Landings based on GPS will eliminate
many of the time and fuel-consuming maneuvers currently
in use. Additionally, GPS can enable the addition
of vertical guidance to landing scenarios where this
capability did not formally exist. Vertical guidance
is a key component to increasing safety.
There are varying types of traditional
approaches to the airfield. These include:
Non-precision (300-500 feet
above touchdown)
Category I approaches (200
feet above touchdown)
Category II/III approaches
(100 feet above touchdown), and
Missed approaches.
As GPS use has evolved, new approach
categories have been developed to make better use
of satellite navigation and its unique capabilities.
These new approaches are referred to as RNAV approaches.
(RNAV stands for area navigation.) These new types
of approaches include:
Lateral Navigation (LNAV)
approach - similar to the traditional non-precision
approach,
Lateral Navigation/Vertical
Navigation (LNAV/VNAV) approach - similar the traditional
non-precision approach with the addition of vertical
guidance,
GNSS Landing System (GLS) approach,
and
GLS-Precision approach - similar
to traditional Category I approaches.
LNAV (Lateral Navigation)
Lateral navigation is the new
terminology for a GPS non-precision approach. The
approach minimums for LNAV are higher than other types
of area navigation (RNAV) due to the lack of vertical
guidance. Aircraft relying on LNAV instrumentation
must descend incrementally rather than following a
fixed glide slope down to the decision height (DH).
Consequently, the DH for LNAV approaches will, in
most cases, be higher than for most LNAV/VNAV approaches.
In some cases, though, such as when there is an obstacle
close to the runway, LNAV's DH will be lower than
in LNAV/VNAV approaches. Aircraft flying an LNAV approach
descend directly after passing over an obstacle whereas
on flying an LNAV/VNAV approach must continue on its
glide slope.
The LNAV/VNAV terminology was
first used in 1998 to describe situations in which
Flight Management Systems (FMS) avionics were used
for certain specialized approaches. LNAV/VNAV is an
approach in which a vertical glide slope guides the
aircraft to a distance about 3800 meters before the
threshold at an average DH of 350 meters. Given that
LNAV/VNAV approaches slope upwards, approach minima
will be lower than for those for LNAV approaches when
there is a controlling obstacle that is far from the
runway. LNAV/VNAV approaches have a vertical alert
limit, or an accuracy calculation, of 20-50 meters.
With the advent of satellite navigation, and because
the FAA has concluded that vertically guided approaches
are safer than purely lateral approaches, these approaches
will soon become widespread.
GLS
GNSS (Global Navigation Satellite
System) Landing System (or GLS) includes the traditional
Category I precision approaches (PAs) such as those
with instrument landing systems (ILS) technology,
as well as approaches using new WAAS and LAAS technology.
In order to make these precision approaches available
to most airports, there will be two levels of GLS
service. The PA will indicate to pilots that they
can expect to see a precision runway environment when
they break out of poor weather conditions. Airports
must qualify for PA status by meeting general lighting,
satellite, and clear obstruction zone criteria. GLS
PA (and then GLS) will therefore provide the lowest
WAAS minimums available. However, until the augmented
GPS signals reach a vertical alert limit of 12 meters,
satellite navigation will continue to function under
the more stringent vertical alert limit of 50 meters
with LNAV/VNAV approaches.
Missed Approach
When an aircraft is caused to
abort a landing after it has already started its landing
approach, the aircraft has to follow a set path to
leave the airspace surrounding the terminal. GPS adds
more flexibility to its path back out of the terminal
area easing congestion and airspace conflicts with
other aircraft in the area.
Also, GPS can afford the pilot
a more flexible path to navigate back around for the
follow-on approach.
