IAG/SSG 2.192 

Report from the members (in alphabetic order):


Georg Beyerle (on behalf of him):
Radio holographic analysis shows that around 1/3 of the GPS/MET occultation events have multibeam propagation in the lower troposphere. Raytracing simulation of the time history of the direction of arrival of direct (GPS--LEO) and reflected (GPS -- earth's surface --LEO) signal explains the observed multipath structure in detail. This finding may improve refractivity retrieval in the lower troposphere and may indicate a future application of radio occultation for sounding of boundary layer, ocean and ice surface.

Ben Chao:
A loosely defined "geodesy team" for the COSMIC Mission intend to use COSMIC (2004 launch) orbit data (from GPS) to infer Earth's gravity field and its temporal variations. Strawman simulations have indicated possible great improvements over our present solutions.

Jennifer Haase:
The usual geometry for GNSS (Global Navigation Satellite System) based on radio occultation sounding has the receiver placed on a LEO (Low Earth Orbit) satellite. We investigated a new geometric approach, assuming an airborne rather than a spaceborne receiver. Information on the refractivity structure and, in turn, on atmospheric variables (most notably temperature and humidity) can be retrieved from accurate airborne measurements of the amplitude and phase delay of the signals occulted by the troposphere. We present advantages and disadvantages of making measurements from commercial aircraft equipped with proper GNSS receivers and antennae. Using the EGOPS4 software (see Kirchengast et al. paper in this session), we simulated realistic airborne occultation observations and assessed their characteristics, such as geometric properties, dynamic range, and error
sources. Findings include that an airborne system has the potential to provide many more profiles below 10 km height than LEO systems (though not with similar global coverage), that the profile characteristics are much more sensitive to the geometry determined by the GNSS satellites than in the LEO case, and that horizontal refractivity structures are expected to be a main error source. Finally, planned future work on the topic is outlined. The project is supported by the European Space Agency.

George Hajj, Manuel de la Torre Juarez, Rob Kursinski, Tony Mannucci, et al.: 
Occultation Research at JPL

The GPS receiver on CHAMP has recently begun producing a limited amount of occultation data. Working with the German occultation team and the U.S. GPS Earth Observatory (GEO) occultation team, the receiver group at JPL is assessing the performance of the receiver and updating the receiver software on the GPS receiver on the CHAMP satellite to fix various bugs and improve performance overall. The GEO group at JPL has begun deriving occultation profiles from CHAMP and assessing the performance of the CHAMP GPS receiver. Initial results are promising in that the Signal to Noise Ratios (SNR) are higher than in previous receivers, particularly the L2 SNR with AS-on. The receiver appears to be tracking occulted GPS signals very deep into the troposphere. Initial retrieval results will be reported on at the Spring AGU meeting in Boston.

The GEO group is developing a retrieval scheme that uses only the GPS CA signal. The work is driven by the very poor L2 data quality of most of the Oersted and GPS/MET occultation data sets. As a result, the ionospheric effects could not be estimated and removed using the normal L1 versus L2 approach. By using the group delay minus phase delay both derived from the GPS CA signal, the effect of the ionosphere can be estimated and removed. The penalty is of course that the ionosphere estimated this way is quite noisy because the group delay estimates are roughly two orders of magnitude noisier than the phase delay estimates. Results and an assessment of the utility of this approach will be presented at the Spring AGU.

Research is proceeding on assimilating GPS occultation data into ionospheric models to study the upper atmosphere.

Yunck et al. (2000) described the Atmospheric Moisture and Ocean Reflection Experiment (AMORE) using high frequency occultation observations and GPS surfaces reflections.

A collaborative research effort is proceeding with Toshi Tsuda et al. in Japan on profiling the atmosphere using occultation observations from mountain tops based on the Zuffada et al. (1999) concept.

NOTE Spring AGU session: there will be a special session at the AGU in Boston at the end of May chaired by Tony Mannucci and John LaBrecque where many new GPS occultation results will be presented.

Sean Healy:
Work at the Met Office for the GRAS-SAF:
We have been deriving the statistics bending angle and impact parameter errors caused by horizontal gradient errors by simulating occultations within the domain of the Met Office mesoscale model. In addition, we are developing a fast bending angle forward model for the direct assimilation of bending angle into numerical weather prediction systems.

Ben Herman:
We are working on an advanced radio occultation experiment to independently monitor water vapor and ozone, as well as refractivity, temperature, pressure, and geopotential height. This will be accomplished with transmitters in space which will have transmitting capbilities at ozone and water vapor absorbing frequencies, as well as the standard L1 and L2 frequencies. We are completing software development and working with JPL on breadboarding the hardware. The experiment, when completed,will operate in a cross-link mode with each space instrument having a transmitter and a receiver.

Klemens Hocke:
By means of the GPS/MET data base and statistical analysis, the 4-D small-scale fluctuation field of the earth's atmosphere and ionosphere
has been preliminary inspected. In particular the detection of thin ionization layers in the lower ionosphere by GPS radio occultation seems to be promising. First results indicate a relationship between earth's topography, atmospheric and ionospheric fluctuations, suggesting a coupling of the whole atmosphere by upward energy/momentum flux of atmospheric waves and showing the potential of GPS radio occultation for correlative studies of lower, middle and upper atmosphere.

Gottfried Kirchengast:
Please see for publications and ARSCliSys Research Group:

Rob Kursinski:
We looked at initial results of combining GPS and ECMWF analysis information within a 1Dvar framework (Kursinski et al., 2000). We are working with Paul Poli and Joanna Joiner at the GSFC Data Assimilation Office (DAO) on assimilating the GPS occultation data into the DAO analyses. In Kursinski and Hajj (2001) we report on initial results of deriving water vapor from GPS/MET data. Comparisons with ECMWF and NCEP analyses and the classic climatology of Peixoto and Oort revealed general similarity but also some important systematic biases. In particular the rounding off of the tradewind inversion in the analyses was very apparent in the comparisons.

At the fall AGU we reported on the dryness of subtropical free troposphere derived from GPS/Met occultations and some implications regarding the water vapor feedback in climate. A manuscript is in preparation.

We have been able to observe the mixed layer in the Planetary Boundary Layer using GPS/MET data. This represents the first time this layer has been observed from space. We will report on this at the Spring AGU and a manuscript is in preparation.

We are evaluating the accuracy of water vapor and temperature derived from high frequency (10 - 200 GHz) occultations and determining the optimum frequencies (see Ben Herman's report above).

NOTE: The special March 2000 issue of TAO dedicated to the COSMIC mission is now out as a book from Springer. (see Lee et al., 2001 in references below)

Christian Marquardt:
Development of variational retrieval schemes of RO soundings in the neutral stratosphere; validation of meteorological products from RO measurements; application of RO data in upper tropospheric and stratospheric dynamics

Manuel Martin-Neira:
 The European Space Agency, within the Payload Systems Division (Directorate of Technical and Operational Support), is researching on the use of reflected GPS or generally, GNSS signals, for altimetry, wind, wave height and TEC retrievals over ocean. We conduct this research through contracts with external institutes, universities or industries, as well as through experiments we perform ourselves
at ESTEC (ESA center in Holland). We proposed the use of GNSS ocean reflected signals for ocean altimetry for the first time in 1993, under a concept called "PARIS" [Martin-Neira, 1993]. The main contracts we have had in the recent past were dedicated to model
the ocean surface as well as the reflected GNSS signals. Right now we finished a study on the use of carrier phase for ocean altimetry and there is an on-going activity to develop algorithms for signal processing from aircraft altititude.
On the experimental side we carried out an experiment from a bridge were an rms height accuracy of 1% of the chip length was achieved (3 m for C/A code) [Martin-Neira et al. 2001].
Last year we also performed a test on a pond to investigate on the use of carrier phase for altimetry and obtained millimetric height error
[Martin-Neira et al., 2000]. As for the future we intend to place industrial contracts to define a "PARIS" instrument for Earth remote sensing from space.

Alexander Pavelyev:

The efficiency of radio holography has been confirmed by direct observation of multi beam propagation and reflected from the sea signal
using MIR/GEO and GPS/MET radio occultation data. This demonstrates high-technology level of the radio holography approach and opened new perspectives for radio occultation experiments: observation natural processes in the atmosphere, mesosphere and ionosphere, measurements of the parameters of the sea surface by means of analysis of reflected signal. The radio holograms of D-layer of the ionosphere revealed wave structures with vertical periods about of 1-2 km in the altitude dependence of the vertical gradient of electron density. Observation of wave structures in D-layer and E-layer of the ionosphere is important for understanding the momentum and energy interchange between lower and upper atmosphere and study fine structure of mesopause region. The main conclusion consists in possibility of qualitative measurements of wind velocity in the lower ionosphere using radio occultation data. Directions of the future progress is outlined. These directions are:

bistatic scatterometry using combined phase and amplitude radio occultation data; this may be considered also in the context of elaborating new international small satellite system for observation of the effect of radiowave propagation on the telecommunication link between two satellite (K. Igarashi et al. 2000);

measurements of parameters of the boundary layer disposed near the sea surface, using reflected signal for more precise evaluation parameters of the lower troposphere, revealing features in the humidity distribution;

elaboration of models for revealing wave phenomena in the upper atmosphere on the phone on more powerful contribution of the F-layer of the ionosphere;

measurements of parameters of natural processes in the upper ionosphere using combined amplitude and phase data analysis;

development of special models to account for ionospheric influence on the atmospheric refractivity restoration and temperature measurements.


Giulio Ruffini:
 I am now working at Starlab (www.starlab.net), and continuing my previous activities in the area of GNSS-R at the newly created company and with a new group (5 people are now in this group). We are working on several aspects of GNSS-R, researching applications in scatterometry as well as in altimetry (using code and phase ranging). Starlab is officially involved in Paris-Alpha, and is responsible for the science aspects of the project. Paris-alpha (ESA project, technical officer is M. Martin-Neira) is interested in retrieving GNSS-R altimetric data from aircraft, and we are considering both code and phase ranging aspects of the problem.

We are also leading Paris-Beta (ESA project, technical officer is P. Silvestrin) which is researching the applications of PARIS for spaceborne bistatic altimetry.

 We have also participated in several experimental campaign related to GNSS-R: MEATEX (MEditerranean Aircraft Tracking EXperiment), MEBEX (MEditerranean Balloon EXperiment), and Zeeland II.

Sergey Sokolovskiy:

At UCAR/GST we looked into the GPS/MET data and found that almost 1/2 of the occultations are affected by receiver tracking errors at the end (sometimes those errors are very large).

"Worst case" RO signals were simulated based on high-resolution radiosondes and forward wave propagation model.

An open loop tracking technique was outlined and tested by the simulated "worst case" RO signals. The "worst case" signals can be tracked without the corruption common for PLL tracking.

For the radio optics (sliding spectral) inversion method an option which overcome the problem of identification and selection of the local maxima in the spectrum of RO signal (which can be very complicated), was introduced and tested by the simulated "worst case" signals.

Additionally: The sliding window Fourier analysis detects reflected signals in the GPS/MET data identically to the MUSIC reported by Beyerle and Hocke.

Andrea Steiner :
 A.K. Steiner and G. Kirchengast

Radio occultation observations using the Global Navigation Satellite System (GNSS) have great potential to contribute to the Global Climate Observing System (GCOS). Besides such benefit for climate monitoring and modeling, the assimilation of high-quality GNSS occultation data into numerical weather prediction (NWP) models could lead to improved weather forecasts and analyses.
 In this context we present results of an empirical error analysis of GNSS radio occultation data, which is based on a realistically simulated
 occultation dataset produced by the End-to-end GNSS Occultation Performance Simulator (EGOPS).
 In order to involve realistic atmospheric profiles and error characteristics we used a T213L50 ECMWF analysis field. The ionosphere was prescribed with the NeUoG model, a global empirical 3D model of the ionospheric electron density field.
 Radio occultation observations were simulated for one observational day adopting the planned European Meteorological Operational satellite (METOP) as Low Earth Orbit (LEO) platform and its GNSS Receiver for Atmospheric Sounding (GRAS) as sensor. Involving a sub-millimetric precision 3D ray tracer, excess phase path profiles were computed for an ensemble of 300 occultation events equally distributed over the globe and in time. With an rms error of ~2 mm at 10 Hz sampling rate they closely mimic the expected METOP/GRAS sensor performance. Atmospheric (troposphere/stratosphere) profiles were retrieved with a state-of-the-art occultation data processing chain and were compared to the "true" co-located profiles. An error analysis was performed at each retrieval step to empirically obtain realistic bias profiles and covariance matrices. We show biases, standard deviations, and correlation functions for each main retrieval product, including bending angle, refractivity, pressure, geopotential height, temperature, and specific humidity. We compare our empirical results with theoretical results and discuss the utility of the empirical covariance matrices for specifying observation error covariance matrices in data assimilation systems ingesting GNSS occultation data.

Jens Wickert:
First occultation measurements of CHAMP have been taken during measurement intervals since February 11, 2001. Vertical profiles of dry temperature and specific humidity were derived and compared with meteorological analyses of ECMWF and NCEP. They show a good agreement. In spite of anti-spoofing on mode of the GPS satellite system, the new BlackJack GPS receiver allows for atmospheric sounding with high accuracy and vertical resolution. It is found that the CHAMP measurements have the potential to reach the Earth's surface. 
It is expected, that "AS on" will have no limiting influence on the data processing, Therefore it is expected that CHAMP will provide about 200 vertical profiles of atmospheric parameters daily after the commissioning phase.

Development of automatically working occultation processing system at GFZ Potsdam.

 Xiaolei Zou:
 Following is a short statement from us: With a raytracing procedure and variational data assimilation techniques, hundreds of GPS/MET bending angle profiles were assimilated into the global atmospheric analysis. A small but consistent improvement in the short-range (6-h) and medium-range (1-5 days) forecast skills, especially in the Southern Hemisphere, were obtained.


Beyerle, G., K. Hocke:
Observation and simulation of direct and reflected GPS signals in radio occultation experiments, Geophysical Research Letters, in press, 2001

Engeln, A.v., S. Buehler, G. Kirchengast, and K. Kuenzi, 
Temperature profile retrieval from surface to mesopause by combining GNSS radio occultation
and passive microwave limb sounder data, Geophys. Res. Lett., 28, 775-778, 2001.

Foelsche, U., and G. Kirchengast, 
An improved mapping function for the hydrostatic delay at GPS frequencies, Phys. Chem. Earth (A), 26, 153-157, 2001.

Garrison, J.L, G. Ruffini, A. Rius, E. Cardellach, D. Masters, M. Armatys, V.U Zavorotny, 
Preliminary results from the GPSR Mediterranean Balloon Experiment (GPSR-MEBEX), Proceedings of ERIM 2000,
Remote Sensing for Marine and Coastal Environments, Charleston, 1-3 May 2000. 

Hartmann, G.K., G. Kirchengast, A. von Engeln, M.L. Richards, J. Ramsauer,and Ch. Jarchow, 
MAS-GRAS sensor combination and optimal estimation retrieval of temperature and HO profiles, Phys. Chem. Earth (A), 25, 625-628, 2000.

S. B. Healy and J. R. Eyre, 2000, 
Retrieving temperature, water vapour and surface pressure information from refractive-index profiles derived by
radio occultation: A simulation study.
Q. J. R. Met. Soc., 126, pp. 1661-1683.

S.B. Healy, 2001, 
Radio occultation bending angle and impact parameter errors caused by horizontal refractive index gradients in the troposphere: A simulation study. J. Geophys. Res. (In press).

S. B. Healy, 2001,
Smoothing radio occultation bending angles above 40km. Annales Geophysicae (In press).

Hocke, K., A.G. Pavelyev: 
General aspect of GPS data use for atmospheric science, Advances in Space Research, in press, 2001

Igarashi, K., A. Pavelyev, K. Hocke, O. Yakovlev: 
Radio holographic principle for observing natural processes in the atmosphere and retrieving meteorological parameters from radio occultation data, Earth, Planets and Space 52, 893-899, 2000

Kursinski, E.R., S. B. Healy and L. J. Romans, 2000, 
Initial results of combining GPS occultations with ECMWF global analyses within a 1DVar framework, Earth Planets Space, 52, 885-892.

Kursinski and Hajj, 2001, 
A comparison of water vapor derived from GPS occultations and global weather analyses, J. Geophys. Res., 106, D1, p. 1113-1138, anuary 16, 2001.

L.-C. Lee, C. Rocken and E.R. Kursinski Eds., 
Applications of Constellation Observing System for Meteorology, Ionosphere and Climate, Springer-Verlag, Hong Kong, 2001, 384 pp.

Liu, H., X. Zou, R. A. Anthes, J. C. Chang, J.-H. Tseng, and B.Wang,
2000: Extended assimilation and forecast experiments using hundreds of GPS/MET bending angle profiles. J. Geoph. Res., Atmosphere,
(submitted), pp22.

Marquardt C., K. Labitzke, Ch. Reigber, T. Schmidt and J. Wickert, 
An assessment of the quality of GPS/MET radio limb soundings during February 1997, Phys. Chem. Earth, 26, 125-130, 2001.

"A Passive Reflectometry and Interferometric System (PARIS): Application to Ocean Altimetry", ESA Journal, Vol.17, pp. 331-355, Dec 1993.

Martin-Neira et al, 
"The PARIS Concept: An Experimental Demonstration of Sea Surface Altimetry Using GPS Reflected Signals", TGARS Vol.39, No.1,
pp.142-150, January 2001.

Martin-Neira et al, 
"Ocean Altimetry Using the Carrier Phase of GNSS Reflected Signals", CERSAT Bulletin, Issue 11, Scientific Topic 22, November 2000.

Pavelyev, A.G., K. Igarashi, K. Hocke, S.S. Matugov, a.I. Kucherjavenkov, D.A. Pavelyev, O.I. Yakovlev:
 First application of radioholographic method to wave observations in the upper atmosphere, Radio Science, in press, 2001

Rieder, M.J., and G. Kirchengast, 
Error analysis for mesospheric temperature profiling by absorptive occultation sensors, Ann. Geophys., 19, 71-81, 2001.

Rieder, M.J., and G. Kirchengast, 
An inversion algorithm for nonlinear retrieval problems extending Bayesian optimal estimation, Radio Sci., 35,45-56, 2000.

( References for G. Ruffini: (see http://foobar.starlab.net/~giulio)

Ruffini, G., Soulat, F, 
Paris Interferometric Processor Theoretical Feasibility Study part I and Part II. , for ESA's PIPAER-IEEC-TN-1100/2100,
ESTEC Contract No. 14071/99/NL/MM, August 2000. Courtesy of ESA. The authors wish to thank Manuel Martin-Neira for very useful comments. The best site to dowload this is http://arXiv.org/ps/physics/0011027
IGARSS 2000 slides: G. Ruffini(1), J.L. Garrison(2), E. Cardellach(1), A. Rius(1), M. Armatys(3), D. Masters(3) , Inversion of GPSR
Delay-Doppler Mapping Waveforms for wind retrieval, Honolulu, July 2000.

Ruffini, G., J.L Garrison, E. Cardellach, A. Rius, M. Armatys, D. Masters, 
Inversion of GPSR Delay-Doppler Mapping Waveforms for wind retrieval, transparencies of talk presented at IGARSS2000, Honolulu, July

Ruffini, G., 
GNSS-OPPSCAT WP1000 ESA Report : Remote Sensing of the Ocean by Bistatic Radar Observations: a Review here:WP1000.ps.Z The author wishes to thank Pierluigi Silvestrin for very useful comments. Courtesy of ESA, GNSS-OPPSCAT project.

Sokolovskiy, S.V., 
Modeling and inverting radio occultation signals in the moist troposphere, Radio Science, accepted, 2001.

Sokolovskiy, S.V., Tracking tropospheric radio occultation signals from low Earth orbit, Radio Science, accepted, 2001.

Steiner, A.K., and G. Kirchengast, 
Gravity wave spectra from GPS/MET occultation observations, J. Atmos. Oceanic Technology, 17, 495-503, 2000.

Steiner, A.K., G. Kirchengast, and H.-P. Ladreiter, 
Inversion, error analysis, and validation of GPS/MET occultation data, Ann. Geophys., 17, 122-138, 1999.

Steiner, A.K., G. Kirchengast, U. Foelsche, L. Kornblueh, E. Manzini, and L. Bengtsson, 
GNSS occultation sounding for climate monitoring, Phys. Chem. Earth (A), 26, 113-124, 2001.

Wickert, J., R. Galas, G. Beyerle, R. König, Ch. Reigber, 
GPS Ground Station Data for CHAMP Radio Occultation Measurements, 
Phys. and Chem. of the Earth, in press, 2001.

Wickert J., Reigber C., Beyerle G., König R., Marquardt C., Schmidt T., Grunwaldt L., Galas R., Meehan T.K, Melbourne W.G., Hocke K.,
Atmosphere sounding by GPS radio occultation: First Results from CHAMP, submitted to GRL, 2001.

Yunck TP, Hajj GA, Kursinski ER, LaBrecque JA, Lowe ST, Watkins MM, McCormick C, 
AMORE: An autonomous constellation concept for atmospheric and ocean observation, 
ACTA ASTRONAUTICA, 46: (2-6) 355-364 JAN-MAR 2000

Zou, X., F. Vandenberghe, B. Wang, M. E. Gorbunov, Y.-H. Kuo, S. Sokolovskiy, J. C. Chang, J. G. Sela, and R. Anthes, 1999:
A raytracing operator and its adjoint for the use of GPS/MET refraction angle measurements. J. Geoph. Res., Atmosphere, 104, 22,301-22,318.

Zou, X., B. Wang, H. Liu, R. A. Anthes, T. Matsumura, and Y.-J. Zhu, 2000: 
Use of GPS/MET refraction angles in 3D variational analysis. Quart. J. Roy. Meteor. Soc., (in press), pp29.

Zou, X., H. Liu, and R. A. Anthes, 2000:
 A Statistical Estimate of Forward Model Errors and Observational Errors Caused by the Calculation of Radio Occultation Bending Angle from Doppler Shift. J. A. S., (submitted), pp25.

Zuffada, C., G. A. Hajj and E. R. Kursinski (1999), 
A novel approach to atmospheric profiling with a mountain-based or airborne GPS receiver, J. Geophys. Res., 104, D20, 24435-24447, October 27, 1999. 



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