Viavi 87339, Руководство по эксплуатации

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Principals of Operation
6 -24
6.1.8 GPS Modernization Signals
A process of GPS system modernization is now underway, which involves the introduction
of new signals to provide improvements in accuracy and integrity.
Fig. 6-27 GPS Modernization Signals
6.1.8.A L2CS
The L2CS system provides a civilian-use signal transmitted on a frequency other than the
L1 frequency used for the Coarse Acquisition (C/A) signal, and broadcast on the L2
frequency. Because it requires new hardware onboard the satellite, it is only transmitted
by the so-called Block IIR-M and later design satellites. The L2CS signal is tasked with
improving accuracy of navigation, providing an easy to track signal, and acting as a
redundant signal in case of localized interference.
Unlike the C/A code, L2CS contains two distinct PRN code sequences to provide ranging
information; the Civilian Moderate length code (called CM), and the Civilian Long length
code (called CL). The CM code is 10,230 bits long, repeating every 20 ms. The CL code
is 767,250 bits long, repeating every 1500 ms. Each signal is transmitted at 511,500 bits
per second (bit/s); however, they are multiplexed together to form a 1,023,000 bit/s
signal.
CM is modulated with the CNAV Navigation Message (see below), whereas CL does not
contain any modulated data and is called a data-less sequence . The long, data-less
sequence provides for approximately 24 dB greater correlation (~250 times stronger) than
L1 C/A-code.
When compared to the C/A signal, L2CS has 2.7 dB greater data recovery and 0.7 dB
greater carrier-tracking, although its transmission power is 2.3 dB weaker.
L 2 L1 L5
P(Y) P(Y)
C/A
C /A
P(Y) P(Y)
Present Signals
Signals After
2009 (Block IIF)
P(Y) P(Y)
C/A
C/A
P(Y) P(Y)
CS
CS
M M M M
P(Y) P(Y)
CS
CS
M
M
Signals After
2013 (Block III)
C/A C/A
P(Y)
P(Y)
M M
C
C
1227 MHz 1575 M Hz 1176 MHz
L 2 L1 L5
P(Y) P(Y)
C/A
C /A
P(Y) P(Y)
Present Signals
L 2 L1 L5
P(Y) P(Y)
C/A
C /A
P(Y) P(Y)
Present Signals
Signals After
2009 (Block IIF)
P(Y) P(Y)
C/A
C/A
P(Y) P(Y)
CS
CS
M M M M
P(Y) P(Y)
C/A
C/A
P(Y) P(Y)
CS
CS
M M M M
P(Y) P(Y)
CS
CS
M
M
Signals After
2013 (Block III)
C/A C/A
P(Y)
P(Y)
M M
C
C
1227 MHz 1575 M Hz 1176 MHz 1227 MHz 1575 M Hz 1176 MHz