A ray tracing technique for radio occultation

Abstract. Atmospheric profiles retrieved from GNSS (Global Navigation Satellite System) radio occultation (RO) measurements are increasingly used to validate other measurement data. For this purpose it is important to be aware of the characteristics of RO measurements. RO data are frequently compared with vertical reference profiles, but the RO method does not provide vertical scans through the atmosphere. The average elevation angle of the tangent point trajectory (which would be 90° for a vertical scan) is about 40° at altitudes above 70 km, decreasing to about 25° at 20 km and to less than 5° below 3 km. In an atmosphere with high horizontal variability we can thus expect noticeable representativeness errors if the retrieved profiles are compared with vertical reference profiles. We have performed an end-to-end simulation study using high-resolution analysis fields (T799L91) from the European Centre for Medium-Range Weather Forecasts (ECMWF) to simulate a representative ensemble of RO profiles via high-precision 3-D ray tracing. Thereby we focused on the dependence of systematic and random errors on the measurement geometry, specifically on the incidence angle of the RO measurement rays with respect to the orbit plane of the receiving satellite, also termed azimuth angle, which determines the obliquity of RO profiles. We analyzed by how much errors are reduced if the reference profile is not taken vertical at the mean tangent point but along the retrieved tangent point trajectory (TPT) of the RO profile. The exact TPT can only be determined by performing ray tracing, but our results confirm that the retrieved TPT – calculated from observed impact parameters – is a very good approximation to the "true" one. Systematic and random errors in RO data increase with increasing azimuth angle, less if the TPT is properly taken in to account, since the increasing obliquity of the RO profiles leads to an increasing sensitivity to departures from horizontal symmetry. Up to an azimuth angle of 30°, however, this effect is small, even if the RO profiles are assumed to be vertical. For applications requiring highest accuracy and precision it is advisable to exclude RO profiles with ray incidence angles beyond an azimuth of 50°. Errors in retrieved atmospheric profiles decrease significantly, by up to a factor of 2, if the RO data are exploited along the retrieved TPT. The tangent point trajectory of RO profiles should therefore be exploited whenever this is possible.

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Meteorological information in GPS-RO reflected signals

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-4-1199-2011 ◽

2011 ◽

Vol 4 (1) ◽

pp. 1199-1231 ◽

Cited By ~ 1

Author(s):

K. Boniface ◽

J. M. Aparicio ◽

E. Cardellach

Keyword(s):

Refractive Index ◽

Ray Tracing ◽

Electromagnetic Waves ◽

Radio Occultation ◽

Good Description ◽

Weather Prediction ◽

Three Dimensional ◽

Perturbation Approach ◽

Vertical Profiles ◽

Reflected Signal

Abstract. Vertical profiles of the atmosphere can be obtained globally with the radio-occultation technique. However, the lowest layers of the atmosphere are less accurately extracted. A good description of these layers is important for the good performance of Numerical Weather Prediction (NWP) systems, and an improvement of the observational data available for the low troposphere would thus be of great interest for data assimilation. We outline here how supplemental meteorological information close to the surface can be extracted whenever reflected signals are available. We separate the reflected signal through a radioholographic filter, and we interpret it with a ray tracing procedure, analyzing the trajectories of the electromagnetic waves over a three-dimensional field of refractive index. A perturbation approach is then used to perform an inversion, identifying the relevant contribution of the lowest layers of the atmosphere to the properties of the reflected signal, and extracting some supplemental information to the solution of the inversion of the direct propagation signals. The methodology is applied to one reflection case.

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Errors in GNSS radio occultation data: relevance of the measurement geometry and obliquity of profiles

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-3-4261-2010 ◽

2010 ◽

Vol 3 (5) ◽

pp. 4261-4283 ◽

Cited By ~ 1

Author(s):

U. Foelsche ◽

S. Syndergaard ◽

J. Fritzer ◽

G. Kirchengast

Keyword(s):

Ray Tracing ◽

Radio Occultation ◽

Elevation Angle ◽

Measurement Data ◽

Azimuth Angle ◽

Random Errors ◽

Tangent Point ◽

Atmospheric Profiles ◽

Systematic And Random Errors ◽

Measurement Geometry

Abstract. Atmospheric profiles retrieved from GNSS (Global Navigation Satellite System) radio occultation (RO) measurements are increasingly used to validate other measurement data. For this purpose it is important to be aware of the characteristics of RO measurements. RO data are frequently compared with vertical reference profiles, but the RO method does not provide vertical scans through the atmosphere. The average elevation angle of the tangent point trajectory (which would be 90° for a vertical scan) is about 40° at altitudes above 70 km, decreasing to about 25° at 20 km and to less than 5° below 3 km. In an atmosphere with high horizontal variability we can thus expect noticeable representativeness errors if the retrieved profiles are compared with vertical reference profiles. We have performed an end-to-end simulation study using high-resolution analysis fields (T799L91) from the European Centre for Medium-Range Weather Forecasts (ECMWF) to simulate a representative ensemble of RO profiles via high-precision 3-D ray tracing. Thereby we focused on the dependence of systematic and random errors on the measurement geometry, specifically on the incidence angle of the RO measurement rays with respect to the orbit plane of the receiving satellite, also termed azimuth angle, which determines the obliquity of RO profiles. We analyzed by how much errors are reduced if the reference profile is not taken vertical at the mean tangent point but along the retrieved tangent point trajectory (TPT) of the RO profile. The exact TPT can only be determined by performing ray tracing, but our results confirm that the retrieved TPT – calculated from observed impact parameters – is a very good approximation to the "true" one. Systematic and random errors in RO data increase with increasing azimuth angle, less if the TPT is properly taken in to account, since the increasing obliquity of the RO profiles leads to an increasing sensitivity to departures from horizontal symmetry. Up to an azimuth angle of 30°, however, this effect is small, even if the RO profiles are assumed to be vertical. For applications requiring highest accuracy and precision it is advisable to exclude RO profiles with ray incidence angles beyond an azimuth of 50°. Errors in retrieved atmospheric profiles decrease significantly, by up to a factor of 2, if the RO data are exploited along the retrieved TPT. The tangent point trajectory of RO profiles should therefore be exploited whenever this is possible.

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Improvements to a GPS radio occultation ray-tracing model and their impacts on assimilation of bending angle

Journal of Geophysical Research Atmospheres ◽

10.1029/2002jd003160 ◽

2003 ◽

Vol 108 (D17) ◽

Cited By ~ 25

Author(s):

H. Liu

Keyword(s):

Ray Tracing ◽

Radio Occultation ◽

Bending Angle ◽

Gps Radio Occultation ◽

Ray Tracing Model

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Simulating GPS Radio Occultation Using 3-D Ray Tracing

International Association of Geodesy Symposia - Earth on the Edge: Science for a Sustainable Planet ◽

10.1007/978-3-642-37222-3_4 ◽

2013 ◽

pp. 27-30

Author(s):

R. Norman ◽

J. Le Marshall ◽

K. Zhang ◽

C. S. Wang ◽

B. A. Carter ◽

...

Keyword(s):

Ray Tracing ◽

Radio Occultation ◽

Gps Radio Occultation

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Variational Assimilation of Radio Occultation Observations into Numerical Weather Prediction Models: Equations, Strategies, and Algorithms

Remote Sensing ◽

10.3390/rs11242886 ◽

2019 ◽

Vol 11 (24) ◽

pp. 2886 ◽

Cited By ~ 1

Author(s):

Michael Gorbunov ◽

Razvan Stefanescu ◽

Vladimir Irisov ◽

Dusanka Zupanski

Keyword(s):

Ray Tracing ◽

Radio Occultation ◽

Prediction Models ◽

Weather Prediction ◽

Standard Form ◽

Plane Curve ◽

Variational Assimilation ◽

Observation Operator ◽

Global Atmospheric Circulation ◽

Bending Angles

We review different approaches to the variational assimilation of radio occultation (RO) observations into models of global atmospheric circulation. We derive the general equation for the bending angle that reduces to the Abel integral for a spherically layered atmosphere. We review the full 3-D observation operator for bending angles, which provides the strictest solution, but is also most computationally expensive. Commonly used is the 2-D approximation that allows treating rays as plane curve. We discuss a simple 1-D approach to the assimilation of bending angles. The observation operator based on the standard form of the Abel integral has a disadvantage, because it cannot account for waveguides. Alternative approaches use 1-D ray-tracing. The most straightforward way is to use the same framework as for the 3-D observation operator, with the refractivity field reduced to a single profile independent from the horizontal coordinates. An alternative 1-D ray-tracing approach uses the form of ray equation in a spherically layered medium that uses an invariant. The assimilation of refractivity has also 1-D and 3-D options. We derive a new simple form of the refractivity-mapping operator. We present the results of numerical tests of different 3-D and 1-D observation operators, based on Spire data.

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