Study of tropospheric delay over Indian region from MODIS, NCEP/NCAR data and ground based water vapor measurements at Kolkata

Abstract

A priori estimation of delays with error variance of atmospheric radio wave propagation is important for precise position estimation and integrity of the system like Global Positioning System (GPS). Tropospheric effects are less severe than the ionospheric counterpart, but behave in a complex manner, making it difficult to isolate them. In this present paper, a ground based radiometer has been utilized to study the characteristics of tropospheric delay and compared it with the MODIS satellite observations over Kolkata (22.57°N, 88.37°E). Results indicate a good agreement between radiometer and MODIS data except the monsoon months. A climatology of spatial and temporal variation of dry and wet tropospheric delay over the Indian subcontinent have also been estimated using MODIS data and NCEP–NCAR reanalysis data for 2008–2012. The spatial variations of dry delay over Indian region are observed to be in the range of 120–250cm with limited seasonal variability. However, the wet delay varies from 20cm in winter months to 45cm during monsoon period in the coastal and central Indian region. Further analysis reveals that the contribution of wet delays in total delay is significant only along the Indo-Gangetic plain. This indicates that extra precaution is needed in handling tropospheric delay for this region due to fast varying nature of water vapor.