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 previously exist. Vertical guidance
is a key component to increased safety.
There are varying types of traditional
approaches to an airfield. These include (with decision
height):
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 added 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,
after arriving at the Final Approach Fix, must descend
incrementally rather than following a fixed glide
slope down to the Minimum Descent Altitude (MDA) while
depending on their barometric altimeter. Consequently,
the MDA 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 MDA 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.
Another type of non-precision
approach is termed LP (Localizer Precision). It takes
advantage of the lateral precision associated with
a WAAS-augmented GPS receiver, but cannot implement
vertical guidance because obstacle or ground infrastructure
limitations prevent publication of a vertically guided
approach. An MDA for an LP approach can be as low
as 300 feet.
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 several thousand feet before
the threshold at an average DH of 350 feet. 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
are becoming more 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 GBAS 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. In the United States, these
precision approaches are termed LPV (Localizer Precision
with Vertical Guidance) or LPV-200, with decision
heights as low as 250 feet or 200 feet, respectivelys.
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 a
follow-on approach.
