Global Positioning System Reference
In-Depth Information
3
Unambiguous Processing Techniques of
Binary Offset Carrier Modulated Signals
Zheng Yao
Tsinghua University
China
1. Introduction
In recent years, applications of global navigation satellite systems (GNSS) are developing
rapidly. The growing public demand for positioning and location services has generated
higher requirements for system performance. However, the performance of the traditional
GPS is constrained by its inherent capability. In order to cope with both the civil and
military expectations in terms of performance, several projects are launched to promote the
next generation of GNSS (Hegarty & Chatre, 2008). GPS is undergoing an extensive
modernization process (Enge, 2003), while the European satellite system, Galileo, is also
under construction. In addition, Russia is restoring their GLONASS (Slater et al., 2004), and
China is in the midst of launching Compass (Gao et al., 2007).
Based on the experience gained during the traditional GPS design and operation, signal
structures of these new navigation systems have been well designed (ARINC, 2005, 2006). A
large number of modifications have been made intended to address the main weakness of
traditional GPS, and to enhance its inherent performance. The accuracy and reliability of
those modernized signals and the compatibility between new signals and already-existing
signals have been simultaneously taken into account in the design.
Binary offset carrier (BOC) (Betz, 2001) and multiplexed binary offset carrier (MBOC)
modulations (Hein et al., 2006) have been chosen as the chief candidate for several future
navigation signals, for example, GPS L1C, GPS M-code, and Galileo open service (OS)
signals. BOC modulation is a square-wave modulation scheme. It moves signal energy away
from the band center and thus achieves a higher degree of spectral separation between BOC
modulated signals and other signals which use traditional binary phase shift keying (BPSK)
modulation, such as the GPS C/A code, in order to get a more efficient sharing of the
L-band spectrum. Besides, many studies (Avila-Rodriguez, et al., 2007; Betz, 2001; Hein, et
al., 2006) show that BOC modulation also provides better inherent resistance to multipath
and narrowband interference.
However, despite these advantages, some problems remain with the use of BOC
modulation. According to the theory of matched filtering (Proakis, 2001), when the
waveform of the local signal is as same as the received one, the output of the correlator has
the highest signal-noise-ratio (SNR). For this reason, in traditional GPS receivers, both of the
acquisition and tracking are based upon the auto-correlation function (ACF) of the received
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