Global Positioning System Reference
In-Depth Information
•
Some geographic regions (i.e. equatorial or high latitude) will require a denser network
than in the mid latitudes due to poorer-satellite geometry, satellite availability, and
ionosphere disturbance etc.
In practice, a main factor affecting the choice of station distances and network configuration
is finding suitable sites for the reference stations. The main considerations are availability of
communication infrastructure and obtaining approval of site owners (whether a
government or a private sector).
It is also possible to integrate observation corrections estimated from networks of different
sizes. For instance, errors of regional or even global nature, such as satellite orbital errors,
clocks and the regional behavior of the ionosphere, which slowly change, can be estimated
from regional networks. The local ionospheric and tropospheric errors on the other hand
can be estimated from local networks. Thus, RTK networks can be configured such that
areas of heavy usage can be covered by a close-meshed reference station network for highest
accuracy and reliability in positioning, whereas less important areas are covered by a wide-
meshed network of regional or national extension (Wübbena
et al
., 2001).
3.2 Communication method between the processing centre and the user
Real-time applications require a communication link between a service provider and the
user. Currently, there are two main modes of communication that can be used in network
RTK; either a duplex (bi-directional) communication or one-direction communication. Each
method has its advantages and disadvantages. In choosing which communication method to
use, the designer has to consider economical factors such as: operational cost by the user,
cost of maintenance of existing infrastructure and/or building new one, and the amount of
computations needed by the rover and the processing centre. The technical aspects that need
to be addressed include:
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expected signal strength at different locations,
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number of users,
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range and coverage (Wu, 2009) ,
•
transmission bandwidth,
•
protocol,
•
reliability and error correction,
•
latency (one second and shorter data transmission latencies are required for cm level
positioning accuracy).
At present, the duplex communication mode is the mostly used method. In this mode, a
cellular modem such as a General Packet Radio Service (GPRS) or Global System for Mobile
Communications (GSM) are used. GPRS is usually preferred as it is more economical than
GSM since the user only pays for the data packets received, not for the entire call duration
when using GSM. GPRS can provide a stable and reliable connection with latencies less than
one second (Hu et al., 2002). The duplex approach has a restriction on the number of users,
as this number is limited by the ability of the NRTK processing centre to simultaneously
perform calculations for all users. This may also result in extended latency in receiving the
network information. For a limited number of users this latency is usually less than three
seconds. On the other hand, the one-direction communication method mainly employs VHF
or UHF broadcasting or encodes the RTK corrections into a broadcast TV audio sub-carrier
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