Occultation Technique Research Articles

Applications of GNSS-RO to Numerical Weather Prediction and Tropical Cyclone Forecast

The global navigation satellite system (GNSS) radio occultation (RO) technique is an atmospheric sounding technique that originated in the 1990s. The data provided by this approach are playing a consistently significant role in atmospheric research and related applications. This paper mainly summarizes the applications of RO to numerical weather prediction (NWP) generally and specifically for tropical cyclone (TC) forecast and outlines the prospects of the RO technique. With advantages such as high precision and accuracy, high vertical resolution, full-time and all-weather, and global coverage, RO data have made a remarkable contribution to NWP and TC forecasts. While accounting for only 7% of the total observations in European Centre for Medium-Range Weather Forecasts’ (ECMWF’s) assimilation system, RO has the fourth-largest impact on NWP. The greater the amount of RO data, the better the forecast of NWP. In cases of TC forecasts, assimilating RO data from heights below 6 km and from the upper troposphere and lower stratosphere (UTLS) region contributes to the forecasting accuracy of the track and intensity of TCs in different stages. A statistical analysis showed that assimilating RO data can help restore the critical characteristics of TCs, such as the location and intensity of the eye, eyewall, and rain bands. Moreover, a non-local excess phase assimilation operator can be employed to optimize the assimilation results. With denser RO profiles expected in the future, the accuracy of TC forecast can be further improved. Finally, future trends in RO are discussed, including advanced features, such as polarimetric RO, and RO strategies to increase the number of soundings, such as the use of a cube satellite constellation.

Compact and high isolation microstrip diplexer for future radio science planetary applications

A novel electrically compact and high isolation microstrip diplexer is engineered for Radio Occultation (RO) System for future planetary systems. The radio occultation technique is being utilized extensively in examining properties of a planetary atmosphere. A detection of shift in microwave signal is primary method to measure the occultation. The On-board RO transmitter utilizes the frequency synthesizers at two distinct frequency bands. The proposed microstrip diplexer has self-symmetrical structure that permits reduction in the tradeoff between miniaturization and port isolation. The balanced geometry of two intrinsic filters permits several transmission zeros creating sharp edge, wider stopband and significant port isolation. The further tuning of the diplexer is possible without junction matching however; the insertion loss and isolation may vary based on the frequency shift. The electrical size of the proposed diplexer is 0.52λ × 0.625λ at lower frequency. The optimized structure provides fraction bandwidth of around 4.6% at 6.7 GHz and 4.34% at 8.4 GHz frequencies. The isolation is better than 27 dB for both frequencies. The validation of simulated results is carried out through measurements on fabricated prototype.

Ionospheric S4 Scintillations from GNSS Radio Occultation (RO) at Slant Path

Ionospheric scintillation can significantly degrade the performance and the usability of space-based communication and navigation signals. Characterization and prediction of ionospheric scintillation can be made from the Global Navigation Satellite System (GNSS) radio occultation (RO) technique using the measurement from a deep slant path where the RO tangent height (ht) is far below the ionospheric sources. In this study, the L–band S4 from the RO measurements at ht = 30 km is used to infer the amplitude scintillation on the ground. The analysis of global RO data at ht = 30 km shows that sporadic–E (Es), equatorial plasma bubbles (EPBs), and equatorial spread–F (ESF) produce most of the significant S4 enhancements, although the polar S4 is generally weak. The enhanced S4 is a strong function of local time and magnetic dip angle. The Es–induced daytime S4 tends to have a negative correlation with the solar cycle at low latitudes but a positive correlation at high latitudes. The nighttime S4 is dominated by a strong semiannual variation at low latitudes.

Performance evaluation of model-assisted data inversion technique for ionospheric radio occultation

Radio occultation (RO) has proved to be a powerful tool for weather forecasting and ionospheric modelling. It is a truly global technique for the observation in the lower as well as in the upper atmosphere. With the evolution of radio occultation technique, the data inversion algorithms used to extract data from the raw RO measurements also evolved over the period of last two decades. In this work, a thorough investigation has been conducted on the performance of model-assisted inversion technique proposed by Shaikh et al. (2017). A combination of low, mid and high latitude geographical locations was chosen to analyze the performance of the technique. It has shown that the proposed technique has significantly improved the data inversion results in high solar activity conditions when the horizontal gradients of the ionosphere are expected to be stronger than normal (for example 24% at King Salmon and 21% at Jeju in high solar activity). The longitudinal sector-wise analysis shows that the American longitudinal sector has slightly weaker performance measures for model-assisted inversion compared to the Asian longitudinal sector.

An Evaluation of Fengyun-3C Radio Occultation Atmospheric Profiles Over 2015–2018

Fengyun-3C (FY-3C) is the first Chinese satellite that is capable of using the Radio Occultation (RO) technique to retrieve atmospheric profiles. This research evaluates the quality of FY-3C RO profiles including refractivity, temperature, and specific humidity by comparing with corresponding information from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim) data over the period of 2015–2018. The evaluation is carried out by calculating and analyzing mean systematic differences between FY-3C and ERA-Interim profiles and corresponding standard deviations over a selected spatial and temporal domain. Results show that the FY-3C RO profiles are overall with good agreements with the ERA-Interim data. Global mean refractivity systematic differences are within ±0.2% from 5 to 30 km altitude range with relative standard deviations of less than 2%. Global temperature mean systematic differences vary within ±0.2 K from a 10- to 20-km altitude range with standard deviations of less than 2 K. Global mean specific humidity differences are found to be within ±0.2 g/kg from 2 to 20 km with standard deviations of less than 1 g/kg. FY-3C profiles show visible latitudinal and altitudinal variations, while the seasonal variations are minor. Sampling errors of refractivity and temperature are also found to be larger at higher latitudinal regions due to RO events being less sampled in the polar region.

New Vary‐Chap Scale Height Profile Retrieved From COSMIC Radio Occultation Data

AbstractThe Global Navigation Satellite System radio occultation technique provides materials for the validation of a mathematical description of the topside ionosphere up to the low Earth orbit (LEO) satellite altitude. An attempt to represent the topside electron density profile is using α‐Chapman function with a continuously varying scale height. In this study, the Vary‐Chap scale height profiles are obtained based on Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) electron density profiles from 1 January 2008 to 31 December 2013 and fitted by a shape function composed of two weighted patterns representing the ion and electron contributions of lower and higher altitudes. The associated fitting parameters a and b are analyzed to reveal their temporal and spatial features and variations along with enhancement of solar activity. The prominent dependence on latitudes, longitudes, the local time, the season, and the solar cycle shown in the parameters inspires future modeling of the Vary‐Chap scale height in constructing empirical topside ionospheric models.

Ionospheric responses to the 21 August 2017 great American solar eclipse – A multi-instrument study

Herein, we report on the ionospheric responses to a total solar eclipse that occurred on 21 August 2017 over the US region. Ground-based GPS total electron content (TEC) data along with ground-based measurements (Millstone Hill Observatory (MHO) and digital ionosondes) and space-based measurements (COSMIC radio occultation (RO) technique) allowed us to identify eclipse-associated ionospheric responses. TEC data at ~20°, ~30°, and ~40°N latitudes from the west to east longitudes show not only considerable depression but also wave-like characteristics in TEC both in the path of totality and away from it, exclusively on the day of eclipse. Interestingly, the observed depressions are associated with lesser (higher) magnitudes at stations over which the solar obscuration percentage was meager (significant), a clear indication of bow-wave-like features. The MHO observes a 30% reduction in F2-layer electron densities between 180 and 220 km on eclipse day. Ionosonde-scaled parameters over Boulder (40.4°N, 100°E) and Austin (30.4°N, 94.4°E) show a significant decrease in critical frequencies while an altitude elevation is seen in the virtual heights of the F-layer only during the eclipse day and that decreases are associated with wave-like signatures, which could be attributed to eclipse-generated waves. The estimated vertical electron density profile from the COSMIC RO-based technique shows a maximum depletion of 40%. Relatively intense and moderate depths of TEC depression, considerable reductions in the F2-layer electron densities measured by the MHO and COSMIC RO-measured densities at the F2-layer peak, and elevations in virtual heights and reduction in the critical frequencies measured by ionosondes during the eclipse day could be due to the eclipse-induced dynamical effects such as gravity waves (GWs) and their associated electro-dynamical effects (modification of ionospheric electric fields due to GWs).

Localized Ionization Hypothesis for Transient Ionospheric Layers

AbstractThe persistent two‐peaked vertical structure of the Martian ionosphere is created by extreme and far ultraviolet radiation whose energies, respectively, determine their ionization altitude. A third low‐altitude transient layer (previously referred to as M3 or Mm) has been observed by radio occultation techniques and attributed to meteor ablation. However, recent remote sensing and in situ observations disfavor a meteoric origin. Here we propose an alternative hypothesis for these apparent layers associated with impact ionization from penetrating solar wind ions, previously observed as proton aurora. Localized ionization, occurring nonglobally at a given altitude range, breaks the symmetry assumed by the radio occultation technique, and creates electron layers apparently lower in the ionosphere than their true altitude. This may occur when the upstream bow shock is altered by a radial interplanetary magnetic field configuration, which allows the solar wind to penetrate directly into the thermosphere. This localized ionization hypothesis provides an explanation for apparent layers' wide variation in heights and their transient behavior. Moreover, this hypothesis is testable with new observations by the Mars Atmospheric and Volatile EvolutioN Radio Occultation Science Experiment in future Mars years. This hypothesis has implications for the ionospheres of Venus and Titan, where similar transient layers have been observed.

Analysis of synchronism and response velocity in instrumental assemblies for the observation of stellar occultations

Currently the stellar occultation technique is one of the most effective methods for observing minor bodies of the Solar System. Due to the collaborative nature of these observations, precise synchronization and short response times are necessary in instrumental assemblies. This paper presents the development of a methodology that allows for the evaluation of the temporal performance of the detectors used in the observation of stellar occultations. The software “TimesCCam” was developed, which, when being photographed by astronomical cameras, graphically represents the parameters of exposure time, dead time, and jitter referenced to UTC. The process of alignment and focus is obtained, thanks to a structure built with movement in the three axes. The methodology was applied in the evaluation of STF-8300M, ST-2000XM and QHY174M-GPS cameras, using different driver software, synchronization methods, and exposure times. In real exposure times, insignificant variations were identified. Dependency was found between the dead times and the configurations used for the CCD cameras, and it was determined that the dead time and its instability in the ST-2000XM camera prevent its use for the observation of stellar occultations. The results show that the STF-8300M camera is convenient for the observation of asteroid occultations, while the QHY174M-GPS camera given its dead time and synchronization, is suitable for the observation of occultations by trans-Neptunian objects.

3-D Ionospheric Tomography Using Model Function in the Modified L-Curve Method

Ionospheric tomography based on the observed total electron content along different satellite-receiver rays is a typically ill-posed inverse problem. Incorporating the electron density profiles data from COSMIC radio occultation technique and ground ionosondes, the Tikhonov regularization method is adopted to reconstruct the 3-D ionospheric electron density, and a regularization parameter is used to balance the weights between the prior (or background) information and real measurements. To determine the optimal regularization parameter, the model function in the modified L-curve method is used. This new method combines the advantages of the model function approach and L-curve criterion, and it has not only high accuracy, but also rapid convergence. To validate the effectiveness of this reconstruction algorithm, both the ideal test and real measurements test are carried out, and the results show that this algorithm can significantly improve the background model outputs.

Periodic Orbits for Sounding Mars’ Atmosphere by Down- and Limb-Looking Measurements

The Periodic Multi-Sun-Synchronous Orbit allows the planning of observational cycles in which the solar illumination of the observed planet varies regularly, and returns to the initial condition at the beginning of each cycle. In the classical down-looking geometry of observation, this leads to the possibility of reconstructing the temporal evolution of all planetary parameters that are characterised by a typical variation in the solar day. In this paper, such an orbit is proposed to analyse the evolution of Mars’ atmosphere time-varying parameters taking into account, in addition to the down-looking configuration, a limb-looking geometry that works with a solar occultation technique.

Quarterdiurnal signature in sporadic E occurrence rates and comparison with neutral wind shear

Abstract. The GPS radio occultation (RO) technique is used to study sporadic E (Es) layer plasma irregularities of the Earth's ionosphere on a global scale using GPS signal-to-noise ratio (SNR) profiles from the COSMIC/FORMOSAT-3 satellite. The maximum deviation from the mean SNR can be attributed to the height of the Es layer. Es are generally accepted to be produced by ion convergence due to vertical wind shear in the presence of a horizontal component of the Earth's magnetic field, while the wind shear is provided mainly by the solar tides. Here we present analyses of quarterdiurnal tide (QDT) signatures in Es occurrence rates. From a local comparison with mesosphere/lower thermosphere wind shear obtained with a meteor radar at Collm (51.3∘ N, 13.0∘ E), we find that the phases of the QDT in Es agree well with those of negative vertical shear of the zonal wind for all seasons except for summer, when the QDT amplitudes are small. We also compare the global QDT Es signal with numerical model results. The global distribution of the Es occurrence rates qualitatively agrees with the modeled zonal wind shears. The results indicate that zonal wind shear is indeed an important driving mechanism for the QDT seen in Es.

Assimilation of GPSRO Bending Angle Profiles into the Brazilian Global Atmospheric Model

The Global Positioning System (GPS) Radio Occultation (RO) technique allows valuable information to be obtained about the state of the atmosphere through vertical profiles obtained at various processing levels. From the point of view of data assimilation, there is a consensus that less processed data are preferable because of their lowest addition of uncertainties in the process. In the GPSRO context, bending angle data are better to assimilate than refractivity or atmospheric profiles; however, these data have not been properly explored by data assimilation at the CPTEC (acronym in Portuguese for Center for Weather Forecast and Climate Studies). In this study, the benefits and possible deficiencies of the CPTEC modeling system for this data source are investigated. Three numerical experiments were conducted, assimilating bending angles and refractivity profiles in the Gridpoint Statistical Interpolation (GSI) system coupled with the Brazilian Global Atmospheric Model (BAM). The results highlighted the need for further studies to explore the representation of meteorological systems at the higher levels of the BAM model. Nevertheless, more benefits were achieved using bending angle data compared with the results obtained assimilating refractivity profiles. The highest gain was in the data usage exploring 73.4% of the potential of the RO technique when bending angles are assimilated. Additionally, gains of 3.5% and 2.5% were found in the root mean square error values in the zonal and meridional wind components and geopotencial height at 250 hPa, respectively.

Utility of GNSS Radio Occultation technique for tropopause height investigation over Egypt

ABSTRACTGlobal Navigation Satellite System (GNSS) Radio Occultation (RO) is an active limb sounding technique, where GNSS satellites transmitted signals passing through the atmosphere of the Earth and received by a GNSS receiver on low earth orbiter (LEO) satellite. RO provides accurate atmospheric refractivity profile. RO technique has been widely used to study the atmosphere of planets. This paper investigates the use of GNSS RO for tropopause height (TPH) estimation as one of the key climate parameters over Egypt. TPH is also very important in determining the wet delay in GNSS analysis. Two years (2016 and 2017) of MetOP A and B satellites data are used. ROPP software package is used in the analysis. For validation of the results, RO-derived TPH is compared with European Centre for Medium-Range Weather Forecast (ECMWF) model as well as radiosonde (RS). Good agreement and high correlation are found between TPH from RO and ECMWF and RS on the other hand. TPH varies between 14 and 16 km over Egypt. It decreases with latitude and shows no clear trend with longitude. Tropopause temperature is found to increase with latitude.

Introduction to the Techniques of Interferometry and Lunar Occultation in Radio Astronomy

Radio telescopes allow a 24 × 7 observation time window of those regions of the space which appear dark in the visible region. But, the major problem with a single radio telescope is its poor resolution. In this article, the method of interferometry in radio astronomy is discussed by citing the example of a two-element interferometer. Interferometry provides improved resolution.With a single telescope as well, the resolution can be improved for the radio sources along the ecliptic which are smaller in angular spread as compared to the Moon. The theoretical principle behind the technique of occultation is discussed using its optical analog of diffraction.

Stratosphere Perturbed by the 2011 Mw9.0 Tohoku Earthquake

AbstractThis study examined the FORMOSAT‐3/COSMIC radio occultation soundings of total electron content in the ionosphere (from 150 to 550 km) and refractivity index in the lower atmosphere (from 0 to 60 km) after/during the 2011 Mw9.0 Tohoku earthquake/tsunami. The refractivity index observations show the first evidence of the Tohoku earthquake/tsunami‐induced vertical oscillations in the lower atmosphere. The atmospheric oscillations with vertical wavelength ranging from 0.5 to 8 km and from 10 to 40 km, respectively, appear in the stratosphere and ionosphere after the earthquake onset. The short vertical wavelengths suggest that the oscillations are the atmospheric oscillatory tail due to the wavefront of the earthquake/tsunami. The radio occultation technique is a tool for detecting the atmospheric waves induced by earthquake or some other sources.

Measuring the global 21-cm signal with the MWA-I: improved measurements of the Galactic synchrotron background using lunar occultation

We present early results from a project to measure the sky-averaged (global), redshifted $21\,$cm signal from the Epoch of Reionisation (EoR), using the Murchison Widefield Array (MWA) telescope. Because interferometers are not sensitive to a spatially-invariant global average, they cannot be used to detect this signal using standard techniques. However, lunar occultation of the radio sky imprints a spatial structure on the global signal, allowing us to measure the average brightness temperature of the patch of sky immediately surrounding the Moon. In this paper we present one night of Moon observations with the MWA between 72 - 230 MHz and verify our techniques to extract the background sky temperature from measurements of the Moon's flux density. We improve upon previous work using the lunar occultation technique by using a more sophisticated model for reflected `earthshine' and by employing image differencing to remove imaging artefacts. We leave the Moon's (constant) radio brightness temperature as a free parameter in our fit to the data and as a result, measure $T_{\rm{moon}} = 180 \pm 12 $ K and a Galactic synchrotron spectral index of $-2.64\pm0.14$, at the position of the Moon. Finally, we evaluate the prospects of the lunar occultation technique for a global EoR detection and map out a way forward for future work with the MWA.

Simulation of liquid water and ice contributions to bending angle profiles in the radio occultation technique

Inversion of radio occultation profiles usually relies on the Abel transform in a spherically symmetric atmosphere. The main contribution to the refractive index comes from gaseous constituents such as pressure, temperature and water vapor. However, the impact of other particles in the troposphere, neglected in the retrieval, can become important during specific weather events causing the commonly applied assumptions to be invalid. In this study, Global Forecast System model is used to simulate the effect of cloud water on bending angle profiles. The information of solid and liquid fractions was assessed from the cloud mixing ratio based on a temperature-dependent separation. The signal was propagated between GPS satellite and low-Earth orbiter through tangent point placed in the vicinity of cloud water. The assumption of a spherically symmetric distribution for cloud refractivity fields is validated based on 2-dimensional simulations with multiple phase screen method. Using one year of forecast data for 2016, the most statistically probable examples were selected and analyzed as well as the uppermost impact of cloud significance was determined in terms of bending angle errors. Typical refractivity values induced by liquid clouds are within 2 ppm and result in single-spike profile structures. Corresponding fractional errors of the bending angle can reach −4% with respect to retrievals with zero clouds contributions. In the presence of unusual weather phenomena, simulations in a horizontally inhomogeneous atmosphere suggest a large dependency on propagation paths that should be considered in the modeling of cloud impact. Bending angles can be affected by −10% fractional error, which inverted with Abel transform corresponds to refractivity error of −2%. The highest fractional difference induced by the ice water is generally below −1% in the bending angle and −0.5% in the refractivity. The most significant cloud content was observed in the first 6 km above the Earth’s surface, whereas ice clouds can affect radio occultation profiles up to the top height of 16 km.

Analysis of ionospheric structure influences on residual ionospheric errors in GNSS radio occultation bending angles based on ray tracing simulations

Abstract. The Global Navigation Satellite System (GNSS) radio occultation (RO) technique is widely used to observe the atmosphere for applications such as numerical weather prediction and global climate monitoring. The ionosphere is a major error source to RO at upper stratospheric altitudes, and a linear dual-frequency bending angle correction is commonly used to remove the first-order ionospheric effect. However, the higher-order residual ionospheric error (RIE) can still be significant, so it needs to be further mitigated for high-accuracy applications, especially from 35 km altitude upward, where the RIE is most relevant compared to the decreasing magnitude of the atmospheric bending angle. In a previous study we quantified RIEs using an ensemble of about 700 quasi-realistic end-to-end simulated RO events, finding typical RIEs at the 0.1 to 0.5 µrad noise level, but were left with 26 exceptional events with anomalous RIEs at the 1 to 10 µrad level that remained unexplained. In this study, we focused on investigating the causes of the high RIE of these exceptional events, employing detailed along-ray-path analyses of atmospheric and ionospheric refractivities, impact parameter changes, and bending angles and RIEs under asymmetric and symmetric ionospheric structures. We found that the main causes of the high RIEs are a combination of physics-based effects – where asymmetric ionospheric conditions play the primary role, more than the ionization level driven by solar activity – and technical ray tracer effects due to occasions of imperfect smoothness in ionospheric refractivity model derivatives. We also found that along-ray impact parameter variations of more than 10 to 20 m are possible due to ionospheric asymmetries and, depending on prevailing horizontal refractivity gradients, are positive or negative relative to the initial impact parameter at the GNSS transmitter. Furthermore, mesospheric RIEs are found generally higher than upper-stratospheric ones, likely due to being closer in tangent point heights to the ionospheric E layer peaking near 105 km, which increases RIE vulnerability. In the future we will further improve the along-ray modeling system to fully isolate technical from physics-based effects and to use it beyond this work for additional GNSS RO signal propagation studies.

A variational regularization of Abel transform for GPS radio occultation

Abstract. In the Global Positioning System (GPS) radio occultation (RO) technique, the inverse Abel transform of measured bending angle (Abel inversion, hereafter AI) is the standard means of deriving the refractivity. While concise and straightforward to apply, the AI accumulates and propagates 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 measurement and thus precludes the error propagation. The variational regularization (VR) proposed in this study approximates the inversion of the forward Abel transform by an optimization problem in which the regularized solution describes the measurement as closely as possible within the measurement'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 measurement space to take advantage of the posterior height determination 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 sounding with a known true solution. The competency of VR relative to AI is validated with a large number of actual RO soundings. The comparison to nearby radiosonde observations 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 supposedly small, VR follows AI very closely in the mean refractivity deserting the first guess. In the lowest few kilometers that AI produces large negative refractivity bias, VR reduces the refractivity bias substantially with the aid of the background, which in this study is the operational forecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF). It is concluded based on the results presented in this study that VR offers a definite advantage over AI in the quality of refractivity.