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25 wide area ground reference systems, two master stations,
and four ground uplink stations.
General Requirements
WAAS avionics must be certified in accordance with
TSO-C145A, Airborne Navigation Sensors Using the GPS
Augmented by the WAAS; or TSO-146A for stand-alone
systems. GPS/WAAS operation must be conducted in
accordance with the FAA-approved aircraft flight manual
(AFM) and flight manual supplements. Flight manual
supplements must state the level of approach procedure that
the receiver supports.
7-35
Figure 7-30. WAAS Satellite Representation.
7-36
Figure 7-32. LAAS Representation.
Figure 7-31. WAAS Satellite Representation.
Instrument Approach Capabilities
WAAS receivers support all basic GPS approach functions
and will provide additional capabilities with the key benefit
to generate an electronic glide path, independent of ground
equipment or barometric aiding. This eliminates several
problems such as cold temperature effects, incorrect altimeter
setting or lack of a local altimeter source, and allows approach
procedures to be built without the cost of installing ground
stations at each airport. A new class of approach procedures
which provide vertical guidance requirements for precision
approaches has been developed to support satellite navigation
use for aviation applications. These new procedures called
Approach with Vertical Guidance (APV) include approaches
such as the LNAV/VNAV procedures presently being flown
with barometric vertical navigation.
Local Area Augmentation System (LAAS)
LAAS is a ground-based augmentation system which uses
a GPS reference facility located on or in the vicinity of
the airport being serviced. This facility has a reference
receiver that measures GPS satellite pseudo-range and
timing and retransmits the signal. Aircraft landing at
LAAS-equipped airports are able to conduct approaches to
Category I level and above for properly equipped aircraft.
[Figures 7-32 and 7-33]
Inertial Navigation System (INS)
Inertial Navigation System (INS) is a system that navigates
precisely without any input from outside of the aircraft. It is
fully self-contained. The INS is initialized by the pilot, who
enters into the system the exact location of the aircraft on the
ground before the flight. The INS is also programmed with
WPs along the desired route of flight.
7-37
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;
therefore, 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
it is working but may be 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
　
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