Global Positioning System Reference
In-Depth Information
Differential GPS (DGPS) is a technique whereby the overall performance
locally is improved, typically, by an order of magnitude. A local base station
receives GPS signals and calculates offsets for each satellite with respect to real
position that has been surveyed (this is known, a priori). This set of offsets, or
differentials, is then communicated to local GPS receivers that often have an extra
input for the DGPS service. DGPS can be part of an AGPS service or offered to
specialist users, for example, fishing fleets.
A similar approach is being offered to improve the overall quality of service
by augmentation services. Examples include WAAS in the United States operated
by the Federal Aviation Administration (FAA) and EGNOS in Europe operated by
the European Space Agency (ESA). Networks of ground stations monitor the
GPS signals and calculate corrections for atmospheric disturbances, timing errors,
and other system disturbances and record these so that historic readings can be
improved. EGNOS is the first part of Galileo to be operational.
Land surveying in the construction industry, by mapmakers and geoscientists,
can also use GPS but in a slightly different way. Measurements based on the
phases of GPS carriers known as carrier phase enhancement GPS (or CPGPS) can
provide centimeter accuracy (enough to measure continental drift). This can be
achieved in either real time through delivering correction messages to the user's
receiver via a radio, Internet, and so forth, or in software during postprocessing.
Regional networks are being established that provide dual frequency phase
corrections to user communities, for example, the OS Net™ network in Great
Britain. Although much better than the 0.3m accuracy expected from the military
GPS signals, the trade-off is that it is quite slow and not suitable for guiding
weapons (the main reason why the military encrypts their higher specification
GPS channel).
6.2.6
The Future of GNSS
It is looking likely that by around 2015 there will be at least three fully operational
GNSSs operated by regional powers. GPS III is the planned evolution of GPS
with extra channels for civilian use and enhanced performance (i.e., bringing GPS
up to a similar performance to the E.U.'s Galileo proposals).
The Russian government has committed to upgrading GLONASS until it has
a full working constellation, and discussions on compatibility are ongoing. At the
fall of the Soviet Union the system degraded until coverage was virtually
unusable. The Indian government is cooperating with the upgrade.
Many other nations are participating in the Galileo system, which (at the time
of writing in December 2007) is looking very likely (but not guaranteed). The
original plan for a public-private partnership GNSS failed, and now public funds
will be used to develop the system, ready to be exploited by commercial industry.
Galileo is not a military system and has defined services useful to commerce,
public infrastructures, and the general public. There is a basic unencrypted free
service and then several others aimed at specific user groups. The main difference
