Since the Challenger circumnavigated the globe on its oceanographic survey in the mid-1870s, scientists have mounted extensive interdisciplinary expeditions to study large regions of the Earth. The ability to probe the Earth from remote platforms has resulted in numerous scientific advances, such as the revolutionary images of previously uncharted ocean floor that were produced by marine geophysical technology in the 1960s. These images eventually helped to trigger a major paradigm shift in the earth sciences with the development of a fundamental theory of modern geophysics, plate tectonics.
Scientists' understanding of geophysical processes (including lithospheric, oceanographic, and ice sheet processes) has been limited by their inability to make accurate, precisely-positioned measurements. Many of the measurement tools used historically by earth scientists for regional studies are not sufficiently accurate to model physical processes and thereby to improve the understanding of natural hazards and the distribution of nonrenewable resources. Physical barriers, such as inaccessibility by land or the impediment of a hazardous environment, and limited resources that prevent the surveying of large areas by conventional means, pose other difficulties. Maps of geophysical measurements commonly delineate political and physiographic boundaries rather than geological trends. For example, gravity coverage in Africa is sparse in those regions that are politically unstable; this distribution makes the geophysical interpretation of the gravity data within a global framework difficult, if not impossible.
Collecting data from satellites or aircraft can overcome these problems in sampling strategies. Satellites provide global coverage, but they may
not provide data with sufficiently high resolution (i.e., the ability to discriminate features spatially), or they may require long lead times between design, launch, and data acquisition. Airborne platforms provide an attractive alternative for studying the Earth. Airborne magnetic measurements have long been available to the earth science community and are important for mapping regional geology, identifying regional deformation patterns, studying seismically active faults, and finding mineral and petroleum resources (e.g., NRC, 1993). Despite the appeal of airborne measurements, their widespread use for other applications, such as gravimetry and precise topographic mapping, has been hampered by the inability to position aircraft accurately. Measurements of gravity and terrain have little value if the aircraft obtaining these measurements cannot be positioned in three dimensions to better than 10 meters (m) horizontally and 1 m vertically. High-resolution surveys that require closely-spaced flight lines have also been difficult to carry out successfully because of problems with positioning the aircraft.
The development of the Global Positioning System (GPS) in the past decade enhanced the ability both to navigate and to position an aircraft. GPS consists of 24 satellites that can be used to position an aircraft to decimeter accuracy at any point on the globe using differential techniques. Differential techniques exploit the concept that errors in the GPS signal propagation and timing can be corrected by using a receiver in a fixed location as a reference for the moving receiver. Prior to the development of GPS and its commercialization, precise positioning was possible only with a great investment of money and manpower to install a local land-based navigation network. The airborne measurement platform combined with a precise positioning capability such as GPS holds great potential for new insights and new applications in solid earth geophysics. Although great strides are being made in this direction, the full extent of the potential for widespread application has been largely unexplored.
The purpose of this report is to highlight the advances, both potential and realized, that airborne geophysics and precise positioning have made or can make possible to the solid earth sciences. The report first discusses the state of the art in airborne geophysics as integrated with new precise positioning systems, focusing first on the new technology to map topography and the gravity field and then on recent advances in precise positioning. These technologies are described in Chapter 1. Chapter 2 then outlines the scientific goals of a focused effort in airborne geophysics,
including advances in our understanding of solid earth science, global climate change, the environment, and resources. Chapter 3 identifies the technological advances in measurement, positioning, and aircraft design that will be required to aggressively pursue the scientific goals discussed in Chapter 2. The recommendations of the Committee on Geodesy for achieving the goals and directions outlined in the report are presented in Chapter 4.
Summary of Recommendations
At present the geoscience community has insufficient access to airborne technology to fully realize its potential for studying the Earth. Although airborne platforms exist in many agencies and organizations, their use is commonly restricted to employees and affiliated scientists. In other cases, the complexity, expense, and logistical difficulties of conducting an airborne geophysical campaign may deter scientists from fully utilizing this important geophysical tool. There needs to be a dedicated airborne earth science facility, consisting of aircraft, instruments, and personnel, that any scientist can propose to use. Such a facility could be managed by any of a variety of university, government, nonprofit, and/or private-sector coordinating bodies.
Recommendation 1. The new capabilities in precise positioning and accurate navigation should be made more accessible to the user community. An initiative to increase access should include both the establishment of an airborne earth science facility and a coordinated effort at educating its potential users.
Most airborne missions are equipped with only the instruments that are necessary to meet the specialized goals of the operating organization. If these organizations were to coordinate some of their missions, a great deal of additional data could be collected at a relatively small incremental cost. The collection of multiple data sets, each of which is collected for a specific research objective, would foster interdisciplinary research and make better use of the aircraft.
Recommendation 2. Airborne geophysical measurements should be coordinated across disciplines, programs, and funding agencies to promote interdisciplinary research and to optimize use of the aircraft.
Accurate measurements are needed for both long-wavelength regional studies and short-wavelength process-oriented studies. Some of these studies require multiple measurement systems on a single platform. Future technological developments must address all of these needs to ensure that airborne geophysical methods will be practical for the scientific, resource, and environmental industries.
Recommendation 3. To ensure uniform coverage that is sufficiently accurate to resolve both long-and short-wavelength geologic features, technological developments should aim at integrating GPS with a broad spectrum of well-calibrated measurement systems.
The geophysical applications discussed in the report require access to GPS signals that make it possible to locate the aircraft's antenna to a few cm. These applications, as well as future advances in airborne technology, are being hampered by the Department of Defense's implementation of Antispoofing, which encrypts the P-code for reasons of national security. It is possible for the geophysical community to compensate for the effects of Antispoofing, but the necessary methods are expensive or are still in the developmental stage.
Recommendation 4. In light of the serious impact on airborne geophysics, particularly for emerging industrial applications, the continuous operation of the Antispoofing system should be carefully evaluated.