Airborne Geophysics and Precise Positioning Scientific Issues and Future Directions (1995) / Chapter Skim
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3 Future Directions
3 Future Directions
Pages 49-74
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From page 49... ...
With the advent of new technologies for measurement, positioning, and outfitting airborne platforms, aircraft may well become the geophysical research vessels of the next decade. MEASllREMENT AND POSITIONING TECHNOLOGY Airborne Measurements of the Earth's Gravity field Airborne Gravity.
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From page 50... ...
The stabilized platform commonly introduces errors in airborne gravity measurements. The platform is designed to ensure that the sensor is aligned orthogonal to the geoid so that it measures the vertical component of gravity.
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From page 51... ...
Traditional ground, ship, and aircraft techniques measure the vertical component of the Earth's gravity field. New techniques, such as vector gravimetry (Schwarz et al., 1992)
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From page 52... ...
Since the vertical component of gravity couples with the sine of the error in leveling into the horizontal component, every arcsecond of leveling error corresponds to 5 mGal of horizontal gravity error. Therefore, the natural drift of the gyros that orient the platform requires special attention; for example, Northrop used a star tracker to limit the gyro error growth.
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From page 53... ...
The computed gradients (e.g., Gzz is the vertical gradient of the vertical component of the gravity vector) show greater detail than the normal gravity.
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From page 54... ...
For example, current airborne topographic mapping techniques consist of rigidly mounted laser and radar ranging systems integrated with precise attitude and positioning recovery. These systems recover accuracies of about20 cm (e.g., Garvin, 1993)
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From page 55... ...
developed at NASA,s Goddard Space Flight Center has successfully imaged both the tops of the trees and the underlying ground terrain, even in densely vegetated areas (e.g., Harding et al., 1994~. Radar systems have larger footprints and sample at a slower rate than laser systems, but they are generally less hindered by local weather conditions such as cloud cover.
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From page 56... ...
Gamin, NASA/Goddard Space Flight Center and R Williams, U.S.
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From page 57... ...
Top: Raw laser ranges with altitude and attitude corrections for a portion of a CASERTZ profile with an aircraft speed of about 75 m/s. The aircraft elevation is based on the differential carrier phase GPS solution and the aircraft attitude corrections are derived from the inertial navigation system.
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From page 58... ...
58 car ~ 2 ~ ~ c,)
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From page 61... ...
This process measures the phase change due to differences in surface height, surface speckle interference, and viewing geometry between two SAR images collected at different angles. The interferogram is also used to recover topography from separate passes over the same region with either P-band or L-band.
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From page 62... ...
62 34° 45 Airborne Geophysics and Precise Positioning 34° 00" 117° 15' 117° 00' 116° 45' 116° 30' 116° 15' 1 1 1 O 25 50 km FIGURE 3.3 Detail of coseismic displacements near the June 1993 Landers earthquake determined using interferometric synthetic aperture radar techniques. Each cycle of gray corresponds to a 28-mm displacement toward the spacecraft.
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From page 63... ...
Traditional photogrammetry relies on the use of ground control points that can then be used to relate a photograph to a three-dimensional reference system, the so-called external orientation process. With the development of precise aircraft positioning techniques, this expensive ground control can be minimized or even removed.
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From page 64... ...
This section reviews the role of GPS and the present limitations on accuracy for each of these applications. Positioning Technology The standard approaches to positioning with GPS technology, in order of increasing accuracy, are as follows: pseudorange; differential carrier smoothed pseudorange; and differential carrier phase.
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From page 65... ...
Since the fundamental measurement in precise GPS applications is the carrier phase broadcast by the receiver, any phenomenon that delays or modifies its travel time introduces errors into the final position solution. The three major sources of propagation errors are the ionosphere, the troposphere, and multipath.
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From page 66... ...
66 o on .e - ~ 'e p" e an o *
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From page 67... ...
This is a significant, but largely undocumented source of error in the airborne kinematic positioning solution. The loss of signal from a satellite for even a fraction of a second introduces significant errors into a precise carrier phase solution.
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From page 68... ...
The presence of induced errors in timing that result from SA, and the inability of the civilian community to utilize P-code reliably because of AS, requires that a real-time differential solution, based on pseudorange or carrier phase technology, be employed for very precise navigation. Efforts to address these issues are being driven principally by the aviation industry's interest in implementing airport approach systems based on GPS technology (e.g., Hundley et al., 1993; Rowson et al., 1994~.
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From page 69... ...
Advances should focus on overcoming the following problems: the implementation of AS and SA, which seriously limit the positioning and navigation applications of GPS technology; · the source and magnitude of the propagation errors attributed to ionospheric, tropospheric, and multipath effects; · the limitations inherent in current receiver, antenna, and commun cations systems; the kinematic GPS software, which shouts! be improved to enhance efficiency and accuracy of positioning; and · the integration of INS and GPS technologies to meet the stringent requirements of airborne geophysics.
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From page 70... ...
Tropospheric effects can also be a problem under some conditions when the highest possible accuracy is needed, and compensating techniques should be developed. The GPS receivers that are used to position and navigate airborne platforms should be modified to address the following deficiencies: Specialized antennas or processing techniques must be developed to minimize the problem of multipath on aircraft.
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From page 71... ...
Even in the most demanding case, that of airborne vector gravimetry, current high-precision INS provides an adequate platform orientation to demonstrate the concept. AIRBORNE PLATFORMS The four airborne platforms widely used in geophysical research and development can be categorized as follows: discovery mission aircraft; process-driven mission aircraft; developmental aircraft; and experimental aircraft.
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From page 72... ...
Each of these platforms is important in fostering a strong technological base and a large scientific user community. To apply airborne techniques to scientific problems, the research community must have access to airborne platforms.
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From page 73... ...
Access to airborne platforms for technology development is also somewhat restricted. This use typically requires access to an aircraft for significant periods of time and is most easily accomplished within government laboratories or in joint programs between researchers and commercial or government contractors.
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