The demand for improved weather forecasts has a solid economic basis. The air transportation industry is a case in point. Domestic air traffic is projected to grow 3% annually over the foreseeable future and international traffic is growing 6% annually. This growth puts continuing pressure on the capacity of the Air Traffic Control System (ATCS), particularly in the transoceanic routes. Overall efficiency and safety are inevitably affected by shrinking capacity margins. Aviation safety, capacity, and efficiency are all intimately linked to the weather. From 1970 to 1985, 40% of all scheduled airline accidents were weather related. Sixty-five percent of annual ATCS delays are attributable to weather and account for nearly two billion dollars annually in direct costs to the airline industry. More accurate forecasts of aviation weather are of critical importance in seeking to improve the economics of aviation operations.
Many segments of the economy besides aviation benefit from improved weather forecasts. Accurate forecasts will improve productivity in the land and maritime transportation sectors, as well as in agriculture, water and energy resource management, and other sectors of the economy. Improved forecasts and severe weather warnings of such events as hurricanes, tornadoes, flash floods, and hail would provide great benefit in reducing loss of life, injury, and property damage. Thus, improved forecasts and warnings would have a direct positive economic impact on the insurance industry, and indirectly for the general population.
Although GPS/MET soundings have the potential to enhance the analysis and prediction of weather (and climate), significant research is required to assess this potential in detail and develop capabilities to derive maximum benefit from this new source of atmospheric data. Fundamental studies are needed to quantify the accuracy of retrieved temperature and moisture profiles for the spectrum of real atmospheric conditions that will be encountered. Ultimately, we need to determine how these data can be most effectively used in weather forecast models, and to further enhance the accuracy and resolution of the data by combining these models with the retrieval process. These studies are a critical part of the feasibility assessment emphasized in the GPS/MET Program.
The clearest global change signal expected is change in the global mean temperature. Detection of the expected global warming is confounded by flawed and patchy observations, by natural variability which adds climatic noise to the system, and because observed climate change is not geographically uniform. Therefore, it has not yet been possible to firmly establish that the observed warming is due to greenhouse gas increases.
GPS/MET data could be valuable for climatological studies by providing an accurate way to monitor long term trends, particularly in the upper troposphere and stratosphere. GPS/MET is particularly applicable for global change research because it has the potential to provide accurate, all-weather monitoring through clouds and aerosols on a global scale. In contrast with current nadir viewing radiometer data, GPS/MET soundings could provide relatively high accuracy and vertical resolution for temperature measurements, particularly in the tropopause and lower stratosphere regions.[5] Because GPS/MET relies on active rather than passive methods, the method also has advantages over limb viewing radiometers (e. g., higher measurement S/N).
Another very important indicator of global climate change is an expected increase in water vapor. Because water vapor is a strong greenhouse gas, it has feedback effects that can substantially enhance the original perturbations. The greenhouse effect of water vapor depends most critically on the upper troposphere water vapor that radiates to space. Solid information on water vapor distribution and trends will help validate climate models and enable questions concerning feedback effects of water-vapor and cloud formation to be addressed better than they can be now.
The loss of ozone in the stratosphere is yet another global change issue. The most pronounced example of this is the emergence of the ozone hole over the high latitudes of the Southern Hemisphere in the southern spring. Stratospheric temperatures play a very important role in this process because the polar stratospheric ice cloud particles, which play a key role in facilitating the chemical reactions leading to ozone depletion, form mainly in very low temperatures. Thus, stratospheric cooling from increases in greenhouse gases may exacerbate ozone losses. GPS/MET soundings could provide the needed high vertical resolution measurements of stratospheric temperature to improve understanding of ozone dynamics.
Climate Change has also been linked to volcanic activity. An injection of material from a large volcanic eruption can result in massive amounts of aerosols in the lower stratosphere, which can significantly limit satellite IR observations of this and lower regions. However, because these aerosols change the radiative forcing of the atmosphere, this is precisely the time and place where accurate observations are needed to determine how the atmospheric thermal structure is changing to achieve overall radiative energy balance. Unlike passive IR radiometers, the accuracy of GPS/MET observations will be relatively unaffected by these aerosols.
For most climate research, the real payoff from the GPS/MET soundings will occur when sufficient high quality homogeneous data is accumulated to see small changes over climatic time scales. For climate change questions, the absolute accuracy of the measurements is less important than consistency and absence of changes in bias. Inhomogeneities arising from changes in satellites and instrumentation bias have been the rule with satellite data in the past. Unlike most other instruments, which rely on amplitude measurements, the stability of GPS/MET observations is fundamentally based on frequency measurements which are referenced to the ultra-high stability frequency standards (5 X 10- 13) inherent in the GPS network. Even a few years of GPS/MET data may be sufficient to bring rewards in addressing some key climate questions.
In short, more accurate and consistent long-term measurements with global coverage and high spatial resolution are needed to help define the short-term variability and long-term trends of temperature and water vapor on a global basis. GPS/MET could provide an absolute standard and/or high stability calibration system for other temperature monitoring systems, thereby satisfying the requirement for long-term continuity and stability in global monitoring.
GPS/MET also will provide an opportunity for global mapping of the ionosphere with sufficient temporal and spatial resolution to investigate many important dynamic processes in the ionosphere/thermosphere system, and their relation to processes in the atmosphere and solid Earth. For example, data from GPS/MET could be useful in the study of gravity waves which transport energy and momentum through the neutral atmosphere and ionosphere. Tracing this phenomenon may be possible by mapping the Total Electron Content (TEC) along the ray path between the LEO and GPS satellites. It has been estimated that the accuracy and precision of TEC derived in this way will approach 1015 and 1014 e/m2 respectively.[6] This accuracy corresponds to ~ 0.1 % of the daytime zenith peak in TEC. Information on global energy transport detected in this way could add further to the primary science objectives of global change research and weather forecasting.
Adding a LEO GPS receiver to the growing network of "fiducial sites" used for differential GPS will improve the accuracy of all position fixes, in ways not possible with an exclusively Earth-bound fiducial network. This will occur primarily due to the improved GPS orbits obtained when the "geometry" of a fiducial network includes an orbiting receiver. For example, as shown by R. P. Malla et. al., a detailed covariance analysis indicates that GPS will be more effective in the study of crustal deformation "...when data collected from low Earth-orbiting satellites as well as from ground sites are combined, enhancing the accuracy and resolution for measuring high frequency geophysical signals over time scales less than one day."[7]
It should also be noted that conventional passive space instruments might be able to provide improved results if GPS/MET data were combined with those data.