GROUND- AND SATELLITE- BASED GNSS MEASUREMENTS OF WATER VAPOUR WITH FOCUS ON CLIMATE APPLICATIONS
Conference contribution, 2014
Water vapour is one of the most important green-house gases and the amount of water vapor in the atmosphere is an important indicator for climate change. Due to its high variability, irregular height distribution and poor mixing with other constituents, accurate measurements of water vapour in the atmosphere are difficult and costly to carry out with high temporal and spatial resolution over long time. The delay of Global Navigation Satellite Systems (GNSS) signals when propagating through the atmosphere due to the presence of water vapour is nearly proportional to the quantity of water vapour integrated along the signal path. Hence, using ground-based GNSS receiver networks it is possible to measure the signal delay in the atmosphere, which in turn can be used to derive the atmospheric water vapour content, the Integrated Water Vapour (IWV). Since time interval measurements can be traceable to standards, it is a promising method for providing an observational system for climate monitoring. However, in order to make a proper uncertainty assessment of the measured water vapour, the different error sources affecting the measurements need to be controlled. GNSS occultation measurements are obtained by deploying GNSS receivers onboard Low Earth Orbit (LEO) satellites. The main observable in GNSS occultation measurements is the Doppler shift of the received signal phase. Based on the Doppler shift the vertical profile of the refractive index can be inferred. From the refractive index measurements, we can estimate temperature, pressure, and the amount of water vapour. A key issue in using occultation measurement for climate purposes lies in the methodology of making the measured parameters traceable to references. Here, we present new methods of using booth ground-based and satellite-based GNSS measurements for climate studies. We have used 6 months of GNSS measurements collected both from ground-based networks and from receivers deployed on satellites.