I-Lotus RX Oncore User Manual - 10 Sbas
Document Number: 403
– GPS – 070
Page | 18
10 SBAS
Fig 10.1
SBAS system
SBAS is a system consisting of three groups of geostationary satellites located over three world regions that provide
every visible GPS satellite with one single correction to have a more accurate user position. This correction is
compound by three contributions that permit to correct the error due to three different effects related to the
ionosphere, the troposphere and the satellite, respectively.
The SBAS signal is acquired and elaborated by the RX Oncore receiver in two phases:
Front – End
Back – End
The Front – End phase is used to search the correction messages received from the SBAS system by using a GPS
channel dedicated to the SBAS satellite signal receipt. It is composed of the Viterbi module and Framer module.
The First module uses the Viterbi algorithm to decode the input stream. The second module uses a state machine
and pattern matching algorithm (Knuth Morris Pratt) to search the SBAS message in the input stream.
The Back – End inputs are a sequence of 250-bit messages provided every second. There are 62 SBAS message
types but the system elaborates just 10 message types. The SBAS message is decoded by the SBAS message
decoder and then the information is stored in two databases, depending on the type of the correction contained in
the message.