Global Positioning System Reference
In-Depth Information
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where
n
is a positive integer, and the normalized power spectral density of the BOC
modulation can be written as
/
1
f
c
tan
(
2
π∆
f/
2
f
s
)
cos
(
π∆
f/f
c
)
if
n
is odd
π∆
f
g(
f
s
,f
c
,
∆
f)
=
(3.90)
f
c
tan
(
2
2
π∆
f/
2
f
s
)
sin
(
π∆
f/f
c
)
if
n
is even
π∆
f
W
e further adopt Betz's abbreviated notation BOC
(
α
,
β
)
to specify the frequencies,
i.e.,
f
s
MHz. For example, the modulation
B
OC(10,5) uses the subcarrier frequency and the spreading code rate of 10.23 MHz
an
d 5.115 MHz, respectively.
A characteristic difference between the BOC and the conventional rectangular
sp
reading code modulation is seen in the power spectral densities of Figure 3.13. The
de
nsities for BOC, in this case BOC(10,5), are maximum at the nulls of the P(Y)-
co
des. Such a property is important for increasing the spectral separation of modu-
la
tions. The sum of the number of mainlobes and sidelobes between the mainlobes
is
equal to
n
, i.e., twice the ratio of the subcarrier frequency to the code rate (3.89).
A
s in conventional PSK the zero crossings of each mainlobe are spaced by twice the
co
de rate, while the zero crossings of each sidelobe are spaced at the code rate. For
ex
ample, with
n
=
1
.
023
α
MHz and
f
c
=
1
.
023
β
[86
Lin
—
2.8
——
Nor
PgE
=
5 the BOC(5,2) modulations have three sidelobes between two
m
ainlobes; with
n
10 the BOC(5,1) modulations have eight sidelobes between two
m
ainlobes. In the case of
n
=
1, that is the case of BOC
(f
c
/
2
,f
c
)
, Equations (3.90)
an
d (3.86) give the same power spectral density.
The new military M-codes will use BOC(10,5), which means the subcarrier fre-
qu
ency and the spreading code rate will be 10.23 MHz and 5.115 MHz, respectively,
as
well as quadraphase modulated, i.e., they share the same carrier with the civilian
signals.
=
[86
3.3 GLONASS
The Russian Global'naya Navigatsionnaya Sputnikkovaya Sistema (GLONASS—
global navigation satellite system [GNSS]) system traces its beginnings to 1982, when
the first satellite of this navigation satellite system was launched. A time line of the
space segment is shown in Figure 3.12. The technical information about GLONASS
can be found in the interface control document GLONASS (1998). Additional details
on the system and its use, plus many references to relevant publications on the subject,
are available in Roßbach (2001).
Like GPS, GLONASS was planned to contain at least twenty-four satellites. The
nominal orbits of the satellites are in three orbital planes separated by 120°; the satel-
lites are equally spaced within each plane with nominal inclination of 64.8°. The
nominal orbits are circular with each radius being about 25,500 km. This translates
into an orbital period of about 11 hours and 15 minutes.
