GALILEO - Understanding GPS: Principles and Applications - page 574

Chip rate: R p
N Chips

p

Period 1 Period 2 Period N s Period 1

N *N chips
ps

N s Chips

N : Primary code length
N : Secondary code length

Chip rate: R =R /N
s XOR modulo-2-addition

p
s

pp

Figure 10.5 Structure of tiered codes.

Table 10.8 Main Characteristics of Spreading Codes

Code Sequence
Duration (ms)

Primary Code
Length (chips)

Secondary Code
Length (bits) Channel

E5a data 20 10,230 20

E5a pilot 100 10,230 100

E5b data 4 10,230 4

E5b pilot 100 10,230 100

E6P Not public Not public Not public

E6C data 1 5,115 N/A

E6C pilot 100 5,115 100

L1P Not public Not public Not public

L1F data 4 4,092 N/A

L1F pilot 100 4,092 25

GALILEO Message Frame Structure
The complete navigation messages will be transmitted on each data channel as a
sequence of superframes. A superframe is composed of several frames, and a frame
is composed of several subframes. The subframe is the basic structure for building
the navigation message and contains the following fields:

• A synchronization word called unique word (UW);
•

Cyclic redundancy check (CRC) bits for error detection;

Tail bits for the FEC encoder, in a defined fixed state, containing all zeros.

•

All subframes are protected by rate half convolutional FEC encoding and block
interleaving. The resultant symbol sequence is then summed with the corresponding
PRN code and used to modulate the navigation signal component.
The GALILEO message frame structure is illustrated in Figure 10.7. The current
baseline is to use a fixed frame format that allows flexibility in allocation of the
subframes to a given message data content (integrity, almanac, ephemeris, clock cor-
rection data or ionospheric correction data). Parallel investigations are ongoing for