For any meaningful analysis of geostationary satellite data images, it is necessary to have accurate geographic locations
of each pixel which requires accurate information about the satellite altitude, attitude and scanning geometry. Except
above, in general, users are provided with the information about the count values of satellite image, sub-satellite point
geographic location, image size and scanning direction. Sometimes, the geographic location data is available at coarse
interval which is not adequate for the purpose of geophysical parameter retrieval, validation and other applications. The
problem is further compounded by data dropouts and garbling affecting the automated detection of earth disk required
for deriving geographic locations of the pixels. An automated procedure has been developed to find the geographic
locations of the earth view pixels. For the situations with noisy data and attitude errors in roll and pitch but with correct
and stable sub-satellite point geographic location, methods have been developed for space view noise rejection, earth
disk detection, proper overlaying of continental boundaries and finally for determining geographic locations of desired
pixels. These procedures, except the yaw attitude error, automatically correct other attitude errors for each image. The
basic assumption made here is that the sub-satellite point location is correct and stable. The method is useful in the
absence of any information except the image specifications. Examples of INSAT and METEOSAT images will be
presented.
Radiative transfer simulation based study was carried for developing sea surface temperature algorithms for
ISRO's next geostationary satellite INSAT-3D that will be similar to GOES-9 configuration. Characterization of Indian
tropical marine atmosphere was done by utilising the surface and atmospheric parameters like temperature, pressure and
humidity observed onboard research vessels, covering entire Indian oceans. These parameters were further perturbed in
order to achieve the full temporal and spatial variability in the Indian region. 1392 such atmospheric profiles were
generated as input to the radiative transfer model. Brightness temperatures for INSAT-3D imager and sounder channels
were simulated for these profiles. Various combinations of the channels suitable for sea surface temperature and total
water vapor estimation were considered and depending on the statistical parameters and retrieval errors, daytime and
nighttime SST retrieval equations were finalised. These equations were applied to GOES-9 data over eastern pacific and
the retrieved SST fields were validated with insitu ship observations. The rms error achieved was ~ 0.68 K. Finally SST
retrieval equations were suggested for INSAT-3D. The advantage of frequent sampling by geostationary satellites was
also demonstrated by studying the diurnal variability of SST and improving the cloud free SST fields using INSAT-3A
data. It was found that cloud free fields can be increased to ~ 25% in a day by compositing eight images for that day.
Interpretation of microwave radiometric measurements over land for atmospheric studies requires representative
information about surface emissivity. A simulation study has been performed to derive some of the meteorological
parameters like atmospheric water vapour content over land using TRMM TMI data through assumed land surface
emissivity satisfying the radiative transfer model. The process of minimization of simulated and observed radiation at
TMI frequencies using a large number of simulated atmospheric and surface conditions simultaneously yields many
atmospheric and surface parameters over land. Preliminary analysis of TRMM TMI data over India and adjoining land
region for few days during different season has been carried out. The derived surface emissivity difference of vertical and horizontal polarization is found highly resembling with Quikscat radar backscatter of land surface over the same area
corroborating the estimation of land surface emissivity parameters. The study has importance in synergic use of
microwave radiometer and scatterometer for studying the surface features as well as retrieval of geophysical parameters
over land in view of forthcoming Megha-Tropiques and Oceansat-2 satellites. Typical examples of TRMM-TMI and
Quikscat scatterometer are presented here.
The forthcoming Indian satellite Oceansat-2 to be launched in 2007 will carry a microwave scatterometer and an
ocean colour monitor onboard. The scatterometer, a Ku-band pencil beam sensor similar to that onboard Quikscat
satellite, will provide surface vector winds over global oceans with a two days repetivity. An algorithm for retrieving
wind vector from scatterometer has been developed with a solution ranking criteria of minimum normalized standard
deviation (NSD) of wind speeds derived using backscatter measurements through a geophysical model function
(GMF). Using Quikscat observational geometry and QSCAT-1 GMF, simulation based evaluation of algorithm
performance under different noise conditions and its comparison with standard algorithm known as Maximum
Likelihood Estimator (MLE) algorithm have been performed. Besides having retrieval performance closely
comparable with MLE, the present algorithm has quality and rain flagging provisions. Moreover, it is
computationally efficient with least subjectivity on various retrieval related parameters. These features are equally
desirable for the operational implementation. Results of simulation studies related to retrieval, quality control and
rain flagging along with its implementation to limited Quikscat data are presented.
Emission based radiative transfer simulations have been carried out to study the impact of atmospheric humidity on
clear-sky microwave emissions at various channels of Megha-Tropiques SAPHIR and NOAA AMSU-B sounders.
Detailed investigations reveal that under cool and drier conditions, water vapour channels in the far wing region like
183.31±7 GHz of AMSU-B and others behave like microwave imagers in contradiction to these being sounding
channels. This feature affects their utility for sounding the lower atmosphere. Simulation study confirms that the layer
average relative humidity is retrieved better as compared to its other forms requiring temperature information and has
logarithmic dependency on radiation. Present study deals with development of retrieval algorithms using multi-channel
sounder data for deriving average relative humidity for different layers of the atmosphere. AMSU-B data during June
and October 2002 over Indian region have been used for testing the algorithms to derive relative humidity in three layers
between 300 to 1000 hpa. The satellite derived humidity fields have been compared and found to be in good agreement
with those from NCEP data.
Spaceborne radar scatterometers operating in microwave frequency bands have several science and operational applications in Oceanography, meteorology, agricultural and geophysical sciences. The basic parameter measured by a scatterometer is the Backscattering coefficient (&psigma;^0) for a certain frequency, polarization and observational geometry. Before addressing a specific application, it is needed that the &psigma;^0 signatures be analyzed over natural, undisturbed and uniform/quasi-uniform target areas. As a prelude to ISRO's forthcoming OceanSat-II mission, carrying a Ku-band scatterometer, QuikSCAT scatterometer measured &psigma;^0 data are analyzed over its dynamical range using the global natural targets.
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