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|YearOfPublication=2011
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The Wide Area Augmentation System ([[WAAS General Introduction|WAAS]]) is the United States [[SBAS General Introduction|Satellite Based Augmentation System]]. The programme, started in 1992, is being carried out by the [http://www.faa.gov/ Federal Aviation Agency (FAA)]<ref name="FAA_NAV_HISTORY">[http://www.faa.gov Navigation Services - History - Satellite Navigation,] [http://www.faa.gov/ FAA.]</ref> and is specially developed for the civil aviation community.<ref name="FAA_WAAS">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/ Navigation Services - Wide Area Augmentation System (WAAS)], [http://www.faa.gov/ FAA.]</ref> The system, which was declared operational in late 2003,<ref name="STANFORD_WAAS">[https://gps.stanford.edu/research/currentcontinuing-research/waas-sbas Wide Area Differential GPS (WADGPS), Stanford University]</ref> currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada.<ref name="APPROACHES">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/approaches/index.cfm GNSS - GPS/WAAS Approaches,] [http://www.faa.gov/ Federal Aviation Agency (FAA).]</ref> WAAS service area includes CONUS, Alaska, Canada and Mexico.<ref name="WAASExpanded">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/news/ WAAS Service Expanded into Canada and Mexico, September 28, 2007,] [http://www.faa.gov/ Federal Aviation Agency (FAA).]</ref>  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.<ref name="EXTENSION">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/satnav/media/SatNav_March08.pdf SatNav News, Vol. 33, March 2008,] [http://www.faa.gov/ Federal Aviation Agency (FAA).]</ref>


The Wide Area Augmentation System (WAAS) is an [[GNSS Augmentation|GPS Augmentation]] system developed by the [http://www.faa.gov 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 Signal==


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.<ref>[http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System WAAS in Wikipedia]</ref>
The Wide Area Augmentation System ([[WAAS General Introduction|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.
==WAAS Signal==
* '''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 objectives of the WAAS are to provide improved: [[Integrity|integrity]],[[Accuracy|accuracy]],[[Availability|availability]], and [[Continuity|continuity]] of service to the [[GPS Services|GPS Standard Positioning Service]] (SPS). The ultimate objective is to provide a navigation system for all phases of flight through precision approach.<ref> FAA.[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/documents/media/waas/2892bC2a.pdf ''Specification for the Wide Area Augmentation System(WAAS)'']. FAA-E- 2892b. August 13, 2001.</ref>
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 Messages|WAAS message]]. The WAAS signal is broadcast by three [[WAAS Space Segment|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:<ref name="ICAO_SARPS">[http://www.icao.org ICAO] Standards and Recommended Practices, Annex 10, Volume 1 Radio Navigation Aids, July 2006</ref>


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.
{| class="wikitable"
|-
!|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
|}


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.
Another relevant WAAS signal characteristics described in the Minimum Operational Performance Standards (MOPS) for the airborne equipment and modified relative to GPS are:<ref name=" RTCA MOPS DO 229">Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airbone Equipment</ref><ref> FAA.[http://www.faa.gov ''Specification for the Wide Area Augmentation System(WAAS)'']. FAA-E- 2892b. August 13, 2001.</ref>
*'''Continuous Service Mode''':  In the normal state, the WAAS shall possess an operational mode of continuous service meeting all of the [[Work in Progress:WAAS Performances|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''<ref>[[SBAS Fundamentals]]</ref>. 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.
* '''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.


On July 10, 2003, the WAAS signal was activated for safety-of-life aviation, covering 95% of the United States, and portions of Alaska <ref>[http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System Wide Area Augmentation System]</ref>. 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.
[[WAAS Messages|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.


==Notes==
==Notes==

Latest revision as of 09:08, 16 February 2021


WAASWAAS
Title WAAS Signal Structure
Edited by GMV
Level Basic
Year of Publication 2011
Logo GMV.png

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)[1] and is specially developed for the civil aviation community.[2] The system, which was declared operational in late 2003,[3] currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada.[4] WAAS service area includes CONUS, Alaska, Canada and Mexico.[5] 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.[6]

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:[7]

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:[8][9]

  • 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.

Notes

References

  1. ^ Navigation Services - History - Satellite Navigation, FAA.
  2. ^ Navigation Services - Wide Area Augmentation System (WAAS), FAA.
  3. ^ Wide Area Differential GPS (WADGPS), Stanford University
  4. ^ GNSS - GPS/WAAS Approaches, Federal Aviation Agency (FAA).
  5. ^ WAAS Service Expanded into Canada and Mexico, September 28, 2007, Federal Aviation Agency (FAA).
  6. ^ SatNav News, Vol. 33, March 2008, Federal Aviation Agency (FAA).
  7. ^ ICAO Standards and Recommended Practices, Annex 10, Volume 1 Radio Navigation Aids, July 2006
  8. ^ Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airbone Equipment
  9. ^ FAA.Specification for the Wide Area Augmentation System(WAAS). FAA-E- 2892b. August 13, 2001.
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