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Singapore, Singapore

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Singapore, Singapore
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Lee Y.H.,Nanyang Technological University | Natarajan V.,Nanyang Technological University | Meng Y.S.,Agency for Science, Technology and Research Singapore | Yeo J.X.,Nanyang Technological University | Ong J.T.,C2N Pte. Ltd.
IEEE Antennas and Propagation Society, AP-S International Symposium (Digest) | Year: 2014

In this paper, preliminary results and analysis of cloud attenuation on a Ka-band satellite propagation link in tropical island of Singapore are reported. The received signal by a ground-based beacon receiver at 18.9 GHz was processed with a sliding window technique to minimize the system and scintillation effects. The results show that the Ka-band cloud attenuation is highly correlated with the observed solar radiation. © 2014 IEEE.


Zhou X.X.,Nanyang Technological University | Lee Y.H.,Nanyang Technological University | Teong O.J.,C2N Pte. Ltd
Asia-Pacific Microwave Conference Proceedings, APMC | Year: 2010

Hourly rainfall rate data recorded in Singapore over 30 years are processed for analysis. The results obtained from the analysis of diurnal variations of rainfall rate and rain attenuation are presented in this paper. The diurnal cycle of monthly rainfall frequency is presented. From the results, it can be concluded that for the different seasons in Singapore, there are significant variations in diurnal cycles. Afternoon showers are highly frequent throughout the year, especially during the Northeast monsoon season. Early morning rainfall is observed during the pre-monsoons and the Southwest monsoon seasons due to the Sumatra. Since earth-space communication systems working in the Ku and Ka bands are severely affected by rainfall, this paper presents the effects of rain attenuation for different seasons in the year for tropical countries such as Singapore. Analysis of the diurnal variation of rain attenuation shows that the hour with the highest rain attenuation in a season might not be the hour with the most frequent rainfall. © 2010 IEICE Institute of Electronics Informati.


Kumar L.S.,Nanyang Technological University | Lee Y.H.,Nanyang Technological University | Yeo J.X.,Nanyang Technological University | Ong J.T.,C2N Pte. Ltd.
Progress In Electromagnetics Research B | Year: 2011

A Z-R relation is derived using a data set which consists of nine rain events selected from Singapore's drop size distribution. Rain events are separated into convective and stratiform types of rain using two methods: the Gamache-Houze method, a simple threshold technique, and the Atlas-Ulbrich method. In the Atlas-Ulbrich method, the variability of the rain integral parameters R; Z, Nw, D0 and gamma model parameter μ are used for the classification of rain into convective, stratiform and transition. Z-R relations are derived for each type of rain after classification. The changes in the coefficients of the Z-R relations for different rain events are plotted and analyzed. The Z-R relations of the different methods using the Singapore data are compared and analyzed. It is concluded that the coefficient A of the Z-R relation is higher for the convective stage followed by the stratiform and transition stages. The coefficient b values are higher for the transition stage followed by the stratiform and convective stages. Reflectivities are extracted from RADAR data above NTU site for rain events and compared with the reflectivities derived from the distrometer data. Rain rates retrieved from RADAR data using the proposed relations from Singapore's data set are compared with the distrometer rain rates. The RADAR extracted rain rates are found to be constantly lower than the distrometer derived rain rates but matches well.


Kumar L.S.,Nanyang Technological University | Lee Y.H.,Nanyang Technological University | Ong J.T.,C2N Pte. Ltd.
IEEE Transactions on Geoscience and Remote Sensing | Year: 2011

Gamma model is fitted using the second, fourth, and sixth moments to model the rain drop size distribution (DSD) of Singapore. As the Joss distrometer measures the number of rain drops between the drop diameters from 0.3 to 5 mm, the truncated moment fitting between these drop diameter ranges is also used for modeling the DSD. Gamma DSD requires three-parameter estimation: N0, the intercept parameter; μ, the shape parameter; and Λ, the slope parameter. The aim of this paper is to find a suitable fixed μ and derive an appropriate μ-Λ relation for the tropical region in order to form a two-parameter gamma model. To find an appropriate μ value, observed DSDs are fitted with different μ values to estimate the rain rates, which are assessed by rain rate observations of the distrometer. Shape-slope relationships are fitted for different categories according to the rain rate and the number of drops. The derived μ-Λ relationships for the Singapore region are compared to the published results from two other regions, and the analysis is presented. Two-parameter gamma models are compared by retrieving the rain rate using the polarimetric radar variables. The effect of truncation on rain rate retrieval is also studied, and the use of the μ-Λ relationship for rain retrieval is recommended for the tropical region. The μ-Λ relation using the truncated moment method for the rain category R ≥ 5mm/hr and rain counts ≥ 1000 drops retrieves the rain rates well compared to other μ-Λ relations. © 2011 IEEE.


Kumar L.S.,Nanyang Technological University | Lee Y.H.,Nanyang Technological University | Ong J.T.,C2N Pte. Ltd
IEEE Transactions on Antennas and Propagation | Year: 2010

A model that is less sensitive to errors in the extreme small and large drop diameters, the gamma model with central moments (3, 4 and 6), is proposed to model the rain drop size distribution of Singapore. This is because, the rain rate estimated using measured drop size distribution shows that the contributions of lower drop diameters are small as compared to the central drop diameters. This is expected since the sensitivity of the Joss distrometer degrades for small drop diameters. The lower drop diameters are therefore removed from the drop size data and the gamma model is redesigned for its moments. The effects of the removal of a particular rain drop size diameter on the specific rain attenuation (in dB) and the slant-path rain attenuation calculations with forward scattering coefficients for vertical polarization are analyzed at Ku-band, Ka-band and Q-band frequencies. It is concluded that the sensitivity of the Joss distrometer although affects the rain rate estimation at low rain rates, does not affect the slant path rain attenuation on microwave links. Therefore, the small drop diameters can be ignored completely for slant path rain attenuation calculations in the tropical region of Singapore. © 2006 IEEE.


Yuan F.,Nanyang Technological University | Lee Y.H.,Nanyang Technological University | Meng Y.S.,Agency for Science, Technology and Research Singapore | Ong J.T.,C2N Pte. Ltd.
International Geoscience and Remote Sensing Symposium (IGARSS) | Year: 2015

A new method is proposed in this paper to determine the cloud vertical structure using water vapor pressure estimated from radiosonde profile. The presence of a cloud depends on the following criteria: the measured water vapor pressure being larger than the critical water vapor pressure at the same level. The estimated results of cloud vertical structures using the proposed method are compared with the Salonen and Uppala model and ceilometer (CL31) data. Results show good agreement between the proposed model, the existing model and the measured data. © 2015 IEEE.


Kumar L.S.,Nanyang Technological University | Lee Y.H.,Nanyang Technological University | Ong J.T.,C2N Pte. Ltd.
Progress In Electromagnetics Research B | Year: 2010

Gamma model parameters using 2nd, 3rd and 4th moments are calculated from the drop size data of Singapore. The gamma model is simplified into two parameter model by finding a relation between the shape and slope parameters, μ and Λ. Due to the poor correlation found between μ and Λ, the drop size data is filtered based on their rain rates before a good correlation between the two parameters can be found. The μ-Λ relations are then fitted for the different ranges of rain rate filtering. Scatter plots of μ and Λ are plotted with constant median volume diameter (D0) lines. The μ-Λ relations for the different rain types for the tropical island of Singapore are proposed and compared with the μ-Λ relations from three other countries of different climatic zones. T-Matrix calculations are performed to find the polarimetric variables at S-band by using the gamma DSD calculated from the Singapore's drop size data. The calculated differential reflectivity and horizontal reflectivity are used along with the best μ-Λ relations to find the gamma model parameters. The retrieved rain rate using polarimetric variables is compared with the distrometer's measured rain rate. Results show a good agreement between the retrieved rain rate and the measured rain rate. Therefore, the proposed shape slope relationship is found to be suitable for rain rate retrieval.


Yeo J.X.,Nanyang Technological University | Lee Y.H.,Nanyang Technological University | Ong J.T.,C2N Pte. Ltd.
International Journal of Satellite Communications and Networking | Year: 2015

Site diversity is an effective rain attenuation mitigation technique, especially in the tropical region where high rainfall rates are common. According to our previous study, site diversity gain is found to be dependent on the site separation distance and path elevation angle while it is independent of signal frequency, baseline angle and polarization angle. Therefore, using 28 months of radar data, a simple site diversity gain prediction model is proposed. The proposed model is compared with the existing ITU-R models. The seasonal wind direction, another factor that might affect the site diversity gain prediction model, is shown to have negligible effect on site diversity gain. Triple-site diversity is also investigated, and although it is found to provide gain improvement over dual-site diversity, this gain improvement is too little to justify for the cost of setting up an additional site. This study is useful for the implementation of site diversity as a rain attenuation mitigation technique in the tropical region. Copyright © 2014 John Wiley & Sons, Ltd.

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