2.1. Continuously Operating Reference Stations
(CORS)Many CORS networks have been developed by a number
of governmental, private, and academic organizations to
provide support for precise positioning and navigation
for applications ranging from ordinary engineering work,
to robust target location identification, and scientific research.
For illustration purposes, this section presents the
National Geodetic Survey (NGS) CORS network, which
covers the United States. CORS is an active wide-area
GNSS network managed by the National Geodetic Survey
(NGS) in cooperation with the United States Coast
Guard, Federal Aviation Administration, and US Army
Corps of Engineers, and it offers DGPS service to the
public free of charge. The development of CORS was
motivated by the accuracy requirements for many activeties
in surveying, mapping, engineering and related fields.
In CORS network, there are always two sets of stations:
1) network stations, which are precisely known locations
within the network, which continuously collect GPS data,
and 2) user stations; fixed or mobile, which are unknown
or approximate locations. The data collected at the
network stations are used to compute GPS corrections for
the network including the user stations within the network
area. The user commonly receives these corrections
after sending the GPS data file to NGS.
A typical CORS network master control facility collects,
stores, and processes GPS data acquired for the
stations within the network. The GPS coordinates of unknown
(new) points are then estimated based on data
collected at a CORS station and processed by the master
CORS facility based on the relative positioning concept
[9]. Besides being utilized for relative positioning, some
reference stations within CORS network depending on its
location and other specific characteristics may be used by
NGS and other governmental agencies in the US for
clock correction, precise ephemerides, crustal motion
monitoring and atmospheric and earth rotation studies [10].
NGS provides the coordinates of CORS stations in both
of North American Datum of 1983 (NAD83-2002_epoch)
and The International Reference Frame of 2000 (ITRF00),
which has the same ellipsoid’s scale and shape parameters
of the well-known World Geodetic System of 1984
(WGS84).
GPS data collected for a time period of up to a fullobservation-day
is transferred from a CORS site to the
master station. Once the GPS data from the CORS station
is received by the master station, it gets processed to
generate files of same format that are sent back to the
user and sometimes archived for further processing. NGS
also makes precise ephemeris data available to the GPS
user community as it computes precise orbits based on
orbital data acquired at the six GPS master stations. Each
orbit covers a one-day period at a 0.25-hour epoch including
satellite clock offsets. Currently CORS data is
not available in real-time streaming mode, but NGS is
planning to broadcast data from some CORS stations in
the near future. NGS is now supporting international and
federal agencies through near real-time GPS data steam ing through the International Global Navigation Satellite
Systems (GNSS) Service known as IGS [11].
There are many requirements need to be met before a
given site can be chosen as a CORS station. These include:
location, availability of basic utilities such as power source
and backup, continuous network connection, ease of
mounting the GPS receiver and peripheral equipment,
adequate sky visibility from the CORS antenna location,
availability of cable and/or telephone line, climate control
capability, and security and ease of access. In wide-area
networks, the GPS receiver type, model, software, data
acquisition rate, ect.; is the backbone of the positioning or
navigation process. An ideal CORS receiver should be
able efficiently collect the pseudo-ranges and the two
carrier phases of the P- and CA-codes [12].
The major benefit of CORS network is the ability of
the GPS user to acquire his/her unknown position within
the CORS network by performing relative positioning
with no need for a base station. At a CORS station, NGS
provides the GPS coordinates based on L1 phase center as
well as the Antenna Reference Point (ARP). The ARPbased
coordinates are those estimated based on the bottom
mount of the GPS antenna from which the phase
center offsets are computed. For precise positioning using
CORS network, the post-processed orbital GPS data can
be obtained through the internet or wireless connections
from NGS. One major issue with CORS-based positioning
is quality control in the context of the utilized CORS
station. These issues are mainly related to multi-path errors,
receiver errors, cycle slip errors, issues with the positional
stability of the GPS antenna, ect.. Although monitoring of
the positional stability of the GPS antenna can be carried
out by repeated inter-station baseline calculations, it is
rarely considered a serious issue by the users. All of these
issues have to be addressed by the GPS user locally; otherwise
the estimated GPS coordinates will have low quality.