Global Positioning System Reference
In-Depth Information
12.3.1 Surveying
The huge economic advantage of using GPS in surveying applications drove the
development of very sophisticated GPS equipment and tools to predict GPS cover-
age and derive position with centimeter accuracy. Delays in the launch of GPS satel-
lites caused by the Challenger disaster in 1986 further strengthened the head start
that surveying applications got over navigational uses of the system, and significant
refinements in the use of carrier-phase, dual-frequency, postprocessed, differential
positions were made. Extreme accuracy is possible by applying information on satel-
lite positions available after the fact to the data obtained in the field. The value of the
technology in the surveying business stems from the availability of absolute posi-
tions with respect to a universal coordinate system (WGS-84) and from the fact that
they can be determined with a much smaller survey crew. A single surveyor can col-
lect data in the field, where it would take a two- or three-person crew to achieve the
same results using some conventional methods. Collected data can be processed to
the required accuracy using inexpensive computing facilities, and the GPS equip-
ment in the field can be used by the surveyor for rough surveys or the location of
benchmarks or other features. Differential and kinematic techniques can provide
accurate real-time information in the field and obviate the need for postprocessing
the data, further reducing the cost of surveying operations. A great deal of sophisti-
cation has been brought to products in this area, and to a large extent the market is
mature, with a handful of suppliers well entrenched. While the market for simple
surveying by GPS may well be saturated, the use of GPS as an aid for position-based
data collection for geographical information systems (GIS) continues to fuel growth
in the market for sophisticated receivers.
12.3.2 Mapping
A major early implementation of GPS was in the provision of ground truthing, or
orientation of aerial photogrammetry. Aircraft or spacecraft are used to photograph
large areas of the Earth's surface. Index marks are often surveyed on the ground to
provide reference locations on these photographs, which can be used in determining
their scale and orientation. GPS can be used to survey these references. Further, the
use of these references can be eliminated altogether if the position of the camera can
be known accurately enough at the precise moment it took the picture. This technol-
ogy has been developed using GPS augmented by accurate INS. Inertial systems have
excellent short-term stability but tend to drift over time and require recalibration. By
contrast, GPS has its inherent absolute referencing capabilities and can provide
excellent augmentation for an INS. The two can be used together in this kind of
application; the INS to help resolve cycle ambiguities inherent in the kinematic
method of GPS use and to carry positioning duties over the short periods of GPS out-
age that may occur. The generation of road maps, or any other kind of feature map,
is now extremely easy, achieved simply by recording a series of positions as a
receiver is moved over the area to be mapped. Any degree of postprocessing neces-
sary to achieve desired accuracy is available. Specific locations recorded may be
annotated with location-specific information, such as street address, elevation, or
vegetation type. This type of data collection is particularly useful for the building of
data for GIS.
 
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