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GNSS Signals
The Global Navigation Satellite System (GNSS) is the standard
term used for the worldwide satellite
radio-navigation systems that provide autonomous geo-spatial
positioning with global coverage. GNSS
includes the United States of Americas Global Positioning System
(GPS), Russias GLObal NAvigation
Satellite System (GLONASS), the future Chinese COMPASS system
and the future European Galileo. AllGNSS systems operate by the
basic principle of calculating the users position by establishing
the
distance relative to the satellites with known positions. The
distance is calculated from the travel time of
radio waves transmitted from the satellites.
Through the modernization of GPS, the development of Galileo and
COMPASS and the reconstruction of
GLONASS the aim to have a complete technically interoperable and
compatible GNSS will have been
achieved. It will be possible for civilians to use the full
system without considering the nationalities of
any given system in order to promote the safety and convenience
of life (GALILEO, 2003; Feng, 2003).
Presently the core satellite navigation systems are the GPS and
GLONASS. It is impossible to put a single
figure on the accuracy of these systems as it depends on several
ever-changing factors, many of which
affect the ionosphere, the biggest single source of error. They
are: position, time of day, season and
solar activity (which affect the ionosphere), the number of
operating satellites in the constellation and
their angular spacing from the aircraft, update of satellite
clocks and ephemeris data, reflection from
buildings and terrain (multipath) and receiver performance [gnss
booklet]. The error budget of GPS is
summarized in Table 1.
Table 1: GPS error budget [gnss booklet].
Satellite clock 3 m
Satellite ephemeris 3 m
Ionospheric delay 10 m
Tropospheric delay 3 m
Multipath 3 m
Receiver noise 1.5 m
Total 12 m
The accuracy available through these core systems has been found
inadequate for precision positioning
requirements. Consequently, they have been augmented through
integrity monitoring systems in three
major ways, namely: Aircraft Based Augmentation System (ABAS)
compares navigation solutions
received from GNSS systems with information available on-board
the aircraft, Ground Based
Augmentation System (GBAS) provide integrity monitoring through
data obtained from the ground and
transmitting the corrections to an aircraft through a suitable
data link. Space Based Augmentation
System (SBAS) refers to having GEO satellite based GPS
compatible navigation payloads transmitting in
L1 and L5 bands over a region supported by the necessary ground
segment and uplink earth stations.
Several SBAS systems have been developed globally to offer
regional positioning accuracy solutions.
While some are currently fully operational, some are still under
development. They include: European
Geostationary Navigation Overlay Service (EGNOS), the Indian GPS
Aided GEO Augmented Navigation
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system (GAGAN), the Japanese MTSAT Satellite-based Augmentation
System (MSAS) and the US Wide
Area Augmentation System (WAAS). All these systems use
frequencies in the L-band of the radio
frequency spectrum.
Figure 1: Space based augmentation systems (source:
http://www.dlr.de/kn).
Each satellite navigation system has specific signal
characteristics and through modernization each
system attempts to be compatible with the others while also
avoiding interference and attenuation
between the signals. Table 2 provides an overview of the signals
currently being used by GNSS systems.
Table 2: GNSS signals currently in use.
GPS: L1 - (1575.42MHz)(C/A, P(Y), L1M), L2 -
(1227.60MHz)(P(Y),
L2C, L2M), L3 (1381.05MHz)(Used by NUDET), L5 (1176.45MHz)
(New civilian (safety-of-life) signal)
GLONASS: L1 - (1602.2MHz)(FDMA Civilian & Military),L2 -
(1246.00MHz)(FDMA Civilian & Military)
GALILEO: L1- (1575.42MHz), E5- (1189MHz), E6-(1278.75)
COMPASS: E1 - (1589 MHz), E2 - (1561 MHz), E5b - (1207 MHz)
& E6
- (1268 MHz)
The L-band is now crowded and researchers have explored possible
new allocations for use in GNSS,
notably the C- and S-band. C-band navigation will be the scope
of this work.
C-band refers to the portion of the electromagnetic spectrum in
the microwave range of frequencies
between 4 GHz and 8 GHz. Figure 2 shows the current radio bands
as categorized in wavelength and
frequency domains. C-band lies between UHF and SHF while L-band
lies in the UHF region.
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Figure 2: electromagnetic spectrum.
The need for the use of C-band is driven by the need to
introduce alternative and complimentary
capabilities to those services already being offered by systems
now in operation or under development.
The World Radio Communications Conference 2000 (WRC-2000)
allocated the portion of C-band
between 5010 and 5030 MHz for RNSS space-to-Earth
applications.
Currently, the available spectrum which can be used for the
development of Radio-Navigation Satellite
Systems (RNSS) as allocated by the International
Telecommunications Union (ITU) is shown in Figure 3[GNSS signals].
GPS, GLONASS, COMPASS, Galileo, the constituent elements of GNSS,
and future GNSS
augmentations, are planned to operate in this bands.
Figure 3: Radio-Navigation Satellite Systems (RNSS) frequency
spectrum defined for GNSS signals
(Galileo 2005).
Previously, the C-band spectrum has been considered and rejected
due to higher free space losses due
to limitations on the higher signal frequency [Gunter Hein
C-band]. Research has also shown increased
signal attenuation of C-band signals due to fog and rain.
However, researchers are hoping that much
smaller ionospheric errors for standard single frequency
applications (as compared to the L-band) and
decrease payload due to the small sized C-band antenna could be
highly advantageous and reasons
enough to warrant reconsideration on using C-band for
GNSS[Schmitz-Peiffer, Gunter Hein et al].
The next section looks at the effects of the atmosphere on radio
signals with particular attention to the
L- and C-band.
