Electron density retrieval from occulting GNSS signals using a gradient-aided inversion technique

In the coming years, opportunities for remote sensing of electron density in the Earth’s ionosphere will expand with the advent of Galileo, which will become part of the global navigation satellite system (GNSS). Methods for accurate electron density retrieval from radio occultation data continue to improve. We describe a new method of electron density retrieval using total electron content measurements obtained in low Earth orbit. This method can be applied to data from dual-frequency receivers tracking the GPS or Galileo transmitters. This simulation study demonstrates that the method significantly improves retrieval accuracy compared to the standard Abel inversion approach that assumes a spherically symmetric ionosphere. Our method incorporates horizontal gradient information available from global maps of Total Electron Content (TEC), which are available from the International GNSS Service (IGS) on a routine basis. The combination of ground and space measurements allows us to improve the accuracy of electron density profiles near the occultation tangent point in the E and F regions of the ionosphere.     

Validation of FORMOSAT-3/COSMIC radio occultation electron density profiles by incoherent scatter radar data

Low Earth Orbiting satellites carrying a dual frequency GPS receiver onboard offer a unique opportunity to remote sensing of the global ionosphere on a continuous basis. No other profiling technique unifies profiling through the entire F2-layer with global coverage. The FORMOSAT-3/COSMIC data can make a positive impact on the global ionosphere study providing essential information about the height electron density distribution and particularly over regions that are not accessible with ground-based measuring instruments such as ionosondes and GPS dual frequency receivers. Therefore, it is important to verify occultation profiles with other techniques and to obtain experience in the reliability of their derivation. In the given study we present results of comparison of the electron density profiles derived from radio occultation measurements on-board FS-3/COSMIC and from the Kharkov incoherent scatter radar sounding.     

Harmonic analysis of the ionospheric electron densities retrieved from FORMOSAT-3/COSMIC radio occultation measurements

In order to investigate the regular variations of the ionosphere, the least-squares harmonic estimation is applied to the time series of ionospheric electron densities derived from about five years of Global Positioning System radio occultation observations by FORMOSAT-3/COSMIC satellites. The analysis is done for different latitudes and altitudes in the region of Iran. The least-squares harmonic estimation is found to be a powerful tool for the frequency analysis of the completely unevenly spaced time series of radio occultation measurements. Although the obtained results are slightly different from the exact expected cycles (i.e. annual and diurnal components with their Fourier decompositions, and the 27-day period) due to the low horizontal resolution of radio occultation measurements, high vertical resolution of the observations enables us to detect not only the total electron content variations but also periodic patterns of electron densities at different altitudes of the ionosphere. The dominant diurnal and annual signals together with their Fourier series decompositions are obtained, which are consistent with the previous analyses on the total electron content. In the equatorial anomaly band, the annual component is weaker than its Fourier decomposition periods. In particular, the semiannual period dominates the annual component, indicating the relationship between the semiannual variation of the electron densities and the ionospheric equatorial anomaly. From detection of the phases of the components, it is revealed that the annual signal generally has its maximum value in summers at high altitudes, and in the winters at low altitudes. This is probably due to the higher [O/N₂] ratios in winter than in the summer in the lower ionosphere. Furthermore, the semiannual component mostly peaks around solstices or about a month before/after them.     

Higher order ionospheric propagation effects on GPS radio occultation signals

With the increasing number of remote sensing satellites using the GPS radio occultation technique for atmospheric sounding, the estimation of higher order ionospheric effects and their mitigation have become relevant and important. Due to long ionospheric limb paths, GPS signals are strongly affected by ionospheric refraction during radio occultation. Standard dual-frequency GPS measurements may be used to estimate the first order term of the refractive index. However, non-linear terms such as the second and third order ionospheric terms and ray path bending effects are not considered in occultation measurements so far. Analysing selected CHAMP–GPS occultation events different higher order ionospheric terms are estimated and their effects on dual-frequency range estimation and total electron content (TEC) estimation are discussed. We have found that the separation between the GPS L1 and L2 ray paths exceeds the kilometer level during occultation for a vertical TEC level of more than 160 TEC units. Corresponding errors in the GPS dual-frequency range estimation and TEC estimation are found to exceed the meter and 10 TEC units level, respectively.     

Case study of inclined sporadic E layers in the Earth’s ionosphere observed by CHAMP/GPS radio occultations: Coupling between the tilted plasma layers and internal waves

We have used the radio occultation (RO) satellite data CHAMP/GPS (Challenging Minisatellite Payload/Global Positioning System) for studying the ionosphere of the Earth. A method for deriving the parameters of ionospheric structures is based upon an analysis of the RO signal variations in the phase path and intensity. This method allows one to estimate the spatial displacement of a plasma layer with respect to the ray perigee, and to determine the layer inclination and height correction values. In this paper, we focus on the case study of inclined sporadic E (E_s) layers in the high-latitude ionosphere based on available CHAMP RO data. Assuming that the internal gravity waves (IGWs) with the phase-fronts parallel to the ionization layer surfaces are responsible for the tilt angles of sporadic plasma layers, we have developed a new technique for determining the parameters of IGWs linked with the inclined E_s structures. A small-scale internal wave may be modulating initially horizontal E_s layer in height and causing a direction of the plasma density gradient to be rotated and aligned with that of the wave propagation vector k. The results of determination of the intrinsic wave frequency and period, vertical and horizontal wavelengths, intrinsic vertical and horizontal phase speeds, and other characteristics of IGWs under study are presented and discussed.     

Assessment of precision in ionospheric electron density profiles retrieved by GPS radio occultations

The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) is a six satellite radio occultation mission that was launched in April 2006. The close proximity of these satellites during some months after launch provides a unique opportunity to evaluate the precision of Global Positioning System (GPS) radio occultation (RO) retrievals of ionospheric electron density from nearly collocated and simultaneous observations. RO data from 30 consecutive days during July and August 2006 are divided into ten groups in terms of daytime or nighttime and latitude. In all cases, the best precision values (about 1%) are found at the F peak height and they slightly degrade upwards. For all daytime groups, it is seen that electron density profiles above about 120 km height exhibit a substantial improvement in precision. Nighttime groups are rather diverse: in particular, the precision becomes better than 10% above different levels between 120 and 200 km height. Our overall results show that up to 100–200 km (depending on each group), the uncertainty associated with the precision is in the order of the measured electron density values. Even worse, the retrieved values tend sometimes to be negative. Although we cannot rely directly on electron density values at these altitudes, the shape of the profiles could be indicative of some ionospheric features (e.g. waves and sporadic E layers). Above 200 km, the profiles of precision are qualitatively quite independent from daytime or latitude. From all the nearly collocated pairs studied, only 49 exhibited a difference between line of sight angles of both RO at the F peak height larger than 10°. After analyzing them we find no clear indications of a significant representativeness error in electron density profiles due to the spherical assumption above 120 km height. Differences in precision between setting and rising GPS RO may be attributed to the modification of the processing algorithms applied to rising cases during the initial period of the COSMIC mission.     

Comparative analysis of Venusian ionosphere dual-frequency radio soundings with the satellites Venera-9, 10 and Pioneer-Venus

A comparison of the Venusian ionosphere electron density profiles obtained by the dual-frequency radio occultation method for the satellites Venera-9,10 (1975) and the Pioneer-Venus orbiter (1978–1979) has been carried out. It is shown that the general nature of the profiles, the main maximum heights and electron densities, the ionopause height positions determined with the satellites Venera-9, 10 on the one hand and Pioneer-Venus orbiter on the other hand are in good agreement. This fact testifies to the reliability of the experimental dual-frequency radio occultation data. An attempt to compare the radio occultation data with the direct measurements points out the essential contradictions between them, which need detailed analysis and discussion.     

Recent results on the Venus atmosphere from pioneer Venus radio occultations

Radio occultation measurements of the temperature structure of the Venus atmosphere have been obtained during seven occultation “seasons” extending from December 1978 to December 1983. Approximately 123 vertical profiles of temperature from about 40 km to about 85 km altitudes have been derived. Since these measurements cover latitudes from both poles to the equator, they have shown the latitudinal dependence of thermal structure. There is a smooth transition from the troposphere to the mesosphere at latitudes below about 45°, with the tropopause at about 56 km. The troposphere then rises to about 62 km in the “collar cloud” region between about 60° and 80° latitude, where a strong temperature inversion (up to 30 K) is present. In the polar areas, 80°–90°, the mesosphere becomes isothermal and there is no inversion. This latitudinal behavior is related to the persistent circulation pattern, in which a predominantly zonal retrograde motion at latitudes below 45° gradually changes to a circumpolar vortex at the “collar cloud” latitudes. Indeed, the radio occultation data have been used in a cyclostrophic balance model to derive zonal winds in the Venus atmosphere, which showed a mid-latitude (50°–55°) jet with a speed of about 120–140 ms^?1 at about 70 km altitude /1,2/. The observations obtained in 1983 and 1984 have shown that above the tropopause there is considerable temporal variability in the detailed thermal structure, suggesting that the persistent circulation pattern is subject to weather-like variability.     

Low-latitude ionospheric scintillations and total electron content obtained with the CITRIS instrument on STPSat1 using radio transmissions from DORIS ground beacons

The primary objective of the Scintillation and Tomography Receiver in Space (CITRIS) is to detect ionospheric irregularities from space at low latitude. For this purpose, the satellite receiver uses the UHF and S-Band transmissions of the ground network of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) beacons. CITRIS, developed at the Naval Research Laboratory, differs from the normal DORIS receiver by being able to capture and store the complex amplitude of the 401.25 and 2036.25 MHz transmissions at 200 Hz sample rate. Ground processing of the CITRIS data yields total electron content (TEC) and both phase and amplitude scintillations. With CITRIS flying on the US Space Test Program (STP) satellite STPSat1, 2 years of data were collected and processed to determine the fluctuations in ionospheric TEC and radio scintillations associated with equatorial irregularities. CITRIS flights over DORIS transmitters yield direct measurements of the horizontal plasma density fluctuations associated with equatorial plasma bubbles. Future flights of CITRIS can provide valuable complements to other satellite instruments such as GPS occultation receivers used to estimate vertical electron density profiles in the ionosphere.     

New field of application of the IRI modeling – Determination of ionosphere transfer characteristic for radio astronomical signals

We suggest a new field of application of IRI modeling – determination of ionosphere transfer characteristic (ITC) for radio astronomical signals (RAS). VHF and HF RAS are widely used for observations of the Sun and pulsars. It is necessary to take into account possible distortions of RAS in the Earth ionosphere. However, in contrast to modern navigation systems (GPS, GLONASS, GALILEO), where very accurate reconstruction of ionosphere parameters is a built-in function, in present-day radio astronomy a retrieve of ITC has not been appropriately worked out yet. It collides with increasing requirements to accuracy of the analysis of RAS amplitude profile and to the angular and polarizing resolution of radio telescopes of new generation. We have developed a method and software for calculation of the ionosphere measure of rotation (RM) and the measure of dispersion (DM). We used the ionosphere model IRI-2001, magnetic-field model IGRF-10 and values of ionosphere total electron content as deduced from GPS measurements. The obtained values of the ionosphere DM and RM were recalculated into characteristics of phase delay, Faraday amplitude modulation and polarization changes. We made calculations for different levels of geomagnetic activity and for different angular position of radio sources as well.     

Evaluation of the IRI-2016 and NeQuick electron content specification by COSMIC GPS radio occultation,ground-based GPS and Jason-2 joint altimeter/GPS observations

We examined performance of two empirical profile-based ionospheric models, namely IRI-2016 and NeQuick-2, in electron content (EC) and total electron content (TEC) representation for different seasons and levels of solar activity. We derived and analyzed EC estimates in several representative altitudinal intervals for the ionosphere and the plasmasphere from the COSMIC GPS radio occultation, ground-based GPS and Jason-2 joint altimeter/GPS observations. It allows us to estimate a quantitative impact of the ionospheric electron density profiles formulation in several altitudinal intervals and to examine the source of the model-data discrepancies of the EC specification from the bottom-side ionosphere towards the GPS orbit altitudes. The most pronounced model-data differences were found at the low latitude region as related to the equatorial ionization anomaly appearance. Both the IRI-2016 and NeQuick-2 models tend to overestimate the daytime ionospheric EC and TEC at low latitudes during all seasons of low solar activity. On the contrary, during high solar activity the model results underestimated the EC/TEC observations at low latitudes. We found that both models underestimated the EC for the topside ionosphere and plasmasphere regions for all levels of solar activity. For low solar activity, the underestimated EC from the topside ionosphere and plasmasphere can compensate the overestimation of the ionospheric EC and, consequently, can slightly decrease the resulted model overestimation of the ground-based TEC. For high solar activity, the underestimated EC from the topside ionosphere and plasmasphere leads to a strengthening of the model underestimation of the ground-based TEC values. We demonstrated that the major source of the model-data discrepancies in the EC/TEC domain comes from the topside ionosphere/plasmasphere system.     

A theoretical assessment of the feasibility of potential Lunar Reconnaissance Orbiter radio occultation observations of the lunar ionosphere

The existence of a “dense” lunar ionosphere has been controversial for decades. Positive ions produced from the lunar surface and exosphere are inferred to have densities that are $?$${10}^6$ – ${10}^7$ m^?3 near the surface and smaller at higher altitudes, yet electron densities derived from radio occultation measurements occasionally exceed these values by orders of magnitude. For example, about 4% of the single-spacecraft radio occultation measurements from Kaguya/SELENE were consistent with peak electron densities of $~$$3\times {10}^8$ m^?3. Space plasmas should be neutral on macroscopic scales, so this represents a substantial discrepancy. Aditional observations of electron densities in the lunar ionosphere are critical to resolving this longstanding paradox. Here we theoretically assess whether radio occultation observations using two-way coherent S-band radio signals from the Lunar Reconnaissance Orbiter (LRO) spacecraft could provide useful measurements of electron densities in the lunar ionosphere. We predict the uncertainty in a single LRO radio occultation measurement of electron density to be $~$$3\times {10}^8$ m^?3, comparable to occasional observations by Kaguya/SELENE of a dense lunar ionosphere. Thus an individual profile from LRO is unlikely to reliably detect the lunar ionosphere; however, averages of multiple ($~$10) LRO profiles acquired under similar geophysical and viewing conditions should be able to make reliable detections. An observing rate of six ingress occultations per day ($~$2000 per year) could be achieved with minimal impact on current LRO operations. This rate compares favorably with the 378 observations reported from the single-spacecraft experiment on Kaguya/SELENE between November 2007 and June 2009. The large number of observations possible for LRO would be sufficient to permit wide-ranging investigations of spatial and temporal variations in the poorly understood lunar ionosphere. These findings strengthen efforts to conduct such observations with LRO.     

An alternative Shell inversion technique - Analysis and validation based on COSMIC and ionosonde data

Multi-channel Global Positioning System (GPS) carrier phase signals, received by the six low Earth orbiting (LEO) satellites from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) program, were used to undertake active limb sounding of the Earth’s atmosphere and ionosphere via radio occultation. In the ionospheric radio occultation (IRO) data processing, the standard Shell inversion technique (SIT), transformed from the traditional Abel inversion technique (AIT), is widely used, and can retrieve good electron density profiles. In this paper, an alternative SIT method is proposed. The comparison between different inversion techniques will be discussed, taking advantage of the availability of COSMIC datasets. Moreover, the occultation results obtained from the SIT and alternative SIT at 500 km and 800 km, are compared with ionosonde measurements. The electron densities from the alternative SIT show excellent consistency to those from the SIT, with strong correlations over 0.996 and 0.999 at altitudes of 500 km and 800 km, respectively, and the peak electron densities (N_mF₂) from the alternative SIT are equivalent to the SIT, with 0.839 vs. 0.844, and 0.907 vs. 0.909 correlation coefficients when comparing to those by the ionosondes. These results show that: (1) the N_mF₂ and h_mF₂ retrieved from the SIT and alternative SIT are highly consistent, and in a good agreement with those measured by ionosondes, (2) no matter which inversion technique is used, the occultation results at the higher orbits (∼800 km) are better than those at the lower orbits (∼500 km).     

Analysis of NASA’s DSN Venus Express radio occultation data for year 2014

The Venus Express Radio Science Experiment (VeRa) was part of the scientific payload of the Venus Express (VEX) spacecraft and was targeted at the investigation of Venus’ atmosphere, surface, and gravity field as well as the interplanetary medium. This paper describes the methods and the required calibrations applied to VEX-VeRa raw radio occultation data used to retrieve vertical profiles of Venus’ ionosphere and neutral atmosphere. In this work we perform an independent analysis of a set of 25 VEX, single-frequency (X-band), occultations carried out in 2014, recorded in open-loop at the NASA Deep Space Network. Our temperature, pressure and electron density vertical profiles are in agreement with previous studies available in the literature. Furthermore, our analysis shows that Venus’ ionosphere is more influenced by the day/night condition than the latitude variations, while the neutral atmosphere experiences the opposite. Our scientific interpretation of these results is based on two major responsible effects: Venus’ high thermal inertia and the zonal winds. Their presence within Venus’ neutral atmosphere determine why in these regions a latitude dependence is predominant on the day/night condition. On the contrary, at higher altitudes the two aforementioned effects are less important or null, and Venus’ ionosphere shows higher electron density peaks in the probed day-time occultations, regardless of the latitude.     

Measurements of the upper troposphere and lower stratosphere during tropical cyclones using the GPS radio occultation technique

Water vapour transport to the upper troposphere and lower stratosphere by deep convective storms affects the radiation balance of the atmosphere and has been proposed as an important component of climate change. The aim of the work presented here is to understand if the GPS radio occultation technique is useful for characterization of this process. Our assessment addresses the question if severe storms leave a significant signature in radio occultation profiles in the upper troposphere/lower stratosphere. Radio occultation data from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) were analyzed, focusing on two particular tropical cyclones with completely different characteristics, the hurricane Bertha, which formed in the Atlantic Basin during July 2008 and reached a maximum intensity of Category 3, and the typhoon Hondo, which formed in the south Indian Ocean during 2008 reaching a maximum intensity of Category 4. The result is positive, suggesting that the bending angle of a GPS radio occultation signal contains interesting information on the atmosphere around the tropopause, but not any information regarding the water vapour. The maximum percentage anomaly of bending angle between 14 and 18 km of altitude during tropical cyclones is typically larger than the annual mean by 5–15% and it can reach 20% for extreme cases. The results are discussed in connection to the GPS radio occultation receiver which will be part of the Atomic Clock Ensemble in Space (ACES) payload on the International Space Station.     

利用COSMIC数据研究顶部电离层O+扩散流的统计特征

从等离子体运动方程出发, 利用COSMIC星座的掩星数据, 借助相关经验模式, 计算了太阳活动低年顶部电离层O+场向扩散速度和扩散通量, 并分析了其全球分布和日变化特征. 结果表明, 白天等离子体扩散速度的方向随高度增加由向下(极向)逐步变为向上(赤道向), 方向转变的高度一般在hmF2+80 km以下. 在白天较高高度, 南北磁纬10o ~20 o存在着向上方向的最大扩散速度和扩散通量; 而在夜间, 南北磁纬30o~40 o存在向下方向的最大扩散速度和扩散通量. 在分点, 南北半球的扩散通量和扩散速度大致对称; 而在至点, 扩散通量存在着明显的南北半球不对称现象. 另外, 不同纬度的扩散速度有着不同的日变化特征.      

GRAS radio occultation on-board of Metop

The GRAS radio occultation instrument is flying on Metop-A and belongs to the EPS (EUMETSAT Polar System). GRAS observes GPS satellites in occultation. Within this work, validation of GRAS closed-loop bending angle data against co-located ECMWF profiles extracted from model fields and occultations from the COSMIC constellation of radio occultation instruments is shown. Results confirm the high data quality and robustness, where GRAS shows lower bending angle noise against ECMWF than COSMIC and in terms of occultations per day, one GRAS ≈ two COSMIC satellites. This is partly due to the operational setup of EPS. For the investigation we focus on two observation periods where updates in the ECMWF (March 2009) and COSMIC processing (October 2009) have improved the statistics further. Bending angles biases agree to within 0.5% against ECMWF and to within 0.1% against COSMIC after the updates for altitudes between 8 and 40 km. In addition, we also analyze the impact of the Metop orbit processing on the derived GRAS bending angle data, where different GPS and Metop orbit solutions are analyzed. Results show that a batch based orbit processing would improve in particular the bending angle bias behavior at higher altitudes. Requirements for the operational processing of GRAS data are briefly outlined, options to ease the use of other positioning system satellites in the near future are discussed. A simplified analysis on the observation of several of these systems, e.g. GPS and Galileo, from one platform shows that about 16% of occultations are found within 300 km, ±3 h, thus providing similar information. A constellation of 2 GRAS like instruments would have only about 10% close-by.     

An introduction of mountain-based GPS radio occultation experiments in China

The mountain-based GPS radio occultation is a novel approach to lower atmospheric profiling. The experiments of the mountain-based GPS radio occultation were conducted on the top of Mt. Yaogu (29.38°N, 113.68°E, ∼1240 m) on December 17, 2003, and on the top of Mt. Jiugong (29.39°N, 114.65°E, ∼1550 m) on July 24, 2004. Based on these observation data, the scientific data processing software has been developed and is used to retrieve successfully the atmospheric refractivity profiles. The validation experiment was performed on the top of Mt. Wuling (40.60°N, 117.48°E, ∼2118 m) during August 1–29, 2005. Collocated automatic weather station and the radiosondes nearby were operated simultaneously for the comparison campaign. Results show that the radio occultation technique obtained about 40 profiles every day with the receiver antenna pointing to the south. Comparisons show that the refractivity measured by occultation agree well with those by the radiosondes, but not well with those by the automatic weather station due to their much different geographic locations of measurements. Results of these experiments suggest that the mountain-based GPS radio occultation is an economic reliable novel technique monitoring temporal and spatial variations of local lower atmospheric environments.     

Analysis of electron content variations over Japan during solar minimum: Observations and modeling

Comparative analysis of GPS TEC data and FORMOSAT-3/COSMIC radio occultation measurements was carried out for Japan region during period of the extremely prolonged solar minimum of cycle 23/24. COSMIC data for different seasons corresponded to equinox and solstices of the years 2007–2009 were analyzed. All selected electron density profiles were integrated up to the height of 700 km (altitude of COSMIC satellites), the monthly median estimates of Ionospheric Electron Content (IEC) were retrieved with use of spherical harmonics expansion. Monthly medians of TEC values were calculated from diurnal variations of GPS TEC estimates during considered month. Joint analysis of GPS TEC and COSMIC data allows us to extract and estimate electron content corresponded to the ionosphere (its bottom and topside parts) and the plasmasphere (h > 700 km) for different seasons of 2007–2009. Percentage contribution of ECpl to GPS TEC indicates the clear dependence from the time and varies from a minimum of about 25–50% during day-time to the value of 50–75% at night-time. Contribution of both bottom-side and topside IEC has minimal values during winter season in compare with summer season (for both day- and night-time). On average bottom-side IEC contributes about 5–10% of GPS TEC during night and about 20–27% during day-time. Topside IEC contributes about 15–20% of GPS TEC during night and about 35–40% during day-time. The obtained results were compared with TEC, IEC and ECpl estimates retrieved by Standard Plasmasphere–Ionosphere Model that has the plasmasphere extension up to 20,000 km (GPS orbit).     

Regional 4-D modeling of the ionospheric electron density

The knowledge of the electron density is the key point in correcting ionospheric delays of electromagnetic measurements and in studying the ionosphere. During the last decade GNSS, in particular GPS, has become a promising tool for monitoring the total electron content (TEC), i.e., the integral of the electron density along the ray-path between the transmitting satellite and the receiver. Hence, geometry-free GNSS measurements provide informations on the electron density, which is basically a four-dimensional function depending on spatial position and time. In addition, these GNSS measurements can be combined with other available data including nadir, over-ocean TEC observations from dual-frequency radar altimetry (T/P, JASON, ENVISAT), and TECs from GPS-LEO occultation systems (e.g., FORMOSAT-3/COSMIC, CHAMP) with heterogeneous sampling and accuracy.