What is RAIM, and why does it matter for GPS navigation?

1-1-19. Ground Based Augmentation System (GBAS) Landing System (GLS)
a. A GBAS ground installation at an airport can provide localized, differential augmentation to the Global
Positioning System (GPS) signal-in-space enabling an aircraft's GLS precision approach capability. Through
the GBAS service and the aircraft's GLS installation a pilot may complete an instrument approach offering
three-dimensional angular, lateral, and vertical guidance for exact alignment and descent to a runway. The
operational benefits of a GLS approach are similar to the benefits of an ILS or LPV approach operation.
Navigation Aids 1-1-39

AIM 2/20/25
NOTE-
To remain consistent with international terminology, the F AA will use the term GBAS in place of the former term Local Area
Augmentation System (LAAS).
b. An aircraft's GLS approach capability relies on the broadcast from a GBAS Ground Facility (GGF)
installation. The GGF installation includes at least four ground reference stations near the airport's runway(s),
a corrections processor, and a VHF Data Broadcast (VDB) uplink antenna. To use the GBAS GGF output and
be eligible to conduct a GLS approach, the aircraft requires eligibility to conduct RNP approach (RNP APCH)
operations and must meet the additional, specific airworthiness requirements for installation of a GBAS receiver
intended to support GLS approach operations. When the aircraft achieves GLS approach eligibility, the aircraft's
onboard navigation database may then contain published GLS instrument approach procedures.
c. During a GLS instrument approach procedure, the installation of an aircraft's GLS capability provides the
pilot three-dimensional (3D) lateral and vertical navigation guidance much like an ILS instrument approach.
GBAS corrections augment the GPS signal-in-space by offering position corrections, ensures the availability
of enhanced integrity parameters, and then transmits the actual approach path definition over the VDB uplink
antenna. A single GBAS ground station can support multiple GLS approaches to one or more runways.
d. Through the GBAS ground station, a GLS approach offers a unique operational service volume distinct
from the traditional ILS approach service volume (see FIG 1-1-9). However, despite the unique service volume,
in the final approach segment, a GLS approach provides precise 3D angular lateral and vertical guidance
mimicking the precision guidance of an ILS approach.
e. Transitions to and segments of the published GLS instrument approach procedures may rely on use of
RNA V 1 or RNP 1 prior to an IAF. Then, during the approach procedure, prior to the aircraft entering the GLS
approach mode, a GLS approach procedure design uses the RNP APCH procedure design criteria to construct
the procedural path (the criteria used to publish procedures titled "RNA V (GPS)" in the US). Thus, a GLS
approach procedure may include paths requiring turns after the aircraft crosses the IAF, prior to the aircraft's
flight guidance entering the GLS approach flight guidance mode. Likewise, the missed approach procedure for
a GLS approach procedure relies exclusively on the same missed approach criteria supporting an RNP APCH.
f. When maneuvering the aircraft in compliance with an ATC clearance to intercept a GLS approach prior to
the final approach segment (e.g. "being vectored"), the pilot should adhere to the clearance and ensure the aircraft
intercepts the extended GLS final approach course within the specified service volume. Once on the GLS final
approach course, the pilot should ensure the aircraft is in the GLS approach mode pr ior to reaching the
procedure's glidepath intercept point. Once the aircraft is in the GLS flight guidance mode and captures the GLS
glidepath, the pilot should fly the GLS final approach segment using the same pilot techniques they use to fly
an ILS final approach or the final approach of an RNA V (GPS) approach flown to LPV minimums. See also the
Instrument Procedures Handbook for more information on how to conduct a GLS instrument approach
procedure.
1-1-40 Navigation Aids

AIM2/20/251/22/26 AIM
FIG 1-1-9
GLS Standard Approach Service Volume