Global Positioning System Reference
In-Depth Information
signals. Nevertheless, because of the square-wave modulated symbol, a BOC modulated
signal has a sawtooth-like, piecewise linear ACF which has multiple non-negligible side
peaks along with the main peak. Since there are significant amount of signal energy located
at side peaks of BOC ACF, in acquisition stage, under the influence of noise it is quite likely
that one of side peak magnitudes exceeds the main peak, and false acquisition will happen.
If false acquisition occurs, the code tracking loop will initially lock on the side peak.
Similarly, due to the side peaks of ACF, in code tracking loop, the discriminator
characteristic curve of a BOC modulated signal has multiple stable false lock points. Once
the loop locks on one of the side peaks, it would result in intolerable bias in pseudorange
measurements, which is unacceptable for GNSS aiming to provide accurate navigation
solution. This problem is reputed as the ambiguity problem
for BOC modulated signal
acquisition and tracking. And in order to employ BOC modulated signals in the next
generation GNSS, solutions have to be found to minimize this bias threat.
In this Chapter, the ambiguity problem of BOC modulated signals as well as its typical
solutions is systematically described. An innovative design methodology for future
unambiguous processing techniques is also proposed. Some practical design examples on
this methodology are also given to show the practicality and to provide reference to further
algorithm development.
The rest of the Chapter is organized as follows. In Section 2, the concept and some main
characteristics of BOC modulated signals are given, and the ambiguity problem is also
described. In Section 3, some existing representative solutions to ambiguity problem are
reviewed. Then in Section 4, we present a parameterized chip waveform pattern, and on this
basis, give the analytic design framework for side-peak cancellation (SC) based
unambiguous BOC signal processing algorithm development. As two application examples
of the proposed design framework, the design process of an SC unambiguous acquisition
algorithm as well as an SC unambiguous tracking loop is described in Section 5 and Section
6, respectively. Finally, some conclusions are drawn in Section 7.
2. BOC modulated signals
2.1 Definitions and main characteristics
In order to take advantage of the frequent phase inversions in the spreading waveform to
realize the precise ranging, and to obtain excellent multiple access capability, the majority of
GNSS employ direct sequence spread spectrum (DSSS) technique. DSSS can be regarded as
an extension of binary phase shift keying (BPSK). The transmitting signal yielded by this
technique can be expressed as the product of the un-modulated carrier, data
d
(
t
), as well as
the baseband spreading signal
g
(
t
), that is
( )
( ) ( ) (
)
st
=
Adt gt
cos 2
π
ft
+
θ
(1)
s
0
where
A
s
is the amplitude of signal,
f
0
is the carrier frequency in Hz, and θ is the carrier
phase in radians. The baseband spreading signal
g
(
t
) can be further represented as
∞
()
∑
( ) (
c
)
g t
=− −
1
i
p tiT
(2)
c
i
=−∞
Search WWH ::
Custom Search