Global Positioning System Reference
In-Depth Information
GNSS Antennas and Front Ends
4.1
Background
Although the focus of this text is on the algorithms for software signal process-
ing of the Global Navigation Satellite System (GNSS) signals, it is important to
consider the source of that data stream to be processed. Since “software” signal
processing is stated, it implies an input digital data stream. Thus, the purpose of
this chapter is to provide some insight into how the satellite signals propagat-
ing through space result in this digital data stream. This is done, of course, via
a GNSS antenna/front end. There are numerous topics completely devoted to the
topics of antenna and others to front-end design; see Balanis (1996) and Tsui
(2000). The purpose of this chapter is to illustrate functional designs for GNSS,
discuss the tradeoffs associated with different designs, and provide a basic un-
derstanding of the source of the digital data to be processed. The focus is on the
narrowband GNSS L1 signals, primarily the Global Positioning System (GPS)
Coarse/Acquisition (C/A) code, but references are made to the Galileo BOC(1,1)
code where appropriate. At the end of the chapter, multiple-band GNSS front ends
are introduced.
The process begins with the GNSS signal, propagating through space, which
is incident on a user's GNSS antenna. This, in turn, induces a voltage within the
element. That voltage is extremely weak, corresponding to a guaranteed signal
power of
160 dBW in the case of the Global Positioning System (GPS) [see
ICD-GPS-200 (1991)] and has a carrier frequency of 1575.42 MHz. Considering
a bandwidth of 2 MHz (the approximate null-to-null bandwidth the GPS C/A code
signal), the received GPS signal power is actually below that of the thermal noise
floor, as defined by Equation (4.1) with a simplified illustration in Figure 4.1.
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