WAAS Signal Structure

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

The Wide Area Augmentation System (WAAS) is a 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

Every SBAS provides ranging signals transmitted by GEO satellites, differential corrections on the wide area and additional parameters aimed to guarantee the integrity of the GNSS user:

• GEO Ranging: transmission of GPS-like L1 signals from GEO satellites to augment the number of navigation satellites available to the users.
• Wide Area Differential (WAD): differential corrections to the existing GPS/GLONASS/GEO navigation services computed in a wide area to improve navigation services performance. This includes corrections to the satellite orbits and clocks, as well as information to estimate the delay suffered from the signal when it passes through the ionosphere.
• GNSS/Ground Integrity Channel (GIC): integrity information to inform about the availability of GPS/GLONASS/GEO safe navigation service.

The SBAS satellite shall transmit a GPS-like L1 (1574.42 MHz) signal, modulated with a Coarse/Acquisition Pseudo-Random Noise (PRN) code. The SBAS L1 radiofrequency characteristics are:

Parameter	Description
Modulation	Bi-phase shift key (BPSK) modulated by a bit train comprising the PRN code and the SBAS data (modulo-2 sum).
Bandwidth	L1 ±30.69 MHz. At least 95% of the broadcast power will be contained within the L1 ±12 MHz band.
Ranging Codes	A PRN Code (Gold code) of 1 millisecond in length at a chipping rate of 1023 Kbps.
SBAS Data	500 symbols per second, module-2 modulated (250 effective bits per second)
Power	Minimum power –131 dBm at 5 degrees elevation Maximum power –119,5 dBm

A convolutional encoding of the bits is performed with the following parameters:

Code Parameter	Value
Coding Rate	1/2
Coding Scheme	Convolutional
Constraint Length	7
Generator Polynomials	G1 = 171(oct); G2 = 133(oct)
Encoding Sequence	G1 then G2
Flush	No

The WAAS delivers to the user the corrections and integrity data as well as some ancillary information (timing, degradation parameters, etc.) through messages encoded in the signal. The format of the messages is thoroughly explained in the article The EGNOS SBAS Message Format Explained, applicable to WAAS Messages.