Till the year 2015 there will be a constant
and radical change in the existing GNSS
data processing techniques
GNSS
is a global navigation satellite system comprising
of network of satellites that transmit ranging
signals used for positioning and navigation anywhere
around the globe; on land, in the air or at sea.
The US Global Positioning System (GPS/Navstar
GPS), the Russian Global Navigation Satellite
System (GLONASS) and the upcoming European GALILEO
system, Data communications satellites with navigation
payloads and Augmentation systems are all part
of GNSS.
IGS the offi cial website of voluntary federation
of more than 200 worldwide agencies that pool
resources and permanent GPS station data to generate
precise products formerly known as International
GPS service has now been renamed as International
GNSS service. The conventional GPS data processing
techniques are also being modified to incorporate
the new satellite systems of GNSS which pose major
challenges of different satellite signals, different
reference frames and different time scales to
be incorporated for combined processing.
Most of the academic and commercial existing data
processing softwares process the code and phase
pseudo ranges obtained from L1 and L2 satellite
signals of Navstar GPS / GPS to obtain the positions.
Most of the world wide users are currently dependent
on Navstar GPS technology which is owned, operated
and controlled entirely by one country, the United
States. Bernese 4.2 GPS software has the capability
to process the satellite signals of both Navstar
and Glonass GPS systems and combining them to
give the precise and improved positions. Proposed
Navstar GPS modernisation has thrown new challenges
with two new satellite civil signals L2C, and
L5 which will be available for initial operational
capability by 2010 and for full operational capability
by approximately 2013. The current NAVSTAR GPS
data processing software’s now should be
able to absorb these new civil signals for positioning.
The world wide GNSS network of tracking systems
now recognises the demand for real time processing
of 1 Hz GNSS data for early warning and rapid
damage assessment. Potential user groups who would
benefi t from the availability of GNSS data and
products include from among others; geodetic agencies
mandated to provide access to a globally consistent
reference frame for all position applications,
precision navigation users (LEO), agencies involved
in natural hazards monitoring, prediction, warning
and response, structural engineering monitoring,
near/real-time atmospheric monitoring for weather
prediction, real-time earthquake seismology (simultaneously
with seismological analysis), and time transfer
and dissemination. The IGS Real-time Working Group
(RTWG) is presently assessing and addressing issues
that pertain to the IGS developing realtime infrastructure
and processes..
GNSS data processing
Currently,
there are only two satellite navigation systems
in operation, the Global Positioning System (GPS/
Navstar GPS), and the Russian equivalent GLONASS
system. Galileo satellite navigation system due
to be launched by European Union in 2006, will
be made available to public by 2010. Table 1 gives
the comparison of present and future Global navigation
satellite systems and Table 2 gives the present
and future services of GNSS. In addition to the
these main GNSS infrastructures, Quasi-Zenith
Satellite System of Japan is under development
(QZSS), with three satellites placed in a special
orbit that maximizes coverage over Japan and Beidou
satellite navigation and positioning system of
china consisting of two geosynchronous satellites
to complement the existing GNSS systems. Space
Based Augmentation Systems (SBAS) and Ground-Based
Augmentation Systems (GBAS) have been built (or
are being deployed) by the United States, Europe,
India, Japan, Australia and China to augment the
existing GNSS.
Currently majority of GPS users worldwide are
using only GPS/ Navstar GPS for their application
and the major scientifi c GPS data processing
software only process the data form Navstar satellites.
Only exception to this is the BERNESE 4.2 software
which has the capability to combine the GPS and
GLONASS satellite data.
The success story of BERNESE group of being able
to combine the two different satellite navigation
systems with different satellite signals, reference
frames and time scales gives confi dence for the
inclusion of Galileo and future satellite systems.
GNSS data processing will now become a cost-effective
and precise approach of high-accuracy software
simulations which are reproducible and totally
controlled by the user, and parameters can be
changed individually if necessary for an in-depth
analysis of the combing the various GNSS systems.
The immediate need of the GPS community is to
be able to include the new satellite signals L2C
and L5 of Navstar GPS in to the data processing
software’s as it effects majority of the
GPS users worldwide.
Table 1: Main
features of GNSS
Real time GNSS data
processing
Many very
demanding applications and systems now require
GPS raw data and products in real time with greatly
reduced delays which underlines a need for the
availability of real time data and products required
for the real time GNSS data processing. The GPS
Earth Observation Network System (GEONET) in Japan
consisting of 1200 GPS stations with an average
spacing of about 20 km has been successful in
real-time observations and analysis. For real
time analysis
GPS sites should sample data at 1 Hz and transmit
the measurements in real time to the analysis
center which are then processed for instantaneous
coordinates and baseline components which are
then adjusted with absolute ITRF coordinates of
the stations suffi ciently remote from the region
of analysis. Instantaneous coordinate accuracy
is about 1-2 cm in the horizontal and 10-20 cm
in the vertical, compared to the known ITRF coordinates
of the stations. Real time instantaneous (horizontal)
position changes detected by a dense GPS network
(like GEONET) could be used as part of an early
warning system for mitigating natural hazards.
To fully serve the multi-disciplinary scientifi
c user community the IGS real-time working group
(RTWG) is working towards enhancing its standards
for infrastructure, data and product availability.
The primary products of such a system will be
GPS/GNSS station data, satellite orbits and clocks,
made available to the user by Internet and other
economical and available streaming technologies.
This will also ensure a standardization of the
real time networks and would cater to multiple
user community as non-IGS near/real time networks
are emerging without standards. Real time water
vapor estimation (SuomiNet and GeoNET) is already
been integrated in to Weather Research and Forecasting
(WRF) model which is a new generation advanced
mesoscale (10km-1km) model for both operational
numerical weather prediction (NWP). Improved rainfall
forecast due to assimilation of GPS Precipitable
Water Vapor were observed for several hours into
the forecast. Similarly, Ionospheric perturbations
associated with atmospheric disturbances, such
as earthquakes, large explosions, or rocket launches
have been detected through fi ltering of dual-
frequency real time GPS phase data.
Table 2 Current
and future services of GNSS
It is important to track the ionosphere which
is a dynamic region capable of adversely impacting
a variety of space-based systems.
Future
Recent
advances highlight the huge potential that exists
for future navigation and positioning applications.
The vast majority of the world will be users of
these existing systems which pose a whole lot
of questions like: Which system or systems should
a country use? Which combination of systems? What
are the benefi ts and respective merits of those
systems? What kind of integration should be followed?
What kind of real time GNSS network should be
used? Which data processing
software to use? Etc... There is no simple answer
to these questions, as
the best solution will undoubtedly depend on the
application, which has its own requirements in
terms of accuracy, reliability, robustness, cost,
and other application-specifi c criteria. What
can be provided, however,
is a means whereby parameters that describe these
performance requirements can be computed. Software
needs to be developed to conduct a qualitative
assessment of the performance characteristics
of future GNSS infrastructure.
From now till the year 2015 there will be a constant
and radical change in the existing GNSS data processing
techniques though the underlying principles will
not change much. As a user , one needs to keep
abreast with all the latest developments in this
fi eld as the current processing software’s
will be adopting to the new satellite signals,
new satellite systems , different reference frames
and real time availability of the data and the
products
U.Hugentobler, S.Schaer and P.Fridez,
Bernese GPS software 4.2 Manual
Heinz Habrich and G.Beutler, Geodetic Applications
of the GLONASS and of GLONASS/ GPS combination.
Günter W. Hein, Jeremy Gödel, Jean-Luc
Issuer, Jean-Christopher Martin, Philippe Erhard,
Rafael Lucas-Rodriguez and Tony Pratt, Status
of Galileo frequency and system design, Members
of the Galileo Signal Task Force of the European
Commission, Brussels Jan skaloud and Patrick Viret
, GPS/INS Integration, European Journal of Navigation,
Vol 2, 2004 C.Seynat, a Performance Analysis of
Future Global Navigation Satellite Systems, the
2004 International Symposium on GNSS/GPS.
January 2006
Sridevi Jade Scientist, CSIR
Center for Mathematical Modelling
and Computer Simulation (C-MMACS),
Bangalore, India sridevi@cmmacs.ernet.in
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