WAAS Performances

WAAS
Title	WAAS Performances
Author(s)	GMV
Level	Basic
Year of Publication	2011

The Wide Area Augmentation System (WAAS) is the United States Satellite Based Augmentation System. The programme, started in 1992, is being carried out by the Federal Aviation Agency (FAA) and is specially developed for the civil aviation community. The system, which was declared operational in late 2003, currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada. WAAS service area includes CONUS, Alaska, Canada and Mexico. The WAAS programme is continuously in evolution; two development phases have been already covered, a third is in progress, and there are plans to improve the capability of the system in parallel with the evolution of the SBAS standards towards a dual-frequency augmentation service.

WAAS Performances

Satellite Based Augmentation Systems (SBAS) performances are usually described in terms of integrity, accuracy, availability, and continuity. The WAAS specification performance requirements depend on the phase of the flight: 1) en route through non-precision approach (NPA) and 2) precision approach.

According to its specification document, WAAS performance requirements are the following:

1. WAAS Performance Requirements for En Route through 'Non-Precision Approach'

Performances Requirements for En Route through Non-Precision Approach

Performance Requirement	Total System	Navigation System	WAAS Signal-in-Space	Airborne
Availability	0.99999	0.99999	0.99999	N/S
Accuracy 95% Horizontal Position 99.999% Horizontal Position 95% Vertical Position 95% Pseudorange	N/S N/S N/A N/A	100 m 500 m N/A N/S	N/S N/S N/A N/S	N/S N/S N/A 1.2 m
Integrity Integrity Risk Time-to-Alarm	N/S 10 s	N/S 10 s	10-7/hour 8 s	N/S 2 s
Continuity Continuity of Navigation Continuity of Fault Detection (1)	1 - 10-5/hour 1 - (2x10-5)/hour	1 - 10-5/hour 1 - (2x10-5)/hour	1 - 10-8/hour 1 - 10-5/hour	1 - 10-5/hour 1 - 10-5/hour

(1) Excluding outages of less than 5 minutes.

2. WAAS Performance Requirements for Precision Approach

Performances Requirements for Precision Approach

Performance Requirement	Total System	Navigation System	GPS/WAAS Signal-in-Space	Airborne
Availability	N/S	N/S	0.999	N/S
Accuracy 95% Horizontal Position 95% Vertical Position 95% Pseudorange	33.5 m 9.8 m N/A	7.6 m 7.6 m N/S	N/S N/S N/S	N/S N/S 1.2 m
Integrity Integrity Risk Time-to-Alarm	N/S N/S	N/S N/S	10-7/approach 5.2 s	N/S N/S
Continuity Continuity of Navigation Continuity of Fault Detection	1 - 10-4/approach N/S N/S	1 - 10-4/approach N/S N/S	1 - (5.5x10-5)/approach N/S N/S	1 - (4.5x10-5)/approach N/S N/S

N/S stands for Not Specified.

N/A stands for Not Applicable.

On July 10, 2003, the WAAS system was certified for 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. Typical performances provided by WAAS System when considering operating in LPV (Horizontal Alarm Limit = 40 m , Vertical Alarm Limit =50 m) are shown in next figure:

Typical WAAS LPV Coverage

WAAS Signal-In-Space Performance Requirements

Satellite Based Augmentation Systems (SBAS) performances are usually described in terms of integrity, accuracy, availability, and continuity. According to the WAAS peformance standard WAAS service area is divided in five coverage zones:

• Zone 1: CONUS.
• Zone 2: Alaska.
• Zone 3: Hawaii.
• Zone 4: Puerto Rico and some other Caribbean islands.
• Zone 5: US territory excluding zones 1 to 4.

The volume covered includes a region up to 100.000 feet above the surface. For zones 1 to 4, the area extends up to 30 nm outside the land.

WAAS specification distiguishes the following flight phases:

• En route
• Terminal
• LNAV (Lateral Navigation)
• LNAV/VNAV (Lateral Navigation/Vertical Navigation)
• LPV (Localizer Performance with Vertical guidance)
• LPV 200 (LPV 200 foot minimum)

The requirements considering the different flight phases and the different coverage zones are summarised in the next table:

WAAS Navigation Performance Requirements

	En Route	Terminal	LNAV	LNAV/VNAV	LPV	LPV 200
TTA	15 s	15 s	10 s	10 s	6.2 s	6.2 s
HAL	2 nm	1 nm	556 m	556 m	40 m	40 m
VAL	N/A	N/A	N/A	50 m	50 m	35 m
Probability of HMI	10-7 per hour	10-7 per hour	10-7 per hour	2 x 10-7 per approach	2 x 10-7 per approach (150 seconds)	2 x 10-7 per approach (150 seconds)
Zone 1 Continuity	1 - 10-5 per hour	1 - 10-5 per hour	1 - 10-5 per hour	1 - 5.5 x 10-5/15 seconds	1 - 8 x 10-6/15 seconds	1 - 8 x 10-6/15 seconds
Horizontal Accuracy (95%)	0.4 nm	0.4 nm	220 m	220 m	16 m	16 m
Vertical Accuracy (95%)	N/A	N/A	N/A	20 m	20 m	4 m
Availability (Zone 1 Coverage)	0.99999 (100%)	0.99999 (100%)	0.99999 (100%)	0.99 (100%)	0.99 (80-100%)	0.99 (40-60%)
Availability (Zone 2 Coverage)	0.999 (100%)	0.999 (100%)	0.999 (100%)	0.95 (75%)	0.95 (75%)	N/A
Availability (Zone 3 Coverage)	0.999 (100%)	0.999 (100%)	0.999 (100%)	N/A	N/A	N/A
Availability (Zone 4 Coverage)	0.999 (100%)	0.999 (100%)	0.999 (100%)	N/A	N/A	N/A
Availability (Zone 5 Coverage)	0.99999 (100%)	0.999 (100%)	0.999 (100%)	N/A	N/A	N/A

In this table, the integrity requirements are given as the probability of HMI, namely Hazardously Misleading Information. In addition to integrity, accuracy, availability and continuity, the table shows the specification for Time To Alert (TTA) and for Horizontal and Vertical Alert Limits (HAL and VAL).