Global Positioning System Reference
In-Depth Information
able receiver. The vast majority of current SBAS users are not involved with aviation
applications—many low-cost, handheld recreational receivers include an SBAS
reception capability.
8.6.1.3 GBAS
In a GBAS, GPS SPS is augmented with a ground reference station to improve the
performance of the navigation services in a local area. The U.S. FAA's GBAS is
referred to as the LAAS and can support all phases of flight within its area of cover-
age, including the precision approach, landing, departure, and surface movement
[57]. The remainder of this section will focus on the LAAS. LAAS is split up in three
separate segments: the space segment, consisting of the GPS satellites; the ground
segment or LAAS ground facility (LGF); and the airborne segment. Figure 8.39
shows a depiction of the LGF. Pseudorange corrections and correction rates are
computed at the local reference station and broadcast to the airborne GBAS receiver
via a communication link. In the aircraft, the corrections and correction rates are
applied to the local pseudorange measurements and used to obtain an improved
position estimate.
The various LAAS segments are reflected in the LAAS development efforts [58].
RTCA has published Minimum Aviation System Performance Standards (RTCA
DO-245) [59] describing the requirements allocation between ground facility and
airborne avionics and Minimum Operational Performance Standards (RTCA
DO-253A) [60] focusing on the airborne equipment requirements. Finally, the FAA
has published LGF specifications in [61].
Pseudorange Correction Computation
Originally, three LAAS position estimation approaches were investigated [62]:
single frequency (L1) carrier-smoothed code-phase DGPS [63, 64], kinematic dual-
Figure 8.39
LAAS ground facility.
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