Geodetic remote sensing satellites

Geodetic remote sensing (RS) satellites are orbital spacecraft equipped with a wide range of sensory technology. These systems conduct detailed surveys of the earth's topography, taking into account changes in landmass positioning and elevation, magnetic fields, and seismic and volcanic activity. The use of RS satellite technology in geodetic studies helps scientists generate invaluable data on variations in the earth's geophysical systems, including postglacial rebound, plate tectonics, and ocean circulation. Using the data collected from such technologies, geodesists can create working models of the earth's geophysical characteristics.

Basic Principles and History

In the study of Earth's shape, topography, and physical features and forces (the field known as geodesy), it is often critical to obtain a comprehensive profile before study can begin. Geodesists have increasingly looked to satellite technology to assist them in analyzing a wide range of geodynamic processes and geological events (such as volcanism and earthquakes). The remote sensing (RS) technologies aboard these spacecraft utilize many different types of survey approaches. Among the RS technologies used are laser and radar altimeters, global positioning systems (GPSs), synthetic aperture radar, and satellite-to-satellite tracking systems.

The geodetic RS satellite is rapidly becoming an invaluable tool in the study of geophysics and geodynamics. For example, RS satellites can be used to study fault zones, the movement of Earth's subterranean plates (as identified by the theory of plate tectonics), and postglacial rebound (the outer crust's ability to return to its original position after being weighed down by glaciers). RS satellites use infrared, thermographic, and three-dimensional imaging systems to give detailed surveys that are virtually impossible to conduct from the ground. These systems can focus on even the tiniest changes in the earth's surface, changes that would otherwise go unnoticed by the naked eye.

Background and History

The size, shape, and profile of the earth have for millennia been points of interest and debate among scientists. The fourth century BCE philosopher Aristotle was the first to attempt to calculate the circumference of Earth. About one century later, Greek astronomer and mathematician Eratosthenes employed a triangulation method, using the sun's rays as a point of reference, to determine the planet's circumference. Eratosthenes' triangulation approach would be used through the centuries that followed, as scientists attempted to better understand not only the earth's size but also its shape.

In the sixteenth century, the Royal Academy of Sciences in Paris looked to end the debate over Earth's shape by sending expeditions to the border between Sweden and Finland and to what is now Ecuador in South America. Once on site, the teams studied the north-south curvature of the planet, concluding that the earth was predominantly spherical.

By the early nineteenth century, the United States and other countries began to establish scientific networks to map their respective coastlines and borders. These networks, however, were often disparate, their means for calculating distances unreliable and imprecise. With the advent of the satellite, however, such calculations were made with exceptional clarity and precision.

In the 1960s, the US Army's Ballistic Research Laboratory used satellite positioning to triangulate an area spanning Maryland to Minnesota to Mississippi. Building on that test, researchers were able to connect each mapping network and generate a precise profile of the United States. Satellites are far more advanced now, and they can carry a wide array of technologies. These technologies can provide comprehensive, multidimensional profiles of the entire Earth and monitor changes and events that occur in specific locations.

Geodesy: Active and Passive Sensor Applications

Two general types of remote sensors are used in this arena. The first type of sensor is passive: It simply detects emissions from the target source. An example of this type of sensor is the radiometer, which detects naturally generated microwave energy radiating from within the earth. The second sensor type is active: It sends a signal to the target area and measures the energy based on the return signal. An example of an active remote sensor is radar, which sends a radio signal to the target area and captures the echo.

There is a wide range of applications for remote sensing technologies to the fields of geology, geophysics, and geodesy. One area involves the study of long-term changes to the earth's outer crust and surface (a process known as deformation). In some cases, researchers use onboard, gravitational field sensors to determine the degree of deformation in a given area over a specific time period.

For example, in 2010, scientists utilized the remote sensors aboard the Gravity Recovery and Climate Experiment (GRACE) satellites to study the thickness of the lithosphere (the layer of rock plates continually moving beneath the earth's outer crust). GRACE compiled more than seven years of data, monitoring fluctuations in the earth's gravitational field as it passed over one geographic location (the region comprising Scandinavia and parts of northern Europe and Eastern Europe known as Fennoscandia). Using these data, researchers created one-and three-dimensional models of the lithosphere and determined the approximate viscosity of the layer of molten rock located between the core and the lithosphere, called the mantle.

RS satellites are also useful for tracking crustal movement. According to the theory of plate tectonics, the massive plates that exist beneath the lithosphere are in constant motion, driven both by the heat radiated from the mantle and core and by the downward force of gravity, in tandem with the weight of oceans and glaciers. Under the weight of glaciers, for example, the lithosphere is pushed down (a process called subduction). When a glacier dissipates, the lithosphere slowly pushes back out, causing deformation in the outer crust as the lithosphere returns to its previous position. This is called postglacial rebound. Scientists use evidence of postglacial rebound to analyze the movement of the earth's crust and to understand the geological history of a given area.

In this arena, geodetic RS satellites have proved highly useful. Researchers have used GPS, coupled with altimeters (which use lasers and radar to calculate the elevation of a surface object) and space-based imaging systems, to calculate the speed at which the crust beneath the Antarctic ice sheet is moving.

In some cases, instead of causing the formation of mountains and ridges, crustal deformation causes rock to pull horizontally along the surface. Using GPS, scientists in the Baltic region calculated the rate of horizontal strain occurring along the crust in three separate areas. However, in each area horizontal deformation occurred at different rates. This information led scientists to conclude that horizontal strain in this region is being caused not by one but by several geodynamic processes.

Sometimes, the geodynamic changes occurring on the Earth's surface are accompanied by the emission of energy, such as energy in seismic waves or the release of heat. While it is possible to detect some form of energy emissions from the ground, geodetic remote sensing satellites can capture the sources and volumes with greater precision. For example, in the late 1990s, scientists used the infrared and other thermal imaging systems aboard the National Aeronautics and Space Administration's Moderate-Resolution Imaging Spectroradiometer satellites to classify the different types of land cover on the earth's surface (types including snow, ice forest, and rock). This study helps geodesists analyze the different forms of energy emissions from beneath the outer crust and how they travel through various types of surface cover.

Radar

Like their ground-and air-based counterparts, satellite radar RS systems are active sensors, sending their targets radio waves that in turn bounce back in the form of an echo. Radar has undergone a significant evolution recently, particularly with regard to its ability to provide a clear and precise image of its target. This characteristic is particularly important for geodetic RS satellite applications, because satellites must be both effective from a great distance and able to operate in all types of weather and at any time of day.

One of the most useful radar RS systems in geodesy is the Synthetic Aperture Radar (SAR). This type of radar emits microwaves at the target, illuminating the target in any weather condition and at any time of day. SAR also is beneficial because its signal coherently returns to the system uncorrupted by the sun's rays. Furthermore, SAR can retain all the information it obtains during a flyover. With repeated flyovers, the satellite can therefore conduct a phase comparison. Such a comparison is useful in tracking even minute changes in the position of the earth's surface.

Thermal/Infrared Imaging

Geodesy is often concerned with the differences in radiation emitted from a target. For example, variations in heat along an ice pack indicate different levels of thickness in the ice pack or activity under the ice. In this regard, many satellites used for geodetic surveys will include thermal and infrared sensors alongside other technologies. Thermal imaging also helps scientists understand fluctuations in temperature in the oceans—fluctuations potentially stemming from seismic or volcanic activity or other types of geodynamic phenomena.

Since the 1970s, for example, scientists have used passive thermal and infrared sensors to map the ice pack in the Antarctic. Their studies have also enabled them to effectively map the various degrees of sea-surface temperatures. In addition, scientists use infrared and thermal imaging sensors to record and study temperature variations in the lithosphere, shedding light on changes in plate tectonics in a given area.

Global Positioning System

GPS has become one the most integral components of geodesy. GPS helps scientists pinpoint changes in the earth's crust down to a few millimeters. This level of accuracy has proved highly useful in monitoring crustal movements and seismic activity. For example, Turkey (a country with a long history of severe earthquakes) has a GPS network of nearly two thousand stations throughout greater Istanbul along a major fault line. This intricate network has helped scientists study with greater effectiveness the activity along that fault zone, calculating any horizontal stress that occurs during a seismic event.

The use of the GPS network in geodetic study is not limited to land-oriented applications. Because of its accuracy, GPS also is used by scientists to study the ocean floor. Such studies are difficult because they often use other forms of sensors, mostly those used for determining ocean depth. However, GPS has been known to provide a greater degree of accuracy. Scientists have combined the use of GPS technologies with acoustic positioning systems to create an effective survey system for mapping the ocean floor and monitoring noteworthy changes.

Relevant Groups and Organizations

Because of the implications of using satellite-based RS systems to create detailed geodetic surveys, a number of groups and organizations are involved.

Government agencies frequently play an important role in organizing and funding geodetic surveys. The US Geological Survey (USGS), for example, operates two RS satellites for geodetic surveys. The Landsat 5 and Landsat 7 crafts have helped the USGS develop one of the most extensive remote sensor data archives in the world. Also, in 2011 the National Oceanic and Atmospheric Administration (NOAA) launched its update of the extensive surveys and maps in its vast archives. Using GPS and other passive and active RS equipment based on satellites, the NOAA project updates data that were first compiled in the early 1980s.

With the ever-increasing availability of GPS technology, a large number of university geology, geography, and earth science departments are using geodetic RS satellite technologies. Based at such institutions, scientists are using the data collected by these satellite systems to study volcanic and seismic activity, climate change (such as rising water levels and shrinking ice packs), and plate tectonics.

Geodetic surveys using RS satellite technologies are not limited to academic or government arenas. Data recorded by satellite-borne RS systems are proving highly beneficial for energy companies in search of new oil, gas, and mineral deposits. These companies often use two-and three-dimensional imaging systems, GPS, and other RS systems aboard satellites like Landsat 7 and Spot-5 to locate new sites for exploratory drilling. These companies also use such technologies for risk assessment and management purposes, calling upon them to locate and mitigate oil spills.

Implications and Future Prospects

Geodesy has seen tremendous evolution, thanks largely to the introduction of GPS and other RS technologies on low- and high-altitude satellites. Whereas approaches to geodesy were until the twentieth century somewhat limited, the advent of the satellite and its many uses have enabled cartographers and geodesists to fill in most of the gaps left by ground-based technologies and approaches.

RS satellite systems are far more precise and powerful than those developed even in the mid-to late twentieth century. According to the US Naval Observatory, thirty-two operating GPS satellites along with hundreds of other orbiting spacecraft are now in orbit. Many of the onboard sensors are capable of penetrating thick cloud cover and hundreds of feet of seawater from high orbit. Hurdles for scientists and engineers still need to be overcome. For example, some GPS satellites are negatively influenced by the ionosphere (part of the earth's upper atmosphere), especially during magnetic storms and other disturbances. Scientists continue to seek ways around these hurdles so that geodetic RS satellites can continue to evolve.

GRACE and other satellite programs implemented during the first decade of the twenty-first century have provided a great deal of geodetic data in a relatively short time, inspiring scientists to build even more state-of-the-art RS satellite systems to examine more of the earth's unexplored areas, such as its lithosphere and other parts of the planet's interior. Indeed, the evolution of RS satellite systems and their application to the exploration of the earth is expected to continue into the long term.

Principal Terms

geodesy: the study of the earth's shape, topography, and physical features and forces

global positioning system (GPS): the network established by satellites to navigate and map the earth

lithosphere: layer of large plates believed to be floating on molten rock beneath the earth's outer crust

mantle: superheated layer of molten rock located between the earth's core and the outer crust

plate tectonics: a theory that states that beneath Earth's outer crust there is a series of plates in constant motion and through which magma flows

post-glacial rebound: the process by which the earth's crust slowly returns to its original position after a glacier dissipates or moves away from the subduction zone

subduction: geodynamic process whereby an extremely heavy object located on the outer crust pushes down on the crust and the tectonic plates beneath it

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