GPS Radio Occultation Measurements Research Articles

A comparison of FORMOSAT-3/COSMIC radio occultation and ionosonde measurements in sporadic E detection over mid- and low-latitude regions

The investigation of sporadic E or Es layers typically relies on ground-based or satellite data. This study compares the Es layers recorded in ionograms with those detected using GNSS L1 signal-to-noise ratio data from FORMOSAT-3/COSMIC radio occultation at mid and low latitudes. GPS radio occultation measurements of Es layers, during an 11-year time span of 2007–2017, within a 2° latitude × 5° longitude grid around each ionosonde site are compared to the Es recordings of the ionosonde. By comparing multi-year radio occultation data with recordings from six ionosonde stations at mid and low latitudes, it was discovered that at least 20% of the Es layer detection results between each ionosonde and its crossing GPS radio occultation measurements did not agree. The results show that the agreement between the two methods in Es detection is highly dependent on the season and local time. This study suggests that Es layer recordings from ground-based ionosonde observations have the best agreement with the Es layers detected by radio occultation data during daytime and local summers. The difference in the Es detection mechanisms between the two methods can explain the inconsistency between Es events measured by these two methods. The detection of Es layers in ionograms relies on the high plasma concentration in the E region, whereas signal scintillations caused by a large vertical gradient of the plasma density in the E region are considered a sign of Es occurrence in satellite techniques.

Equinoctial Asymmetry in the Upper Ionosphere: Comparison of Satellite Observations and Models

AbstractThe terrestrial ionosphere displays significant equinoctial asymmetry despite the upper atmosphere receiving similar levels of solar ionization energy at a given location and local time in March and September during similar solar activity conditions. This intriguing feature is not well understood or modeled, particularly in the upper ionosphere, and causes of the asymmetry are not fully established and quantified. Yet, their study is important to provide better insights into the atmosphere‐ionosphere coupling processes. Analysis of Langmuir probe data from ESA's Swarm B satellite at ∼525 km altitude reveals that the daytime electron density is larger for all latitudes during the March equinox than during the September equinox, while the electron temperature shows an inverted asymmetry except at low latitudes. Simultaneously obtained neutral density data from Swarm GPS accelerations indicate that the thermosphere is denser during the March equinox. The asymmetry seen by Swarm electron density observations is also present in electron densities obtained using GPS radio occultation measurements from the COSMIC satellites. Simulations were performed using physics‐based ionosphere models (SAMI3, WACCM‐X, and TIE‐GCM) to determine their ability to produce the observed asymmetry, understand the generation mechanism(s), and establish the relative role of physical drivers. Results produced by TIE‐GCM are closest to the observations when seasonally varying eddy diffusivity is considered in the model. The asymmetry produced by other models is noticeably weaker. Modeling of the asymmetry by SAMI3 driven with the TIE‐GCM neutral atmosphere shows that both neutral density and winds play a critical role, but the density has a greater effect.

Migrating and nonmigrating tidal signatures in sporadic E layer occurrence rates

Abstract. We analyse sporadic E (ES) layer occurrence rates (OR) obtained from ionospheric GPS radio occultation measurements by the FORMOSAT-3/COSMIC constellation. Maximum OR are seen at 95–105 km altitude. Midlatitude ES layers are mainly due to wind shear in the presence of tides, and the strongest signals are the migrating diurnal and semidiurnal components. Especially in the Southern Hemisphere, nonmigrating components such as a diurnal westward wave 2 and a semidiurnal westward wave 1 are also visible, especially at higher latitudes. Near the equator, a strong diurnal eastward wavenumber 3 component and a semidiurnal eastward wavenumber 2 component occur in summer and autumn. Terdiurnal and quarterdiurnal components are weaker than the diurnal and semidiurnal ones.

Morphology of sporadic E layers derived from Fengyun-3C GPS radio occultation measurements

The Global Positioning System (GPS) radio occultation (RO) data provided by the Fengyun-3C (FY-3C) mission during January 2015 to December 2019 are used to detect the existence of sporadic E (Es) layers over the globe, based on which the spatial and temporal distributions of the Es occurrence rates (ORs) are presented and analyzed. The results are compared with the Es morphology obtained using the RO data from the Constellation Observing System for the Meteorology, Ionosphere, and Climate (COSMIC) mission. It is found that the seasonal variation patterns of the spatial distributions of Es ORs derived from the FY-3C RO data, which show clearly the effects of wind shear mechanism and the Earth’s magnetic field on the formation of Es layers, are basically consistent with those derived from the COSMIC RO data. While the limited local time distribution of the FY-3C RO-detected Es occurrences makes it impossible to resolve the complete diurnal variations of Es layers. Detailed comparisons of the Es morphologies derived from the two different RO missions reveal that the magnitudes of the Es ORs derived by FY-3C data are slightly smaller than those derived by COSMIC data in the middle and low latitudes, which is due to that the top heights of the 50 Hz RO data of FY-3C mission are generally lower than those of the COSMIC mission. In the polar regions, the distinctly low ratio of the FY-3C RO 50 Hz measurements which reach the height of 90 km reduces the capability of these observations for Es-layer detections.Graphical

Monthly Observations of Cold-point Tropopause Temperature and Height for 2008 in the Philippines Using COSMIC GPS Radio Occultations

The cold-point tropopause (CPT) is the thermal boundary between the troposphere and stratosphere defined by a temperature profile’s coldest level. In this paper, the CPT is examined using temperature profiles from Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) GPS radio occultation measurements during 2008 inside the Tropical Cyclone Advisory Domain (TCAD) in the Philippines. The monthly and spatial variability is examined using two CPT properties: CPT temperature (CPT-T) and CPT height (CPT-H). Generally, the coldest CPT temperatures are in the months of December-January-February, which lie within the cool dry season. CPT-T is at its hottest in July-August-September, within the rainy season. CPT-H is observed to decrease (increase) as CPT-T increases (decreases), suggesting an inverse relationship between the two properties. CPT-T is also observed to rise in the upper latitudes, while no discernible spatial pattern can be seen for CPT-H over this domain expect for that the start increase and decrease of CPT-H appear to occur over the Pacific first.

Seasonal and Annual Changes of the Regional Tropical Belt in GPS-RO Measurements and Reanalysis Datasets

AbstractThe width of the tropical belt has been analyzed with a variety of metrics, often based on zonal-mean data from reanalyses. However, constraining the global and regional tropical width requires both a global spatial-resolving observational dataset and an appropriate metric to take advantage of such data. The tropical tropopause break is arguably such a metric. This study aims to evaluate the performance of different reanalyses and metrics with a focus on depicting regional tropical belt width. We choose four distinct tropopause-break metrics derived from global positioning system radio occultation (GPS-RO) satellite data and four modern reanalyses (ERA-Interim, MERRA-2, JRA-55, and CFSR). We show that reanalyses generally reproduce the regional tropical tropopause break to within 10° of that in GPS-RO data—but that the tropical width is somewhat sensitive (within 4°) to how data are averaged zonally, moderately sensitive (within 10°) to the dataset resolution, and more sensitive (20° over the Northern Hemisphere Atlantic Ocean during June–August) to the choice of metric. Reanalyses capture the poleward displacement of the tropical tropopause break over land and equatorward displacement over ocean during summertime, and the reverse during the wintertime. Reanalysis-based tropopause breaks are also generally well correlated with those from GPS-RO, although CFSR reproduces 14-yr trends much more closely than others (including ERA-Interim). However, it is hard to say which dataset is the best match of GPS-RO. We further find that the tropical tropopause break is representative of the subtropical jet latitude and the Northern Hemisphere edge of the Hadley circulation in terms of year-to-year variations.

Global ionosonde and GPS radio occultation sporadic-E intensity and height comparison

A multi-year global comparison between mid-latitude and equatorial sporadic-E (Es) height and intensity derived from COSMIC GPS radio occultation (GPS-RO) data and blanketing E frequency (fbEs) from ground-based Digisonde measurements is performed. The Es layer intensity from GPS-RO measurements was estimated using the S2 scintillation index and total electron content (TEC) assuming constant and variable Es cloud thicknesses individually. GPS-RO crossings were limited to within 150 km and 30 min of fbEs occurrences at each Digisonde. Intensities derived from the TEC with constant thickness method agreed more closely with the Digisonde fbEs measurements in terms of relative error and spread. Es path lengths estimated from the variable thickness method were also significantly larger than the average Es cloud length indicating that tube geometry assumptions between thickness and length do not apply to sporadic-E. Furthermore, intensities calculated from the S2 technique generally placed the frequency of fbEs between 2.7 and 5.3 MHz. Mean fbEs predictions from the S2 and constant thickness TEC methods are found to be statistically similar to the mean Digisonde fbEs. Both TEC and S2 methods underestimated the altitude of the fbEs events, but the S2 technique performed better overall. The mean heights calculated from both of the GPS-RO methods show a statistically significant difference with the mean Digisonde calculated heights, suggesting a bias in the height estimates.

Extraction of electron density profiles with geostationary satellite-based GPS side lobe occultation signals

GPS radio occultation (RO) measurements recorded on the geostationary earth orbit (GEO) satellite TJS-2 have been investigated for electron density profile (EDP) retrieval. The total electron content derived from TJS-2 single-frequency excess phase is refined by a moving average filter to smooth high-frequency errors, which outperforms the single-difference technique. The side lobe GPS RO signals have been used to estimate electron densities up to several thousand kilometers in height for the first time. By comparison with the ground-based digisonde, the IRI 2016 model and the Constellation Observing System for Meteorology, Ionosphere, and Climate satellite (COSMIC) EDPs, the TJS-2 ionospheric EDPs show good agreement with correlation coefficients exceeding 0.8. The TJS-2 average NmF2 differences compared to digisondes and COSMIC results are 12.9% and 1.4%, respectively, while the hmF2 differences are 1.65 km and 1.76 km, respectively. Our results reveal that GEO-based RO signals can estimate EDPs to altitudes up to several thousand kilometers at specific locations with daily repeatability, which makes it a very suitable technique for routinely monitoring EDP variations.

Gravity Wave Behavior in Lower Stratosphere During Tropical Cyclones Over the Bay of Bengal

AbstractGravity waves associated with tropical cyclones over the Bay of Bengal have been studied using Constellation Observing System for Meteorology, Ionosphere, and Climate GPS radio occultation measurements. The sources of gravity waves are located well below the tropopause where the intensity of a tropical cyclone is high. The gravity wave potential energy between 19 and 26 km shows an enhancement in the lower stratosphere during the cyclone. Intense convection associated with tropical cyclone is characterized by low outgoing longwave radiation values. The present study shows an increase in potential energy in the lower stratosphere over a storm path before the actual occurrence of cyclones. The power spectral density of gravity waves shows that the vertical wavelengths in the range 2–2.4 km carry the maximum energy in the lower stratosphere over the cyclone path.

The Spatial and Temporal Variability of Global Stratospheric Gravity Waves and Their Activity during Sudden Stratospheric Warming Revealed by COSMIC Measurements

This study investigates the spatial and temporal variability of global stratospheric gravity waves (GWs) and the characteristics of GW activity during sudden stratospheric warming (SSW) using the GPS radio occultation measurements from the COSMIC mission during September 2006 to May 2013. Corresponding to the COSMIC RO observational window and analysis method, GW potential energy (Ep) with vertical scales no shorter than ∼2 km is resolved. It is found that the distributions of GW Ep over 20–30 km and 30–38 km show similar spatial and seasonal variations. The variations of GW Ep with altitude and latitude along the westerly wind are identified in different seasons over 60◦–80◦W. In the middle and high latitudes, seasonal cycles are distinct in the time–latitude and time–altitude distributions of GW activities, which show larger Ep in winters when westerly wind dominates and smaller Ep in summers when easterly wind dominates. The influence of quasi-biennial oscillation on GW activity is recognized in the tropics. GW Ep enhances closely following the occurrence of minor SSW events; while during major events, GW Ep may not enhance, and sometimes may even weaken, in the regions where reversals of zonal wind occur, probably caused by the filtering impact of the 0 m s−1 wind level on the GWs.

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

Abstract. This work presents new results about sporadic E-layers (Es layers) using GPS (global positioning system) radio occultation (RO) measurements obtained from the FORMOSAT-3/COSMIC satellites and digisonde data. The RO profiles are used to study the Es layer occurrence as well as its intensity of the signal-to-noise ratio (SNR) of the 50 Hz GPS L1 signal. The methodology was applied to identify the Es layer on RO measurements over Cachoeira Paulista, a low-latitude station in the Brazilian region, in which the Es layer development is not driven tidal winds only as it is at middle latitudes. The coincident events were analyzed using the RO technique and ionosonde observations during the year 2014 to 2016. We used the electron density obtained using the blanketing frequency parameter (fbEs) and the Es layer height (h'Es) acquired from the ionograms to validate the satellite measurements. The comparative results show that the Es layer characteristics extracted from the RO measurements are in good agreement with the Es layer parameters from the digisonde. Keywords. Ionosphere (ionosphere–atmosphere interactions)

An intercomparison of stratospheric gravity wave potential energy densities from METOP GPS radio occultation measurements and ECMWF model data

Abstract. Temperature profiles based on radio occultation (RO) measurements with the operational European METOP satellites are used to derive monthly mean global distributions of stratospheric (20–40 km) gravity wave (GW) potential energy densities (EP) for the period July 2014–December 2016. In order to test whether the sampling and data quality of this data set is sufficient for scientific analysis, we investigate to what degree the METOP observations agree quantitatively with ECMWF operational analysis (IFS data) and reanalysis (ERA-Interim) data. A systematic comparison between corresponding monthly mean temperature fields determined for a latitude–longitude–altitude grid of 5° by 10° by 1 km is carried out. This yields very low systematic differences between RO and model data below 30 km (i.e., median temperature differences is between −0.2 and +0.3 K), which increases with height to yield median differences of +1.0 K at 34 km and +2.2 K at 40 km. Comparing EP values for three selected locations at which also ground-based lidar measurements are available yields excellent agreement between RO and IFS data below 35 km. ERA-Interim underestimates EP under conditions of strong local mountain wave forcing over northern Scandinavia which is apparently not resolved by the model. Above 35 km, RO values are consistently much larger than model values, which is likely caused by the model sponge layer, which damps small-scale fluctuations above ∼ 32 km altitude. Another reason is the well-known significant increase of noise in RO measurements above 35 km. The comparison between RO and lidar data reveals very good qualitative agreement in terms of the seasonal variation of EP, but RO values are consistently smaller than lidar values by about a factor of 2. This discrepancy is likely caused by the very different sampling characteristics of RO and lidar observations. Direct comparison of the global data set of RO and model EP fields shows large correlation coefficients (0.4–1.0) with a general degradation with increasing altitude. Concerning absolute differences between observed and modeled EP values, the median difference is relatively small at all altitudes (but increasing with altitude) with an exception between 20 and 25 km, where the median difference between RO and model data is increased and the corresponding variability is also found to be very large. The reason for this is identified as an artifact of the EP algorithm: this erroneously interprets the pronounced climatological feature of the tropical tropopause inversion layer (TTIL) as GW activity, hence yielding very large EP values in this area and also large differences between model and observations. This is because the RO data show a more pronounced TTIL than IFS and ERA-Interim. We suggest a correction for this effect based on an estimate of this artificial EP using monthly mean zonal mean temperature profiles. This correction may be recommended for application to data sets that can only be analyzed using a vertical background determination method such as the METOP data with relatively scarce sampling statistics. However, if the sampling statistics allows, our analysis also shows that in general a horizontal background determination is advantageous in that it better avoids contributions to EP that are not caused by gravity waves.

Convectively Driven Tropopause‐Level Cooling and Its Influences on Stratospheric Moisture

AbstractCharacteristics of the tropopause‐level cooling associated with tropical deep convection are examined using CloudSat radar and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) GPS radio occultation measurements. Extreme deep convection is sampled based on the cloud top height (>17 km) from CloudSat, and colocated temperature profiles from COSMIC are composited around the deep convection. Response of moisture to the tropopause‐level cooling is also examined in the upper troposphere and lower stratosphere using microwave limb sounder measurements. The composite temperature shows an anomalous warming in the troposphere and a significant cooling near the tropopause (at 16–19 km) when deep convection occurs over the western Pacific, particularly during periods with active Madden‐Julian Oscillation (MJO). The composite of the tropopause cooling has a large horizontal scale (~6,000 km in longitude) with minimum temperature anomaly of ~ −2 K, and it lasts more than 2 weeks with support of mesoscale convective clusters embedded within the envelope of the MJO. The water vapor anomalies show strong correlation with the temperature anomalies (i.e., dry anomaly in the cold anomaly), showing that the convectively driven tropopause cooling actively dehydrate the lower stratosphere in the western Pacific region. The moisture is also affected by anomalous Matsuno‐Gill‐type circulation associated with the cold anomaly, in which dry air spreads over a wide range in the tropical tropopause layer (TTL). These results suggest that convectively driven tropopause cooling and associated transient circulation play an important role in the large‐scale dehydration process in the TTL.

Оптимизация прогнозирования температуры атмосферы с помощью GPS-радиозатменных измерений

The matters of optimum forecasting atmospheric temperature using GPS radio occultation measurements are considered. The analysis of the available data regarding to the comparison of temperature measurements using radio occultation method and radiosondes was made. As a result it was concluded that the mean value of those results’ difference and also the mean quadratic deviation of these difference increases in common by increase of the forecasting time. In order to prevent surplus loading of telemetry channels and broadcasting inaccurate forecast values via them the optimization of general procedure of radio occultation temperature measurements are carried out using fine functions method. For optimization the concurrent parameters, changing on antiphase order are determined. It is found out that utilization of fine function method taking into account the applied optimization criterion and some limitation conditions make it possible to optimize the whole procedure of forecasting atmospheric temperature using the GPS radio occultation measurements.

Tropical temperature variability and Kelvin-wave activity in the UTLS from GPS RO measurements

Abstract. Tropical temperature variability over 10–30 km and associated Kelvin-wave activity are investigated using GPS radio occultation (RO) data from January 2002 to December 2014. RO data are a powerful tool for quantifying tropical temperature oscillations with short vertical wavelengths due to their high vertical resolution and high accuracy and precision. Gridded temperatures from GPS RO show the strongest variability in the tropical tropopause region (on average 3 K2). Large-scale zonal variability is dominated by transient sub-seasonal waves (2 K2), and about half of sub-seasonal variance is explained by eastward-traveling Kelvin waves with periods of 4 to 30 days (1 K2). Quasi-stationary waves associated with the annual cycle and interannual variability contribute about a third (1 K2) to total resolved zonal variance. Sub-seasonal waves, including Kelvin waves, are highly transient in time. Above 20 km, Kelvin waves are strongly modulated by the quasi-biennial oscillation (QBO) in stratospheric zonal winds, with enhanced wave activity during the westerly shear phase of the QBO. In the tropical tropopause region, however, peaks of Kelvin-wave activity are irregularly distributed in time. Several peaks coincide with maxima of zonal variance in tropospheric deep convection, but other episodes are not evidently related. Further investigations of convective forcing and atmospheric background conditions are needed to better understand variability near the tropopause.

Ionospheric F-layer global scintillation index variation using COSMIC during the period of 2007–2013

The three-dimensional global morphology and seasonal characteristics of the ionospheric scintillation index of the F-layer between 150 and 550 km altitudes are analyzed using the GPS radio occultation measurements from the Constellation Observing System for Meteorology, Ionosphere and Climate during the 7-year period of low and high sunspot activity from 2007 to 2013. The results show that the prominent scintillation intensity, which is confined within ±30° geomagnetic latitude, starts at post-sunset, reaches a maximum at around pre-midnight, and often persists until postmidnight. Moderate scintillation activity can be observed in the high-latitude region almost at any time, whereas weak scintillation prevails in the midlatitude region. The noticeable scintillation peak near midnight occurs at an altitude of approximately 250 km in most cases. However, the peak of the scintillation activity during the solar maximum extends to higher altitudes than observed during the solar minimum. Additionally, the local variation in time and altitude of the scintillation intensity is closely correlated with ionospheric HmF2. Statistical analysis indicates that an increase in solar activity or geomagnetic activity enhances the occurrence rate of scintillation and results in intense scintillation. The current research is beneficial for directly studying global ionospheric irregularities at GHz frequency based on high-rate L1 data and constructing a global scintillation model.

Assessment of Radiometer Calibration with GPS Radio Occultation for the MiRaTA CubeSat Mission.

The Microwave Radiometer Technology Acceleration (MiRaTA) is a 3U CubeSat mission sponsored by the NASA Earth Science Technology Office (ESTO). The science payload on MiRaTA consists of a tri-band microwave radiometer and Global Positioning System (GPS) radio occultation (GPSRO) sensor. The microwave radiometer takes measurements of all-weather temperature (V-band, 50-57 GHz), water vapor (G-band, 175-191 GHz), and cloud ice (G-band, 205 GHz) to provide observations used to improve weather forecasting. The Aerospace Corporation's GPSRO experiment, called the Compact TEC (Total Electron Content) and Atmospheric GPS Sensor (CTAGS), measures profiles of temperature and pressure in the upper troposphere/lower stratosphere (∼20 km) and electron density in the ionosphere (over 100 km). The MiRaTA mission will validate new technologies in both passive microwave radiometry and GPS radio occultation: (1) new ultra-compact and low-power technology for multi-channel and multi-band passive microwave radiometers, (2) the application of a commercial off the shelf (COTS) GPS receiver and custom patch antenna array technology to obtain neutral atmospheric GPSRO retrieval from a nanosatellite, and (3) a new approach to spaceborne microwave radiometer calibration using adjacent GPSRO measurements. In this paper, we focus on objective (3), developing operational models to meet a mission goal of 100 concurrent radiometer and GPSRO measurements, and estimating the temperature measurement precision for the CTAGS instrument based on thermal noise. Based on an analysis of thermal noise of the CTAGS instrument, the expected temperature retrieval precision is between 0.17 K and 1.4 K, which supports the improvement of radiometric calibration to 0.25 K.

The impact of gravity waves and cloud nucleation threshold on stratospheric water and tropical tropospheric cloud fraction

AbstractUsing the Modern Era Retrospective‐Analysis for Research and Applications (MERRA) and MERRA‐2 reanalysis winds, temperatures, and anvil cloud ice, we explore the impact of varying the cloud nucleation threshold relative humidity (RH) and high‐frequency gravity waves on stratospheric water vapor (H2O) and upper tropical tropopause cloud fraction (TCF). Our model results are compared to 2008/2009 winter TCF derived from Cloud‐Aerosol Lidar with Orthogonal Polarization and H2O observations from the Microwave Limb Sounder (MLS). The RH threshold affects both model H2O and TCF, while high‐frequency gravity waves mostly impact TCF. Adjusting the nucleation RH and the amplitude of high‐frequency gravity waves allows us to tune the model to observations. Reasonable observational agreement is obtained with a nucleation threshold between 130% and 150% RH consistent with airborne observations. For the MERRA reanalysis, we lower the tropopause temperature by 0.5 K roughly consistent with GPS radio occultation measurements and include ~0.1 K high‐frequency gravity wave temperature oscillations in order to match TCF and H2O observations. For MERRA‐2 we do not need to adjust the tropopause temperature nor add gravity waves, because there are sufficient high‐frequency temperature oscillations already present in the MERRA‐2 reanalysis to reproduce the observed TCF.

The ionosphere electron density spatio-temporal modeling based on the Slepian basis functions

Radio occultation (RO) is one of the most novel satellite approaches that can be used to study ionospheric electron densities, by combining the observations of global navigation satellite systems and low earth orbiting (LEO) satellites to determine ionospheric electron density profiles with high vertical resolutions. This study aims at developing a multi-dimensional model of the electron density derived from ionospheric GPS RO measurements by the FORMOSAT-3/COSMIC satellites. In order to develop a 4D local model for the electron density, a Slepian function was used to account for longitudinal and latitudinal variations, the B-spline was utilized for time variations and the Chapman profile function was used to express the height variations of the electron density, in this way the parameters of the Chapman function, namely the maximum electron density of the F2 layer (NmF2) and the corresponding F2 peak height (hmF2) were modeled. The coefficients of the two sets of the Slepian functions were estimated using the least-squares adjustment technique. In order to validate the accuracy of the proposed technique, two approaches are considered: (1) The estimated F2-peak values were compared with the IRI model and ionosonde stations values, (2) Comparing the modeled electron density profile against the one estimated using RO methods.

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

Abstract. A modification to the standard bending-angle correction used in GPS radio occultation (GPS-RO) is proposed. The modified approach should reduce systematic residual ionospheric errors in GPS radio occultation climatologies. A new second-order term is introduced in order to account for a known source of systematic error, which is generally neglected. The new term has the form κ(a) × (αL1(a)-αL2(a))2, where a is the impact parameter and (αL1, αL2) are the L1 and L2 bending angles, respectively. The variable κ is a weak function of the impact parameter, a, but it does depend on a priori ionospheric information. The theoretical basis of the new term is examined. The sensitivity of κ to the assumed ionospheric parameters is investigated in one-dimensional simulations, and it is shown that κ ≃ 10–20 rad−1. We note that the current implicit assumption is κ=0, and this is probably adequate for numerical weather prediction applications. However, the uncertainty in κ should be included in the uncertainty estimates for the geophysical climatologies produced from GPS-RO measurements. The limitations of the new ionospheric correction when applied to CHAMP (Challenging Minisatellite Payload) measurements are noted. These arise because of the assumption that the refractive index is unity at the satellite, made when deriving bending angles from the Doppler shift values.