Global Positioning System Reference
In-Depth Information
1.4
GPS Modernization Program
In January 1999, the U.S. government announced a new GPS modernization initia-
tive that called for the addition of two civil signals to be added to new GPS satellites
[11]. These signals are denoted as L2C and L5. The L2C signal will be available for
nonsafety of life applications at the L2 frequency; the L5 signal resides in an aero-
nautical radionavigation service (ARNS) band at 1,176.45 MHz. L5 is intended for
safety-of-life use applications. These additional signals will provide SPS users the
ability to correct for ionospheric delays by making dual frequency measurements,
thereby significantly increasing civil user accuracy. By using the carrier phase of all
three signals (L1 C/A, L2C, and L5) and differential processing techniques, very
high user accuracy (on the order of millimeters) can be rapidly obtained. (Iono-
spheric delay and associated compensation techniques are described in Chapter 7,
while differential processing is discussed in Chapter 8.) The additional signals also
increase the receiver's robustness to interference. If one signal experiences high
interference, then the receiver can switch to another signal. It is the intent of the U.S.
government that these new signals will aid civil, commercial, and scientific users
worldwide. One example is that the combined use of L1 (which also resides in an
ARNS band) and L5 will greatly enhance civil aviation.
During the mid to late 1990s, a new military signal called M code was devel-
oped for the PPS. This signal will be transmitted on both L1 and L2 and is spectrally
separated from the GPS civil signals in those bands. The spectral separation permits
the use of noninterfering higher power M code modes that increase resistance to
interference. Furthermore, M code will provide robust acquisition, increased accu-
racy, and increased security over the legacy P(Y) code.
Chapter 4 contains descriptions of the legacy (C/A code and P(Y) code) and
modernized signals mentioned earlier.
At the time of this writing, it was anticipated that both M code and L2C will be
on orbit when the first Block IIR-M (“R” for replenishment, “M” for modernized)
satellite is scheduled to be launched. (The Block IIR-M will also broadcast all legacy
signals.) The Block IIF (“F” for follow on) satellite is scheduled for launch in 2007
and will generate all signals, including L5. Figure 1.1 provides an overview of GPS
signal evolution. Figures 1.2 and 1.3 depict the Block IIR-M and Block IIF satellites,
respectively.
At the time of this writing, the GPS III program was underway. This program was
conceived in 2000 to reassess the entire GPS architecture and determine the necessary
architecture to meet civil and military user needs through 2030. It is envisioned that
GPS III will provide submeter position accuracy, greater timing accuracy, a system
integrity solution, a high data capacity intersatellite crosslink capability, and higher
signal power to meet military antijam requirements. At the time of this writing, the
first GPS III satellite launch was planned for U.S. government fiscal year 2013.
1.5
GALILEO Satellite System
In 1998, the European Union (EU) decided to pursue a satellite navigation system
independent of GPS designed specifically for civilian use worldwide. When com-
 
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