Radar Instrument Research Articles

CMOD5.H—A High Wind Geophysical Model Function for C-Band Vertically Polarized Satellite Scatterometer Measurements

The Advanced Scatterometer (ASCAT) on the MetOp-A satellite is a radar instrument designed specifically to retrieve the ocean surface wind speed and direction. The ASCAT wind vector products are produced and utilized operationally in support of the National Oceanic and Atmospheric Administration (NOAA)'s weather forecasting and warning mission. The standard ASCAT winds at NOAA are produced using the ASCAT wind data processor developed at the Royal Netherlands Meteorological Institute (KNMI) utilizing the CMOD5.n geophysical model function (GMF). Recent validation of the ASCAT wind retrievals revealed a low bias at high wind speeds when compared to both the QuikSCAT winds and the National Centers for Environmental Prediction numerical weather prediction (NWP) model winds. The goal of this paper is to investigate the ASCAT high-wind-speed performance and to modify, as appropriate, the high-wind-speed portion of CMOD5.n GMF. This effort would potentially improve the utility of ASCAT wind retrievals in supporting wind warning and analysis and thus better mitigate the loss of QuikSCAT data products. Traditionally, the GMF is developed empirically by collocating scatterometer measurements and other truth data such as buoy and NWP model winds. However, NWP models are known to underestimate the intensity of higher wind speeds, and data sources such as ship-based or buoy-based observations provide an inadequate quantity of measurements for empirical GMF development. In this paper, a method utilizing aircraft-based scatterometer measurements in the high-wind-speed regimes is used in conjunction with satellite scatterometer measurements to refine the satellite GMF. As a result of this paper, a high wind C-band satellite GMF, CMOD5.h, was developed and implemented in NOAA's ASCAT processor. The validation comparison of the high wind and standard ASCAT wind products revealed 0.6-m/s reduction in the wind speed bias for winds greater than 15 m/s with respect to QuikSCAT, WindSat, and Step Frequency Microwave Radiometer high wind measurements.

Whistler wave-induced ionospheric plasma turbulence: Source mechanisms and remote sensing

We report a series of experiments conducted at Arecibo Observatory in the past, aimed at the investigation of 40.75kHz whistler wave interactions with ionospheric plasmas and the inner radiation belts at L=1.35. The whistler waves are launched from a Naval transmitter (code-named NAU) operating in Aguadilla, Puerto Rico at the frequency and power of 40.75kHz and 100kW, respectively. Arecibo radar, CADI, and optical instruments were used to monitor the background ionospheric conditions and detect the induced ionospheric plasma effects. Four-wave interaction processes produced by whistler waves in the ionosphere can excite lower hybrid waves, which can accelerate ionospheric electrons. Furthermore, whistler waves propagating into the magnetosphere can trigger precipitation of energetic electrons from the radiation belts. Radar and optical measurements can distinguish wave–wave and wave–particle interaction processes occurring at different altitudes. Electron acceleration by different mechanisms can be verified from the radar measurements of plasma lines. To facilitate the coupling of NAU-launched 40.75kHz whistler waves into the ionosphere, we can rely on naturally occurring spread F irregularities to serve as ionospheric ducts. We can also use HF wave-created ducts/artificial waveguides, as demonstrated in our earlier Arecibo experiments and recent Gakona experiments at HAARP. The newly constructed Arecibo HF heater will be employed in our future experiments, which can extend the study of whistler wave interactions with the ionosphere and the magnetosphere/radiation belts as well as the whistler wave conjugate propagation between Arecibo and Puerto Madryn, Argentina.

A Novel Technique for the Automatic Detection of Surface Clutter Returns in Radar Sounder Data

One of the most critical problems that affect the analysis of orbiting radar sounder data is the presence of spurious surface clutter returns. These are due to off-nadir echoes related to surface topography which may be detected as (or mask) actual subsurface targets. The detection of such returns is usually carried out manually through a visual comparison between actual radargrams and surface clutter simulations obtained using available digital elevation models (DEMs). This is an inherently subjective and time-consuming task, which may reduce the scientific return of the data. In this paper, we address this problem by proposing a novel technique for the automatic detection of surface clutter returns in radar sounder data. The proposed method is made up of three steps: 1) the simulation of surface clutter returns using available DEMs; 2) the automatic coregistration between radargrams and simulations; and 3) the extraction of surface clutter returns from the coregistered radargrams. The coregistration step is performed in two phases: 1) a coarse registration based on the detection of the first return line on both input radargrams and 2) a fine registration based on B-spline deformation. The proposed technique is robust to radargram geometric deformations (e.g., due to ionospheric effects) and allows the generation of different types of outputs (e.g., coregistered simulations, binary clutter maps, and false-color compositions) that can both greatly support the scientific community in the manual analyses of radar sounder data and drive the development of reliable automatic methods for high level processing. The effectiveness of the proposed method is proven on two data sets acquired on different areas of Mars by the Shallow Radar instrument.

Modeling the SAR backscatter of linear dunes on Earth and Titan

We modeled the Synthetic Aperture Radar (SAR) response of linear dunes of the Great Sand Sea in Egypt using a single surface scattering term, based on Integral Equation and Physical Optics Models. Using multi-frequency SIR-C/X-SAR radar scenes and topography obtained from the Shuttle Radar Topography Mission (SRTM), we were able to estimate reasonable values for the parameters describing the surface roughness of the dunes. As the linear dunes of the Great Sand Sea are relevant analogs for the linear dunes observed on Titan by the Cassini Radar instrument, these results were thus used as a starting point to simulate the radar response of Titan’s dunes, as imaged by the Radar instrument onboard the Cassini spacecraft during the T8 flyby in October 2005. We show that a single surface scattering term is not sufficient to simulate the radar signal backscattered by the dunes on Titan: one has to add a diffuse scattering term, indicating that Titan’s dunes are likely to have somewhat inhomogeneous internal structures related to porosity and/or internal layering. Our results also indicate that the dunes on Titan should be close to perfectly smooth, possibly because of the formation of smaller ripples than on Earth, plus smoothing resulting from precipitation events.

High-overtone bulk acoustic resonator as passive ground penetrating RADAR cooperative targets

RAdio-frequency Detection And Ranging instruments—RADARs—are widely used for applications aimed at measuring passive target velocity or ranging for various metrology applications such as ground position and localization. Within the context of using piezoelectric acoustic passive sensors as cooperative targets to RADARs probed through a radiofrequency link, this paper reports on investigating the compatibility of narrowband resonator architectures with the classical operation mode of wideband RADAR instruments. Since single mode resonators are hardly compatible due to the limited bandwidth of their spectrum, the investigation has been extended to High-overtone Bulk Acoustic Resonator (HBAR) whose comb of modes appears appropriate to the use with RADAR instruments. This analysis leads to consider HBARs as delay lines providing a comb of echos in the time domain rather than through the usual frequency comb considerations. Experimental measurements of HBAR responses are demonstrated using Ground Penetrating RADAR instruments fitted with a variety of antennas, and thus, operating in various frequency ranges, as well as the identification of the device temperature through the echo time delay computed as the cross correlation maximum position. Finally, the use of such cooperative targets for single reflector identification within a clutter of reflectors is theoretically considered with the proposal of a Finite-Difference Time-Domain-based simulation method encompassing both passive dielectric reflectors and the contribution of buried passive acoustic sensors.

Compact Wideband Corrugated Feedhorns With Ultra-Low Sidelobes for Very High Performance Antennas and Quasi-Optical Systems

The corrugated or scalar feedhorn has found many applications in millimeter wave and sub-millimeter wave systems due to its high beam symmetry, relatively low sidelobe levels and strong coupling to the fundamental mode Gaussian beam. However, for applications such as millimeter wave cosmology, space-based experiments, or even high performance imaging, there is a generic requirement to reduce the size of horns whilst maintaining very high levels of performance. In this paper we describe a general analytic methodology for the design of compact dual-profiled corrugated horns with extremely low sidelobe levels. We demonstrate that it is possible to achieve -50 dB sidelobe levels, over wide bandwidths with short horns, which we believe represents state-of-the-art performance. We also demonstrate experimentally a simple scalar design that operates over wide bandwidths and can achieve sidelobes of better than -40 dB, whilst maintaining a frequency independent phase center. This design methodology has been validated experimentally by the successful manufacture and characterization of feedhorns at 94 GHz and 340 GHz for both radar and quasi-optical instrumentation applications.

Relationship of local incidence angle with satellite radar backscatter for different surface conditions

This paper examines the relationship of C-band radar backscatter from the Advanced Synthetic Aperture Radar on board the ENVISAT satellite with the local angle of incidence, whose influence on the received signal is significant, particularly in the modes of sensor operation that use the full swath of the orbit track. Linear regression is carried out for each pixel throughout a large time series of radar data over the whole of the state of Queensland, Australia, and at Great Salt Lake, Utah, USA. In the first case, the resultant coefficients are analysed for correlation against various parameters, with regolith showing the highest correlation. Class separability analysis shows the potential to use the resultant coefficients as a supplement to absolute threshold values in order to distinguish between classes of vegetation and/or geology, where cloud cover may preclude the use of optical data. It is observed that the separability between water and land is greatly higher using the slope coefficient B than using backscatter σ0, which may be of great benefit in the remote sensing of water where cloud cover is present (from which radar is largely independent). This is especially the case when considering the observed overlapping of backscatter values from water, with values from aeolian sand and lacustrine and alluvial sediments, rendering the use of backscatter alone problematic. In order to test the potential use of B to map water extents, the study over the Great Salt Lake compares the classification accuracy of B against that of σ0. It is found that the σ0 classification misrepresents desert, salt flat and dry lake basin areas, where the B classification differentiates these regions accurately. The resultant classification achieves a kappa statistic around 0.9, which shows very high conformance. An accurate and novel method to classify water is therefore demonstrated, which awaits the launch of anticipated improved synthetic aperture radar instruments on satellite missions in the coming few years.

Radar detection and tracking of German V-2 rocket launches in WW21

For CH, while no useful warning to Londoners could be achieved, the important function of trajectory backtracking to identify V-2 launch points could be made, aiding site attack planning. For the See-Elefant, it could possibly measure V-2 impact points on London, which would aid assessment of V-2 in-flight performance. The former technique is used today by counterbattery radars and the latter by range instrumentation radars.

New Radar Interferometric Time Series Analysis Toolbox Released

Interferometric synthetic aperture radar (InSAR) has become an important geodetic tool for measuring deformation of Earth's surface due to various geophysical phenomena, including slip on earthquake faults, subsurface migration of magma, slow‐moving landslides, movement of shallow crustal fluids (e.g., water and oil), and glacier flow. Airborne and spaceborne synthetic aperture radar (SAR) instruments transmit microwaves toward Earth's surface and detect the returning reflected waves. The phase of the returned wave depends on the distance between the satellite and the surface, but it is also altered by atmospheric and other effects. InSAR provides measurements of surface deformation by combining amplitude and phase information from two SAR images of the same location taken at different times to create an interferogram. Several existing open‐source analysis tools [Rosen et al., 2004; Rosen et al., 2011; Kampes et al., 2003; Sandwell et al., 2011] enable scientists to exploit observations from radar satellites acquired at two different epochs to produce a surface displacement map.

High-Altitude Radar Measurements of Ice Thickness Over the Antarctic and Greenland Ice Sheets as a Part of Operation IceBridge

The National Aeronautics and Space Administration (NASA) initiated a program called Operation IceBridge for monitoring critical parts of Greenland and Antarctica with airborne LIDARs until ICESat-II is launched in 2016. We have been operating radar instrumentation on the NASA DC-8 and P-3 aircraft used for LIDAR measurements over Antarctica and Greenland, respectively. The radar package on both aircraft includes a radar depth sounder/imager operating at the center frequency of 195 MHz. During high-altitude missions flown to perform surface-elevation measurements, we also collected radar depth sounder data. We obtained good ice thickness information and mapped internal layers for both thicker and thinner ice. We successfully sounded 3.2-km-thick low-loss ice with a smooth surface and also sounded about 1-km or less thick shallow ice with a moderately rough surface. The successful sounding required processing of data with an algorithm to obtain 56-dB or lower range sidelobes and array processing with a minimum variance distortionless response algorithm to reduce cross-track surface clutter. In this paper, we provide a brief description of the radar system, discuss range-sidelobe reduction and array processing algorithms, and provide sample results to demonstrate the successful sounding of the ice bottom interface from high altitudes over the Antarctic and Greenland ice sheets.

Analysis, Simulation and Testing of RF Reception System of a RIR Type Radar

This work presents the analysis of an RF (radio-frequency) receiver system which is part of a RIR (Range instrumentation Radar) type Radar. This radar has been unused for over 20 years, which is why a method to validate the correct performance of the reception chain and study modernization is required. The following consist of the simulation process created over LabVIEW systems with the objective of having a mathematical model of the receiver which is used to verify the correct functioning of the system. The results of the simulation are contrasted with testing performed over the system, obtaining very satisfactory results in the analysis of the antenna reception lobes and the process that measures the shifting in the RF axis.

Marketing de Relacionamento e Inovação: Um Estudo em Organizações do Segmento Gráfico Em Natal/Rn

Este trabalho busca analisar, por meio de uma pesquisa quantitativa, realizada através da aplicação de questionários com perguntas fechadas, o uso da dimensão Relacionamento, uma das 13 dimensões que compõem o instrumento Radar da Inovação, método utilizado para mensurar o grau de inovação das empresas. As empresas analisadas, em sua totalidade sete gráficas atendidas pelo Programa Agentes Locais de Inovação do Rio Grande do Norte, não possuem ações eficazes para manter os clientes, conhecem suas necessidades e expectativas com relação ao produto/serviço, contudo não inovaram em práticas para aperfeiçoar e firmar relacionamentos duradouros. Foi possível perceber que os empresários estão absorvendo aos poucos essa cultura de propiciar facilidades e recursos que atendam aos desejos e necessidades dos seus clientes. Conclui-se que as empresas estão abertas a novos conhecimentos, conceitos e mudanças, facilitando o incentivo à cultura da inovação e a novas abordagens que propiciem o crescimento e expansão do setor, por meio de práticas para melhorar o relacionamento com os clientes.

Meteor studies applying incoherent scatter radar instruments

AbstractOne of the interesting aspects of optical meteor studies is that the spectral composition of the brightness reveals information about the element composition of the solid particles that enter Earthfs atmosphere from interplanetary space. Deriving composition from optical spectra requires understanding the entry process during which the entering solid interacts with atmospheric species. This is especially so, because most meteors are observed at altitudes where the mean free path changes from tens of meters to millimeters, that is in the 120 km to 80 km altitude range within the atmosphere. The ionization that causes optical emission also reflects radio waves, so that meteors are observed with different kinds of radar instruments. Incoherent scatter radar facilities are in particular designed to study the upper atmosphere by using the backscattering from free electrons and are basically High Power Large Aperture radars. During the past 15 years they have been increasingly used for meteor studies. The phased-array incoherent scatter radars that are currently under development, such as the planned EISCAT-3Dsystem in northern Scandinavia, will further improve the spatial and time resolution of radar observations and will allow simultaneous measurements of the meteors and of the parameters of the surrounding ionosphere. Radar backscattering is also sensitive to objects that are smaller than those detected optically, so that the observations also permit studying the extension of the meteoroid size distribution to smaller sizes. In this presentation we consider the possibilities for measurements with the future EISCAT-3D as a new path of studying the physics of meteor phenomena with high accuracy.

Evaluation of Clouds and Precipitation in the ECHAM5 General Circulation Model Using CALIPSO and CloudSat Satellite Data

Abstract Observations from Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat satellites are used to evaluate clouds and precipitation in the ECHAM5 general circulation model. Active lidar and radar instruments on board CALIPSO and CloudSat allow the vertical distribution of clouds and their optical properties to be studied on a global scale. To evaluate the clouds modeled by ECHAM5 with CALIPSO and CloudSat, the lidar and radar satellite simulators of the Cloud Feedback Model Intercomparison Project’s Observation Simulator Package are used. Comparison of ECHAM5 with CALIPSO and CloudSat found large-scale features resolved by the model, such as the Hadley circulation, are captured well. The lidar simulator demonstrated ECHAM5 overestimates the amount of high-level clouds, particularly optically thin clouds. High-altitude clouds in ECHAM5 consistently produced greater lidar scattering ratios compared with CALIPSO. Consequently, the lidar signal in ECHAM5 frequently attenuated high in the atmosphere. The large scattering ratios were due to an underestimation of effective ice crystal radii in ECHAM5. Doubling the effective ice crystal radii improved the scattering ratios and frequency of attenuation. Additionally, doubling the effective ice crystal radii improved the detection of ECHAM5’s highest-level clouds by the radar simulator, in better agreement with CloudSat. ECHAM5 was also shown to significantly underestimate midlevel clouds and (sub)tropical low-level clouds. The low-level clouds produced were consistently perceived by the lidar simulator as too optically thick. The radar simulator demonstrated ECHAM5 overestimates the frequency of precipitation, yet underestimates its intensity compared with CloudSat observations. These findings imply compensating mechanisms in ECHAM5 balance out the radiative imbalance caused by incorrect optical properties of clouds and consistently large hydrometeors in the atmosphere.

Edge detection applied to Cassini images reveals no measurable displacement of Ontario Lacus' margin between 2005 and 2010

[1] Ontario Lacus is thus far the largest flat-floored topographic depression of Titan's southern hemisphere interpreted as a permanent or ephemeral lake. From 2005 to 2010, it was imaged several times and at various wavelengths by ISS, VIMS and RADAR instruments onboard Cassini's spacecraft. We analyze the position and uncertainty of Ontario Lacus' margin in all these images using an edge detection method based on image derivation. We find that, given the range of uncertainties in contour locations derived from images, no measurable displacement of Ontario Lacus' margin can be detected between 2005 and 2010 at the actual image spatial resolutions. The discrepancy between this result and previous ones is attributable to differences in (1) the basics behind the methods used, (2) the actual spatial resolutions and contrasts of the available images due to differential atmospheric scattering effects at different wavelengths, and (3) the geomorphological interpretation of contours derived from images acquired at different wavelengths. This lack of measurable displacement in the images suggests that the imaged contour corresponds either (1) to the border of a surface liquid body, provided that potential changes in its extent over five terrestrial years were not sufficiently large to be measured, or (2) to the stationary topographic border between Ontario Lacus' depression and the surrounding alluvial plain. Potential displacements of Ontario Lacus' margin between 2005 and 2010 are thus below the actual resolution of currently available images or have to be sought for within the extent of the topographic depression rather than along its borders.

Morphotectonic features on Titan and their possible origin

Spectro-imaging and radar measurements by the Cassini–Huygens mission suggest that some of the Saturnian satellites may be geologically active and could support tectonic processes. In particular Titan, Saturn's largest moon, possesses a complex and dynamic geology as witnessed by its varied surface morphology resulting from aeolian, fluvial, and possibly tectonic and endogenous cryovolcanic processes. The Synthetic Aperture Radar (SAR) instrument on board Cassini spacecraft, indicates the possibility for morphotectonic features on Titan's surface such as mountains, ridges, faults and canyons. The mechanisms that formed these morphotectonic structures are still unclear since ensuing processes, such as erosion may have modified or partially obscured them. Due to the limitations of Cassini–Huygens in the acquisition of in situ measurements or samples relevant to geotectonics processes and the lack of high spatial resolution imaging, we do not have precise enough data of the morphology and topography of Titan. However we suggest that contractional tectonism followed by atmospheric modifications has resulted in the observed morphotectonic features. To test the possibility of morphotectonics on Titan, we provide in this work a comparative study between Cassini observations of the satellite versus terrestrial tectonic systems and infer suggestions for possible formation mechanisms.

Evaluation of the predicted error of the soil moisture retrieval from C-band SAR by comparison against modelled soil moisture estimates over Australia

The Sentinel-1 will carry onboard a C-band radar instrument that will map the European continent once every four days and the global land surface at least once every twelve days with finest 5×20m spatial resolution. The high temporal sampling rate and operational configuration make Sentinel-1 of interest for operational soil moisture monitoring. Currently, updated soil moisture data are made available at 1km spatial resolution as a demonstration service using Global Mode (GM) measurements from the Advanced Synthetic Aperture Radar (ASAR) onboard ENVISAT. The service demonstrates the potential of the C-band observations to monitor variations in soil moisture. Importantly, a retrieval error estimate is also available; these are needed to assimilate observations into models. The retrieval error is estimated by propagating sensor errors through the retrieval model.In this work, the existing ASAR GM retrieval error product is evaluated using independent top soil moisture estimates produced by the grid-based landscape hydrological model (AWRA-L) developed within the Australian Water Resources Assessment system (AWRA). The ASAR GM retrieval error estimate, an assumed prior AWRA-L error estimate and the variance in the respective datasets were used to spatially predict the root mean square error (RMSE) and the Pearson's correlation coefficient R between the two datasets. These were compared with the RMSE calculated directly from the two datasets. The predicted and computed RMSE showed a very high level of agreement in spatial patterns as well as good quantitative agreement; the RMSE was predicted within accuracy of 4% of saturated soil moisture over 89% of the Australian land mass. Predicted and calculated R maps corresponded within accuracy of 10% over 61% of the continent. The strong correspondence between the predicted and calculated RMSE and R builds confidence in the retrieval error model and derived ASAR GM error estimates.The ASAR GM and Sentinel-1 have the same basic physical measurement characteristics, and therefore very similar retrieval error estimation method can be applied. Because of the expected improvements in radiometric resolution of the Sentinel-1 backscatter measurements, soil moisture estimation errors can be expected to be an order of magnitude less than those for ASAR GM. This opens the possibility for operationally available medium resolution soil moisture estimates with very well-specified errors that can be assimilated into hydrological or crop yield models, with potentially large benefits for land-atmosphere fluxes, crop growth, and water balance monitoring and modelling.

Development of a Global Backscatter Model in support to the Sentinel-1 mission design

The high data throughput of spaceborne Synthetic Aperture Radar (SAR) instruments introduces stringent requirements on the end-to-end system, from the on-board storage capacity and data downlink to processing power and archiving capacity in the ground segment. An efficient raw data compression scheme on-board the satellite is therefore required to maximise the return on investment for SAR missions. The next generation of European SAR satellites, the Sentinel-1 mission, will employ the Flexible Dynamic Block Adaptive Quantization (FDBAQ) compression scheme. In order to validate the implementation of the FDBAQ, expected radar reflectivity levels for the global land surface were needed.The paper presents the development of the Global Backscatter Model (GBM) which has been used to simulate realistic radar reflectivity acquisitions in support of performance validation of the Sentinel-1 FDBAQ implementation. The GBM consists of a parameter database and SAR image simulation software. The model parameter database characterises C-band radar reflectivity as a function of local incidence angle with a resolution of 1km for 85% of the global land surface. The parameter database was derived from a global multi-temporal coverage of ENVISAT Advanced Synthetic Aperture Radar (ASAR) data. The SAR image simulation software uses the parameter database together with orbit propagation and swath determination algorithms in order to account for the instrument acquisition geometry and the location specific backscattering characteristics. The model has the potential to simulate acquisitions for other SAR instruments. Furthermore, the parameter database could be a valuable source of information for global land cover studies.

Observations of Wintertime U.S. West Coast Precipitating Systems with W-Band Satellite Radar and Other Spaceborne Instruments

Abstract The potential of CloudSat W-band radar for observing wintertime storms affecting the West Coast of North America is evaluated. Storms having high hydrological impact often result from landfalls of “atmospheric rivers” (“ARs”), which are the narrow elongated regions of water vapor transport from the tropics. CloudSat measurements are used for retrievals of rain rate R and cloud ice water path (IWP) along the satellite ground track over ocean and land. These retrievals present quasi-instantaneous vertical cross sections of precipitating systems with high-resolution information about hydrometeors. This information is valuable in coastal areas with complex terrain where observations with existing instrumentation, including ground-based radars, are limited. CloudSat reflectivity enhancements [i.e., bright band (BB)] present a way to estimate freezing levels, indicating transitions between rainfall and snowfall. CloudSat estimates of these levels were validated using data from radiosonde soundings and compared to model and microwave sounder data. Comparisons of CloudSat retrievals of rain rates with estimates from ground-based radars in the areas where measurements from these radars were available indicated an agreement within retrieval uncertainties, which were around 50%. The utility of CloudSat was illustrated for case studies of pronounced AR events at landfall and over ocean. Initial analysis of CloudSat crossings of ARs during the 2006/07 season were used for rainfall regime prevalence assessment. It indicated that stratiform rain, which often had BB features, warm rain, and mixed rain were observed with about 26%, 24%, and 50% frequency. Stratiform regions generally had higher rain rates. Significant correlation (~0.72) between mean values of IWP and rain rate was observed for stratiform rainfall.

Performance evaluation of the TRMM precipitation estimation using ground-based radars from the GPM validation network

The Global Precipitation Measurement (GPM) mission is scheduled to fly in the year of 2013 to measure the earth's precipitation structure. Since the precipitation measurement by the GPM platform will be very similar to its predecessor, the Tropical Rainfall Measuring Mission (TRMM), hence, the development of GPM algorithms to improve precipitation retrievals can be addressed through the lessons learnt from the former TRMM mission in terms of precipitation retrievals and its associated uncertainty. To support the future GPM mission, this paper assesses the performance of the TRMM precipitation estimation using ground-based radars from the GPM validation network. A total of 22 significant overpass instantaneous events from 22 different radar sites has been evaluated in view of different surface and rain type flags. The overall results show that attenuation corrected reflectivity from the TRMM precipitation radar agrees well to the measured reflectivity from ground based radars with correlation coefficients r=0.91 (without frequency adjustment) and r=0.92 (with frequency adjustment). However, the correlation decreases by 10–30%, once the reflectivity are transformed to rainfall rates. The lower correlations on the basis of precipitation estimation by the TRMM are exhibited over the coast than those of ocean and land surface terrain. Taking into account the rain type flags, the analysis shows a poor correlation during convective precipitations, in particular, those retrieved from the TRMM precipitation radar. In contrast, the combined algorithm, which utilizes both radar and microwave imager instrument on-board TRMM, outperforms throughout the analysis, yet, there is a scope to improve the precipitation retrievals.