SXBlue GPS Reference Manual
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Upon the successful completion of a 21-day test on August 24, 2000, the FAA
announced that WAAS would be running 24 hours per day, seven days per week from
then on. Testing has shown since that this signal is accurate and reliable, however,
since no official statement on it’s Initial Operating Capability has been issued, this signal
is to used at your risk.
Other government agencies are in the process of developing compatible SBAS systems
for their respective geographic regions. In Europe, the European Space Agency, the
European Commission, and EUROCONTROL are jointly developing the European
Geostationary Overlay System (EGNOS). In Japan, the MTSAT Satellite-based
Augmentation System (MSAS) is in progress of development by the Japan Civil Aviation
Bureau (JCAB). China has a similar program for a SBAS and the service is named the
Chinese Satellite Navigation Augmentation System (SNAS). The SXBlue GPS is
capable of receiving correction data from all compatible SBAS.
EGNOS is currently in a prototyping phase, referred to as the EGNOS System Test Bed
(ESTB) and which has been broadcasting a test signal since February 2000. EGNOS
should be used at your risk only. MSAS has yet to begin transmitting data publicly.
SNAS is transmitting correction data currently on a military communication channel and
is expected to become publicly available in the near future.
Warning – Although WAAS has successfully passed a 21-day test, and is publicly
available; its use is at your risk and discretion. EGNOS is not currently
broadcasting with any form of certification or approval. It may produce misleading
information, and its use is entirely at your risk and discretion.
MSAS may begin broadcasting a preliminary signal as early as the end of 2002.
How it Works
A SBAS incorporates a modular architecture, similar to GPS, comprised of a Ground
Segment, Space Segment, and User Segment:
•
The Ground Segment includes reference stations, processing centers, a communication
network, and Navigation Land Earth Stations (NELS)
•
The Space Segment includes geostationary satellites (For example, WAAS and EGNOS
use Inmarsat-III transponders).
•
The user segment consists of the user equipment, such as a SXBlue GPS receiver and
antenna
A SBAS uses a state-based approach in their software architecture. This means that a
separate correction is made available for each error source rather than the sum effect of
errors on the user equipment’s range measurements. This more effectively manages
the issue of spatial decorrelation than some other techniques, resulting in a more
consistent system performance regardless of geographic location with respect to
reference stations. Specifically, SBAS calculates separate errors for the following:
•
The ionospheric error
•
GPS satellite timing errors
•
GPS satellite orbit errors
