Ionospheric correction of GPS radio occultation data in the   troposphere

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 by 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 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 in the presence of a sharp top of the boundary layer reveals differences that can be explained by shifts in the impact parameter. The ionosphere-induced vertical shifts of the bending angle profiles require further investigation.

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Ionospheric correction of GPS radio occultation data in the troposphere

Atmospheric Measurement Techniques ◽

10.5194/amt-9-335-2016 ◽

2016 ◽

Vol 9 (2) ◽

pp. 335-346 ◽

Cited By ~ 13

Author(s):

Z. Zeng ◽

S. Sokolovskiy ◽

W. Schreiner ◽

D. Hunt ◽

J. Lin ◽

...

Keyword(s):

Radio Occultation ◽

Neutral Atmosphere ◽

Ionospheric Correction ◽

Bending Angle ◽

Gps Radio Occultation ◽

Strong Inversion ◽

Occultation Data ◽

Horizontal Inhomogeneity ◽

L1 And L2 ◽

Bending Angles

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.

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

Atmospheric Measurement Techniques ◽

10.5194/amt-8-3385-2015 ◽

2015 ◽

Vol 8 (8) ◽

pp. 3385-3393 ◽

Cited By ~ 20

Author(s):

S. B. Healy ◽

I. D. Culverwell

Keyword(s):

Impact Parameter ◽

Radio Occultation ◽

Order Term ◽

Weather Prediction ◽

Implicit Assumption ◽

Ionospheric Correction ◽

Gps Radio Occultation ◽

Uncertainty Estimates ◽

The Impact ◽

Bending Angles

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 &simeq; 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.

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The Impact of Vertical Resolution in the Assimilation of GPS Radio Occultation Data

Weather and Forecasting ◽

10.1175/waf-d-17-0061.1 ◽

2018 ◽

Vol 33 (4) ◽

pp. 1033-1044 ◽

Cited By ~ 1

Author(s):

Ji-Hyun Ha ◽

Jeon-Ho Kang ◽

Suk-Jin Choi

Keyword(s):

Geopotential Height ◽

Radio Occultation ◽

Three Dimensional ◽

Vertical Resolution ◽

Bending Angle ◽

Gps Radio Occultation ◽

Angle Data ◽

Upper Level ◽

Occultation Data ◽

The Impact

Abstract The sensitivity of GPS radio occultation (GPSRO) bending angle assimilation to vertical resolution was studied within a global three-dimensional variational data assimilation (3DVAR) system. The sensitivity experiments were performed using different vertical resolutions of GPSRO data at 2 km, 1 km, 500 m, and 200 m. The assimilation of the higher vertical resolution GPSRO data showed better consistency in the analysis–forecast cycle in terms of the differences between GPSRO bending angle data and 6-h forecasts (O-F). This resulted in an improved analysis of the temperature, geopotential height, and wind in the mid-/upper-level troposphere by the hydrostatic response and the related model dynamics. It should be noted that the highest vertical resolution of the GPSRO data (200 m in this study) improved the forecasting skill level in terms of the root-mean-square error (against the European Centre for Medium-Range Weather Forecasts analysis) and the anomaly correlation of the geopotential height forecasting at 500 and 200 hPa in both the Northern and Southern Hemispheres. The benefits of assimilating higher vertical resolution GPSRO data were more pronounced in the upper-level troposphere, which was in agreement with previous studies using real GPSRO observations.

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

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-8-1177-2015 ◽

2015 ◽

Vol 8 (1) ◽

pp. 1177-1201 ◽

Cited By ~ 2

Author(s):

S. B. Healy ◽

I. D. Culverwell

Keyword(s):

Impact Parameter ◽

Radio Occultation ◽

Order Term ◽

Weather Prediction ◽

Implicit Assumption ◽

Ionospheric Correction ◽

Gps Radio Occultation ◽

Uncertainty Estimates ◽

The Impact ◽

Bending Angles

Abstract. A modification to the standard bending angle correction used in GPS radio occultation 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)-αL1(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 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 &simeq; 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 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.

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Estimation of the Added Value of the Absolute Calibration of GPS Radio Occultation Data for Numerical Weather Prediction

Monthly Weather Review ◽

10.1175/mwr-d-14-00153.1 ◽

2015 ◽

Vol 143 (4) ◽

pp. 1259-1274 ◽

Cited By ~ 7

Author(s):

Josep M. Aparicio ◽

Stéphane Laroche

Keyword(s):

Bias Correction ◽

Radio Occultation ◽

Weather Prediction ◽

Added Value ◽

Atmospheric Measurements ◽

Gps Radio Occultation ◽

Correction Scheme ◽

Occultation Data ◽

The Impact ◽

Radio Occultation Data

Abstract An analysis of the impact of GPS radio occultation observations on Environment Canada’s global deterministic weather prediction system is presented. Radio occultation data, as any other source of weather observations, have a direct impact on the analyses. Since they are assimilated assuming that they are well calibrated, they also impact the bias correction scheme employed for other data, such as satellite radiances. The authors estimate the relative impact of occultation data obtained from, first, their assimilation as atmospheric measurements and, second, their influence on the bias correction for radiance data. This assessment is performed using several implementations of the thermodynamic relationships involved, and also allowing or blocking this influence to the radiance bias correction scheme. The current implementation of occultation operators at Environment Canada is presented, collecting upgrades that have been detailed elsewhere, such as the equation of state of air and the expression of refractivity. The performance of the system with and without assimilation of occultations is reviewed under conditions representative of current operations. Several denial runs are prepared, withdrawing only the occultation data from the assimilation, but keeping their influence on the radiance bias correction, or assimilating occultations but denying their impact on the bias correction procedure, and a complete denial. It is shown that the impact of occultations on the analysis is significant through both paths—assimilation and radiance bias correction—albeit the first is larger. The authors conclude that the traceability link of the ensemble of occultations has an added value, beyond the value of each datum as an atmospheric measurement.

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Improved model for correcting the ionospheric impact on bending angle in radio occultation measurements

Atmospheric Measurement Techniques ◽

10.5194/amt-11-2213-2018 ◽

2018 ◽

Vol 11 (4) ◽

pp. 2213-2224 ◽

Cited By ~ 7

Author(s):

Matthew J. Angling ◽

Sean Elvidge ◽

Sean B. Healy

Keyword(s):

Zenith Angle ◽

Solar Zenith Angle ◽

Radio Occultation ◽

Residual Error ◽

Electron Content ◽

Neutral Atmosphere ◽

Ionospheric Correction ◽

Bending Angle ◽

Angle Difference ◽

Bending Angles

Abstract. The standard approach to remove the effects of the ionosphere from neutral atmosphere GPS radio occultation measurements is to estimate a corrected bending angle from a combination of the L1 and L2 bending angles. This approach is known to result in systematic errors and an extension has been proposed to the standard ionospheric correction that is dependent on the squared L1 ∕ L2 bending angle difference and a scaling term (κ). The variation of κ with height, time, season, location and solar activity (i.e. the F10.7 flux) has been investigated by applying a 1-D bending angle operator to electron density profiles provided by a monthly median ionospheric climatology model. As expected, the residual bending angle is well correlated (negatively) with the vertical total electron content (TEC). κ is more strongly dependent on the solar zenith angle, indicating that the TEC-dependent component of the residual error is effectively modelled by the squared L1 ∕ L2 bending angle difference term in the correction. The residual error from the ionospheric correction is likely to be a major contributor to the overall error budget of neutral atmosphere retrievals between 40 and 80 km. Over this height range κ is approximately linear with height. A simple κ model has also been developed. It is independent of ionospheric measurements, but incorporates geophysical dependencies (i.e. solar zenith angle, solar flux, altitude). The global mean error (i.e. bias) and the standard deviation of the residual errors are reduced from -1.3×10-8 and 2.2×10-8 for the uncorrected case to -2.2×10-10 rad and 2.0×10-9 rad, respectively, for the corrections using the κ model. Although a fixed scalar κ also reduces bias for the global average, the selected value of κ (14 rad−1) is only appropriate for a small band of locations around the solar terminator. In the daytime, the scalar κ is consistently too high and this results in an overcorrection of the bending angles and a positive bending angle bias. Similarly, in the nighttime, the scalar κ is too low. However, in this case, the bending angles are already small and the impact of the choice of κ is less pronounced.

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Optimal Noise Filtering for the Ionospheric Correction of GPS Radio Occultation Signals

Journal of Atmospheric and Oceanic Technology ◽

10.1175/2009jtecha1192.1 ◽

2009 ◽

Vol 26 (7) ◽

pp. 1398-1403 ◽

Cited By ~ 29

Author(s):

S. Sokolovskiy ◽

W. Schreiner ◽

C. Rocken ◽

D. Hunt

Keyword(s):

Radio Occultation ◽

Residual Effects ◽

Small Scale ◽

Neutral Atmosphere ◽

Simple Method ◽

Ionospheric Correction ◽

Weather Forecasts ◽

Gps Radio Occultation ◽

Medium Range ◽

L1 And L2

Abstract GPS radio occultation remote sensing of the neutral atmosphere requires ionospheric correction of L1 and L2 signals. The ionosphere-corrected variables derived from radio occultation signals—such as the phase, Doppler, and bending angle—are affected by small-scale ionospheric effects that are not completely eliminated by the ionospheric correction. They are also affected by noise from mainly the L2 signal. This paper introduces a simple method for optimal filtering of the L4 = L1 − L2 signal used to correct the L1 signal, which minimizes the combined effects of both the small-scale ionospheric residual effects and L2 noise on the ionosphere-corrected variables. Statistical comparisons to high-resolution numerical weather models from the European Centre for Medium-Range Weather Forecasts (ECMWF) validate that this increases the accuracy of radio occultation inversions in the stratosphere.

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Use of the L2C signal for inversions of GPS radio occultation data in the neutral atmosphere

GPS Solutions ◽

10.1007/s10291-013-0340-x ◽

2013 ◽

Vol 18 (3) ◽

pp. 405-416 ◽

Cited By ~ 9

Author(s):

S. V. Sokolovskiy ◽

W. S. Schreiner ◽

Z. Zeng ◽

D. C. Hunt ◽

Y.-H. Kuo ◽

...

Keyword(s):

Radio Occultation ◽

Neutral Atmosphere ◽

Gps Radio Occultation ◽

Occultation Data ◽

L2c Signal ◽

Radio Occultation Data

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Improved model for correcting the ionospheric impact on bending angle in radio occultation measurements

10.5194/amt-2017-162 ◽

2017 ◽

Cited By ~ 1

Author(s):

Matthew J. Angling ◽

Sean Elvidge ◽

Sean B. Healy

Keyword(s):

Zenith Angle ◽

Solar Zenith Angle ◽

Radio Occultation ◽

Neutral Atmosphere ◽

Night Time ◽

Bending Angle ◽

Height Range ◽

Improved Model ◽

The Impact ◽

Bending Angles

Abstract. The standard approach to remove the effects of the ionosphere from neutral atmosphere GPS radio occultation measurements is to estimate a corrected bending angle from a combination of the L1 and L2 bending angles. This approach is known to result in systematic errors and an extension has been proposed to the standard ionospheric correction that is dependent on the squared L1/L2 bending angle difference and a scaling term (κ). The variation of κ with height, time, season, location and solar activity (i.e. the f10.7 flux) has been investigated by applying a 1D bending angle operator to electron density profiles provided by a monthly median ionospheric climatology model. As expected, the residual bending angle is well correlated (negatively) with the vertical TEC. However, κ is more strongly dependent on the solar zenith angle. Furthermore, over the height range of interest (40–80 km) κ is approximately linear with height. A simple κ model has also been developed. It is independent of ionospheric measurements, but incorporates geophysical dependencies (i.e. solar zenith angle, solar flux, altitude). The global mean error (i.e. bias) and the standard deviation of the residual errors are reduced to −2.2 × 10−10 rad and 2.0 × 10−9 rad respectively. Although a fixed scalar κ also reduces bias for the global average the selected value of κ (14 rad−1) is only appropriate for a small band of locations around the solar terminator. In the day time, the scalar κ is consistently too high and this results in an over correction of the bending angles and a positive bending angle bias. Similarly, in the night time, the scalar κ is too low. However, in this case, the bending angles are already small and the impact of the choice of κ is less pronounced.

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