Global Positioning System Reference
In-Depth Information
signal (Petrovski
et al
., 2001). For VHF broadcasting, allocation of suitable broadcast radio
frequency and obtaining its license is an important issue in the early development of a
network RTK. The main advantage of this method is that there is no restriction on the
number of users concurrently using the NRTK service. However, the main disadvantage of
the method is the high cost of the infrastructure needed to build radio signal repeaters, if
needed, to cover the whole area. In addition, some problems can be experienced due to the
possibility of receiving signals of varying strength in different locations, and possible
frequency jamming. A mix of both communication methods is however possible (Cruddace
et al
., (2002).
The data transmission from the reference stations to the control centre server and from the
control centre server to the user for RTK corrections is mostly carried out via the Network
Transport of RTCM via Internet Protocal (Ntrip), BKG, 2011. Ntrip is an open source and
can be downloaded from the internet (LENZ, 2004). Ntrip was built over the TCP/IP
foundation and is an application
-
level protocol for streaming GNSS data over the internet.
It was first developed by the German Federal Agency for Cartography and Geodesy (BKG).
Ntrip uses HTTP and has three components: Ntrip Client, Server and Ntrip Caster. Ntrip is
designed for disseminating differential correction data (e.g. in the RTCM-SC104 format) or
other kind of GNSS streaming data to stationary or mobile users over the internet. It allows
simultaneous PC, Laptop, PDA, or receiver connections to a broadcasting host. Ntrip
supports wireless internet access through mobile IP networks like GSM, GPRS, EDGE, or
UMTS (BKG, 2011).
To reduce latency, the amount of data transmitted to the rover should be minimised. One
possible solution is to change (optimise) the update rates for the different parameters to
follow their physical behaviour. Distance dependent errors can thus be separated into a
dispersive component, consisting mainly of the ionospheric refraction, and a non-dispersive
component consisting of the tropospheric refraction and orbit errors. Different proposals for
optimising the update rates have been made. An update rate of 15 seconds seems reasonable
for non-dispersive correction differences, while an update rate of only 10 seconds may be
sufficient for the dispersive contribution (Euler
et al
., 2004). However, the impact of these
rates on the Time-To-First-Fix (TTF) of carrier phase ambiguities should be carefully
studied, as it lies at the top of the user interests (El-Mowafy, 2005).
The type of communications used also affects the network algorithm and the amount of
calculations required at the processing centre and by the user. For instance, if a bi-
directional communication is used, the processing centre can individualise the network
information for a user based on his/her approximate location. Thus, the computations made
at the user receiver are minimised. On the other hand, if the data link is one-directional, the
user has to make the necessary interpolation of errors at his location and has to identify a
suitable reference station to use.
3.3 NRTK solution methods
Currently several solution methods can be applied in Network RTK, including the Virtual
Reference Station (VRS), Pseudo-Reference Station (PRS), individualised Master-Auxiliary
corrections (iMAX), Area-Parameter Corrections (Flächenkorrekturparameter -FKP- in its
German origin), and the Master-Auxiliary (MAC) method. In VRS, PRS and iMAX
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