IAG/SSG
2.192
"SPACEBORNE GNS ATMOSPHERE SOUNDING"
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:
GNSS RADIO OCCULTATION FOR AIRBORNE SOUNDING OF THE TROPOSPHERE
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:
http://www.uni-graz.at/igam
------------------------------------------------------------
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 :
EMPIRICAL ERROR ANALYSIS OF GNSS RADIO OCCULTATION DATA
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.
-------------------------------------------------------------------------
References:
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.
Martin-Neira,
"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.
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M.J., and G. Kirchengast,
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(
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
2000.
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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