Figure 7-33. LAAS Representation.

INS Components
INS is considered a stand-alone navigation system, especially
when more than one independent unit is onboard. The
airborne equipment consists of an accelerometer to measure
acceleration which, when integrated with time, gives
velocity and gyros to measure direction.

Later versions of the INS, called inertial reference systems
(IRS) utilize laser gyros and more powerful computers;
there lore, the accelerometer mountings no longer need to
be kept level and aligned with true north. The computer
system can handle the added workload of dealing with the
computations necessary to correct for gravitational and
directional errors. consequently, these newer systems are
sometimes called strap down systems, as the accelerometers
and gyros are strapped down to the airframe, rather than being
mounted on a structure that stays fixed with respect to the
horizon and true north.

INS Errors
The principal error associated with INS is degradation of
position with time. INS computes position by starting with
accurate position input, which is changed continuously as
accelerometers and gyros provide speed and direction inputs.
Both accelerometers and gyros are subject to very small
errors; as time passes, those errors probably accumulate.

While the best INS/IRS display errors of 0.1 to 0,4 NM after
flights across the North Atlantic of 4 to 6 hours, smaller and
less expensive systems are being built that show errors of 1
to 2 NM per hour. This accuracy is more than sufficient for
a navigation system that can be combined with and updated
by GPS. The synergy of a navigation system consisting of an
INS/IRS unit in combination with a GPS resolves the errors
and weaknesses of both systems. GPS is accurate all the time
ii is working but may he subject to short and periodic outages.
INS is made more accurate because it is continually updated
and continues to function with good accuracy if the GPS has
moments of lost signal.

Instrument Approach Systems
Most navigation systems approved for en route and terminal
operations under IFR, such as VOR, NDB, and GPS, may
also be approved to conduct IAPs. The most common
systems in use in the United States are the ILS, simplified
directional facility (SDF), localizer directional aid (LDA),
and microwave landing system (MLS). These systems
operate independently of other navigation systems. There are
new systems being developed, such as WAAS and LAAS.
Other systems have been developed for special use.

Instrument Landing Systems (ILS)

The ILS system provides both course and altitude guidance
to a specific runway. The ILS system is used to execute
a precision instrument approach procedure or precision
approach. [Figure 7-34] The system consists of the following
components

1. A localizer providing horizontal (left/right) guidance
along the extended centerline of the runway.

2. A glide slope (GS) providing vertical (up/down)
guidance toward the runway touchdown point, usually
at a 3° slope.

3. Marker beacons providing range information along
the approach path.

4. Approach lights assisting in the transition from
instrument to visual flight.

The following supplementary elements, though not specific
components of the system, may be incorporated to increase
safety and utility:

1. Compass locators providing transition from en route
NAVAIDs to the ILS system and assisting in holding
procedures, tracking the localizer course, identifying
the marker beacon sites, and providing a FAF for ADF
approaches.

2. DME collocated with the GS transmitter providing
positive distance-to-touchdown information or DME
associated with another nearby facility (VOR or stand-
alone), if specified in the approach procedure.

ILS approaches are categorized into three different types of
approaches based on the equipment at the airport and the
experience level of the pilot, Category I approaches provide
for approach height above touchdown of not less than 200 feet.
Category II approaches provide for approach to a height above
touchdown of not less than 100 feet. Category III approaches
provide lower minimums for approaches without a decision
height minimum. While pilots need only be instrument rated
and the aircraft be equipped with the appropriate airborne
equipment to execute Category I approaches, Category II
and III approaches require special certification for the pilots,
ground equipment, and airborne equipment.