WAAS Signal Structure

WAAS
Title	WAAS Signal Structure
Edited by	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; three development phases have been already covered, and there are on-going plans to improve the capability of the system in parallel with the evolution of the SBAS standards towards a dual-frequency augmentation service.

WAAS Signal

The Wide Area Augmentation System (WAAS) 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 and 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 and GEO safe navigation service.

The WAAS signal-in-space (SiS) has been designed to minimize standard GPS receiver hardware modifications. The signal interface characteristics comprise carrier and modulation radio frequency, message structure, protocol and content of the WAAS message. The WAAS signal is broadcast by three GEO satellites that transmit a GPS-like L1 (1574.42 MHz) signal, modulated with a Coarse/Acquisition Pseudo-Random Noise (PRN) code. The WAAS 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.
Carrier Phase Noise	The phase noise spectral density of the unmodulated carrier is such that a phase locked loop of 10Hz one-sided noise bandwidth is able to track the carrier to an accuracy of 0.1 radians rms.
SBAS Data	500 symbols per second, module-2 modulated (250 effective bits per second)
Power	Minimum power –161 dBW at 5 degrees elevation Maximum power –155 dBW

Another relevant WAAS signal characteristics described in the Minimum Operational Performance Standards (MOPS) for the airborne equipment and modified relative to GPS are:

• Doppler Shift: The Doppler shift, as perceived by a stationary user, on the signal broadcast by WAAS GEOs is less than 40 meters per second (≈210 Hz at L1) in the worst case (at the end of life of the GEOs). The Doppler shift is due to the relative motion of the GEO.
• Carrier Frequency Stability: The short term stability of the carrier frequency (square root of the Allan Variance) at the input of the user´s receiver antenna will be better than 5x10-11 over 1 to 10 seconds, excluding the effects of the ionosphere and Doppler.
• Polarization: The broadcast signal is right-handed circularly polarized. The ellipticity will be no worse than 2 dB for the angular range of ±9.1o from boresight.
• Code/Carrier Frequency Coherence: The lack of coherence between the broadcast carrier phase and the code phase shall be limited. The short term (<10sec) fractional frequency difference between the code phase rate and the carrier frequency shall be less than 5x10-11 (one sigma). Over the long term (<100 sec), the difference between the change in the broadcast code phase (convert to carrier cycles) and the change in the broadcast carrier phase shall be within one carrier cycle (one sigma).
• Correlation Loss: Correlation loss is defined as the ratio of output powers from a perfect correlator for two cases: 1) the actual receiver WAAS signal correlated against a perfect unfiltered PN reference, or 2) a perfect unfiltered PN signal normalized to the same total power as the WAAS signal in case 1. The correlation loss resulting from modulation imperfections and filtering inside the WAAS satellite payload is less than 1 dB.

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.