Global Positioning System Reference
In-Depth Information
3. Network design parameters
Establishing a network RTK usually starts after a thorough cost/benefit analysis. At the
design stage, the following main factors should be considered:
1.
Baseline lengths (distances between the reference stations) station locations and
network configuration (number and geometric distribution of reference stations).
2.
The communication method between the computing centre and the user.
3.
Calculations required by the network control and by the user (network algorithm).
These factors are discussed in the following sections.
3.1 Distance between the reference stations and network configuration
The main advantage of the network approach is that it improves modelling of the
distance-dependent errors over long distances (El-Mowafy
et al
., 2003 and Euler
et al
.,
2004). The observation corrections (computed as the same value of the errors but with
opposite sign) can be generated after removing cycle slips and determining double
differenced phase ambiguities between the reference stations. A major technical challenge
in NRTK is ambiguity resolution within a reasonable short period over such large
distances between reference stations. In order to achieve a fast and reliable ambiguity
resolution, the distance between the reference stations is better chosen not to exceed 100 km
(Wübbena and Willgalis, 2001). Typically, baseline lengths in NRTK range between 20 km
and 100 km (70 km on average).
In principle, a minimum of three stations is required to generate RTK network corrections,
but in practice this number should not be less than five. The increased redundancy of
reference stations improves positioning accuracy and ambiguity resolution and helps to
sustain network availability and reliability in the case of a temporary failure of any reference
station. However, the degree of redundancy should be evaluated by means of a cost/benefit
analysis, balancing the need to improve the economical aspects of establishing and running
the network and keeping the required degree of redundancy. Hence, selection of baseline
lengths by the network designers should satisfy the following conditions:
•
covering of the whole area of interest with reliable corrections;
•
maintaining sufficient station redundancy;
•
achieving a reliable ambiguity resolution with acceptable confidence level at every
location within the network area as long as a minimum of five satellites are observed;
•
Ensuring reliable communications between the reference stations and the network
centre (mostly via the internet through land lines, and in remote areas through satellite
communication);
•
choosing sites free from multipath and radio frequency interference. It is also preferable
to have the references stations situated at a similar altitude.
In network configuration, the following can be taken into consideration:
•
For a limited number of reference stations, it is recommended to shape the network as a
polygon with one or more central stations.
•
A compact shape of the network is preferable (i.e. a circular network is better than a
rectangular network).
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