Global Positioning System Reference
In-Depth Information
NRTK usually requires a minimum of three reference stations to generate corrections for the
network area. In general there is no restriction concerning the network size, it can be
regional, national, or even international. However, reference station separation is usually
restricted to less than 100 km to allow for quick and reliable ambiguity resolution. As the
number of stations increases, redundancy increases, and better corrections can be estimated.
If one or two reference stations fail at the same time, their contribution can be eliminated
from the solution and the remaining reference stations can still provide the user with
corrections and give reliable results (El-Mowafy
et al
., 2003, Hu
et al
., 2003). Typically, a
NRTK server system would consist of the following components (e.g. Leica Geo systems,
2011):
•
A site server connected to each reference station receiver,
•
A network server that acquires the data from the site servers and sends it to the
processing centre,
•
A cluster server that hosts the network processing software. The software performs
several tasks including: quality check of data, apply antenna phase centre corrections,
ambiguity fixing, modelling and estimation of systematic errors, interpolation of errors
(corrections) in some techniques (e.g. VRS, PRS) and generation of virtual observations,
model coefficients in others (FKP), or Mac data.
•
A firewall is usually established to protect the above servers from being accessed by a
user.
•
RTK proxy server to deal with requests from the users and send back network
information.
•
The user interface to send/receive data from the NRTK centre.
The main advantages of the Network RTK can be summarised as follows:
•
Cost and labour reduction, as there is no need to set up a base reference station for each
user.
•
Accuracy of the computed rover positions are more homogeneous and consistent as
error mitigation refers to one processing software, which uses the same functional and
stochastic modelling and assumptions, and use the same datum.
•
Accuracy is maintained over larger distances between the reference stations and the
rover.
•
The same area can be covered with fewer reference stations compared to the number of
permanent reference stations required using single reference RTK. The separation
distances between network stations are tens of kilometres, usually kept less than 100
km.
•
NRTK provides higher reliability and availability of RTK corrections with improved
redundancy, such that if one station suffers from malfunctioning a solution can still be
obtained from the rest of the reference stations.
•
Network RTK is capable of supporting multiple users and applications.
Network RTK has though some disadvantages, which are:
•
The cost of subscription with a NRTK provider.
•
The cost of wireless communication with the network (typically via a wireless mobile
using for instance GPRS technology).
•
The dependence on an external source to provide essential information.
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