Vector Sensor Reference Manual 133
than some other techniques, resulting 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
Provided that a GPS satellite is available to the SBAS reference station network for tracking
purposes, orbit and timing error corrections will be available for that satellite. Ionospheric
corrections for that satellite are only available if the signal passes through the ionospheric map
provided by SBAS (for example, the WAAS ionospheric map covers the majority of North
America). As an example, if a satellite is South of your current location at a low elevation angle, the
pierce point of the ionosphere will be considerably South of your location since the ionosphere is
at an altitude of approximately 60 km. There must be sufficient ionospheric map coverage beyond
your location in order to have ionospheric correctors for all satellites.
To enhance the information provided by SBAS, the Vector Sensor system extrapolates the
ionospheric information beyond the broadcast information. This increases the usable geography
for WAAS and is discussed in Section 1.5.5. This feature helps to improve the usable coverage
area of a SBAS service.
Signal Information
A SBAS transmits correction data on the same frequency as GPS from a geostationary satellite (the
space segment), allowing the use of the same receiver equipment used for GPS. Another
advantage of having SBAS transmit on the same frequency is that only one antenna is required.
Reception
Since SBAS broadcast in the L-band, the signal requires a line of sight in the same manner as GPS
to maintain signal acquisition.
Currently, two commercial marine communication satellites are transmitting WAAS data for
public use, and one each is located above both the Pacific Ocean and Northern Brazil. Due to
their location, these satellites may appear lower on the horizon, depending on your geographic
position on land. In regions where the satellites appear lower on the horizon, they may be more
prone to being masked by terrain, foliage, buildings or objects, resulting in signal loss. The further
that you are away from the equator and the satellite’s longitude will cause the satellite to appear
lower on the horizon. Fortunately, the CSI Wireless COAST Technology helps alleviate this
problem by maintaining system performance when WAAS (SBAS) signal loss occurs for extended
periods of time. More information on COAST is provided in Section 1.7.
The EGNOS System Test Bed (ESTB), also referred to as EGNOS in this document, uses two
geostationary satellites (separate from WAAS), however, in this case, they are located over the
Atlantic and Indian Oceans. Similar to WAAS, the satellites may appear lower on the horizon,
depending on your geographic position on land. The further that you are away from the equator
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