WAAS Terms and Definitions
- Wide Area Augmentation System (WAAS)
The WAAS is made up of an integrity reference monitoring network,
processing facilities, geostationary satellites, and control
facilities. Wide area reference stations and integrity monitors
are widely dispersed data collection sites that contain GPS/WAAS
ranging receivers that monitor all signals from the GPS, as well
as the WAAS geostationary satellites. The reference stations
collect measurements from the GPS and WAAS satellites so that
differential corrections, ionospheric delay information, GPS/WAAS
accuracy, WAAS network time, GPS time, and UTC can be determined.
The wide area reference station and integrity monitor data are
forwarded to the central data processing sites. These sites
process the data in order to determine differential corrections,
ionospheric delay information, and GPS/WAAS accuracy, as well as
verify residual error bounds for each monitored satellite. The
central data processing sites also generate navigation messages
for the geostationary satellites and WAAS messages. This
information is modulated on the GPS-like signal and broadcast to
the users from geostationary satellites.
- Precision Approach (PA) Navigation Mode
The Precision Approach navigation mode refers to the navigation
solution operating with a minimum of four satellites with all WAAS
corrections (fast, long term, and ionospheric ) available.
- Non-Precision Approach (NPA) Navigation Mode
The Non-Precision Approach navigation mode refers to the
navigation solution operating with a minimum of four satellites
with fast and long term WAAS corrections (no WAAS ionospheric
- Geometric Dilution of Precision (GDOP)
The magnitude of user position errors can be determined from the
combination of satellite ranging errors and the geometry of
selected satellites. The effect of geometry is expressed by the
geometric dilution of precision (GDOP). The GDOP may be represented
in any user local coordinate desired. Examples are HDOP for local
horizontal, VDOP for local vertical, PDOP for all three coordinates,
and TDOP for time.
PDOP X Satellite Range Error (1 sigma) = Radial error in user
position (1 sigma) in 3 dimensions
Therefore, small values of the GDOP parameters indicate good
arrangements in the geometry of the selected satellites and
correspondingly small user position errors.
- Horizontal Protection Level (HPL)
The Horizontal Protection Level is the radius of a circle in the
horizontal plane (the plane tangent to the WGS-84 ellipsoid), with
its center being at the true position, which describes the region
that is assured to contain the indicated horizontal position. It
is based upon the error estimates provided by WAAS.
- Vertical Protection Level (VPL)
The Vertical Protection Level is half the length of a segment on
the vertical axis (perpendicular to the horizontal plane of WGS-84
ellipsoid), with its center being at the true position, which
describes the region that is assured to contain the indicated
vertical position. It is based upon the error estimates provided
- LPV Service (Solid Red Line)
Area encompassed meets WAAS LPV operational service level with
horizontal alert limit (HAL) equal to 40 meters and a vertical
alert limit (VAL) equal to 50 meters.
- LNAV/VNAV Service (Dashed Black Line)
Area encompassed meets WAAS LNAV/VNAV operational service level
with horizontal alert limit (HAL) equal to 556 meters and a
vertical alert limit (VAL) equal to 50 meters.