GNSS Signal Simulation - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 156

chronization to the start of the subframe (similar to the preamble in GPS).
The unique synchronization word is followed by the data part, a checksum
field, and tail bits. It is expected that the construction of the data part will
be different from that of the GPS messages.
Ephemerides and Almanac The satellite orbit parameters have the same field size
and scale in both systems. The time parameters have different field size and
scales (except clock correction coefficients in the almanac).
SynchronizationWord (Preamble) GPS is using an 8-bit (symbol) pattern. Galileo
is likely to use a 10-symbol pattern.
Error Detection Galileo will use cyclic redundancy check (CRC) to detect data
corruptions inside subframes. It is expected that the CRC will be computed
over the 24 data bits in the subframe. The specification does not indicate
that any bits from the previous subframe should be used in computation of
the CRC.
Channel Coding In addition to CRC, Galileo will use forward error correction
(FEC) to detect data corruptions and correct corruptions to a certain ex-
tent. This will facilitate correction of a much larger amount of corruptions
compared to GPS where only one bit per subframe can be corrected. Block
interleaving will be used to make the Galileo data even more corruption
resistant.
FEC is used already in WAAS and EGNOS signals.
Data Authentication Galileo is likely to use a data authentication technique to
make the GNSS signal tracking secure. The purpose of the data authentica-
tion is to provide means for the user to distinguish genuine Galileo signals
from simulated signals (the technical term is signal spoofing). Signal spoof-
ing is an intentional malicious provision of faulty signals that can lead to
malicious location spoofing.
Only certified receivers will be able to decode authentication information,
but the authentication will be provided by the OS signals. P(Y) code en-
cryption is the means of signal authentication in GPS.
Modulation Galileo uses BOC(1,1) modulation (which in effect perform man-
chester encoding of the data and pilot channel) and the CASM multiplexing
scheme to combine three signals into a hexaphase representation whereas
GPS uses BPSK.
Time Reference Galileo will use a reference time called Galileo System Time
(GST) whereas GPS uses GPS Time (GPST). GPS Time is a composite
clock; the average of a number of GPS clocks computed in a Kalman filter.
Galileo System Time is a master clock; the output of a steered active H-
maser. Galileo System Time will be steered to International Atomic Time
(TAI) at the Galileo Precise Time Facility (PTF). GPS Time is steered to
a real-time representation of Coordinated Universal Time (UTC) by the