Measuring the scintillation, electron density, and total electron content of the ionosphere is important to both atmospheric research and forecasting the ionosphere's effect on electric power grids and space-based activities, such as communications, manned space flight, ballistic missile early warning, and, potentially, ballistic missile defense. David Anderson discussed global ionospheric specification and forecasting and the Parameterized Real-time Ionospheric Specification Model (PRISM), the primary tool for integrating data from a number of satellite-based and ground-based measurement systems. PRISM forecasts the ion and electron density profile of the ionosphere from 90 to 1600 kilometers altitude on a grid spacing of 2 degrees latitude and 5 degrees longitude. The auroral boundary and altitude of the high latitude ionospheric trough are also identified. The PRISM code is driven with near-real-time data from a network of ground-based GPS receivers as well as other sensors.
Characterizing the properties of the ionosphere using GPS and other sensors will also improve the accuracy of GPS-based navigation. Next to SA, ionospheric delay is the greatest source of error for a single frequency GPS receiver.2 Therefore, the FAA's WAAS is being designed to broadcast ionospheric error corrections to its users. Yi-chung Chao described research at Stanford University to design an accurate model for estimating these corrections. . At the heart of the model is an interfrequency bias calibration algorithm that separates interfrequency bias from the actual ionospheric delay observed by dual-frequency reference station receivers that will be part of the WAAS network.
GPS-based remote sensing produces data about the atmosphere itself as well as data that can be used to increase the accuracy of positioning information used by geophysical researchers. Therefore, the remote sensing working group was comprised of both atmospheric researchers interested in numerical weather prediction, local weather forecasting, climate monitoring, and space weather (ionospheric scintillation, electron density, and total electron content); and geophysical researchers who study crustal deformations related to earthquakes and volcanic activity.
The working group began by discussing opportunities for atmospheric scientists to apply GPS-derived data to their work, which ranges from short-range weather forecasting to research associated with water vapor distributions on global scales and time scales ranging from seasonal to decadal. A ground-based receiver with a collocated meteorological station, which adds perhaps $30,000 to $50,000 to the cost of the installation, can potentially provide continuous measurements of precipitable water vapor with accuracies on the order of 1 to 2 millimeters out of typically measured total values of 20 to 50 millimeters of equivalent water content. Continuous observations have the potential to improve short term, small scale weather forecasts and to monitor climate variables globally over land. As the resolution of North American numerical weather prediction models approaches 10 kilometers, higher spatial and temporal resolution in the initial data fields will be needed. GPS ground-based observations offer this potential.
It also was noted that the global change and climate communities should be apprised, as soon as possible, of the potential opportunities GPS offers. The group agreed that GPS observations should be considered as one of the variables to be measured as part of the Global Climate Observing System.
The group also discussed the need for global measurements for weather, space weather, and climate applications. International cooperation to make ground-based GPS observations acquired in countries outside the United States available to the global climate research and operational weather forecasting communities was suggested as a topic for future study. If the necessary cooperation cannot be fostered, the group decided that space-based approaches, such as GPS/MET, may offer a useful and cost-effective alternative to ground-based observations. However, both ground- and space-based GPS sensing systems must offer cost and performance advantages over existing approaches in order to play an important role in the weather and climate observing system of the future.
Geophysical applications are moving to real-time operational needs, both in the case of ionospheric delay and signal delays caused by tropospheric water vapor. The time delay of a GPS signal due to water vapor is small compared to time delays caused by ionospheric propagation effects. However, water vapor can induce larger errors than those resulting from uncertainties in GPS orbits. Therefore, all sources of error in GPS measurements must be characterized. Corrections based on these errors must be computed quickly and must be accurate. Related data handling and distribution systems must improve accordingly in order to provide better spatial resolution for
Because the total electron content of the ionosphere has large diurnal variations and large variations over the 11-year solar cycle, signal delays for single frequency GPS receivers can vary widely. A typical SPS (Standard Positioning System) receiver has an algorithm that can remove about 50 percent of the ionospheric error, leading to an error ranging from less than 1 meter to 35 meters. However, 7 meters is often used as an average error value.