Global Positioning System Reference
In-Depth Information
(LAAS) implemented only in some major airports. NRTK systems can also be used in
search and rescue operations, emergency landing, road traffic monitoring from the air,
as well as emergency response.
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Compared to LAAS, no significant additional infrastructure cost is involved as the
hardware and software of the GNSS-NRTK are available in most developed countries
and the establishment of new networks is currently underway or planned in different
regions worldwide.
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Network RTK provides cm to decimetre positioning accuracy even in the case of
malfunctioning of some reference stations, particularly for dense networks.
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Network RTK can give better runway utilisation by improving airport surface
navigation. It can also enhance air traffic management by increasing dynamic flight
planning.
The use of the VRS technique in the airborne mode is not generally recommended since in
this high velocity environment continuously updated approximate coordinates have to be
used for the VRS computation. This is similar to having a moving reference station. A
system reset should thus be frequently performed when the VRS coordinates are changing,
which will result in frequent initialisation of the carrier-phase ambiguities. Therefore, it is
preferable to keep the VRS location for the longest possible range. An alternative approach
would be to apply the PRS technique, where the PRS points are chosen along the path of the
final approach and close to and at the airport. Furthermore, the duplex communication
mode used in the VRS technique is limited by the ability of the processing centre to
simultaneously perform calculations for all users. As this number grows, extended latency
in receiving the corrections may result. Additionally, the possibility of signal breaks in the
duplex communication mode is more than the case of using a one-direction communication.
Thus, the use of a one-directional communication method, e.g. applying the FKP method,
would be more appropriate for the airborne mode. The PRS and Mac techniques can also be
implemented in the one-directional mode, whereby the PRS or the Master-Auxiliary stations
are selected to cover a specific area, such as the airport. The establishment of ground
transmitters at the airport can improve availability of the corrections.
The feasibility of using real-time reference networks for positioning in the airborne mode
was examined using the DVRS NRTK over the city of Dubai. Flight tests using a helicopter
and a small fixed-wing airplane were carried out. The trajectory of the fixed-wing aircraft
test is illustrated in Figure 9. The main parameters under investigation were the achievable
accuracy and availability of VRS measurements. In these tests, aircraft positions were
determined using a dual-frequency GPS receiver (Leica SR530). The data were processed in
real time at one-second intervals. The DVRS reference stations collect and process data at
five-second intervals. Thus, the NRTK data were interpolated in time for the rover receiver
to compute positions at the one-second interval.
To assess performance of NRTK approach for this test, the results were compared with
positions determined from a standard double-difference technique whereby the
observations of the aircraft receiver were stored and processed in a post-mission mode. The
aircraft data in this case were referenced to one of the DVRS network stations located within
a range of a few kilometres from the flight route. Precise IGS orbits were used in the post-
mission processing. The differences between the two methods (NRTK and post-mission
processing) are given in Figure 10. For the test at hand, the DVRS data were lost for some
periods, which ranged from a few seconds to three minutes. The periods when the DVRS
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