Academic literature on the topic 'RO data inversion'

Abstract. For inversions of the GPS radio occultation (RO) data in the neutral atmosphere, this study investigates an optimal 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 processing 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 ionospheric 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 inhomogeneity of the ionosphere.

Abstract. GNSS Radio Occultation (RO) refractivity climatologies for the stratosphere can be obtained from the Abel inversion of monthly average bending-angle profiles. The averaging of large numbers of profiles suppresses random noise and this, in combination with simple exponential extrapolation above an altitude of 80 km, circumvents the need for a "statistical optimization" step in the processing. Using data from the US-Taiwanese COSMIC mission, which provides ~ 1500–2000 occultations per day, it has been shown that this Average-Profile Inversion (API) technique provides a robust method for generating stratospheric refractivity climatologies. Prior to the launch of COSMIC in mid-2006, the data records rely on data from the CHAMP mission. In order to exploit the full range of available RO data, the usage of CHAMP data is also required. CHAMP only provided ~ 200 profiles per day, and the measurements were noisier than COSMIC. As a consequence, the main research question in this study was to see if the average bending angle approach is also applicable to CHAMP data. Different methods for suppression of random noise – statistical and through data quality pre-screening – were tested. The API retrievals were compared with the more conventional approach of averaging individual refractivity profiles, produced with the implementation of statistical optimization used in the EUMETSAT Radio Occultation Meteorology Satellite Application Facility (ROM SAF) operational processing. In this study it is demonstrated that the API retrieval technique works well for CHAMP data, enabling the generation of long-term stratospheric RO climate data records from August 2001 and onward. The resulting CHAMP refractivity climatologies are found to be practically identical to the standard retrieval at the DMI below altitudes of 35 km. Between 35 km to 50 km the differences between the two retrieval methods started to increase, showing largest differences at high latitudes and high altitudes. Furthermore, in the winter hemisphere high latitude region, the biases relative to ECMWF were generally smaller for the new approach than for the standard retrieval.

Abstract. Vertical profiles of the atmosphere can be obtained globally with the radio-occultation technique. However, the lowest layers of the atmosphere are less accurately extracted. A good description of these layers is important for the good performance of Numerical Weather Prediction (NWP) systems, and an improvement of the observational data available for the low troposphere would thus be of great interest for data assimilation. We outline here how supplemental meteorological information close to the surface can be extracted whenever reflected signals are available. We separate the reflected signal through a radioholographic filter, and we interpret it with a ray tracing procedure, analyzing the trajectories of the electromagnetic waves over a three-dimensional field of refractive index. A perturbation approach is then used to perform an inversion, identifying the relevant contribution of the lowest layers of the atmosphere to the properties of the reflected signal, and extracting some supplemental information to the solution of the inversion of the direct propagation signals. The methodology is applied to one reflection case.

Abstract The spatial and temporal variability of the marine boundary layer (MBL) over the southeastern Pacific is studied using high-resolution radiosonde data from the VAMOS Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx), lidar cloud measurements from the CALIOP instrument on the CALIPSO satellite, radio occultation (RO) data from the COSMIC satellites, and the ERA-Interim. The height of the MBL (MBLH) is estimated using three RO-derived parameters: the bending angle, refractivity, and water vapor pressure computed from the refractivity derived from a one-dimensional variational data inversion (1D-VAR) procedure. Two different diagnostic methods (minimum gradient and break point method) are compared. The results show that, although a negative bias in the refractivity exists as a result of superrefraction, the spatial and temporal variations of the MBLH determined from the RO observations are consistent with those from CALIOP and the radiosondes. The authors find that the minimum gradient in the RO bending angle gives the most accurate estimation of the MBL height.

Abstract. Global Navigation Satellite System Radio Occultation (GNSS-RO) refractivity climatologies for the stratosphere can be obtained from the Abel inversion of monthly average bending-angle profiles. The averaging of large numbers of profiles suppresses random noise and this, in combination with simple exponential extrapolation above an altitude of 80 km, circumvents the need for a "statistical optimization" step in the processing. Using data from the US–Taiwanese COSMIC mission, which provides ~1500–2000 occultations per day, it has been shown that this average-profile inversion (API) technique provides a robust method for generating stratospheric refractivity climatologies. Prior to the launch of COSMIC in mid-2006, the data records rely on data from the CHAMP (CHAllenging Mini-satellite Payload) mission. In order to exploit the full range of available RO data, the usage of CHAMP data is also required. CHAMP only provided ~200 profiles per day, and the measurements were noisier than COSMIC. As a consequence, the main research question in this study was to see if the average bending-angle approach is also applicable to CHAMP data. Different methods for the suppression of random noise – statistical and through data quality prescreening – were tested. The API retrievals were compared with the more conventional approach of averaging individual refractivity profiles, produced with the implementation of statistical optimization used in the EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) Radio Occultation Meteorology Satellite Application Facility (ROM SAF) operational processing. In this study it is demonstrated that the API retrieval technique works well for CHAMP data, enabling the generation of long-term stratospheric RO climate data records from August 2001 and onward. The resulting CHAMP refractivity climatologies are found to be practically identical to the standard retrieval at the DMI (Danish Meteorological Institute) below altitudes of 35 km. Between 35 and 50 km, the differences between the two retrieval methods started to increase, showing largest differences at high latitudes and high altitudes. Furthermore, in the winter hemisphere high-latitude region, the biases relative to ECMWF (European Centre for Medium-range Weather Forecasts) were generally smaller for the new approach than for the standard retrieval.

Abstract. Vertical profiles of the atmosphere can be obtained globally with the radio-occultation technique. However, the lowest layers of the atmosphere are less accurately extracted. A good description of these layers is important for the good performance of Numerical Weather Prediction (NWP) systems, and an improvement of the observational data available for the low troposphere would thus be of great interest for data assimilation. We outline here how supplemental meteorological information close to the surface can be extracted whenever reflected signals are available. We separate the reflected signal through a radioholographic filter, and we interpret it with a ray tracing procedure, analyzing the trajectories of the electromagnetic waves over a 3-D field of refractive index. A perturbation approach is then used to perform an inversion, identifying the relevant contribution of the lowest layers of the atmosphere to the properties of the reflected signal, and extracting some supplemental information to the solution of the inversion of the direct propagation signals. It is found that there is a significant amount of useful information in the reflected signal, which is sufficient to extract a stand-alone profile of the low atmosphere, with a precision of approximately 0.1 %. The methodology is applied to one reflection case.

Abstract Accurate temperature and water vapor profiles in the middle and lower troposphere (LT) are crucial for understanding the water cycle, cloud systems, and energy balance. Global positioning system (GPS) radio occultation (RO) is the first technique that can provide a high-vertical-resolution all-weather refractivity profile, which is a function of pressure, temperature, and moisture. However, in the moist LT over the Tropics, the refractivity retrievals from GPS RO data are often significantly negatively biased because of tracking errors and propagation effects related to sharp vertical moisture gradients that may result in superrefraction (SR). The Atmospheric Infrared Sounder (AIRS) is a nadir-viewing sounder that can measure vertical temperature and moisture profiles with about 1–2-km vertical resolution. However, AIRS observations cannot usually obtain accurate temperature and water vapor profiles in the planetary boundary layer (PBL) because of the poor resolving power in the LT. This study uses simulations based on radiosonde profiles by combining the AIRS and the GPS RO measurements to obtain the best temperature and moisture retrievals in the LT. Different approaches are used for the drier LT and the moist LT. For the drier LT, where GPS RO data are not affected by SR errors, a multivariable regression algorithm for inverting the combined AIRS and GPS RO measurements is used. In the moist LT (e.g., SR on top of PBL), the combined AIRS and GPS RO regression inversion above the LT is used as the first guess for AIRS-only physical retrieval, which is extended into the LT. The results show that combining AIRS and GPS RO data effectively constrains the individual solutions, and therefore significantly improves inversion results. The algorithm is also applied for all available radiosonde profiles (19 profiles) over a 1-month period from the site characterized by strong SR on top of the PBL. Retrieved temperature and water vapor profiles yield unbiased low-resolution refractivity profiles in the PBL.

Abstract. The Global Navigation Satellite System (GNSS) atmospheric radio occultation (RO) has been an effective method for exploring Earth's atmosphere. RO signals propagate through the ionosphere before reaching the neutral atmosphere. The GNSS signal is affected by the ionospheric irregularity including the sporadic E (Es) and F region irregularity mainly due to the multipath effect. The effect of ionospheric irregularity on atmospheric RO data has been demonstrated by several studies in terms of analyzing singe cases. However, its statistical effect has not been investigated comprehensively. In this study, based on the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) RO data during 2011–2013, the failed inverted RO events occurrence rate and the bending angle oscillation, which is defined as the standard deviation of the bias between the observed bending angle and the National Center for Atmospheric Research (NCAR) climatology model bending angle between 60 and 80 km, were used for statistical analysis. It is found that at middle and low latitudes during the daytime, the failed inverted RO occurrence and the bending angle oscillation show obvious latitude, longitude, and local time variations, which correspond well with the Es occurrence features. The F region irregularity (FI) contributes to the obvious increase of the failed inverted RO occurrence rate and the bending angle oscillation value during the nighttime over the geomagnetic equatorial regions. For high latitude regions, the Es can increase the failed inverted RO occurrence rate and the bending angle oscillation value during the nighttime. There also exists the seasonal dependency of the failed inverted RO event and the bending angle oscillation. Overall, the ionospheric irregularity effects on GNSS atmospheric RO measurement statistically exist in terms of failed RO event inversion and bending angle oscillation. Awareness of these effects could benefit both the data retrieval and applications of RO in the lower atmosphere.

AbstractRadio occultation (RO) can provide high-vertical-resolution thermodynamic soundings of the planetary boundary layer (PBL). However, sharp moisture gradients and strong temperature inversion lead to large gradients in refractivity N and often cause ducting. Ducting results in systematically negative RO N biases resulting from a nonunique Abel inversion problem. Using 8 years (2006–13) of Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) RO soundings and collocated European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-I) data, we confirm that the large lower-tropospheric negative N biases are mainly located in the subtropical eastern oceans and we quantify the contribution of ducting for the first time. The ducting-contributed N biases in the northeast Pacific Ocean (160°–110°W; 15°–45°N) are isolated from other sources of N biases using a two-step geometric-optics simulation. Negative bending angle biases in this region are also observed in COSMIC RO soundings. Both the negative refractivity and bending angle biases in COSMIC soundings mainly lie below ~2 km. Such bending angle biases introduce N biases that are in addition to those caused by ducting. Following the increasing PBL height from the southern California coast westward to Hawaii, centers of maxima bending angles and N biases tilt southwestward. In areas where ducting conditions prevail, ducting is the major cause of the RO N biases. Ducting-induced N biases with reference to ERA-I compose over 70% of the total negative N biases near the southern California coast, where strongest ducting conditions prevail, and decrease southwestward to less than 20% near Hawaii.

Abstract The global positioning system (GPS) radio occultation (RO) technique has demonstrated the ability to precisely probe earth’s atmosphere globally with high vertical resolution. However, the lowermost troposphere still presents some challenges for the technique. Over moist marine areas, especially in subtropical regions, a very large negative moisture gradient often exists across the thermal inversion capping the marine boundary layer (MBL), which frequently causes superrefraction (SR), or ducting. In the presence of SR, the reconstruction of refractivity from RO data becomes an ill-posed inverse problem. This study shows that one given RO bending angle profile is consistent with a continuum (an infinite number) of refractivity profiles. The standard Abel retrieval gives the minimum refractivity solution of the continuum and thus produces the largest negative bias, consistent with a negative bias often present in the retrieved refractivity profiles in the moist lower troposphere. By applying a simple linear parameterization of the refractivity structure within and just below the SR layer, an analytical relation between the Abel-retrieved refractivity and a continuum of solutions is derived. Combining the Abel retrieval and the analytical relation with some physical constraints, a novel approach is developed to reconstruct the vertical refractivity structure within and below the SR layer. Numerical simulation studies in this paper have demonstrated the great potential of the reconstruction method to provide a much-improved retrieval in the presence of SR, and the method should greatly enhance the ability to measure the MBL structure globally using the GPS RO technique.

The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km to more than 1000 km. It owes its existence primarily to ultraviolet radiation from the sun. The ionosphere is becoming more relevant to human society with its reliance on modern technology, since the accuracy of navigation and quality of telecommunication is influenced by ionospheric conditions. The free electrons in the ionosphere affect the propagation of radio waves. Below about 30 MHz the ionosphere acts like a mirror, bending the path traveled by a radio wave back toward the Earth. At higher frequencies, such as those used by GPS, radio waves pass right through the ionosphere. They are, nevertheless, affected by it. Disruption of communications and navigation systems can have severe societal consequences. Even though the ionospheric observational techniques and the ionospheric models have gone through considerable development sustained over many decades, accurate monitoring and forecasting of the ionosphere conditions still presents stubborn challenges. The global navigation satellite system (GNSS)-based radio occultation (RO) has been proven to be a powerful technique for remotely sensing the earth’s troposphere, stratosphere, and ionosphere in the past decade [8]. Radio occultation is a relatively new technique that can be used to study the ionosphere, offering potentially global and continuous measurements. MARINER IV first applied the RO observation technique to observe the Mars atmosphere and ionosphere in 1965 [48]. MicroLab-1 GPS/MET was launched in 1995 and applied to monitor the Earth’s atmosphere and ionosphere by using GPS RO technique [37,50]. The Global Positioning System (GPS) to Low Earth Orbit (LEO) satellite paths essentially make long, near-horizontal measurements of the integrated content of ionospheric electron density; namely total electron content (TEC). These measurements are not simple to interpret, since the satellite transmission paths map out a complicated and continuously changing measurement geometry. Nevertheless, a strong advantage of this system is that it provides measurements over the oceans and into remote polar caps, thus enabling the ionosphere to be studied on a truly global-scale. The FORMOSAT-3/COSMIC (the most recent and advanced RO mission in operation) was launched in April 2006, and has six micro satellites in different orbital planes. The GPS radio occultation experiment (GOX) is one of the satellite mission objectives, and observes the ionosphere and atmosphere vertical structure by using the RO observation technique. RO observations, particularly from FORMOSAT-3/COSMIC, have significantly improved our capability of monitoring the global ionosphere. In the ionosphere, the important scientific RO data product is the retrieved electron density profile (Ne(h)) along the tangent points during an occultation event. The Abel inverse transform is the conventional method to analyze the tropospheric occultations and it was natural to adopt this approach for the ionospheric studies. It allows the vertical profile of electron concentration to be obtained, nominally at a single location between the GPS and LEO (onboard RO receiver). The resulting profile is therefore some average of the ionosphere traversed by the occulting ray paths between the two satellites. However, the classical approach of the Abel inversion assumes spherical symmetry of the electron density field in the vicinity of an occultation. In practice, the footprint of an occultation generally covers wide regions and averages any spatial variations connected with variable declinations of the magnetic field from the horizontal direction along the occulted ray path. Indeed, inhomogeneous electron density in the horizontal direction for a given occultation is believed to be the main source of error when using the Abel inversion. Large amount of research has been done, in last couple of decades, to improve the Abel inversion by removing or reducing the effect of ionospheric asymmetry. One potential and frequently studied and revised method is the Abel inversion aided by other horizontal information such as the global ionospheric map (GIM) [9,30,32,36,42,49,72]. However, due to variability of available maps and the fact that not much attention was paid to the large-scale Abel retrieval error, as illustrated by [54,92,93], no standard procedure have been globally accepted and Abel inversion is still the most widely used technique to produce the ionospheric products using RO technique. In our research, we have thoroughly investigated the spherical symmetry problem of Abel inversion; qualitatively as well as quantitatively. This was done to first understand what actually is happening when we apply such algorithm for RO data inversion. In the process of this investigation, we were able to find an effective way of quantifying the impact of ionospheric asymmetry on the final product of RO data inversion, i.e., vertical electron density profile. The asymmetry index is based on the electron density variation along the occulted ray path. It efficiently incorporates electron density gradients along the RO ray path to find a number that sums-up the impact of prevailing ionospheric condition, on the final RO product, by giving it a number on a scale from 0 to 1. Our results, based on model simulations, show that the designed algorithm is proving to be an effective technique to find such information quickly and accurately. Using the knowledge gained during this thorough investigation, to mitigate the impact of spherical symmetry hypothesis from RO data inversion, we have also implemented a very effective technique based on the NeQuick2 (electron density model) adaptation to RO-derived TEC. It relies on the minimization of a cost function involving experimental and model-derived TEC data to determine the NeQuick2 input parameters (local ionization parameters) at the wanted locations and time. These parameters are then used to evaluate the electron density profile along the ray perigee positions associated to the relevant RO event. The results indicate that the technique significantly improved the RO inversion product and is able to avoid the presence of negative electron density values in the reconstructed profiles. Furthermore, no external data, such as GIM maps or other data, is required to apply the technique. The technique is currently under development and, when fully developed, will not only provides a solution to a long awaited problem (spherical symmetry hypothesis) to be solved related to RO data processing, but also defines some new concepts on which RO technique may be used in future research related to the ionosphere. This will be a significant contribution for RO scientific community, specifically, when a large increase in RO observations (from approximately 1,500 to 12,000 per day) is expected with the launch of COSMIC-2 RO mission in the next few years.

Following the success in the exploration drilling campaign in the last few years, Pertamina EP puts the recently discovered Wol Structure into the appraisal stage. The exploration wells Wol-001 and Wol-002 were spudded in 2017 and 2019 respectively, and both flowed a significant gas rate from an excellent reservoir of Miocene Reef of Minahaki Formation. A good understanding of the reservoir distribution was essential in such a stage. Therefore, a proper reservoir characterization was then carried out for further appraisal purposes. Using the improved quality data from the latest 5D interpolation-PSDM as input, integration of amplitude versus offset (AVO) techniques and rock physics analysis was conducted to investigate the hydrocarbon extent. The AVO class IIp was observed at the boundary between overlying Kintom Shale and gas saturated Minahaki limestone. It is indicated by a positive intercept (Ro), decreased amplitudes with offsets, and negative amplitudes in the far offsets. This polarity reversal characteristic is clearly seen from both AVO modeling and actual CDP in the well locations. Several CDPs inside and outside the closure were also examined to check the consistency. The slice of partial stack volumes has also exhibited a similar trend within the closure where class IIp is suggestive. Since the AVO attributes such as intercept and gradient solely were not able to visualize the reservoir extent properly, the pre-stack seismic inversion was performed to obtain a more accurate reservoir distribution through quantitative interpretation. A cross plot of P-impedance (Ip) over S-impedance (Is) differentiates the gas zone clearly from the wet linear trend. A depth slice at GWC (gas water contact) level describes that most of the Wol Structure is gas-saturated including the newly identified closure in the northwest. It is a three-way dip closure formed by limestone that was dragged upward by a thrust fault. Interestingly, it has a similar AVO response to the main Wol Structure which suggests a gas-bearing reservoir. This work brings an added value to the use of AVO analysis and pre-stack inversion for hydrocarbon mapping for appraisal purposes. Not only it has largely reduced the subsurface uncertainty, but also revealed an upside potential that is worth considering in future exploration.