Galileo Signal - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 49

TABLE 3.7. Almanac parameters

Parameter No. of Bits Scale Factor Unit

SV ID 7 1 dimensionless

√ a

2 − 11 m 1 / 2 24

2 − 21 e 16 dimensionless

i 0 (relative to 56 ◦ ) 2 − 19 16 semicircle

2 − 23

0 24 semicircle

˙

2 − 38 16 semicircle/s

2 − 23

ω 24 semicircle

2 − 23

µ 24 semicircle

2 − 20

a f0 15 s

2 − 38

a f1 11 s/s

SV SHS 5 — dimensionless

SV DHS 3 — dimensionless

Data ID 2 — dimensionless

IODA 2 — dimensionless

t 0a 8 4096 s

WN a 8 1 week

Two parameters tell the satellite's signal component health SV SHS and the satel-
lite's navigation data health SV DHS . In the almanac the applicable navigation data
structure for each satellite is defined by Data ID . The IODA identifies an almanac
batch unambiguously. The update rate being slow, two bits are sufficient. All pa-
rameters are described in Table 3.7.

3.5 The Received L1 OS Signal

Let the total received power be P , the transmission delay (traveling time) be τ ,

the carrier frequency offset be f (Doppler), and the received phase be θ .Then

the received L1 OS signal can be written as

89 √ 2 P
3

s d ( cos 2

− τ) + θ .

ϕ( t ) = 0 . t − τ) − s p ( t − τ) π( f − f )( t (3.28)

The data channel and the pilot channel are denoted by d and p , respectively. The
coefficients s d and s p are products of code sequences and subcarriers with sine
phasing.
From the observation

ϕ( t ) we want to estimate τ , f ,and θ . The first step is

to find global approximate values of τ and f , which is called signal acquisition .

The second step is a local search for τ , f , and possibly θ .If θ is estimated,

the search is called coherent signal tracking . If the carrier phase θ is ignored, the

search is called noncoherent signal tracking .