Observations of Water Vapor with GPS Reciever, Radiometer and Radiosonde at a Tropical Location. Soiuvik Majumder, Rohit Chakraborty, Saurabh Das, Animesh Maitra # # Department of Radio Physics & Electronics, University of Calcutta S.K. M itra Centre for Research in Space Environment Institute of Radio Physics and Electronics, University of Calcutta # animesh.maitra@gmail.com Abstract— Radio signals transmitted from satellites are refracted, delayed and depolarirized by the ionosphere and the neutral troposphere while they propagate through the atmosphere producing adverse effects in earth-space communication systems and navigational systems. Water vapour, which is a minor constituent of troposphere, is responsible for such propagation hazards. Perceptible water vapour (PWV) serves as a good parameter to quantify the amount of water vapour in the atmosphere and so in this paper we have obtained PWV from GPS observations using retrieval software developed at Kolkata, India for the post monsoon period of September 2012- November 2012. The PWV data are then compared to radiosonde data and radiometric measurements obtained at the same location. A comparison between the data sets show a reasonable extent of matching amongst them indicating the suitability of GPS measurement for futuristic applications. Keywords— Global Positioning System, Global Navigation Satellite System, Precipitible Water Vapour, Radiosonde, Tropical Location, Radiometer. I. INTRODUCTION Navigational systems depend on the propagation effects of signals through atmosphere. The delay imposed by the atmosphere can cause significant errors in precise positioning systems. Aircraft navigation systems need globally interoperable navigational infrastructure which provides safety, efficiency with highest level of accuracy. It is well known to us that the basic atmospheric parameters like temperature, relative humidity are directly affected by very small change in weather condition of tropical region. The main variability in weather conditions arise from various types of precipitation techniques and these variability can have a major contribution to the major navigation hazards at present. Owing to the random variation of climate it is difficult to evaluate a model which can predict perceptible water vapour (PWV) in atmosphere at any given instant of time and hence the only option is to find the method to measure water vapour directly. Hence, the determination of PWV is of prime importance for meteorology, navigation and communication. Global Positioning System (GPS) with its high integrity, continuous availability and reliability has revolutionized the navigation system based on radio ranging. High accuracy can be achieved by correcting the GPS signal with external augmentation system. The use of satellites becomes reliable through the development of this augmentation system like Wide Area Augmented System (WAAS), European Space Based Augmentation System (ESBAS), European Geostationary Navigation Overlay System (EGNOS), Multi- Functional Satellite Augmentation System (MSAS), Quadra Zenith Satellite System (QZSS), COMPASS, BEIDOU etc. India too initiated space-based regional augmentation systems like GPS Aided Geo Augmented Navigation (GAGAN) for civil use and IRNSS for military application. In future, this regional system will act as an essential input to Global Navigation Satellite System (GNSS) to support a broad range of activities in the global navigation sector. The GPS is a very useful tool to study the PWV in a real time basis irrespective of weather conditions. In fact, the positioning error caused mainly by the troposphere serves as a tool to measure the water vapour (PWV). The above mentioned fact makes it a suitable option compared to other techniques such as microwave radiometer (as it fails to provide correct PWV values at heavy rain conditions) and radiosonde (owing to its poor temporal coverage) [1,2]. II. INSTRUMENT AND DATA The GPS constellation consists of 24 satellites inclined at 55 degrees. The satellites are separated into 6 orbital planes with 4 satellites in each plane, with the ascending node of each plane separated by 60 degrees. The satellites orbit at about half of geosynchronous orbit, at near 20,200 km altitude above Earth, giving them a period of about 12 hours. Each satellite transmits coded data on two frequencies, L1 (1575.42 MHz) and L2 (1227.6 MHz). Two distinct codes, Course- Acquisition (C/A) code and Precision (P) code, are superimposed on the carrier frequencies, along with satellite