment must also be considered. The utility of Global Position- ing System (GPS) radio occultation (RO) observations for the detection of convective systems is tested. A collocation database between RO events and convective systems over subtropical to midlatitude mountain regions close to the Alps and Andes is built. From the observation of large-amplitude GW structures in the absence of jets and fronts, subsets of RO profiles are sampled. A representative case study among those considered at each region is selected and analyzed. The case studies are investigated using mesoscale Weather Research and Forecasting (WRF) simulations, ERA-Interim reanalysis data, and measured RO temperature profiles. The absence of fronts or jets during both case studies reveals sim- ilar relevant GW features (main parameters, generation, and propagation). Orographic and convective activity generates the observed GWs. Mountain waves above the Alps reach higher altitudes than close to the Andes. In the Andes case, a critical layer prevents the propagation of GW packets up to stratospheric heights. The case studies are selected also be- cause they illustrate how the observational window for GW observations through RO profiles admits a misleading inter- pretation of structures at different altitude ranges. From re- cent results, the distortion introduced in the measured atmo-
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Study of sporadic E layers based on GPS radio occultation measurements and digisonde data over the Brazilian region

Study of sporadic E layers based on GPS radio occultation measurements and digisonde data over the Brazilian region

Recently, the GPS radio occultation (RO) technique has been used to detect Es layers on a global scale (Arras et al., 2008, 2009; Wickert et al., 2009). In fact, a constella- tion of LEO (low Earth orbit) satellites is required to observe Earth’s atmosphere by the RO technique in a high spatial and temporal resolution. The main LEO satellite missions that provide GPS RO data are CHAMP (Challenging Minisatel- lite Payload), GRACE (Gravity Recovery and Climate Ex- periment) and FORMOSAT-3/COSMIC (FORMOsa SATel- lite mission-3/Constellation Observing System for Meteorol- ogy, Ionosphere and Climate). These missions have accumu- lated an extensive database to study the Es layers between 2001 and 2016, in which there are about 6.7 million RO pro- files available. Some relevant sporadic E characteristics were observed such as the strong summer occurrence at midlati- tudes and the absence of the Es layers along Earth’s magnetic Equator (Arras et al., 2009).
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Characteristics of tropopause parameters as observed with GPS radio occultation

Characteristics of tropopause parameters as observed with GPS radio occultation

However, to investigate tropopause properties based only on observational data, the relatively new global position- ing system (GPS) radio occultation (RO) technique delivers well-suited data. The RO method (Melbourne et al., 1994; Kursinski et al., 1997; Hajj et al., 2002; Kuo et al., 2004) pro- vides near-vertical profiles of atmospheric thermodynamic variables with high vertical resolution (better than 1 km) and global coverage. Other features of RO measurements include all-weather capability, high accuracy, high precision, and long-term stability (Anthes, 2011). A number of studies con- firmed the feasibility and excellent eligibility of RO measure- ments for monitoring the atmosphere (Foelsche et al., 2008, 2009) and for climate change detection (Leroy et al., 2006; Schmidt et al., 2008; Steiner et al., 2011).
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A perspective on the fundamental quality of GPS radio occultation data

A perspective on the fundamental quality of GPS radio occultation data

Observations underpin all areas of numerical modeling, weather analysis and forecasting, and climate monitoring and projections that are closely relevant to each other. Thus, the availability of high-quality observations is of the ut- most importance, carrying broad socioeconomic implica- tions. Recently, Global Positioning System (GPS) radio oc- cultation (RO) (Melbourne et al., 1994; Ware et al., 1996; Kursinski et al., 1997; Anthes et al., 2008) has been receiving a great deal of attention as a promising source of data for both weather and climate applications. The primary observable of RO is the phase path of GPS signals received by an accurate receiver onboard a low Earth orbiting (LEO) satellite. By an- alyzing the time–frequency content in the occulted signals, a profile of the ray’s bending angle and subsequent profiles of atmospheric refractivity, pressure, and temperature can be derived. In past decades, numerous studies have demon- strated the unique strengths of GPS RO, which include high accuracy and vertical resolution, global coverage, all-weather capability, and self-calibration aptitude (e.g., Kursinski et al., 1997; Hajj et al., 2002; Wickert et al., 2004; Kuo et al., 2004). The data are accepted as an operationally reliable source of information by NWP centers worldwide (Poli et al., 2010), and have shown clear positive impacts on weather forecast-
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Temporal and spatial distributions of atmospheric wave energy in the equatorial stratosphere revealed by GPS radio occultation temperature data obtained with the CHAMP satellite during 2001–2006

Temporal and spatial distributions of atmospheric wave energy in the equatorial stratosphere revealed by GPS radio occultation temperature data obtained with the CHAMP satellite during 2001–2006

The global distribution of stratospheric wave activity has also been studied by employing a novel GPS radio occul- tation (RO) technique (Tsuda et al., 2000; Ratnam et al., 2004). The GPS signals received on low Earth orbiting (LEO) satellites are then used for active limb-sounding of the atmosphere. Recent GPS RO missions, such as the Challenging Mini-Payload (CHAMP) satellite and the Con- stellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) missions, have achieved signifi- cant progress in measuring accurate temperature profiles below 35–40 km. The GPS RO data are characterized by a good height resolution, comparable to radiosondes, and they are particularly valuable in the tropics where routine radiosonde observations are sparse.
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Evaluating the lower-tropospheric COSMIC GPS radio occultation sounding quality over the Arctic

Evaluating the lower-tropospheric COSMIC GPS radio occultation sounding quality over the Arctic

Since the advent of the Global Positioning System (GPS) radio occultation (RO) technique in the early 1990s, the RO soundings have demonstrated a high-quality observa- tion with sub-kelvin temperature precision (Kursinski et al., 1997; Melbourne, 2004; Healy et al., 2005). Since the launch of the six-satellite Constellation Observing System for Me- teorology, Ionosphere, and Climate (COSMIC), GPS RO has provided near-real-time, high-vertical-resolution, uniformly distributed global soundings of atmospheric bending angle and refractivity (Anthes et al., 2008) from the stratosphere down to near the surface in all weather conditions. GPS RO bending angle and refractivity measurements have been oper- ationally assimilated into global weather forecasting models and demonstrate significant positive impact especially over the upper troposphere and above the open ocean (Healy et al., 2005; Cucurull et al., 2008). GPS RO measurements are also considered for global climate benchmark monitoring (Ho et al., 2009; Steiner et al., 2013) for their self-calibration and long-term stability (Steiner et al., 2013). Moreover, GPS RO has demonstrated its capability to observe the lower tropo- sphere (Sokolovskiy et al., 2006a, 2010) and the planetary boundary layer (PBL; von Engeln et al., 2005; Sokolovskiy et al., 2007; Ao et al., 2012; Xie et al., 2012; Ho et al., 2015). The GPS RO measurement could fill the gap in observing the lower troposphere over the Arctic (e.g., Ganeshan and Wu, 2015; Chang et al., 2017). Nevertheless, the uncertainty of RO sounding increases in the lower troposphere especially within the PBL, which remains largely uncharacterized over the remote Arctic Ocean.
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A global perspective on atmospheric blocking using GPS radio occultation – one decade of observations

A global perspective on atmospheric blocking using GPS radio occultation – one decade of observations

Abstract. Atmospheric blocking represents a weather pat- tern where a stationary high-pressure system weakens or re- verses the climatological westerly flow at mid-latitudes for up to several weeks. It is closely connected to strong anoma- lies in key atmospheric variables such as geopotential height, temperature, and humidity. Here we provide, for the first time, a comprehensive, global perspective on atmospheric blocking and related impacts by using an observation-based data set from Global Positioning System (GPS) radio occul- tation (RO) from 2006 to 2016. The main blocking regions in both hemispheres and seasonal variations are found to be represented well in RO data. The effect of blocking on ver- tically resolved temperature and humidity anomalies in the troposphere and lower stratosphere is investigated for block- ing regions in the Northern and Southern hemispheres, re- spectively. We find a statistically significant correlation of blocking with positive temperature anomalies, exceeding 3 K in the troposphere, and a reversal above the tropopause with negative temperature anomalies below − 3 K in the lower stratosphere. Specific humidity is positively correlated with temperature throughout the troposphere with larger anoma- lies revealed in the Southern Hemisphere. At the eastern and equatorward side of the investigated blocking regions, a band of tropospheric cold anomalies reveals advection of cold air by anticyclonic motion around blocking highs, which is less distinct in the Southern Hemisphere due to stronger zonal flow. We find GPS RO to be a promising new data set for blocking research that gives insight into the vertical atmo- spheric structure, especially in light of the expected increase in data coverage that future missions will provide.
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Quality aspects of the Wegener Center multi-satellite GPS radio occultation record OPSv5.6

Quality aspects of the Wegener Center multi-satellite GPS radio occultation record OPSv5.6

Abstract. The demand for high-quality atmospheric data records, which are applicable in climate studies, is undis- puted. Using such records requires knowledge of the quality and the specific characteristics of all contained data sources. The latest version of the Wegener Center (WEGC) multi- satellite Global Positioning System (GPS) radio occultation (RO) record, OPSv5.6, provides globally distributed upper- air satellite data of high quality, usable for climate and other high-accuracy applications. The GPS RO technique has been deployed in several satellite missions since 2001. Consis- tency among data from these missions is essential to create a homogeneous long-term multi-satellite climate record. In order to enable a qualified usage of the WEGC OPSv5.6 data set we performed a detailed analysis of satellite-dependent quality aspects from 2001 to 2017. We present the impact of the OPSv5.6 quality control on the processed data and reveal time-dependent and satellite-specific quality characteristics. The highest quality data are found for MetOp (Meteorologi- cal Operational satellite) and GRACE (Gravity Recovery and Climate Experiment). Data from FORMOSAT-3/COSMIC (Formosa Satellite mission-3/Constellation Observing Sys- tem for Meteorology, Ionosphere, and Climate) are also of high quality. However, comparatively large day-to-day varia- tions and satellite-dependent irregularities need to be taken into account when using these data. We validate the con- sistency among the various satellite missions by calculat- ing monthly mean temperature deviations from the multi- satellite mean, including a correction for the different sam- pling characteristics. The results are highly consistent in the altitude range from 8 to 25 km, with mean temperature de-
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A variational regularization of Abel transform for GPS radio occultation

A variational regularization of Abel transform for GPS radio occultation

Abstract. In the Global Positioning System (GPS) radio oc- cultation (RO) technique, the inverse Abel transform of mea- sured bending angle (Abel inversion, hereafter AI) is the standard means of deriving the refractivity. While concise and straightforward to apply, the AI accumulates and prop- agates the measurement error downward. The measurement error propagation is detrimental to the refractivity in lower altitudes. In particular, it builds up negative refractivity bias in the tropical lower troposphere. An alternative to AI is the numerical inversion of the forward Abel transform, which does not incur the integration of error-possessing measure- ment and thus precludes the error propagation. The varia- tional regularization (VR) proposed in this study approxi- mates the inversion of the forward Abel transform by an opti- mization problem in which the regularized solution describes the measurement as closely as possible within the measure- ment’s considered accuracy. The optimization problem is then solved iteratively by means of the adjoint technique. VR is formulated with error covariance matrices, which permit a rigorous incorporation of prior information on measurement error characteristics and the solution’s desired behavior into the regularization. VR holds the control variable in the mea- surement space to take advantage of the posterior height de- termination and to negate the measurement error due to the mismodeling of the refractional radius. The advantages of having the solution and the measurement in the same space are elaborated using a purposely corrupted synthetic sound- ing with a known true solution. The competency of VR rel- ative to AI is validated with a large number of actual RO soundings. The comparison to nearby radiosonde observa- tions shows that VR attains considerably smaller random and systematic errors compared to AI. A noteworthy finding is that in the heights and areas that the measurement bias is
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A simulation analysis to optimize orbits for a tropical GPS radio occultation mission

A simulation analysis to optimize orbits for a tropical GPS radio occultation mission

The GPS RO technique was demonstrated by the proof- of-concept mission GPS/MET (Global Positioning Sys- tem/Meteorology) experiment conducted by UCAR (Uni- versity Corporation for Atmospheric Research) (Ware et al., 1996). GPS/MET has successfully provided the profiles of humidity and atmospheric temperature in the troposphere and stratosphere from April 1995 to February 1997 (Rocken et al., 1997). GPS/MET data are also used to study the elec- tron density fluctuations in the ionosphere (e.g., Tsuda et al., 2004). The success of the GPS/MET experiment moti- vated the scientific community to launch many other LEO satellites to study the Earth’s atmosphere and ionosphere.
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Empirical analysis and modeling of errors of atmospheric profiles from GPS radio occultation

Empirical analysis and modeling of errors of atmospheric profiles from GPS radio occultation

Abstract. The utilization of radio occultation (RO) data in atmospheric studies requires precise knowledge of error characteristics. We present results of an empirical error anal- ysis of GPS RO bending angle, refractivity, dry pressure, dry geopotential height, and dry temperature. We find very good agreement between data characteristics of different missions (CHAMP, GRACE-A, and Formosat-3/COSMIC (F3C)). In the global mean, observational errors (standard deviation from “true” profiles at mean tangent point location) agree within 0.3 % in bending angle, 0.1 % in refractivity, and 0.2 K in dry temperature at all altitude levels between 4 km and 35 km. Above 35 km the increase of the CHAMP raw bend- ing angle observational error is more pronounced than that of GRACE-A and F3C leading to a larger observational er- ror of about 1 % at 42 km. Above ≈ 20 km, the observational errors show a strong seasonal dependence at high latitudes. Larger errors occur in hemispheric wintertime and are asso- ciated mainly with background data used in the retrieval pro- cess particularly under conditions when ionospheric resid- ual is large. The comparison between UCAR and WEGC results (both data centers have independent inversion pro- cessing chains) reveals different magnitudes of observational errors in atmospheric parameters, which are attributable to different background fields used. Based on the empirical error estimates, we provide a simple analytical error model for GPS RO atmospheric parameters for the altitude range of 4 km to 35 km and up to 50 km for UCAR raw bending angle and refractivity. In the model, which accounts for ver- tical, latitudinal, and seasonal variations, a constant error is adopted around the tropopause region amounting to 0.8 % for bending angle, 0.35 % for refractivity, 0.15 % for dry pres- sure, 10 m for dry geopotential height, and 0.7 K for dry
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Case study on complex sporadic E layers observed by GPS radio occultations

Case study on complex sporadic E layers observed by GPS radio occultations

tially too. These cases are mainly found in the middle latitude of Eurasian continent and the Pacific Ocean. This is logical because the relatively higher horizontal component of the ge- omagnetic field over this region makes the wind shear more efficient (Haldoupis, 2011). To evaluate these results, the raw ionograms accessed from the Digital Ionogram DataBase (DIDB) operated by the Lowell Global Ionospheric Radio Observatory (GIRO) Data Center (LGDC) through the SAO Explorer software are used to make an independent com- parison (Galkin et al., 1999; Reinisch and Galkin, 2011). For 2006, a total of about 35–40 ionosonde stations shared their data in DIDB. Generally, Europe and northern America have relatively more and denser ionosonde distributions. We checked the ionograms over these regions during the occur- rence of simultaneous E s over a broad region indicated by
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Remote sensing of tropospheric turbulence using GPS radio occultation

Remote sensing of tropospheric turbulence using GPS radio occultation

servations from a GPS to a COSMIC satellites radio links. We estimated the impact of turbulence strength on L-band signals in terms of scintillation index. During the time frame relevant to this study, the COSMIC satellites provide a sig- nificant number of RO profiles (up to about 2000 profiles per day) observed by the six micro-satellites covering the entire globe. Utilizing the RO profiles, we were able to estimate the global distribution of the effect of scintillation on GPS sig- nals. The technique provides valuable scintillation data es- pecially over the oceans where ground-based measurements are both difficult and expensive to perform. RO data were first used to determine the intensity and location of turbu- lent regions (Cornman et al., 2012). Our study differs in that we aim to study the global climatology of tropospheric tur- bulence. In addition, we suggest that amplitude and phase in the impact parameter domain, rather than raw signal am- plitude and phase, provide a more effective observable for measuring scintillation of interest.
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A modification to the standard ionospheric correction method used in GPS radio occultation

A modification to the standard ionospheric correction method used in GPS radio occultation

This error term arises even for the simplest case of a spher- ically symmetric ionosphere, with no magnetic field. Integra- tion by parts shows that the integral in Eq. (5) is always pos- itive (see Eq. A4), so this error biases the corrected bending angles negative, meaning the values produced by Eq. (3) are consistently too small. The error occurs because the L1 and L2 signals have different ray paths. The dependence on n 2 e indicates that the bias will depend on the solar cycle. The present study is concerned with estimating and correcting this specific source of bias in the bending angles, in order to improve GPS-RO geophysical climatologies. However, we emphasise that this requires some a priori assumptions about the ionospheric state.
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Potential radio frequency interference with the GPS L5 band for radio occultation measurements

Potential radio frequency interference with the GPS L5 band for radio occultation measurements

However, certain aeronautical navigation systems already occupy this frequency range. Distance measuring equipment (DME) and tactical air navigation (TACAN) systems offer potential sources of interference due to coexistence within the L5 band (Kim and Grabowski, 2003). These systems are comprised of an airborne interrogator and a ground-based transponder. A TACAN system is essentially a higher pow- ered DME station used for military purposes. Due to the limited placements available within the Aeronautical Radio Navigation Services (ARNA) radio band for aviation use, the GPS L5 signal was placed within the already existing DME–TACAN band. The premise was that an aircraft using the system would only encounter a limited number of pulsed interfering signals, thereby allowing the interoperability be- tween a GPS L5 receiver and a DME–TACAN signal. How- ever, due to the higher number of interfering stations seen by a GPS radio occultation (RO) satellite in low Earth Orbit (LEO), the possibility for signal degradation for RO applica- tions exists (Kim and Grabowski, 2003).
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