Radio occultation (RO) measurements are sensitive to the small-scale irregularities in the atmosphere. In this study, we present a new technique to estimate tropospheric turbulence strength (namely, scintillation index) by analyzing RO amplitude fluctuations in impact parameter domain. GPS RO observations from the COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellites enabled us to calculate global maps of scintillation measures, revealing the seasonal, latitudinal, and longitudinal characteristics of the turbulent troposphere. Such information are both difficult and expensive to obtain especially over the oceans. To verify our approach, simulation experiments using the multiple phase screen (MPS) method were conducted. The results show that scintillation indices inferred from the MPS simulations are in good agreement with scintillation measures estimated from COSMIC observations.
The JPL author's copyright for this publication is held by the California Institute of Technology. Government sponsorship acknowledged.
Atmospheric turbulence associated with fluctuation of temperature, humidity,
and water vapor are prevalent in the tropospheric region. Irregularities in
the turbulence cause the index of refraction of the tropospheric medium to
fluctuate. Electromagnetic signals transmitted from Global Navigation
Satellite System (GNSS) satellites (for example), carrying communication and
navigation information, propagate through the turbulent troposphere. The
spatial changes of the index of refraction introduce irregular fluctuations
in the intensity and phase of the traversing electromagnetic signals by
causing scintillation
In this paper, we have employed data analysis and model simulation to
investigate and quantify the effects of tropospheric turbulence on L-band
signals propagating from a GPS satellite to a low-Earth orbit (LEO) satellite
such as COSMIC (Constellation Observing System for Meteorology, Ionosphere,
and Climate)
This section presents investigation of the effect of tropospheric turbulence
on L-band propagation utilizing RO observations from a GPS to a COSMIC
satellites radio links. We estimated the impact of turbulence strength on
L-band signals in terms of scintillation index. During the time frame
relevant to this study, the COSMIC satellites provide a significant number of
RO profiles (up to about 2000 profiles per day) observed by the six
micro-satellites covering the entire globe.
Utilizing the RO profiles, we were able to estimate the
global distribution of the effect of scintillation on GPS signals. The
technique provides valuable scintillation data especially over the oceans
where ground-based measurements are both difficult and expensive to perform.
RO data were first used to determine the intensity and location of turbulent
regions
The basic observations of RO soundings on COSMIC are time series of amplitude
(the 1 s voltage signal-to-noise ratio) and phase of L-band signals
transmitted by a GPS satellite (e.g.,
We note that CT operates under the assumption of a spherically symmetric atmosphere where each ray is uniquely identified by its impact parameter. Thus the presence of mesoscale or large-scale horizontal inhomogeneity can result in fluctuations in the CT amplitudes; however, these tend to occur at a larger spatial scales than the turbulent effects being considered so that their contribution to the scintillation estimates is expected to be small.
The scintillation index
Figures 1 and 2 show global maps of
In the global map of scintillation estimates presented in Fig. 1, the
scintillation index is an altitudinal average of
The scintillation maps (Figs. 1 and 2) contain a wealth of valuable
information. Figures 1 and 2 show similar geographic and seasonal patterns;
however, the values of
Global map of altitudinal average (1–4 km altitudes) scintillation index derived from the COSMIC RO data for January, April, July, and October 2008.
Global map of altitudinal average (4–8 km altitudes) scintillation index derived from the COSMIC RO data for January, April, July, and October 2008.
Histogram of
The effects of the tropospheric turbulence on L-band propagation have a very strong seasonal dependence.
Summer hemispheres show significant turbulent activities (measured by
Figures 1 and 2 show that irrespective of seasons, the tropical regions are characterized by a
relatively large Using The scintillation estimates for the equinox seasons (April 2008, Figs. 1b and 2b; October 2008, Figs. 1d and 2d)
are symmetrical about the equator. Scintillation estimates in the
tropics are fairly symmetrical for all seasons. The troposphere over the Sahara region is characterized by low scintillation effects on L-band propagation
compared to the neighboring regions to its east and west. The water vapor content is consistently low and the air is dry
over the Sahara Figures 1a–d and 2a–d clearly demonstrate an ocean-continent contrast of the scintillation estimates in
the Southeast Pacific, South America, and South Atlantic regions. Over these
regions, the average The turbulence strength difference between summer and winter seasons over the Antarctica is small compared
to the turbulence strength difference between summer and winter seasons over
the Arctic. Similar results based on radiosonde, satellite, and atmospheric
reanalyses observation were reported For all seasons,
The index of refraction structure parameter
The index of refraction structure parameter
Assuming that the wavelength of propagation is small compared to the scale of
index of refraction fluctuations,
The
Figure 4 depicts representative
The merit of the phase screen technique to find solution of electromagnetic
wave propagation through turbulent medium has been described in the
literature (e.g.,
The MPS technique involves finding the solution of the parabolic wave
equation
The main input for the MPS model run is an index of refraction profile of the
lower troposphere. The phase screens are constructed as random perturbations
of an exponential background refractivity profile (first term in Eq. 3):
The input parameters for MPS model runs were as follows: the number of phase screens was equal to 4000; vertical spacing was 1 m; spacing between phase screens was 1 km; a Gaussian random number initializes each phase screen differently; on each phase screen, each altitude is initialized differently.
The MPS model runs were performed for two cases where (a)
For comparison and validation purposes, Fig. 5d plots
We have used (i) radio occultation observations on board the COSMIC satellites and (ii) multiple phase screen model calculations to investigate and quantify the effect of tropospheric turbulence on L-band propagation. Instead of regular amplitude and phase data, we have used the CT amplitudes in estimating the scintillation indices from each occultation. This has the advantage of removing signal fluctuations due to atmospheric multi-path and diffraction from sharp vertical layers.
Global maps of scintillation measures across different seasons were obtained
from 1 year of COSMIC RO data. The resulting global scintillation maps
reveal very strong seasonal dependence, with the northern hemispheric summer
exhibiting relatively large turbulent activities compared to the southern
hemispheric summer. Irrespective of seasons, the tropical regions are
generally characterized by a relatively large scintillation index. The maps
also show clear ocean–continent contrast of the scintillation estimates in
the Southeast Pacific, South America, and South Atlantic regions. The
scintillation estimates appear to be positively correlated with water vapor,
precipitation, and convection. We have also presented
We have also performed numerical simulations of radio propagation through a
random phase changing screen (in which refractivity profiles were specified
by the Kolmogorov spectra). Scintillation index profiles inferred from the
MPS technique are in a reasonable agreement with scintillation index profiles
inferred from COSMIC RO data and provide confidence to our estimates of
The COSMIC radio occultation data used in this study were processed at the
Jet Propulsion Laboratory (JPL) and are available from
Esayas Shume and Chi Ao contributed to COSMIC RO data analysis, multiple phase screen simulation, and writing this paper.
The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The authors would like to acknowledge grant support from NASA ROSES GNSS Remote Sensing Team (NNH11ZDA001N-GNSS). We thank the UCAR COSMIC Data analysis and Archive Center for access to the COSMIC raw data. Edited by: S. Malinowsk