Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 155

The remainder of the chapter addresses acquisition (Section 5.8); other func-
tions performed by the receiver including the sequence of initial operations (Section
5.9), data demodulation (Section 5.10), and special baseband functions (Section
5.11) such as SNR estimation and lock detection; and some special topics. The spe-
cial topics include the use of digital processing (Section 5.12), considerations for
indoor use (Section 5.13), and techniques to track the Y code without cryptographic
access to this signal (Section 5.14). Throughout the chapter, extensive use of spread-
sheet approximation equations and some experience-proven, rule-of-thumb, track-
ing threshold criteria are presented that will make it practical for the reader to not
only understand but actually design the baseband portion of a GPS receiver.

5.2 GPS Receiver Code and Carrier Tracking

Most modern GPS receiver designs are digital receivers. These receiver designs have
evolved rapidly toward higher and higher levels of digital component integration,
and this trend is expected to continue. Also, microprocessors and their specialized
cousin, DSPs, are becoming so powerful and cost effective that software defined
receivers (SDRs) are being developed that use no custom digital components. For
this reason, a high-level block diagram of a modern generic digital GPS receiver will
be used to represent a generic GPS receiver architecture, as shown in Figure 5.1. The
GPS RF signals of all SVs in view are received by a RHCP antenna with nearly hemi-
spherical (i.e., above the local horizon) gain coverage. These RF signals are ampli-
fied by a low noise preamplifier (preamp), which effectively sets the noise figure of
the receiver. There may be a passive bandpass prefilter between the antenna and
preamp to minimize out-of-band RF interference. These amplified and signal condi-

Antenna

AGC

RF

N

2

Analog
IF

Digital
IF

1
Digital receiver
channel

Down-
converter

A/D
converter

Preamp

LOs

Frequency
synthesizer

Navigation
processing

Receiver
processing

Reference
oscillator

Regulated
DC power

Power
supply

Unregulated
input power

User
interface

Figure 5.1 Generic digital GPS receiver block diagram.