# 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 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.