GPS/RAIM/WAAS STUDY GUIDE

Table of Contents
1 Global Positioning System (GPS) Functionality ................................................................ 2
2 Receiver Autonomous Integrity Monitoring (RAIM) ......................................................... 4
3 Wide Area Augmentation System (WAAS) ...................................................................... 5
4 GPS CDI Scaling .............................................................................................................. 7
5 References ................................................................................................................... 10

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1 Global Positioning System (GPS) Functionality
GPS is a space-based radio navigation system that uses a constellation of 31 satellites (as of July
3, 2023), with at least 4 always visible worldwide. The constellation is operated and maintained
by the United States Department of Defense.

For FAA guidance, see Satellite Navigation - GPS - How It Works.

GPS satellites continuously send coded radio signals that contain the time stamp the signal was
broadcast. The GPS receiver decodes this signal, then uses the time difference between the
time of signal reception and the broadcast time to compute the distance from the receiver to
the satellite.

Remember that Distance = Speed x Time. By determining the time difference between signal
broadcast and signal reception, the receiver can correct any distortion caused by the Earth’s
atmosphere. With this correction, it is possible for the receiver to use the equation above to
provide a distance calculation.

With the signal from one satellite, our receiver can place our location anywhere on a sphere
with a known radius (our distance) surrounding the satellite, as shown in the figure below. With
just one satellite, our receiver could be in any direction from the satellite at a distance of r.

With the signals from two satellites, it is possible for our receiver to place our location
anywhere on the circle where the two spheres intersect, as shown below.

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By adding a 3rd satellite, our receiver is now able to place our location on one of two points on
the circle from the previous graphic by process of elimination. This provides a 2D position
resolution.

With signals from 3 satellites, our GPS receiver would need an atomic clock synchronized to GPS
to compute altitude from the 3 signals. By taking a measurement from a 4th satellite, the
receiver avoids the need for an atomic clock. By interpreting the signals from 4 satellites, the
receiver can compute latitude, longitude, altitude, and time. This provides a 3D position
resolution.

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2 Receiver Autonomous Integrity Monitoring (RAIM)
According to AIM 1-1-17(a)(3), “RAIM is the capability of a GPS receiver to perform integrity
monitoring on itself by ensuring available satellite signals meet the integrity requirements for a
given phase of flight.” Without RAIM, the pilot would have no assurance of the GPS position
integrity in a non-WAAS equipped aircraft.

We will cover WAAS in the following section of this document, but it is important to note that
RAIM/preflight RAIM predictions are not required on WAAS-equipped aircraft while in WAAS
coverage.

RAIM provides two functions, depending on the receiver and the number of satellites in view at
a given time. The first is called Fault Detection (FD), which uses an additional satellite to
monitor for any integrity anomalies. The second is called Fault Detection and Exclusion (FDE),
which excludes a failed satellite from the position solution and still provides an integrity-
assured position.

The number of satellites required for each RAIM function depends on aircraft equipment. If the
aircraft is non-baro-aiding, then 5 satellites are required for FD and 6 are required for FDE. If
the aircraft is baro-aiding, then 4 satellites are required for FD and 5 are required for FDE.

Baro-aiding is a method of augmenting the GPS integrity solution by using a non-satellite input
source in lieu of a 5th satellite. Plainly speaking, the pitot/static input from the ADAHRS is added
into the GPS position resolution to give additional altitude information that would normally be
provided by a satellite. The Piper Archer TX is approved for baro-VNAV and utilizes baro-aiding
in its RAIM functions. See PA-28-181 ARCHER III Section 1.21 G1000 GNSS (GPS/SBAS)
NAVIGATION SYSTEM EQUIPMENT APPROVALS for further details.

RAIM Requirements Fault Detection Fault Detection and Exclusion

Baro-Aiding 4 Satellites 5 Satellites
Non Baro-Aiding 5 Satellites 6 Satellites

In the simplest terms, the following statements are true regarding GPS and RAIM:
- With 1 satellite in view, you know you are on a sphere with a known radius surrounding
the satellite.
- With 2 satellites, you know you are on a circle between the two satellites.
- With 3 satellites, you know you’re on one of two points on the previous circle (2D
position).
- With 4 satellites, you know where you are (3D position).
- With 5 satellites, you can now tell if you’re getting bad information from one of the
satellites (RAIM FD) - OR 4 satellites with baro-aiding.
- With 6 satellites, you can safely exclude bad information from the solution and monitor
the other 5 for problems (RAIM FDE) - OR 5 satellites with baro-aiding.

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3 Wide Area Augmentation System (WAAS)
WAAS provides navigation services across all of the National Airspace System (NAS). It provides
augmentation information to GPS/WAAS receivers to enhance the accuracy and integrity of
position estimates.

For FAA guidance, see Satellite Navigation – WAAS – How It Works.

3 1
2

GPS information is received by Wide-Area Reference Stations (WRS) sites, show in area 1 above.
The information collected from these sites is transmitted to WAAS Master Stations (WMS) in
area 2, which generate correction messages every second. These messages have solutions to
allow GPS/WAAS receivers to correct errors in the GPS signal, which greatly improve system
accuracy.

The corrections are sent from the WMS to uplink sites, shown in area 3, for transmission to
geostationary communications satellites, shown in area 4. The communications satellites
receive the messages and then broadcast the corrections on GPS-like signals all over the NAS.

The GPS-like signal from the communication satellite can also be used by the GPS/WAAS
receiver to calculate position.

WAAS also provides GPS/WAAS receivers information of areas where the GPS system is
unusable due to system errors or other defects, which is why RAIM is not required when
within SBAS/WAAS coverage areas.

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An example of WAAS coverage area is shown below:

Current WAAS coverage can be viewed at any time by navigating to

https://www.nstb.tc.faa.gov/RT_VerticalProtectionLevel.htm

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4 GPS CDI Scaling
When set to Auto (default), the G1000 GPS CDI will automatically change sensitivity based on
phase of flight. Standard RNP levels are defined in the AIM 1-2-2 and AIM TBL 1-2-1, and are
shown below:

The G1000 Pilot User’s Guide provides the following illustration regarding GPS CDI scaling:

Once a departure procedure is activated, the CDI is scaled for departure (0.3 NM). The system
switches to terminal CDI scaling (1.0 NM) under a variety of conditions. See G1000 NXi Pilot's
Guide for the PA-28-181 Archer digital page 92 for a full list of conditions.

At 30 NM from the departure airport the enroute phase of flight is automatically entered and
the CDI scaling changes to 2.0 NM over 1.0 NM, except:
- If the last departure waypoint has not yet been reached, and that point is greater than
30 NM from the departure airport, OR
- The leg after the last departure waypoint has been activated, OR
- A direct-to waypoint is activated.

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If the nearest airport is more than 200 NM away from the aircraft after completing the
departure procedure and no approach procedure has begun, the CDI is scaled for oceanic flight
(4.0 NM).

Within 31 NM of the destination airport, the CDI scale gradually ramps down from 2.0 NM to
1.0 NM over a distance of 1.0 NM, unless the aircraft reaches the first point of an arrival route
that is greater than 31 NM from the destination airport. Then it will transition at the time that
arrival waypoint is reached, over a distance of 1.0 NM.

During approach, the CDI scale ramps down even further (see below). This transition normally
begins within 2.0 NM of the final approach fix (FAF). The CDI switches to approach scaling
automatically once the approach procedure is activated or if Vector-to-Final (VTF) is selected.

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Normally, for LPV and LP approaches, the specified course width upon crossing the landing
threshold is 700’. This is the same width of a localizer at the same point, which is where the
names “localizer performance” and “localizer performance with vertical guidance” come from.
However, on the GPS, the scaling stops decreasing at the threshold, unlike the angular
sensitivity of the localizer course guidance.

When a missed approach is activated, the CDI scale changes to 0.3 NM.

For further guidance on system annunciations and associated CDI scaling, see the tables below
(sourced from the G1000 NXi Pilot's Guide for the PA-28-181 Archer):

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5 References
PA-28-181, ARCHER III POH - https://docs.thrustflight.com/Fleet%20Docs/4%20-
%20Archer/piper-archer-poh.pdf

GPS System Details and Overview -

https://www.gps.gov/systems/gps/space/#:~:text=This%2024%2Dslot%20arrangement%20ens
ures,satellites%20are%20serviced%20or%20decommissioned.

How GPS Receivers Work - https://gisgeography.com/trilateration-triangulation-gps/

GPS – How It Works

https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navser
vices/gnss/gps/howitworks

RAIM Functionality -
https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html

Performance-Based Navigation Operations -

https://www.faa.gov/aircraft/draft_docs/afs_ac/AC_90-119_Coord_Copy.pdf

Performance-Based Navigation (PBN) and Area Navigation (RNAV) -

https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_2.html

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WAAS – How It Works
https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navser
vices/gnss/waas/howitworks

WAAS Animated Storyboard -

https://www.faa.gov/about/office_org/headquarters_offices/ang/offices/tc/library/Storyboard
/detailedwebpages/waas.html

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