WAAS and its Relation to Enabled Hand-Held GPS Receivers
26 Feb. 2006  (Revised 19 Mar. 2012)
For a discussion of DGPS and Technical WAAS issues, see (HERE)

Statement from the FAA

WAAS is based on a network of approximately 25 ground reference stations that covers a very large service area. Signals from GPS satellites are received by wide area ground reference stations (WRSs). Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist. These WRSs are linked to form the U.S. WAAS network. Each WRS in the network relays the data to the wide area master station (WMS) where correction information is computed. The WMS calculates correction algorithms and assesses the integrity of the stem.  A correction message is prepared and uplinked to a geosynchronous satellite via a ground uplink system (GUS). The message is then broadcast from the satellite on the same frequency as GPS (L1, 1575.42MHz) to receivers on board aircraft (or hand-held receivers) which are within the broadcast
coverage area of the WAAS. These communications satellites also act as additional navigation satellites for the aircraft, thus, providing additional navigation signals forposition determination.

The WAAS will improve basic GPS accuracy to approximately 7 meters vertically and horizontally, improve system availability through the use of geostationary communication satellites (GEOs) carrying navigation payloads, and to provide important integrity information about the entire GPS constellation.  At present there are three geostationary satellites serving the WAAS area.  See graphic below.


The European area will eventually be served by two Inmarsats, AOR-E (Atlantic Ocean Region-East) and IOR (Indian Ocean Region) and the European Space Agency satellite, ARTEMIS.  The footprints of all but ARTEMIS (Aircraft-Based Augmentation System) is shown below.  On the future ARTEMIS satellite, the GPS/GLONASS augmentation is made directly from aircraft based equipment.  Japan will be served by the MSAS system.  The first MSAS satellite was lost on launch.

EGNOS & WAAS do not currently share almanac information, and EGNOS is broadcasting a "do not use" indication.  So it is unlikely that users in Europe will see any response from EGNOS until their systems share more information and allow use of the corrections.

WAAS Information site: http://www.nstb.tc.faa.gov/

Garmin units allow for 19 WAAS/EGNOS/MSAS unique GEO satellites as specified by the FAA TSO C-146.   They are depicted on the GPS as Satellite IDs 33-51 which is actually a NMEA convention.  Each WAAS/EGNOS/MSAS satellite will have its own unique PRN code assigned from the list of 19.  The WAAS satellites are shown on the GPS satellite page with an ID below.  These satellites do not move on the screen as do the other GPS low-earth-orbit satellites.  A short comparison of WAAS hand-held accuracy is shown (HERE).

Garmin receivers use one or two channels to track WAAS satellites and they will use the WAAS satellite in the position solution, if the WAAS system indicates it is OK to use for navigation (sometimes the WAAS satellite is flagged as "do not use for navigation" but the corrections are still useful).

WAAS reference stations for the USA.  WAAS coverage is approximately 200nm around these stations

Another map of the WAAS service area is at: http://www.nstb.tc.faa.gov/RT_VerticalProtectionLevel.htm
This map concentrates on the Vertical Protection Level -the most important measure for enroute aircraft

WAAS GEO Coverage Starting in Fall of 2010
A global map which is not quite up to date is shown (HERE)

Here is the PRN/Satellite ID information for WAAS and EGNOS
NOTE: The satellites IDs are the PRN numbers less 87.

The following PRNs have been allocated to the WAAS system:
   Geostationary        PRN      NMEA Satellite ID       Longitude
        Inmarsat        133             46                   98°W
        Galaxy 15**     135             48                  133°W
        Anik F1R##      138             51                  107°W
** Has drifted out of orbit
## Check (HERE) to see the elevation angle to expect on your unit in the US

The following PRNs have been allocated to the EGNOS system:
        AOR-E           120             33                 15.5°W
       Artemis          124             37                 21.5°E
        IOR-W           126             39                 25.1°E
        IOR-E           131             44                   ?

The following PRNs have been allocated to the MSAS system:
       MTSAT-1          129             42                  140°E
       MTSAT-2          137             50                  145°E

The following PRNs have been allocated to the GAGAN system:
                                                            127                                  40                                                   82°E
                                                            128                                  41                                                   55°E

A Typical Ionospheric-delay Map of the US as Computed by the "GPS Receiver" Sites from all received GPS Signals
A moving graphic of the daily ionospheric delays can be seen (HERE)

Ionospheric Corrections:
The IONO information transmitted by the WAAS system is much more accurate than the basic GPS IONO model.  Also, the WAAS system will generally be more accurate than beacon based DGPS because of the way the corrections are rendered by the WAAS system and applied by the GPS receiver. The primary factor is spatial decorrelation, which is the degradation of corrections due to separation from the reference station.  RTCM based DGPS corrections suffer from spatial decorrelation, but WAAS corrections do not.

This Iono data (and other corrections) are constantly uploaded to the Geo Sats for re-transmission to GPS navigation receivers.  There is no interpolation between ground stations by the receiver.  This is because the WAAS master system computes a "grid of Iono corrections" which are location dependent based on the user's position.  There is an interpolation/extrapolation process to determine the iono correction, but it is not specifically related to the location of ground stations that collect the information. The Iono-corrections grid offered by WAAS are interpolated and applied by the receiver.

GPS receivers must then apply the data for corrections appropriate at their location.  This may take five or ten minutes to complete in a typical receiver.

Here are some interesting links:

Some encouraging results using aircraft testbeds

The FAA 727 performed a series of Category I precision approaches to the runway at Keflavik, Iceland Airport using onboard equipment that received signals from the FAA's National Satellite Test Bed (NSTB), which is a forerunner to WAAS, and the United Kingdom's (U.K.) Northern European Satellite Test Bed (NESTBed). An Iceland Civil Aviation Administration (ICAA) Beechcraft King Air 200 and a U.K. National Air Traffic Services BAC 1-11 also performed Category I approaches using signals from both systems. Category I approaches are used primarily in bad weather where the pilot must see the runway at no less than 200 feet above the ground and at a distance of one-half mile.

EGNOS tests in Europe

The NESTBED results for positioning accuracy gathered during flight trials are promising. Corrected position results show: 95% of horizontal errors less than 2.5m and 95% of vertical errors less than 4.5m, well within the requirement for CAT 1 aircraft landing (CAT 1 is the less demanding of 3 categories or precision approach [ie. landing] for aircraft, where guidance must be provided to a Decision Height of between 200 and 350 feet above the runway at a range of 1 mile).

Map of EGNOS Coverage and Estimated Accuracy

Jack Yeazel