Fundamentals of Satellite Navigation - Understanding GPS: Principles and Applications - page 50

relatively high altitude (which in turn produces good dilution of precision proper-
ties), and a relatively low number of satellites required to provide the redundancy of
coverage required for navigation. It is true that stationkeeping is more frequent at
the GPS 12-hour orbital altitude than other potential altitudes in the 20,000- to
25,000-km range due to the resonance issue discussed is Section 2.3.2.3, and so
newer satellite navigation architectures, such as that for GALILEO, consider crite-
rion (6) and make slight modifications to the exact orbital altitude of the MEO
constellation. (GALILEO is discussed in Chapter 10.)
The robustness considerations of (4) and (5) drove the desire for multiple satel-
lites per orbital plane, versus a more generalized Walker-type constellation that
could provide the same level of coverage with fewer satellites but in separate orbital
planes (see the discussion at the end of Section 2.3.2.3). Ultimately, a 6-plane config-
uration was selected with four satellites per plane. The orbital planes are inclined by
55°, in accordance with Walker's results, but due in part to early plans to use the
Space Shuttle as the primary launch vehicle. The planes are equally spaced by 60° in
right ascension of the ascending node around the equator. Satellites are not equally
spaced within the planes, and there are phase offsets between planes to achieve
improved geometric dilution of precision characteristics of the constellation. Hence,
the GPS constellation can be considered a tailored Walker constellation.
In reality, more than 24 satellites are operated on orbit today, in part to provide
greater accuracy and robustness of the constellation and, at the time of this writing,
in part because a relatively large number of Block IIR satellites exist in storage on the
ground, so “overpopulation” of the constellation has been possible.

2.4 Position Determination Using PRN Codes

GPS satellite transmissions utilize direct sequence spread spectrum (DSSS) modula-
tion. DSSS provides the structure for the transmission of ranging signals and essen-
tial navigation data, such as satellite ephemerides and satellite health. The ranging
signals are PRN codes that binary phase shift key (BPSK) modulate the satellite car-
rier frequencies. These codes look like and have spectral properties similar to ran-
dom binary sequences but are actually deterministic. A simple example of a short
PRN code sequence is shown in Figure 2.14. These codes have a predictable pattern,
which is periodic and can be replicated by a suitably equipped receiver. At the time
of this writing, each GPS satellite broadcasted two types of PRN ranging codes: a
“short” coarse/acquisition (C/A)-code and a “long” precision (P)-code. (Additional
signals are planned to be broadcast. They are described in Chapter 4.) The C/A code
has a 1-ms period and repeats constantly, whereas the P-code satellite transmission
is a 7-day sequence that repeats approximately every Saturday/Sunday midnight.
Presently, the P-code is encrypted. This encrypted code is denoted as the Y-code. The
Y-code is accessible only to PPS users through cryptography. Further details regard-

1 11 1 1 1 1 1 1 1

− 1 − 1 − 1 − 1 − 1 − 1 − 1 − 1 − 1

Figure 2.14 PRN ranging code.