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 system. 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 for
position 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.
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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.
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/vpl.html
This map concentrates on the
Vertical Protection Level -the most important measure for enroute
aircraft
Here is the PRN/Satellite ID information for WAAS and EGNOS:
Inmarsat
PRN Garmin Satellite ID
AOR-E
120
33
AOR-W
122
35
Artemis
124
?
IOR
131
44
POR
134
47
The following PRNs have been allocated to the MSAS system:
Satellite
PRN Garmin Satellite ID
MTSAT-1
129
42
MTSAT-2
137
50
One can readily determine the elevation and bearing of these satellites at their location from these URLs:
Name Series
Location
AOR-W 3F4 Brazil east
coast
http://www.lyngsat.com/tracker/inmar3f4.shtml
AOR-E 3F2 West of
Africa
http://www.lyngsat.com/tracker/inmar3f2.shtml
IOR 2F3 and/or
3F1 Indian Ocean
http://www.lyngsat.com/tracker/inmar3f1.shtml
POR 3F3 and/or
2F1 Pacific Ocean
http://www.lyngsat.com/tracker/inmar3f3.shtml
Comments from a Garmin engineer:
I
have used AOR-E (EGNOS) in the past, but the system broadcasts a "do not use"
indication much of the time during developmental phases. AOR-E is visible
from the East Coast of the US but again the corrections are of little value
because the system does not transmit IONO correction outside the coverage area
of its ground stations. Right now the US WAAS system is the only system
giving reliable correction information.
I have also used POR during some of my flights over the Pacific and have also used AOR-W in the UK. However, when using these satellites outside the coverage area of the ground station network, the WAAS corrections do little to improve the accuracy, because there is no IONO delay information available. Now that S/A is gone, IONO is probably the most significant source of positioning error.
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:
http://waas.stanford.edu/~wwu/rfuller/iongps98/sld001.htm
http://www.sciencenet.org.uk/slup/CuttingEdge/Mar00/navigation.html
http://www.avweb.com/articles/satnav.html
Some encouraging results using aircraft testbeds
EGNOS tests in Europe
Map of
EGNOS Coverage and Estimated Accuracy