WAAS/GPS 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
corrections) available.

**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 by WAAS.

**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.