GPS Meteorology Research Articles

Vertical Wave Number Spectrum of Temperature Fluctuations in the Stratosphere using GPS Occultation Data.

We have analyzed small-scale fluctuations of temperature in the stratosphere using radio occultation data from the GPS/MET (GPS/Meteorology) experiment. From this, we have determined a vertical wave number spectrum of the normalized temperature fluctuations (T'/T) at 20-30 km and 30-40 km during three periods in June/July 1995, October 1995 and February 1997. The spectra at 20-30 km in the equatorial region in February 1997 agreed very well with radiosonde results in Indonesia as well as with a model spectrum assuming a linear saturation of gravity waves. We have found that the GPS radio occultation technique could measure meso-scale temperature perturbations in the lower stratosphere with a vertical wavelength down to 400 m. We have investigated the dependence of the mean wave number spectra on latitude, height and season. At 20-30 km the power law index for short wavelengths (<2 km) was about -3, consistent with a saturated gravity wave model. However, the spectral density was sometimes smaller than that predicted by the model. At 30-40 km both the spectral slope and density agreed well with the model for wavelengths shorter than about 1.5 km, though the slope was more gradual for small m. We have calculated the variance (T'/T) 2 integrating the spectra in two vertical wavelength ranges: 10-2.5 km and 2-0.4 km, and estimated potential energy per unit mass (Ep). Seasonal and latitudinal variations of Ep were evident at 20-30 km, in particular, Ep was highly enhanced near the equator for both long and short wavelength ranges. At 30-40 km the enhancement of Ep at low latitudes became less evident than at 20-30 km.

Controls of the sounding points in space-based GPS/LEO meteorology

GPS radio occultation technique has been developed resulting from remote sensing applications in planetary atmospheric limb sounding pioneered by JPL and Stanford University in 1960s. Since the launch of the experimental satellite MicroLab1 on April 3 1995, the primary results from this Low Earth orbiter (LEO) satellite demonstrated potential scientific values and practical significance of this innovation in GPS meteorology. For MicroLab1, the sounding locations on the Earth's surface were globally pseudo-randomly distributed, which is good for data assimilation in atmospheric and ionospheric research. As another strategy, as similar to mountain-based or airborne GPS receiver, sequential occultation footprints falling within adjacent regions of some specific points on the Earth's surface are expected in the following possible cases: comparing and cross-calibrating radio occultation data with other techniques, such as ground-based GPS, radiosonde and lindar; continuous monitoring of local atmospheric refractivity field and local hazardous weather within a specific time period. In this paper, we discuss a possibility to get sequential atmospheric profiles near a specific ground location by optimally designing a LEO satellite orbit. This may have potential benefits in the applications of GPS/LEO occultation technique to atmospheric science, improving numerical weather prediction with data assimilation, and the monitoring of hazardous weather and atmospheric refractivity field. Such an orbit design of LEO satellite would influence future GPS launch plan, and would influence the design of new generation of LEO satellite.

Application of an optimal estimation inverse method to GPS/MET bending angle observations

Palmer et al. [2000] describes an optimal estimation inverse method for radio occultation (RO) bending angle measurements to retrieve simultaneously temperature, humidity, and surface pressure; outlines quality control procedures for retrieved profiles; and investigates the results from numerical simulations. Here we present retrievals that use bending angle observations from the Global Positioning System Meteorology (GPS/MET) satellite instrument and a priori information from the European Centre for Medium‐Range Weather Forecasts analyses. Retrieved profiles are compared with correlative radiosondes, United Kingdom Meterological Office (UKMO) model analyses, and retrievals from the conventional inverse method. Retrieved temperature profiles are generally colder than analyses but agree with the conventional inverse method to within 1 K. Water vapor retrievals are generally drier than the UKMO analyses and wetter than the radiosonde profiles. Quality of retrieved surface pressure values are related to the extent to which RO observations reach into the troposphere. Low‐latitude retrievals make large adjustments to surface pressure and tropospheric temperatures, which are directly linked to the lack of water vapor above 300 hPa in the inverse model, consistent with previous studies. A study of individual occultations at low and high latitude shows that the optimal retrievals are able to resolve small‐scale atmospheric structure exhibited by the conventional inverse method and collocated radiosondes, not shown by analyses.

Comparisons of GPS/MET retrieved ionospheric electron density and ground based ionosonde data

The Global Positioning System/Meteorology (GPS/MET) mission has been the first experiment to use a low Earth orbiting (LEO) satellite (the MicroLab-1) to receive multi-channel Global Positioning System (GPS) carrier phase signals and demonstrate active limb sounding of the Earth’s atmosphere and ionosphere by radio occultation technique. Under the assumption of spherical symmetry at the locality of the occultation, the dual-band phase data have been processed to yield ray-path bending angle profiles, which have then been used to yield profiles of refractive index via the Abel integral transform. The refractivity profiles can then, in turn, yield profiles of ionospheric electron density and other atmospheric variables such as neutral atmospheric density, pressure, and temperature in the stratosphere and upper troposphere, and water vapor in the lower troposphere with the aid of independent temperature data. To approach a near real-time process, electron density profiles can also be derived by the Abel transform through the computation of total electron content (TEC) assuming straight-line propagation (neglecting bending). In order to assess the accuracy of the GPS/MET ionospheric electron density retrievals, coincidences of ionosonde data with GPS/MET occultations have been examined. The retrieved electron density profiles from GPS/MET TEC observations have been compared with ionogram inversion results derived from digital ionospheric sounders operated by the National Central University (the Chung-Li digisonde; 24.6°N, 121.0°E) and by Utah State University (the Bear-Lake dynasonde; 41.9°N, 111.4°W). A fuzzy classification method for the automatic identification and scaling of ionogram traces has been applied to recorded ionograms, and then bottomside ionospheric electron density profiles are determined from true-height analysis. The comparison results show better agreement for both of the derived electron density profiles and the F2-layer critical frequency ( foF2) at mid-latitude observations than at low-latitude observations. The rms foF2 differences from the GPS/MET retrievals are 0.61 MHz to the Bear-Lake dynasonde measurements and 1.62 MHz to the Chung-Li digisonde measurements.

Determination of weighted mean tropospheric temperature using ground meteorological measurements

The weighted mean tropospheric temperature is a critical parameter in the conversion of wet zenith delay to precipitable water vapor in GPS Meteorology. This parameter can not be calculated from the radiosonde data in real time through the conventional methods. In this study, we first discuss the admissible error of weighted mean temperature to enable the accuracy of the conversion better than 1 mm, then summarize the performance of some of the existing methods. An empirical formula is established that satisfies the real-time requirement in GPS meteorology using Sequential Regression Analysis method. It is shown that this real-time formula as compared with other empirical methods is more accurate for local applications.

GPS meteorology: validation and comparisons with ground-based microwave radiometer and mesoscale model for the Italian GPS permanent stations

Ground-based GPS meteorology has been developed during the last decade. Beside the GPS traditional application for precise geodesy, the GPS system has proved to be a powerful tool in atmospheric studies, such as climatology and meteorology. During 1999, at the Space Geodesy Center (CGS) of the Italian Space Agency (ASI) an operative and automatic system was developed in order to produce GPS tropospheric parameters on a daily basic for the Italian GPS permanent stations. In this work the operational processing of the GPS data is described. Moreover, the validation of the GPS tropospheric parameters through a comparison with other independent GPS solutions as well as independent techniques is presented. The GPS internal validation of zenith tropospheric delay (ZTD) shows a bias of 5 mm (absolute values) and a standard deviation ranging from 4 to 20 mm. For a few stations a comparison is performed between the GPS derived precipitable water vapor (PWV) and both the one computed using the MM5 and the one measured by the water vapor radiometer (WVR) and radiosondes (RAOB). The agreement between GPS and MM5 is within the range of 2–7 mm; for GPS, WVR and RAOB the standard deviation ranges from 0.85 to 1.62 mm.

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

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/m2. NWP assimilation tests will be carried out in the final year of the project.

Instantaneous geodetic positioning at medium distances with the Global Positioning System

We evaluate a new method of Global Positioning System (GPS) data analysis, called instantaneous positioning, at spatial scale lengths typical of interstation spacings in a modern crustal motion network. This method is more precise and versatile than traditional GPS static and kinematic processing of multi‐epoch batches of data. The key to instantaneous positioning is the ability to resolve integer‐cycle phase ambiguities with only a single epoch of dual‐frequency phase and pseudorange data, rendering receiver cycle slips irrelevant. We estimate three‐dimensional relative coordinates and atmospheric zenith delay parameters independently every 30 s over a 12‐week period for baseline distances of 50 m, 14 km, and 37 km. Horizontal precision of a single‐epoch coordinate solution is about 15 mm and vertical precision is about 7–8 times worse. Removing that component of each time series which repeats with a period of exactly 1 sidereal day, and thus manifests signal multipath, reduces the scatter by about 50% in all components. Solution averaging of the high‐frequency time series can be performed using any number of measurement epochs to further improve coordinate precision. We demonstrate that the daily coordinates estimated with instantaneous positioning are more precise (by 20–50% per coordinate component) than those estimated with 24‐hour batch processing. Spectral analysis of the single‐epoch solutions indicates that the flicker noise characteristic of GPS time series observed in lower‐frequency bands also affects GPS solutions in the frequency band 0.01 mHz to 10 mHz. We argue that the flicker noise is induced by tropospheric effects. Since modern GPS receivers are capable of observing at frequencies as high as 10 Hz, our technique significantly overlaps and complements the frequency band of broadband seismology and benefits other research areas such as earthquake geodesy, volcanology, and GPS meteorology.

Improving GPS surveying with modeled ionospheric corrections

We model the ionospheric delay of Global Positioning System (GPS) signals with high precision and use it to correct single frequency (L1) GPS baseline estimations. We find that baselines up to 30 km in length are more precisely determined using corrected L1 data than using dual frequency data. The high resolution ionospheric modeling technique (called HiRIM in this paper) is demonstrated with 195 days of data from a 25‐site GPS network at ∼1 km spacing in central Japan. The network was designed for high vertical precision tectonic studies. We compute ionospheric corrections using data from a surrounding grid of nine GPS sites spaced ∼50 km. Based on the observations from the surrounding grid, epoch and satellite specific ionospheric delays are interpolated to correct L1 observations from the internal sites. HiRIM has potential post‐processing and real‐time applications in navigation, surveying, and GPS meteorology.

Retrieval of Water Vapor Profiles from GPS/MET Radio Occultations

Present Global Positioning System Meteorology (GPS/MET) refractivity profiles cannot distinguish between refractivity effects due to water vapor and those due to air density. Current methods of resolving the ambiguity rely heavily on ancillary upper-air data, such as National Centers for Environmental Prediction and European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. However, the accuracy of these ancillary sources suffers in regions where upper-air data are sparse. A method of separating the water vapor and temperature effects in GPS/MET-derived refractivity profiles with the addition of only ancillary surface pressure and temperature data and the hydrostatic assumption is discussed. Water vapor and temperature data derived from this method are presented and compared with accepted values. This method allows for the construction of temperature profiles with a mean bias of 0.33 K and a mean standard deviation of 1.86 K when compared with ECMWF data from 30 to 1000 mb. Height fields can als...

Gravity Wave Spectra from GPS/MET Occultation Observations

Abstract The potential utility of radio occultation data in general, and of data from the Global Positioning System/Meteorology (GPS/MET) experiment in particular, for studying atmospheric gravity waves is discussed. Based on a validated set of ∼270 GPS/MET-derived temperature profiles, the authors produced and analyzed mean vertical wavenumber power spectra of normalized temperature fluctuations in three latitude bands (low, middle, high) within the lower stratosphere (∼15–30 km), where data accuracy was proven highest. The Fresnel diffraction limited vertical resolution and the limited height range of the dataset restricted this initial investigation to medium- to large-scale waves with vertical wavelengths of about 2–5 km. The deduced vertical wavenumber power spectra were compared with a saturation spectrum predicted by gravity wave saturation theory and generally found consistent with the theoretical saturation limit. The low-latitude power spectra exhibited almost saturation, with spectral power abo...

A Global Morphology of Gravity Wave Activity in the Stratosphere Revealed by the GPS Occultation Data (GPS/MET)

Using temperature profiles obtained by the GPS/MET (GPS Meteorology) experiment from April 1995 to February 1997, we have extracted mesoscale temperature perturbations with vertical wavelengths ranging from 2 to 10 km and background Brunt‐Väisälä frequency squared, N2. For each occultation event, we can evaluate a potential energy Ep which is assumed to be caused by atmospheric gravity waves. The monthly mean values of Ep at 15–20 km around Japan showed an annual variation with an enhancement in winter, which is consistent with the climatological behavior of the kinetic energy of gravity waves observed with the MU (middle and upper atmosphere) radar (34.9°N, 136.0°E) from 1985 to 1989. We have then derived the global distribution of Ep at 20–30 km during Northern Hemisphere winter (from November to February). Our analysis shows that the largest Ep values are generally centered around the equator between 25°N and 25°S with considerable longitude variations. Longitudinal variations of Ep at 20–30 km in a latitude range of 30°–60°N are also analyzed, resulting in larger Ep values over the continents than over the Pacific Ocean. Using GPS/MET data without antispoofing, latitudinal variations of Ep are determined in 15–45 km. Although large Ep values are concentrated near the equator at 20–30 km, Ep tends to become larger at midlatitudes at 30–40 km and higher‐altitude regions. At midlatitudes, Ep is found to be larger in winter months in both hemispheres. Height variations of Ep indicate a decrease at 25–30 km and a monotonic increase above 30 km.

GPS掩蔽法(GPS/MET)により観測された熱帯域対流圏界面の季節・経度変化

The GPS/MET (Global Positioning System/Meteorology) experiment was carried out between April 1995 and February 1997. Height profiles of the atmospheric refractive index were obatined at 1-60 km from the active limb sounding of occulted radio signals from the GPS satellites. By assuming the hydrostatic relation for a dry atmosphere, a new global data set of high resolution temperature profiles has become available. This paper deals with the determination of the thermal structure near the tropopause in the equatorial region using the GPS/MET data. First, we have compared the GPS/MET profiles with nearby radiosonde results at two balloon launch sites in Indonesia; Bandung (6.9°S, 107.6°E) and Pontianak (0.03°N, 109.3°E), and determined that the rms deviation was approximately 1 K in the upper troposphere when effects of humidity are small, and about 2 K in the lower stratosphere. The GPS/MET profiles are found to be very unique in revealing detailed temperature structure, including sharp inversions and step-wise increase of temperature gradient near the tropical tropopause, which has not been achieved by a conventional satellite measurement. The monthly mean value of the minimum temperature (T min ) near the tropopause agreed well between radiosonde profiles at Bandung and the GPS/MET results, showing an annual variation with warm enhancements in August-September, and broader cold periods from January to April. The altitude corresponding to T min became lower/higher for warmer/colder T min . Taking advantage of the global coverage of the GPS/MET data, we have investigated the longitude distribution of T min and H min , which generally agreed well with earlier studies.

Inversions of radio occultation amplitude data

Radio occultation remote sensing of the Earth's atmosphere consists of satellite‐to‐satellite observations of phase and amplitude of radio waves that propagate through the atmosphere. The observed excess phase along with the positions and velocities of the satellites are inverted into bending angle as a function of impact parameter and then into vertical profiles of refractivity, pressure, and temperature in the neutral atmosphere, or into electron density in the ionosphere. The retrieved profiles are assigned to the perigee points of the sounding rays. Amplitude data are normally not used, except when solving diffraction back propagation problems. In this paper a simple method to utilize amplitude radio occultation data is discussed. Equations based on geometric optics are considered for the inversion of an amplitude into bending angle. These inversions do not require high coherence of radio waves or precise orbit determination, as with phase inversions, but they do require precise calibration of the amplitude. Even though amplitude inversions are not so precise as phase inversions, they may still be useful for a number of applications. When compared to phase inversions they allow the optimization of the filter bandwidth for phase inversions, the detection of multipath propagation, and the localization of electron density irregularities in the ionosphere. These applications are demonstrated by processing of the Global Positioning System/Meteorology (GPS/MET) radio‐occultation data collected onboard the satellite Microlab‐1.

Calibration of zenith hydrostatic delay model for local GPS applications

Determination of dry zenith delay, DZD, is important for separating wet zenith delay with high accuracy from Global Positioning System (GPS) measurements. The existing models are usually for the calculation of the zenith hydrostatic delay, ZHD. Because the zenith hydrostatic delay includes the contribution of water vapor and is also not accurate enough, the DZD, approximately replaced by ZHD, will induce ∼2 cm error into local GPS applications. In this study we calibrate a popular ZHD model, namely, the Saastamoinen model for local GPS meteorology, using radiosonde data. We test the calibrated model by comparing the predicted delays from the calibrated model against the observed delays from radiosonde data that are not included in the calibration solution. The calibrated model shows 20 mm improvement over the uncalibrated model predictions over the same period. A comparison of the DZDs calculated from GPS measurements over a month period with calibrated and uncalibrated ZHD model predictions shows 15 mm average improvement.

Remote sensing of water vapor content using ground-based GPS data

Spatial and temporal resolution of water vapor content is useful in improving the accuracy of short-term weather prediction. Dense and continuously tracking regional GPS arrays will play an important role in remote sensing atmospheric water vapor content. In this study, a piecewise linear solution method was proposed to estimate the precipitable water vapor (PWV) content from ground-based GPS observations in Hong Kong. To evaluate the solution accuracy of the water vapor content sensed by GPS, the upper air sounding data (radiosonde) that are collected locally was used to calculate the precipitable water vapor during the same period. One-month results of PWV from both ground-based GPS sensing technique and radiosonde method are in agreement within 1–2 mm. This encouraging result will motivate the GPS meteorology application based on the establishment of a dense GPS array in Hong Kong.

A ray‐tracing operator and its adjoint for the use of GPS/MET refraction angle measurements

The development of small, high‐performance instruments to receive Global Positioning System (GPS) signals has created an opportunity for active remote sounding of the Earth's atmosphere by radio occultation techniques. A prototype demonstration of this capability has been provided by the GPS Meteorology (GPS/MET) experiment. Although it was shown that high vertical resolution profiles of atmospheric refractivity, temperature, and geopotential height of constant pressure levels can be derived from the GPS measurements, with high accuracy under many circumstances, many issues remain. These include the existence of multipath propagation, the ambiguity between water vapor and temperature in moist regions of the atmosphere, and the difficulty in retrieving an accurate refractivity profile from the GPS refraction angle measurements over regions where the horizontal gradient of the refractivity is large. The aim of this paper is to begin the development of a methodology for incorporating the GPS “raw” measurements (refraction angles) directly into numerical weather analysis and/or prediction systems in order to alleviate the above mentioned problems. First, a ray‐tracing observation operator that links the atmospheric state to the GPS refraction angle measurements is developed, the physics and numerics involved are described, and the simulated refraction angles, based on the NOAA National Centers for Environmental Prediction (NCEP) global analysis, are compared with the observed GPS/MET refraction angle measurements. Second, the tangent linear and adjoint of the ray‐tracing operator are developed. These three operators are required for the direct use of GPS refraction angle measurements in a variational data analysis system. A single observation experiment reveals that the direct use of GPS refraction angles in a variational analysis causes changes in the temperature and specific humidity fields that are not limited to the occultation location but in an elongated band of ±300 km in its occultation plane. On σ‐levels, changes from the use of one GPS occultation occur in an area of about 600 km × 600 km large which is centered around the ray perigee point. Finally, the advantages and disadvantages are discussed for the use of the GPS refractivities versus refraction angles. Errors made by using local estimates of refractivity are also assessed.

Remotely Sensing the Earth’s Atmosphere Using the Global Positioning System (GPS)—The GPS/MET Data Analysis

Abstract The Global Positioning System/Meteorology (GPS/MET) project is an active satellite-to-satellite remote sensing experiment using the radio occultation technique. Due to the atmospheric index of refraction and gradient of the index of refraction, GPS signals propagate through the earth’s atmosphere along a slightly curved path and with slightly retarded speeds. When these signals arrive at a receiver aboard a low earth orbit satellite, the receiver records an excess phase delay compared with the phase delay of a straight line propagation in a vacuum. Using the Abel integral equations, the phase delay rates with time can be converted into the atmospheric index of refraction profile; then, using the hydrostatic equation, the pressure and temperature profiles may be derived. This paper describes the principles of the GPS/MET occultation experiment and the detailed data analysis procedure. Data smoothing technique and error analysis are also discussed. Some GPS/MET intermediate and final retrieval resu...

Modeling weather fronts to improve GPS heights: A new tool for GPS meteorology?

The precision of vertical position and atmospheric water vapor content determined by the Global Positioning System (GPS) is limited by errors due to tropospheric delay. One factor is the spatial and temporal variability in tropospheric refractivity caused by passing weather fronts. We can explain some of the temporal characteristics of estimated tropospheric delay in terms of a simple path delay model as a function of frontal parameters. These results suggest that GPS could be used to estimate the geometry and passage time of a frontal zone. We have developed indices which detect tropospheric variability from GPS data alone; the detection rate of fronts with this approach is up to 70%. Once detected, we eliminated days affected by fronts or other tropospheric variability from the time series of station height estimates, resulting in improved long‐term repeatability. The additional variance attributable to fronts is estimated to be up to 10 mm2 at Herstmonceux, England, where fronts occur every 2–3 days. The effect of fronts on the horizontal station component is up to 80% smaller than for the vertical. Studies in the field of GPS meteorology may be improved by estimating frontal parameters.

Inversion, error analysis, and validation of GPS/MET occultation data

Abstract. The global positioning system meteorology (GPS/MET) experiment was the first practical demonstration of global navigation satellite system (GNSS)-based active limb sounding employing the radio occultation technique. This method measures, as principal observable and with millimetric accuracy, the excess phase path (relative to propagation in vacuum) of GNSS-transmitted radio waves caused by refraction during passage through the Earth's neutral atmosphere and ionosphere in limb geometry. It shows great potential utility for weather and climate system studies in providing an unique combination of global coverage, high vertical resolution and accuracy, long-term stability, and all-weather capability. We first describe our GPS/MET data processing scheme from excess phases via bending angles to the neutral atmospheric parameters refractivity, density, pressure and temperature. Special emphasis is given to ionospheric correction methodology and the inversion of bending angles to refractivities, where we introduce a matrix inversion technique (instead of the usual integral inversion). The matrix technique is shown to lead to identical results as integral inversion but is more directly extendable to inversion by optimal estimation. The quality of GPS/MET-derived profiles is analyzed with an error estimation analysis employing a Monte Carlo technique. We consider statistical errors together with systematic errors due to upper-boundary initialization of the retrieval by a priori bending angles. Perfect initialization and properly smoothed statistical errors allow for better than 1 K temperature retrieval accuracy up to the stratopause. No initialization and statistical errors yield better than 1 K accuracy up to 30 km but less than 3 K accuracy above 40 km. Given imperfect initialization, biases &gt;2 K propagate down to below 30 km height in unfavorable realistic cases. Furthermore, results of a statistical validation of GPS/MET profiles through comparison with atmospheric analyses of the European Centre for Medium-range Weather Forecasts (ECMWF) are presented. The comparisons indicate the high utility of the occultation data in that very good agreement of upper troposphere/lower stratosphere temperature (better than 1.5 K rms, &lt;0.5 K bias) is found for a region (Europe+USA) where the ECMWF analyses are known to be good, but poorer agreement for a region (Southern Pacific) where the analyses are known to be degraded.Key words. Atmospheric composition and structure (pressure; density and temperature), Meteorology and atmospheric dynamics (instruments and techniques), Radio science (remote sensing)