Geoscience Reference
In-Depth Information
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Accept user commands and display results via control unit or a PC
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Record the data for postprocessing (optional)
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Transmit the data to another receiver via radio modem for real-time solutions (optional)
An important characteristic of the RF section is the number of channels and hence the number
of satellites that can be tracked simultaneously. Older receivers had a limited number of channels
(even as little as one), which required sequencing through satellites in order to acquire enough
information for 3D positioning. Modern GPS receivers are based on dedicated channel architecture,
where every channel tracks one satellite on L1 or L2 frequency.
9.11 GpS ModeRnIzAtIon And otheR SAtellIte SySteMS
The current plan of GPS modernization is focused on improving the quality of civilian uses of GPS,
primarily through the implementation of a new L2C code on L2 frequency and a new civilian signal
on L5 frequency of 1227.6 MHz (Spilker and Van Dierendonck, 2001). In addition, a new M-code
(encrypted) has been implemented exclusively for military use, ensuring that military and civilian
users will have entirely separate signals and codes. Consequently, AS policy will be abandoned.
(SA has already been turned off.) The new dual-frequency civilian tracking capability is available
on the Block IIR-M GPS satellites (currently, five satellites in orbit, with the most recent launch of
Decembr 20, 2007), and more improvements are planned for Block IIF satellites. As of February
2008, the first GPS Block IIF launch was projected by 2010 (http://facility.unauco.org/science_
tech_gnss_modernization.html) . The GPS Block IIF spacecraft represents the next-generation GPS
system designed to meet both military and civil customer requirements. Among other features, the
GPS Block IIF spacecraft will feature extended design life up to 15 years, modular design, 3 m
spherical error probable (CEP), options for L5 civil signals, increased autonomy from ground seg-
ment, and rapid on-orbit reprogrammability (www.deagel.com/pandora/?p=pm00371004).
In terms of GPS instrumentation, hardware miniaturization, further development of soft-
ware receivers, improvements in reliability, faster sampling rates, lower noise and more multipath
resistance, more real-time operations, and ubiquitous dual-frequency measurement capability are
expected (Rizos, 2002c). The current trend of integrating GPS with other sensors such as inertial,
vision systems, laser scanners, and pseudolites will continue to serve more specialized, customized
applications. It is expected that the increasing number of CORS-based local services will serve
real-time users.
Other GNSS that are complementary to GPS are the Russian Glonass system originally developed
for the Russian military, and the European Galileo system (civilian), currently under implementation
(first satellite was launched on December 28, 2005). Glonass became operational with a full twenty-
four-satellite constellation in 1996. However, as of February 2008, the number of operational satellites
is sixteen, with the most recent launch of December 25, 2007, where four new Glonass satellites were
placed in orbit. Still, the system's long-term stability might be questionable. Nevertheless, GPS/Glonass
receivers, which take advantage of the extended constellation created by the additional satellites in view,
were developed. For more information on Glonass and Galileo, the reader is referred to http://www.
gpsworld.com/; http://www.insidegnss.com/.
9.12 GpS MAppInG pRojeCt And ConneCtIon
to AGRICUltURAl GeophySICS
Modern geophysics uses a variety of sensors and techniques for exploration, probing, and mapping,
in applications such as mining/petroleum exploration, environmental studies, engineering investiga-
tions, and agricultural field assessments. The geophysical techniques presently used most often for
agricultural purposes include resistivity, electromagnetic induction, and ground-penetrating radar.
