Global Positioning System Reference
In-Depth Information
spacing will be increase with more frequencies (up to 80-100 kms), but the rover
receiver will improve the reliability of fixing.
•
The tropospheric biases will not be removed by using three or more frequencies. With a
higher number of satellites tracked by a network of reference stations, instead, these
errors will be estimated with a greater accuracy (Zhang & Lachapelle, 2001).
•
Regardless of the GNSS future improvements, the multipath error in the rover will still
be present. However, this error can be modelled on reference stations, giving the benefit
of a more reliable estimation of the other biases.
To achieve a high real time positioning accuracy, in conclusion, a network solution will be
required. The new GNSS constellations will decrease the time-to-fix the ambiguities, for
both the network reference stations and the rover receiver.
A major technical problem of the network positioning is the correction signal broadcasting.
A GPRS/GSM coverage in the survey site is not always available: in these cases, a radio link
between the master and the rover receiver is a common solution. Otherwise, this solution
does not involve the network positioning. A possible solution, especially for large networks,
would be the integration between the NRTK correction and the SBAS architecture. This
integration, in fact, does not require the use of additional antennas but only the payment of
an access fee to the satellite band. Another solution might be to use digital subcarriers of TV
channels. This solution fits very well, for example, with the MAC technique in the RTCM3
format.
On the other hand, the two-way internet communication could allow the network manager
to offer additional services that are not usually provided. It may be possible, for example, to
broadcast the number of satellite with a fixed network ambiguity, the maps on the survey
site, the geoid undulation, etc… At the same time, the network user could transmit
measurement data, updating in real time its survey, in addition to the quality of the data
and of the fixed ambiguities, and to other parameters that can provide useful information
for increase the reliability of GNSS positioning.
6. The experiments
In the previous sections, the network positioning concept and the different correction
strategies were presented. But what is the effective distance between reference stations that
allows to achieve the precision required for real-time positioning, using both geodetic and
GIS receivers? And how the positional accuracy changes with increasing distances between
Continuously Operating Reference Stations (CORSs)? These are only some of the questions
that the experiments reported in the following try to answer.
The experiments were based on three different networks, with different inter-station
distances: the first one (in the following, “red network” or “small network”), with distances
of about 50 kms, is comparable with the existing GNSS networks in Italy. The second
network (“green network” or “medium network”) is characterized by distances of about
100 kms, which is the average spacing of the national geodetic network which materializes
the Italian reference system (
Rete Dinamica Nazionale
- RDN). The last one (“blue network” or
“large network”) has inter-station distances of about 150 kms and it is used to verify the
possibility of use not too thick networks.
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