Global Positioning System Reference
In-Depth Information
Maximum
(cm)
Average
(cm)
S.D
σ
E
3.60
1.92
3.93
2.11
σ
N
σ
h
9.03
4.10
Table 4. Difference between network RTK and post-mission positioning in machine
automation testing
7.3 Using network RTK in the airborne mode
The network RTK approach is mostly used in static or kinematic ground applications. In this
section, the use of the NRTK approach in the airborne mode is discussed. At present,
positioning by GNSS is a widely used technique in the airborne mode for geo-referencing of
aerial mapping data and surveillance by Unmanned Aerial Vehicles (UAV). In aviation, it is
estimated that from 2015, most new commercial aircraft will be fitted with GNSS to enhance
precise navigation and make it safer (Pedreira, 2009). However, at the moment, GPS is the
only approved system as a stand-alone aid for non-precision approaches (Radišić, 2010), e.g.
as a supplementary navigation system and for positioning in non safety-of-life applications.
This is mainly due to the need to achieve high level of performance in terms of integrity,
availability and reliability in the airborne navigation, which GPS on its own cannot reach
due to the limited number of satellites available in one site at any particular instance. This
situation is expected to improve with the addition of the new systems such as Galileo and
Compass. When using network RTK in the airborne navigation, additional concerns have to
be addressed, which include:
•
Due to the high dynamics involved in the airborne navigation, a high update rate of
sending the corrections is needed compared with the rate implemented for land
applications. This rate has a direct impact on the Time-To-First-Fix of phase
ambiguities, and thus on the overall positioning feasibility and accuracy (El-Mowafy,
2004).
•
The format of GPS measurement corrections should be standardised to ensure that the
system is independent of any single receiver manufacturer. The use of the RTCM
Version 3.1 standards is thus recommended.
The main advantages of using network RTK for precise airborne positioning can be
summarised as follows:
•
No dedicated ground reference stations are needed for post-mission or real-time
applications.
•
Unlike standard differential positioning, the distance between the aircraft receiver and
the nearest reference station does not present a concern as long as the aircraft flies
within the network RTK area of coverage.
•
In navigation, due to the fact that networks RTK usually have an area of coverage that
extends to several hundreds of kilometres, each network can cover more than one
airport, including small airports, unlike the current Local Area Augmentation systems
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