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International Federation of Surveyors
Article of the Month – December 2009
Volker Schwieger, Mikael Lilje, Rob Sarib
GNSS CORS – Reference Frames and Services
1/21
GNSS CORS - Reference Frames and Services
Volker SCHWIEGER, Mikael LILJE, Rob SARIB,
Germany, Sweden, Australia
Key words
: Reference Frames, Geodetic Datums, GNSS, GPS, CORS, Positioning
SUMMARY
Nowadays the time of GPS or even GNSS static baseline determination is almost out-of-date
for everyday applications. The pr
esent trends are to continuous
ly observe and measure using
GNSS, install permanent GNSS base stations
or networks and provide real time accurate
positioning. If you put these three trends together you have an advanced component of
geoscience infrastructure known
as a GNSS CORS network. These networks need to have a
geodetic datum, meaning that they have to be
linked to the available terrestrial reference
frames. Such GNSS CORS networks are or wi
ll be the primary means by which numerous
users can access and realize position that is
based on a geodetic datum. In some regions,
GNSS CORS networks are so well developed a
nd dynamic that they have a more prominent
role than the existing classical passive ge
odetic infrastructure in reference frame
determination or monitoring.
This paper will focus on the relationship of GNSS CORS with global and regional reference
frames. It will provide procedures for rea
lizing the link between the GNSS CORS and the
available reference frames. An overview on the
key aspects of a GNSS
CORS site and CORS
network will be articulated. Some information
regarding the concepts
of virtual reference
station and of area-correction pa
rameters as well as of the master-auxiliary-concept will also
be discussed. In addition, this paper will outline the various positioning services that GNSS
CORS networks could provide.
1.
MOTIVATION
One of the main trends in mode
rn geosciences is the monitoring of the planet earth as a whole
realized by the Global Earth Observing System
(GEOSS) and, as a part of it, the Global
Geodetic Observing System (GGOS). Apart from other topics its task is to maintain the
stability of and to provide open access to the ge
ometric and gravimetric reference frames as
well as time series of data a
nd products, by ensuring the genera
tion of uninterrupted state-of-
the-art global observation related to the th
ree fundamental aspects of geodesy, namely
geometry and kinematics, earth orientation
and rotation, and the gr
avity field and its
variability (IAG 2009). These mon
itoring tasks help to understand
climate change, are usable
to predict and monitor natural disasters, help
to realise sustainable development and may lead
to a global structural policy. One important
module is the establishing, monitoring and
making available of a global accurate and reliable re
ference frame. This is already realised by
the International Reference Frame (ITRF) with its
different realisations
. This reference frame
is non-active but is a near real
time accessible geodetic datum.
International Federation of Surveyors
Article of the Month – December 2009
Volker Schwieger, Mikael Lilje, Rob Sarib
GNSS CORS – Reference Frames and Services
2/21
On the other side there is a strong world-wide need for positions in real time regarding
positioning, navigation as well as guidance and cont
rol. These positions are needed for mass
market applications like car na
vigation systems as well as ge
odetic applications like state
survey, cadastral measurements or engineerin
g surveys. This is a reason that some GNSS
receivers are continuously operated as so ca
lled CORS (Continuously Operating Reference
Stations) to allow the real time availability. A
dditionally CORS can be un
ited into networks to
reduce errors and to have the capability to deli
ver positioning services for a country, a state or
even world-wide.
In any case for high accurate applications esp
ecially in geoscience the connection to global
terrestrial reference frames like ITRF is very
important. Consequently, the authors of this
paper will provide some basic procedures re
garding the linking of CORS networks to
reference frames and also an insight into the
various positioning infras
tructure and services
that can emanate from such networks.
2.
THE FUNDAMENTALS OF REFERENCE FRAMES
From a spatial information perspective, it is common for spatial datasets and geographical
information data to extend over national or
regional boundaries and fo
r the global surveyors
or organisations across c
ontinents. In this situation it is
needed to have a common reference
frame for the collection, storage, visualisation
and exchanging of the information. ITRF is the
most accurate reference frame that exists intern
ationally. ITRF is defi
ned by the International
Earth Rotation and Reference System Service (IE
RS). The present trend is that more and
more regions as well as countries
are using a solution based on ITRF.
ITRF is continuously monitored by geodetic obs
ervations from numerous scientific and
geodetic measuring facilities or networks. The
reference frame can be considered as being
well-defined, long term stable, highly accurate
and easily accessible and is the basis for all
precise positioning on and near the Earth‟s surface.
Coordinates, as well as velocities, in an
International Terrestrial Reference System (ITRS
) are computed at different epochs and the
various solutions are given a year code fo
r identification as ITRF97, ITRF2005 and so on.
Due to improved measurement techniques, more
observations and sites, new models and
analysis tools the coordinates changes for a
certain point between th
e different ITRF. The
latest ITRF solution, ITRF 2005, was determined
from a combination of observations the
following space-based geodetic techniques:

GPS (Global Positioning System) respect
ively GNSS (Global Navigation Satellite
System),

VLBI (very long baselin
e interferometry),

SLR (satellite laser ranging),

LLR (lunar laser ranging) and

DORIS (doppler orbitography and radio
positioning integrated by satellite).
These measurement techniques all have their
strengths and weaknesses but the combination
produces a strong terrestrial reference frame.
This reference frame is determined from
geodetic observations from globally distributed
networks, however the solution could be
improved if there were more spaced based geode
tic observations in the southern hemisphere.
International Federation of Surveyors
Article of the Month – December 2009
Volker Schwieger, Mikael Lilje, Rob Sarib
GNSS CORS – Reference Frames and Services
3/21
Well defined parameters for a 7-parameter thre
e-dimensional similarity transformation are
used for transformation between the different ITRF realisations. The velocities valid in the
respective reference frame have to be transformed as well. In literature this procedure is
referred to as 14 parameter transformation. The pr
ocedure is described in IERS (2004) and the
transformation parameters are given e.g. in
ITRF (2009). A new solution, ITRF 2008, is
expected to be released during 2009.
ITRF coordinates or positions ar
e articulated as three dimensional geocentric or Earth Centred
Cartesian co-ordinates ie “X, Y and Z”. To c
onvert these Cartesian co
ordinates to geographic
respectively ellipsoidal coordinates (latitude
s and longitudes and ellipsoidal height) the
GRS80 ellipsoid is pre-dominantly used as it is
the best fitting analytical model for the earth‟s
surface. However in some cases it is necessary to
describe an ITRF position in plane (grid)
co-ordinates (eg two dimensions – eastings
and northings) hence
a mathematical map
projection is used. A popular
map projection which retains the angle is the Universal
Transverse Mercator
(UTM) projection.
Fig. 1: Recent Realisation of ITRF: ITRF2005 (source: ITRF 2009)
Today it is common to determine a point‟s posit
ion using Global Navigation Satellite Systems
(GNSS). Positions are not absolute quantities.
They are dependent on the reference frame to
which they are referred. If GPS is used then the point‟s position is determined in the reference
system WGS 84. Observing in a good GNSS envir
onment, the absolute accuracy for a single
point position fix is ± 5 - 10 metres in the ho
rizontal – ie 2 dimensions at the 2 sigma (2
σ
)
confidence level.
WGS84 or the World Geodetic System 1984 is th
e geodetic reference system used by the
GNSS - “GPS”. It was developed for the United
States Defence Mapping Agency (DMA),
International Federation of Surveyors
Article of the Month – December 2009
Volker Schwieger, Mikael Lilje, Rob Sarib
GNSS CORS – Reference Frames and Services
4/21
now called NGA (National Geospatial - Intellig
ence Agency). Although the name WGS84 has
remained the same, it has been enhanced on several occasions to a point where it is now
aligned on the cm-level to ITRF2000 at
epoch 2001.0 and referenced as WGS 84 (G1150)
referred to the GPS week no 1150 (NIMA 2006). The
origin of the WGS84
framework is also
the earth‟s centre of mass. For other GNSS we
can mention that Galileo will use ITRF as
reference system whereby the others will consid
er aligning as close as possible to ITRF.
According to ICSM (2009), the ellipsoid reco
mmended by the Internat
ional Association of
Geodesy (IAG) is the Geodetic Reference Syst
em 1980 ellipsoid. This ellipsoid was used by
the United States Defence Mapping Agency
with WGS84. The parame
ters of the WGS84
ellipsoid
“....are identical to those for the Geodeti
c Reference System 1980 (GRS80) ellipsoid
with one minor exception. The form
coefficient J2 used for the second degree zonal is that of
the WGS84 Earth Gravitationa
l Model rather th
an the notation J2 used with GRS80.”
- DMA
(1987). The end result is that
the GRS80 and WGS84 ellipsoids have a very small difference in
the inverse flattening, but this difference is
insignificant for most
practical applications.
Tab. 1: Ellipsoid paramet
ers of GRS80 and WGS84
Ellipsoid
Semi major axis
Inverse flattening
GRS 80 6 378 137 m 298.257222101
WGS 84 6 378 137 m 298.257223563
For all practical purposes, an ITRF based ge
odetic datum and WGS84 are the same for the
epochs defined. The difference is below the cm-l
evel for each coordinate. As a consequence it
is very seldom that the reference frame fo
r GNSS CORS (Continuously Operating Reference
Station) networks is not based on ITRF.
If mm to cm accuracy is require
d then GNSS phase data from points of known position in the
region are needed. The resulting coordinates for th
e point to be determined
will then be in the
same reference frame as the point with given coor
dinates, the reference point. This local point
could be a permanent GNSS station in cont
inuously operating reference station (CORS)
network that is linked to an International Te
rrestrial Reference Frame (ITRF). The same is
valid for non-accurate code data solution on th
e 1 to 10 m level. Here the difference between
WGS84 and any ITRF solution is without importance.
Regarding the height system by using a
GNSS CORS network the surveyor will
normally derive a height based on the reference ellipsoid ie the GRS80. Most users however
are working with physical height
s based on a local height datum
(ie local mean sea level) and
thus need to relate the derived ellipsoid height to this local height datum. This is achieved
by using a geoid model for the subject survey
area or by determining a local geoid by
interpolation. A more sophi
sticated approach is given by Jäger et al. (2003).
3.
GNSS CORS
3.1
Definition and General Technical Background
International Federation of Surveyors
Article of the Month – December 2009
Volker Schwieger, Mikael Lilje, Rob Sarib
GNSS CORS – Reference Frames and Services
5/21
The most known GNSS is the Global Positio
ning System (GPS) developed by the United
States Department of Defence and is currently
managed by the United
States Air Force 50th
Space Wing. Competing and complementary syst
ems are GLONASS and in the future, among
others, the European Galileo and the Chines
e Compass. There ar
e also other GNSS in
operation or being developed such as the
Japanese “QZSS” and the Indian Regional
Navigational Satellite System “IRNSS”.
For all GNSS measurements of geodetic accur
acy you will need at least two simultaneously
measuring receivers and the use of phase obser
vations. In general, the one on a site with
known coordinates is called the re
ference station (also known as the „master‟ or „base‟). The
one located on the site with unkno
wn coordinates is called the r
over. This pro
cedure is used
for code observations as well. In this case
the accuracy is somewh
at worse around 0.5 m or
even between 1 m and 3 m depending on the measur
ing technique (compare section 3.2). This
determination of relative coordinates is called the DGPS-principle. Figure 2 presents the
general idea behind it. The error influences that
are the same or nearly the same at both sites
are eliminated or at least strongly reduced.
This covers all influences but multipath and
measurement noise.
CORS
Rover
actual correction
correction
at CORS
correction
surface
remaining error
rover correction
submitted from CORS
to rover
Fig. 2: Advantage of DGPS - principle
The development of GNSS, esp
ecially of GPS, ha
s lead to the opera
tion of continuous
operating reference stations (CORS) that acq
uire GPS signals without any interruption.
Additionally these CORS have the task to stor
e the data and in some circumstances process
the data and then transmit this data to rover
receivers. CORS help the users by economizing
one GPS receiver as the operation of the referenc
e station is performed by the service provider
of the CORS network. Thereby networks usin
g code or phase obser
vation show different
characteristics.
Every CORS network consists of several GN
SS stations interconnected by reliable
communications to enable real time computations
and control. Each station, as a minimum,
requires a receiver (preferably geodetic quality),
an antenna (affixed to a stable monument),
communications and a power supply. In some cases
a computer is installed additionally for
data transmission and control, however modern
day receivers, with suitable communications
and network management software) now have the ab
ility to stream raw data back to a central
server location. In ideal cases a supplementary conf
iguration is used for reliability or back up
reasons (Refer to Figure 3). A
dditionally a user interface is requi
red to configure and maintain
the network. This may be realized remote
ly e.g. by radio communication or by digital
communication (mobile phone network) or via intern
et connection. If we
are talking about an
International Federation of Surveyors
Article of the Month – December 2009
Volker Schwieger, Mikael Lilje, Rob Sarib
GNSS CORS – Reference Frames and Services
6/21
offline network that provides the information to
the user for post-processing, the stored data
files use “receiver independent ex
change format”, that is RINEX
1
.. For online networks real
time kinematic (RTK) is the application and the RTCM (Radio Technical Commission for
Maritime Services) format is normally used fo
r data transmission. The RTCM format is an
internationally accepted standard format for real
time data transmission of differential (RTK)
GNSS corrections from GNSS CORS (including
networks) to mobile GNSS receivers. Details
of RTCM 3.0 can be found at
:
http://www.rtcm.org/overview.php#Standards
.
Fig. 3. Typical CORS Infrastructure (example taken from SWEPOS)
It is also important to note that as CORS netw
orks continue to be established globally there
will be a need to categorise or develop a hierarchy of CORS networks. Such a concept,
espoused by Rizos (2008), should
also be considered when developing CORS networks or
positioning infrastructure. The proposed hierarc
hy or tier structure by Rizos (2008) is as
follows:

Tier 1 – ultra-high accuracy CORS networks
equipped with geodetic quality receivers
that can track all the broadcast GNSS/RNSS
(regional navigation
satellite systems)
frequencies/signals, have a stable antenna monument, comply with the International
GNSS Service (IGS) Site Guidelines, and are established for ultra-high accuracy
networks to support geo-scien
tific research and global refe
rence frame definition. Tier
1 contributes to the IGS.

Tier 2 – are the high accuracy CORS netw
orks equipped with geodetic quality
receivers, that can track all the broad
cast GNSS/RNSS frequencie
s/signals, have a
stable antenna monument, and comply with
the International GNSS Service (IGS) Site
Guidelines. These networks are normally
operated by National geodetic agencies or
State governments for the purpose of ma
intaining national geodetic datums and
providing the fundamental „bac
kbone‟ of a national geospa
tial reference frame.

Tier 3 – are the CORS networks equipped w
ith “minimum interope
rable configuration
design” receivers that can track the inte
roperable L1-L5 GNSS
/RNSS frequencies /
1
This is an internationally accepted st
andard data format for the interchangi
ng of

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