GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 145

subframe 4 includes ionospheric correction parameters for single-frequency users
(see Section 7.1.2.5) and parameters so that user equipment can relate UTC to GPS
system time (see Section 2.6.3). Page 25 of subframes 4 and 5 provide configuration
and health flags for SVs 1-32. The data payloads of the remaining pages of
subframes 4 and 5 are currently reserved.

4.5 Modernized GPS Signals

At the time of this writing, three additional signals were anticipated to be broadcast
by GPS satellites by 2006. As illustrated in Figure 4.20, these include two new civil
signals, an L2 civil (L2C) signal [10, 16] and a signal at 1,176.45 MHz (115 f 0 )
referred to as L5 [17, 18]. A new military signal, M code, will also be added at L1
and L2 [19]. This section provides an overview of each of these new signals.

4.5.1 L2 Civil Signal
As shown in Figure 4.20, the L2 civil (L2C) signal has a similar power spectrum
(i.e., 2.046 MHz null-to-null bandwidth) to the C/A code. L2C is very different
from the C/A code in many other ways, however. First, L2C uses two different PRN
codes per satellite. The first PRN code is referred to as the civil moderate (CM) code
because it employs a sequence that repeats every 10,230 chips, which is considered
to be of moderate length. The second code, the civil long (CL) code, is extremely
long with a length of 767,250 chips. As shown in Figure 4.21, these two codes are
generated, each at a 511.5-kchip/s rate, and are used in the following manner to
generate the overall L2C signal: First, the CM code is modulated by a 25-bps navi-
gation data stream after the data is encoded into a 50-baud stream with a rate
one-half constraint-length 7 FEC code. The 25-bps data rate is one-half the rate of
the navigation data on the C/A code and P(Y) code signals and was chosen so that
the data on the L2C signal can be demodulated in challenged environments (e.g.,
indoors or under heavy foliage) where 50-bps data could not be. Next, the baseband

C/A code

P(Y) code P(Y) code

C/A code

L2C

P(Y) code

P(Y) code

L5

M code

M code

Frequency

L5
(1,176.45 MHz)

L2
(1,227.6 MHz)

L1
(1,575.42 MHz)

Figure 4.20 Legacy (top) and modernized GPS signals (bottom).