THE PROMISE OF GPS IN ATMOSPHERIC MONITORING

Steven Businger, Michael Bevis, Steven R. Chiswell, and Jingping Duan

University of Hawaii, Honolulu, Hawaii


This page provides an overview of applications of the Global Positioning System (GPS) for active measurement of the Earth's atmosphere. The Global Positioning System consists of a constellation of 24 satellites that transmit L-band radio signals (~19 and 24 cm wavelengths) to large numbers of users engaged in navigation, time transfer, and relative positioning.

There are two primary methods by which GPS can be used to actively sense properties of the Earth's atmosphere. The first technique involves data collected by dual-frequency GPS receivers at fixed locations on the ground (Figure 1). GPS signals are delayed and refracted by the gases comprising the atmosphere as they propagate from GPS satellites to the Earth-based receivers (Figure 2). In particular, a significant and unique delay is introduced by water vapor by virtue of the fact that it is the only common atmospheric constituent that possesses a permanent dipole moment. This dipole moment results from an asymmetric distribution of charge in the water molecule and it retards the propagation of electromagnetic radiation through the atmosphere. Water's permanent dipole moment is also directly responsible for the unusually large latent energy associated with water's changes of phase, which in turn significantly impacts the vertical stability of the atmosphere, the structure and evolution of storm systems, and the meridional and radiational energy balance of the Earth-atmosphere system. Thus, knowledge of the distribution of water vapor is essential to understanding weather and global climate.

The delay in GPS signals reaching Earth-based receivers due to the presence of water vapor is nearly proportional to the quantity of water vapor integrated along the signal path. Measurement of atmospheric water vapor by Earth-based GPS receivers was demonstrated during the GPS/STORM field project to be comparable and in some respects superior to that of ground-based water vapor radiometers (Figure 3 and Figure 4). Increased spatial and temporal resolution of the water vapor distribution provided by the GPS receiver network during GPS/STORM proved useful in monitoring the moisture-flux convergence along a dry line (Figure 3 and Figure 5) and the decrease in integrated water vapor associated with the passage of a midtropospheric cold front (Figure 4 and Figure 6), both of which triggered severe weather over the area during the course of the experiment.

Given the rapid growth in local and regional networks of continuously operating Earth-based GPS receivers, an opportunity exists to observe the distribution of water vapor with increased spatial and temporal coverage, which could prove valuable in a range of operational and research applications in the atmospheric sciences (Figure 7 and Figure 8).



The first space-based GPS receiver designed for sensing the Earth's atmosphere was launched in April 1995. Phase measurements of GPS signals as they are occluded by the atmosphere provide refractivity profiles (Figure 1 and Figure 9). A GPS receiver in low earth orbit with an inclination of at least 65 degrees will provide refractivity profiles from approximately 500 GPS occultations per day distributed over the globe (Figure 10). Water vapor limits the accuracy of temperature recovery below the tropopause, because of uncertainty in the water vapor distribution. However, the sensitivity of atmospheric refractivity to water vapor pressure means that refractivity profiles can in principle yield information on the atmospheric humidity distribution given independent information on the temperature and pressure distribution from NWP models or independent observational data.

A rich array of research opportunities are being actively pursued by the research community to capitalize on the complementary nature of the methods of active atmospheric monitoring by GPS and other observation systems (e.g., Doppler radar, satellite sensors, etc.) for use in weather and climate studies and in numerical weather forecast models.


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The information above was originally published in a poster format. Additional information and copies of this poster are available on a limited basis upon request from Professor Steven Businger. He can be reached at businger@hawaii.edu or at:

Dr. Steven Businger
Department of Meteorology
University of Hawaii
2525 Correa Road
Honolulu, HI 96822