WAAS Signal Structure

WAAS
Title	WAAS Signal Structure
Author(s)	GMV.
Level	Basic
Year of Publication	2011

The Wide Area Augmentation System (WAAS) is an GPS Augmentation system developed by the Federal Aviation Administration (FAA), WAAS is an extremely accurate navigation system developed for civil aviation, it provides service for all classes of aircraft in all phases of flight - including en route navigation, airport departures, and airport arrivals.

WAAS uses a network of ground-based reference stations, in North America and Hawaii, to measure small variations in the GPS satellites' signals in the western hemisphere. Measurements from the reference stations are routed to master stations, which queue the received Deviation Correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the corrections while computing their positions to improve accuracy.

WAAS Signal

The objectives of the WAAS are to provide improved: integrity,accuracy,availability, and continuity of service to the GPS Standard Positioning Service (SPS). The ultimate objective is to provide a navigation system for all phases of flight through precision approach.

The WAAS is a safety-critical system consisting of the equipment and software which augments Global Positioning System (GPS) Standard Positioning Service (SPS). The WAAS provides a signal-in-space to WAAS users to support en route through precision approach navigation. The WAAS users include all aircraft with approved WAAS avionics using the WAAS for any approved phase of flight. The signal in space provides two services: (1) data on GPS and GEO satellites, and (2) a ranging capability. In addition to providing GPS integrity, the WAAS verifies its own integrity and takes any necessary action to ensure that the system meets the WAAS performance requirements. The WAAS also has a system operations and maintenance function that provides information to FAA Airway Facilities (AF) National Airspace System (NAS) personnel.

The WAAS has a single state - normal state. Periodic maintenance and corrective maintenance do not interrupt the normal state. There are two modes for the normal state.

• Continuous Service Mode: In the normal state, the WAAS shall possess an operational mode of continuous service meeting all of the performance requirements, both for the precision approach and non-precision approach phases of flight.
• Military Emergency Mode: In the normal state, the WAAS shall possess a military emergency mode that only meets the performance requirements for en route through non-precision approach services.

In general, the WAAS service operator provides different services aiming at different market sectors, namely an Open Service, a Safety of Life service (SoL) and even a Commercial Service. For the Safety-of-Life (SoL) service, the WAAS user equipment shall be compliant (certified) against several standards, i.e. RTCA MOPS DO 229 (see article SBAS Standards). The SoL civil aviation certified equipment is in the highest rank with respect its cost. The Open Service (OS) targets low cost, general purpose GPS equipment that uses the WAAS Signal-In-Space (SIS) to provide the user with an enhanced accuracy performance in comparison with the one provided by a standalone GPS device. In comparison with the certification requirements of the SoL user equipment, user equipment is not necessarily compliant with the RTCA MOPS DO 229 processing rules, but might only make use of the processing algorithms that render the accuracy corrections provided by the SBAS SIS.

On July 10, 2003, the WAAS signal was activated for safety-of-life aviation, covering 95% of the United States, and portions of Alaska . At present, WAAS supports en-route, terminal and approach operations down to a full LPV-200 (CAT-I like Approach Capability) for the CONUS, Mexico and Canada.