14.3 Satellite Radio Navigation Systems
255
The service of standard positioning SPS and time synchronization is available for
all categories of consumers free of charge and globally and is realized through the
emission of navigation radio signals by all GPS SV, modulated by the rangefinder
code C/A (Coarse / Acquisition). The C/A code is a 1023 symbol Gold pseudorandom sequence (PRS) with a 1.023 MHz clock rate. Thus, the PRS of C/A-code has
a repetition period of T = 1 ms, which corresponds to a pseudo-range unambiguous
measurement interval of about 300 km. The GPS Development Program envisions
the provision of SPS services to civilian customers using L2C, L5 and L1C signals.
The PPS is realized by the emission of navigation radio signals in the L1 and L2
bands, modulated by the ranging P(Y) code, by all space vehicles of the GPS orbital
constellation. The PPS service is intended for use exclusively by the US military, US
federal agencies, and some Allied military forces.
The regular GPS orbital constellation consists of 32 main satellites located in six
circular orbits, denoted by Latin letters from A to F. Additionally, in some orbits there
may be one or two backup satellites designed to maintain the system parameters in
the event of the main satellites failing. The inclination of the orbital planes is 55°, and
the longitudes of the ascending nodes differ by 60°. An orbital altitude of 20,200 km
corresponds to an orbital period of 11 h 58 min, i.e., the orbits of GPS vehicles are
synchronous.
Currently, the replenishment of the orbital constellation is carried out by the launch
of Block IIF space vehicles (“F”—follow on). In accordance with the current plans,
Block IIF satellites must replace Block IIA satellites in orbit, Block III satellites will
replace Block IIR (“R”—replacement). The main task of the Block III NSV is to
provide navigation services using the new L1C navigation radio signal and to improve
the accuracy of ephemeris-time information, the availability of the navigation radio
signal, the radiation power, as well as increase the active lifetime. Table 15.5 shows
the satellites technical characteristics of the GPS. The spectrum of GPS navigation
radio signals is shown in Fig. 14.7 (Fig. 14.6).
The characteristics of the orbital GPS constellation are given in Tables 14.4 and
14.5.
The characteristics of the GPS navigation radio signals are given in Table 14.6.
The introduction of new GPS navigation signals is accompanied by the improvement of the structure of digital information and the use of new types of modulation,
as well as the transition from the structure of the navigation message of the NAV
type to the structures of the CNAV and CNAV-2 type. Navigational messages such as
CNAV are improved versions of the navigation message NAV, allowing more accurate transmission of real-time and non-real-time information about the state of the
GPS. The CNAV navigation message contains information of the same type as the
NAV message (current time, signs of the satellite state, ephemeris-time information,
system almanac, etc.), but this information is transmitted in a new format. Instead
of using a superframe/frame architecture, the message is transmitted as packets of
different lengths. The most significant changes in the CNAV structure are the expansion of the number of space vehicles used for its intended purpose from 32 up to 63,
as well as the ability to promptly transmit data on the performance of a particular
vehicle (integrity) with a delay of less than 6 s.
255
The service of standard positioning SPS and time synchronization is available for
all categories of consumers free of charge and globally and is realized through the
emission of navigation radio signals by all GPS SV, modulated by the rangefinder
code C/A (Coarse / Acquisition). The C/A code is a 1023 symbol Gold pseudorandom sequence (PRS) with a 1.023 MHz clock rate. Thus, the PRS of C/A-code has
a repetition period of T = 1 ms, which corresponds to a pseudo-range unambiguous
measurement interval of about 300 km. The GPS Development Program envisions
the provision of SPS services to civilian customers using L2C, L5 and L1C signals.
The PPS is realized by the emission of navigation radio signals in the L1 and L2
bands, modulated by the ranging P(Y) code, by all space vehicles of the GPS orbital
constellation. The PPS service is intended for use exclusively by the US military, US
federal agencies, and some Allied military forces.
The regular GPS orbital constellation consists of 32 main satellites located in six
circular orbits, denoted by Latin letters from A to F. Additionally, in some orbits there
may be one or two backup satellites designed to maintain the system parameters in
the event of the main satellites failing. The inclination of the orbital planes is 55°, and
the longitudes of the ascending nodes differ by 60°. An orbital altitude of 20,200 km
corresponds to an orbital period of 11 h 58 min, i.e., the orbits of GPS vehicles are
synchronous.
Currently, the replenishment of the orbital constellation is carried out by the launch
of Block IIF space vehicles (“F”—follow on). In accordance with the current plans,
Block IIF satellites must replace Block IIA satellites in orbit, Block III satellites will
replace Block IIR (“R”—replacement). The main task of the Block III NSV is to
provide navigation services using the new L1C navigation radio signal and to improve
the accuracy of ephemeris-time information, the availability of the navigation radio
signal, the radiation power, as well as increase the active lifetime. Table 15.5 shows
the satellites technical characteristics of the GPS. The spectrum of GPS navigation
radio signals is shown in Fig. 14.7 (Fig. 14.6).
The characteristics of the orbital GPS constellation are given in Tables 14.4 and
14.5.
The characteristics of the GPS navigation radio signals are given in Table 14.6.
The introduction of new GPS navigation signals is accompanied by the improvement of the structure of digital information and the use of new types of modulation,
as well as the transition from the structure of the navigation message of the NAV
type to the structures of the CNAV and CNAV-2 type. Navigational messages such as
CNAV are improved versions of the navigation message NAV, allowing more accurate transmission of real-time and non-real-time information about the state of the
GPS. The CNAV navigation message contains information of the same type as the
NAV message (current time, signs of the satellite state, ephemeris-time information,
system almanac, etc.), but this information is transmitted in a new format. Instead
of using a superframe/frame architecture, the message is transmitted as packets of
different lengths. The most significant changes in the CNAV structure are the expansion of the number of space vehicles used for its intended purpose from 32 up to 63,
as well as the ability to promptly transmit data on the performance of a particular
vehicle (integrity) with a delay of less than 6 s.