What is WAAS?
A. The Wide
Area Augmentation System (WAAS) uses a system of ground stations
to provide necessary augmentations to the GPS SPS navigation
signal. A network of precisely surveyed ground reference stations
is strategically positioned across the country including Alaska,
Hawaii, and Puerto Rico to collect GPS satellite data. Using
this information, a message is developed to correct any signal
errors. These correction messages are then broadcast through
communication satellites to receivers onboard aircraft using
the same frequency as GPS. The WAAS is designed to provide the
additional accuracy, availability, and integrity necessary to
enable users to rely on GPS for all phases of flight, from en
route through GLS approach for all qualified airports within
the WAAS coverage area. This will provide a capability for the
development of more standardized precision approaches, missed
approaches, and departure guidance for approximately 4,100 ends
of runways and hundreds heliport/helipads in the NAS. WAAS will
also provide the capability for increased accuracy in position
reporting, allowing for more uniform and high-quality worldwide
Air Traffic Management (ATM). In addition, WAAS will provide
benefits beyond aviation to all modes of transportation, including
maritime, highways, and railroads.
How does WAAS know that the correction it sends is valid to my particular location?
The WAAS supplies two different sets of corrections: 1) corrected GPS parameters
(position, clock, etc) and 2) Ionospheric parameters. The first set of corrections
is user position independent - they apply to all users located within the WAAS
service area. The second set of corrections is area specific. WAAS supplies
correction parameters for a number of points (organized in a grid pattern) across
the WAAS service area. The user receiver computes ionospheric corrections for
the received GPS signals based on algorithms which use the appropriate grid points
for where the user is located. Further, the appropriate grid points may differ
for each GPS satellite received and process by the user receiver as the GPS satellites
are located at various positions in the sky relative to the user. The combination
of the two sets of corrections allows for significantly increased user position
accuracy and confidence anywhere in the WAAS service area.
Will the WAAS provide a performance comparable to ILS? How will
the FAA respond to users who claim to be getting poorer performance
A.Yes. WAAS has been designed and
is being built to provide performance comparable to Category 1
ILS. The Satellite Operational Implementation Team, or SOIT, believes
that WAAS will provide an equivalent level of precision approach
service to that of existing Category 1 ILS when fully deployed.
When the WAAS signal is fully stabilized, and
the FAA accepts the system, we believe actually performance will
exceed system specifications. Preliminary tests using WAAS software
indicate that this is the case.
Does the FAA plan to live up to its original commitment to deliver
the WAAS program with the capability that was envisioned when initial
funding was requested?
A. The FAA remains committed to the
implementation of WAAS because of its safety benefits for the
aviation community and the flying public, and because it is central
to our overall efforts to modernize the NAS.
What is the data collecting method for ionospheric data, solar activity,
etc. to evaluate performance?
WAAS collects GPS data at the reference stations. The system is then be able to estimate the signal delay and error due to the ionosphere. This information is then passed to the user as a part of the WAAS navigation message. In addition, ionospheric data is collected and archived by the WAAS Test Team
for analysis of scintillation and range delay effects by experts in the ionospheric field.
Who manages (and how) the safety analysis of the WAAS system including
the ground component, RF Uplink system, and satellite component?
A Safety Working Group has been
formed to continuously look into the safety performance/operation
for WAAS (including all of the components - reference stations,
uplink stations, and satellites). This working group is comprised
of various representatives from the FAA, Raytheon, and Mitre Corporation.
What is the six-second time to alarm?
WAAS has six seconds to do
one of two actions:
1. Correct user position outside the guaranteed accuracy
protection limits to get back within the protection limits.
If WAAS is able to correct misleading information within
six seconds, there is no lapse in system integrity.
2. Shut-off connections and notify the user not to use. If
the system is unable to correct misleading information in the
six-second timeframe, it becomes Hazardously Misleading Information
(HMI) and should not be used for navigation.
When will the WAAS system be available in Europe?
A. On August 2, 2010, the European Geostationary Navigation Overlay Service (EGNOS) began broadcasting satellite-based navigation service certified to meet the high standards of international aviation. EGNOS is the European equivalent of the U.S. Wide Area Augmentation System (WAAS). Both systems monitor and provide correction updates to Global Positioning System (GPS) signals and support aircraft flight operations including departure climb, enroute flight, arrivals and landing – using only guidance from satellites. This new operational capability will significantly boost the availability and utility of worldwide satellite based augmentation system (SBAS) service. SBAS is the international term for navigation systems such as WAAS and EGNOS. More information on EGNOS can be found on the European Satellite Services Provider (ESSP) website at http://www.essp-sas.eu.
|Q. What approaches
can I fly with GPS? WAAS?
A. This answer is highly dependent upon
your specific equipment and installation. Please check the
Aeronautical Information Manual, Table 1-1-5
and compare this with the documentation provided with your GPS and installation.
This table provides information on TSO-C129 units. LPV approaches require
WAAS Class 3 equipment built to TSO-C145a or TSO-C146a and installed IAW
The short answer is that you cannot use any hand-held receiver
for anything other than situational awareness. Older GPS receivers may
be for VFR-only use, or may be sufficient to fly non-precision GPS approaches.
LNAV/VNAV and LPV approaches require additional equipment beyond the
basic GPS receiver.
OK, so explain what these various Technical Standard Orders and Advisory
Circulars relate to.
TSO-C129 is “Airborne Supplemental Navigation
Equipment Using the GPS.”
TSO-C145a is “Airborne Navigation Sensors
Using the GPS Augmented by the Wide Area Augmentation System.”
TSO-C146a is “Stand-Alone Airborne Navigation
Equipment Using the GPS Augmented by the Wide Area Augmentation System.”
AC 20-130a is “Airworthiness Approval of Navigation
or Flight Management Systems Integrating Multiple Navigation Sensors.”
AC 20-138a is “Airworthiness Approval of GPS
Navigation Equipment for Use as a VFR and IFR Navigation System.”
AC 90-94 is “Guidelines for Using GPS Equipment.”
|Q. What are the differences in capabilities
between the various WAAS TSOs?
A. First, the basic information for GPS. To use GPS for navigation, the
equipment must be certified in accordance with TSO-C129 and the installation
must be done in accordance with AC 20-138 or AC 20-130A. TSO-C115a does
not meet the requirements of TSO-C129.
For WAAS, you must use either TSO-C145a
or TSO-C146a. Most General Aviation WAAS receivers would comply with
TSO-C146a, which applies to panel-mounted navigation equipment (as
opposed to sensors that provide data to a flight management system).
|Q. I already have GPS. Does WAAS require
a separate antenna?
A. No, the WAAS
message is broadcast on the same frequency as the GPS signal. You will just
need a WAAS-capable receiver provided your existing antenna is compatible
with the WAAS equipment. Your current system may be upgradeable. Please contact
your manufacturer directly for information on availability, installation
Can I use GPS to fly RNAV approaches? WAAS?
A. To fly an LNAV or LNAV/VNAV approach,
you must have either (1) WAAS avionics approved for LNAV/VNAV approaches,
(2) a certified Baro-VNAV approach system with IFR-approved GPS, (3) a certified
Baro-VNAV approach system with an IFR-approved WAAS, or (4) an RNP-0.3 certified
Wait—there’s another new term…what is RNP?
A. RNP stands for “Required Navigational
is a metric of system navigational capability. The FAA is moving toward
a performance-based national airspace system. In the future, your ability
to fly in certain areas may be governed by your ability to achieve precise
navigational performance within specific tolerances. RNP-0.3 will be used
for approaches, and it refers to .3 nautical mile accuracy. This accuracy
may be achieved through various means (GPS, WAAS, flight management system
using automatic DME updates), but your aircraft will be certified to a
particular RNP. There are other requirements beyond accuracy that will
be defined for each RNP operation before it is implemented.
What is an LPV approach? Can I fly LPV approaches with WAAS today?
A. LPV is a new category of approach that
uses the WAAS signal and provides vertical guidance. You can fly LPV with
WAAS avionics approved for LPV approaches. WAAS avionics do not automatically
mean that you can fly the LPV minima line. WAAS approaches do not require
any special ground equipment at the destination airport—no localizer
or glide slope transmitters are necessary. To fly an LPV approach, you must
have WAAS Class 3 avionics certified to TSO-C145a or TSO-C146A, and installed
IAW AC 20-138A.
I see comments about WAAS “integrity.” What is meant by “integrity?” How
does integrity differ from availability or continuity? What is “ephemeris?”
Integrity refers to usability of the satellite signal, and
means that the signal has not been corrupted. Integrity is the ability of
a system to provide timely warnings to users when the system should not be
used for navigation as a result of errors or failures in the system. WAAS
improves upon the integrity of the basic GPS signal and detects much smaller
errors more quickly.
Availability refers to the percentage of time in a given
period that the signal is expected to be received and usable.
Continuity differs from availability, in that it refers to the continuous reception
of the signal. A signal could have high availability but numerous short
Ephemeris is the term for the exact position of a GPS satellite
in space at a given time, and it is necessary for GPS calculations.
Can my aircraft be equipped only with GPS for navigation and be legal
for flying in IMC?
A. The FAA has authorized GPS as the primary
means of navigation in certain areas (oceanic and remote). The FAA requires
that the aircraft have the appropriate equipment necessary for the route
of flight. Furthermore, a GPS approach cannot be used for the alternate if
it is also used for the destination when using TSO-C129 equipment. Therefore,
it may be legal to fly a GPS-only equipped aircraft in IMC, but the circumstances
are quite rigorous. In Alaska , IFR-approved and installed WAAS avionics
are legal as the only navigation equipment on board if the route and destination
are identified for GPS/WAAS.
I want to fly an RNAV (GPS) approach. I have an FMS, but no GPS.
The approach says GPS or RNP-0.3 required. It also says DME/DME-0.3
NA. Can I fly this approach legally?
A. Your system updates using ranging from
multiple DMEs. Optimal DME geometry of multiple DME facilities and validation
is required to achieve this performance. At this time, RNP-0.3 approaches
using multiple DMEs are not common. Therefore, there is no way you can fly
this approach without having GPS.
What do I need to know about RAIM?
A. Receiver Autonomous Integrity Monitor
(RAIM) is a form of integrity monitoring performed within the avionics themselves.
It ensures available satellite signals meet the integrity requirements for
a given phase of flight. By comparing the pseudorange measurements of a number
of satellites, the RAIM function can identify a satellite failure and issue
an alert to the pilot. Many VFR GPS receivers and most hand-held receivers
do not have RAIM capability. Without RAIM capability, the pilot has no assurance
of the accuracy of the GPS position. A minimum of five satellites is required
to detect a bad satellite; at least six satellites are required to detect
and exclude a bad satellite from the navigation solution if your receiver
has a fault detection and exclusion (FDE) RAIM algorithm. The GPS receiver
should also tell you when its RAIM function is unavailable, at both present
time/position and at any selected future time/position. You can get information
on satellite outages through the NOTAM system, however the effect of an
outage on the intended operation cannot be determined unless the pilot
has a RAIM availability prediction program which allows excluding a satellite
which is predicted to be out of service.
If you are using GPS to fly an
approach and you receive a RAIM annunciation prior to the final approach
waypoint, you may not have sufficient accuracy to complete the approach.
You can get information on satellite outages through the
NOTAM system, however the effect of an outage on the intended operation
cannot be determined unless the pilot has a RAIM availability prediction
program which allows excluding a satellite which is predicted to be out
my GPS-equipped aircraft considered RNAV-capable?
A. Aircraft operating by IFR-approved GPS
are considered to be area navigation (RNAV) aircraft, and should file the
appropriate equipment suffix.
I have heard about flying an “overlay” approach, and that
it is basically flying a VOR or other approach, but using the GPS instead
of the VOR or ADF. Can I just use the GPS instead of the VOR?
A. No. Overlay approaches can use GPS instead
of the primary designated navigational aid, but the approach must be designated
for GPS and be in the current aircraft database. For example, it must say “VOR
or GPS RWY 16.” You cannot just use GPS in lieu of VOR, Automatic Direction
Finder (ADF) or other navigational source naming the approach. You can,
however, use GPS to determine waypoints during the approach.
What is the sensitivity of my CDI when using GPS?
A. En route, full-scale Course Deviation
Indicator (CDI) deflection is typically 5 miles with an accuracy of +/- 2
miles. Within 30 miles of the arrival and departure airport, GPS CDI sensitivity
typically transitions to one mile. When flying an approach (and the approach
mode is armed), GPS CDI sensitivity transitions from 1 mile to 0.3 miles
approximately 2 miles from the FAWP.
Can I use an alternate with a GPS approach?
A. If your system is a TSO-C129, any required
alternate must have an approved instrument procedure other than GPS, and
your aircraft must have the appropriate equipment to fly the approach. If
you have approved WAAS avionics, you may plan to use any instrument approach
authorized for use with WAAS avionics at a required alternate. You must use
the LNAV minima line for planning purposes in case vertical guidance is not
The GPS approach at my destination had a NOTAM saying “Unreliable.” Can
I file based upon the NOTAM? Can I fly the approach upon arrival?
A. You can file using the GPS, however you
must file an alternate. You can fly the approach—even though listed
as “Unreliable.” The
term UNRELIABLE is an advisory to pilots indicating the expected level
of service may not be available. You should use RAIM and all other sources
to confirm the suitability of the GPS signal upon arrival. Unreliable and
Unavailable have different meanings, and you must be certain of each. Unavailable
indicates a loss or malfunction of the GPS signal in the specified area
during the specified time period.
seen the acronyms TSO and STC used together. What’s the difference?
A. TSO is a Technical Standard Order, and it describes the minimum performance
standard for a system or component. An STC is a Supplemental Type Certificate.
An STC is a document issued by the Federal Aviation Administration approving
a product (aircraft, engine, or propeller) modification. The STC defines
the product design change, states how the modification affects the existing
type design, and lists serial numbers of the component affected by the
change. Safe flight using GPS equipment depends on airworthiness (does
it have a TSO?), installation (was it installed per AC 20-138 or AC 20-130a),
and signoff (was it done properly per TC/STC or Form 337).
I’ve heard the term, “number of nines” what does
A. The FAA uses
metrics in the process of determining the safety of operations. Two nines
would equate to 99%, or one occurrence in 100. Five nines would be .99999,
or one occurrence in 100,000.
What is the problem with flying baro-VNAV in really cold weather?
A. Altimeters are susceptible to
atmospheric-related error, and when the temperature is colder than standard,
this error can become significant, with your altimeter reading higher than
your actual altitude. Some charts list minimum temperatures for certain minimums,
and the reason is that your actual altitude is below the altitude that would
be displayed on your altimeter. Sometimes there are limitations associated
with the availability of a local altimeter as well. The pilot is expected
to account for this error in extremely cold conditions (as discussed in the
I’ve seen the term APV approach. What does that mean?
APV is the International Civil Aviation Organization (ICAO) term for
an approach with vertical guidance, and it refers to specific ICAO criteria
adopted in May 2000. This approach classification allows the use of stabilized
descent using vertical guidance without the accuracy required for traditional
precision approach procedures. The US has developed criteria for lateral/vertical
navigation (LNAV/VNAV) and LPV approach procedures that meet this approach
classification. The LNAV/VNAV and LPV approaches provide guidance in both
the lateral and vertical planes.
What’s an MMR?
A. An MMR is a multi-mode receiver.
It would receive the basic GPS signal and the WAAS signal. It may also
include the Ground Based Augmentation System (GBAS) signal at a future date. It may also receive VHF, UHF,
VOR or other signals.
I’ve heard about Beta, Delta, and Gamma WAAS equipment. What’s
the difference and which one will I need to buy for my single-engine
aircraft? When will I be able to buy them?
A. These designations were used
in the engineering standards for WAAS equipment, but they are not used
operationally. WAAS equipment that provides PVT (position, velocity, time)
into a flight management system, where the flight management system provides
the actual navigation function is marked with a TSO-C145a. Panel-mount
navigation systems that include a sensor and a navigation unit are TSO-C146a
receivers. A special class of TSO-C146a equipment is an MMR that includes
the WAAS capability to conduct LPV approach procedures. It does not support
en route navigation other than providing PVT into an FMC.
I already have a GPS receiver—how do I upgrade it for WAAS? Do
I need to buy a new box or can I upgrade with just a card or with better
software? How much will it cost me?
A. You need to contact the manufacturer
of your system. Many systems can be easily and inexpensively upgraded by
an avionics technician or the manufacturer.
Are LNAV/VNAV or LPV approaches considered RNP approaches?
A. No, although they may share
similar accuracies. LNAV/VNAV and LPV approaches are RNAV approaches. RNP
approach design involves linear obstacle assessments, while LNAV/VNAV and
LPV approaches use angular obstacle assessments.
Why do so many RNAV approaches say RNP-0.3 (DME/DME NA)?
A. DME signal reception is limited to line-of-sight, and for
approach operations, the aircraft typically cannot receive very many DME
signals. As a consequence, DME/DME performance can be poor, and is frequently
inadequate. The FAA is currently working with industry to develop criteria
to implement some DME-based capability for approach procedures using the
I have a WAAS non-aviation receiver - Where can I use it? What types
of accuracies will I get?
A.. WAAS provides better accuracy that GPS alone. Non-aviation
WAAS receivers in the aviation WAAS coverage area (http://www.nstb.tc.faa.gov/RT_VerticalProtectionLevel.htm)
will get far better accuracies due to the WAAS corrections they are receiving.
However, non-aviation users outside of the WAAS aviation coverage area
can also benefit from WAAS corrections. Please refer to the Horizontal
Protection Limit website (http://www.nstb.tc.faa.gov/RT_NPACoverage.htm)
which shows the areas WAAS can be used for non-aviation and what types
of accuracies you may expect.
Can I use WAAS in my vehicle?
A. WAAS is extremely
useful for vehicle navigation, as it increases accuracy from 10-12
meters with GPS alone, to 1-2 meters horizontal. This increase in accuracy
can mean knowing which side of the highway a vehicle is on, lane determination,specific
vehicle location, or where an exit is early enough so that you can
make the turn before the exit is passed.
Because of this, WAAS is becoming more and more popular for use in vehicles.
In fact, public safety departments including police, fire, rescue,
and state transportation authorities are already using WAAS. CALTRANS
has incorporated WAAS into the receivers used to layout construction
and improvements of highways. Also, a number of vehicular navigation services
are investigating WAAS for the future. OnStar will be incorporating
WAAS into 2008 model year GM vehicles. OnStar provides notification of
vehicle location to a given GPS location within 15 seconds of airbag deployment.
The addition of WAAS provides greater accuracy in the location determination
and contributes to this life saving benefit. It has provided vehicle
tracking for stolen as well as carjacked vehicles and led to the recovery
of infants in cars that have been carjacked. Also, DaimlerChrysler has
tested WAAS as a part of an intelligent transportation system in development,
and found it to be the best option to provide the accuracy needed for