Monitoring twenty-first century climate using GPS radio occultation 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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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 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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Analysis of GPS radio occultation data from the FORMOSAT-3/COSMIC and Metop/GRAS missions at CDAAC

Atmospheric Measurement Techniques ◽

10.5194/amt-4-2255-2011 ◽

2011 ◽

Vol 4 (10) ◽

pp. 2255-2272 ◽

Cited By ~ 34

Author(s):

W. Schreiner ◽

S. Sokolovskiy ◽

D. Hunt ◽

C. Rocken ◽

Y.-H. Kuo

Keyword(s):

Noise Level ◽

Radio Occultation ◽

Signal To Noise Ratio ◽

Lower Troposphere ◽

Mean Difference ◽

Bending Angle ◽

Tropical Regions ◽

Gps Radio Occultation ◽

Single Difference ◽

Bending Angles

Abstract. This study investigates the noise level and mission-to-mission stability of Global Positioning System (GPS) radio occultation (RO) neutral atmospheric bending angle data at the UCAR COSMIC Data Analysis and Archive Center (CDAAC). Data are used from two independently developed RO instruments currently flying in orbit on the FORMOSAT-3/COSMIC (F3C) and Metop/GRAS (GNSS Receiver for Atmospheric Sounding) missions. The F3C 50 Hz RO data are post-processed with a single-difference excess atmospheric phase algorithm, and the Metop/GRAS 50 Hz closed loop and raw sampling (down-sampled from 1000 Hz to 50 Hz) data are processed with a zero-difference algorithm. The standard deviations of the F3C and Metop/GRAS bending angles from climatology between 60 and 80 km altitude from June–December 2009 are approximately 1.78 and 1.13 μrad, respectively. The F3C standard deviation reduces significantly to 1.44 μrad when single-difference processing uses GPS satellites on the same side of the spacecraft. The higher noise level for F3C bending angles can be explained by additional noise from the reference link phase data that are required with single-difference processing. The F3C and Metop/GRAS mean bending angles differences relative to climatology during the same six month period are statistically significant and have values of −0.05 and −0.02 μrad, respectively. A comparison of ~13 500 collocated F3C and Metop/GRAS bending angle profiles over this six month period shows a similar mean difference of ~0.02 ± 0.02 μrad between 30 and 60 km impact heights that is marginally significant. The observed mean difference between the F3C and Metop/GRAS bending angles of ~0.02–0.03 μrad is quite small and illustrates the high degree of re-produceability and mission independence of the GPS RO data at high altitudes. Collocated bending angles between two F3C satellites from early in the mission differ on average by up to 0.5% near the surface due to systematically lower signal-to-noise ratio for one of the satellites. Results from F3C and Metop/GRAS differences in the lower troposphere suggest the Metop/GRAS bending angles are negatively biased compared to F3C with a maximum of several percents near the surface in tropical regions. This bias is related to different tracking depths (deeper in F3C) and data gaps in Metop/GRAS which make it impossible to process the data from both missions in exactly the same way.

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Analysis of GPS radio occultation data from the FORMOSAT-3/COSMIC and Metop/GRAS missions at CDAAC

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-4-2433-2011 ◽

2011 ◽

Vol 4 (2) ◽

pp. 2433-2489

Author(s):

W. Schreiner ◽

S. Sokolovskiy ◽

D. Hunt ◽

C. Rocken ◽

Y.-H. Kuo

Keyword(s):

Noise Level ◽

Radio Occultation ◽

Signal To Noise Ratio ◽

Lower Troposphere ◽

Mean Difference ◽

Bending Angle ◽

Tropical Regions ◽

Gps Radio Occultation ◽

Single Difference ◽

Bending Angles

Abstract. This study investigates the noise level and mission-to-mission stability of Global Positioning System (GPS) radio occultation (RO) neutral atmospheric bending angle data at the UCAR COSMIC Data Analysis and Archive Center (CDAAC). Data are used from two independently developed RO instruments currently flying in orbit on the FORMOSAT-3/COSMIC (F3C) and Metop/GRAS (GNSS Receiver for Atmospheric Sounding) missions. The F3C 50 Hz RO data are post-processed with a single-difference excess atmospheric phase algorithm, and the Metop/GRAS 50 Hz closed loop and raw sampling (down-sampled from 1000 Hz to 50 Hz) data are processed with a zero-difference algorithm. The standard deviations of the F3C and Metop/GRAS bending angles from climatology between 60 and 80 km altitude from June–December 2009 are approximately 1.78 and 1.13 μrad, respectively. The F3C standard deviation reduces significantly to 1.44 μrad when single-difference processing uses GPS satellites on the same side of the spacecraft. The higher noise level for F3C bending angles can be explained by additional noise from the reference link phase data that are required with single-difference processing. The F3C and Metop/GRAS mean bending angles differences relative to climatology during the same six month period are statistically significant and have values of −0.05 and −0.02 μrad, respectively. A comparison of ~13 500 collocated F3C and Metop/GRAS bending angle profiles over this six month period shows a similar mean difference of ~0.02 ± 0.02 μrad between 30 and 60 km impact heights that is marginally significant. The observed mean difference between the F3C and Metop/GRAS bending angles of ~0.02–0.03 μrad is quite small and illustrates the high degree of re-produceability and mission independence of the GPS RO data at high altitudes. Collocated bending angles between two F3C satellites from early in the mission differ on average by up to 0.5% near the surface due to systematically lower signal-to-noise ratio for one of the satellites. Results from F3C and Metop/GRAS differences in the lower troposphere suggest the Metop/GRAS bending angles are negatively biased compared to F3C with a maximum of several percents near the surface in tropical regions. This bias is related to different tracking depths (deeper in F3C) and data gaps in Metop/GRAS which make it impossible to process the data from both missions in exactly the same way.

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Ionospheric correction of GPS radio occultation data in the troposphere

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-8-7781-2015 ◽

2015 ◽

Vol 8 (7) ◽

pp. 7781-7803

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 ◽

Occultation Data ◽

L1 And L2 ◽

The Impact ◽

Radio Occultation Data ◽

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 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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