Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 110

CORS Networks in the World

Currently, CORS systems have been established or are being established in many
developed countries worldwide. Here are some typical examples.

CORS Network in the USA

American CORS is in the charge of the National Geodetic Survey (NGS). It initially
consisted of 137 GPS reference stations, such as the NGS tracking network, the
United States Coast Guard (USCG) differential network, the WAAS (Wide Area
Augmentation System) of the US Federal Aviation Administration (FAA), and the
tracking network of the USA Corps of Engineers (USACE).
At present there are more than 2000 continuously operating GPS stations in the
continental USA. They cover America completely (including Alaska) and consti-
tute a new generation of American national kinematic reference system. All the
reference stations in this system are equipped with full-wave double-frequency
GPS receivers and ground rings antennae. Every day they unload the data of the day
and record them in RINEX format of 1 s, 5 s, 15 s, and 30 s. This system offers
reference station coordinates and data from GPS satellite tracking stations to users
throughout the world, including Americans themselves, by means of the Internet. It
also provides other services like geoid and coordinate system transformation. Users
can observe any place in America with a GPS receiver, and can unload data from
reference stations via the Internet to obtain post-processed precise positioning.
NGS can offer users corresponding GPS carrier phase and code distance (within
the period of time users required) of CORS stations (more than three) adjacent to
unknown GPS points through networks to support users' GPS near-real-time or
post-processed positioning. Using networks, the NGS can also provide GPS posi-
tioning calculation services called Online Position User Service (OPUS), which
will be completed within several hours of users offering the observational data of
the unknown points.
Moreover, NGS networks can also provide its users with North American Datum
1983 (NAD83), the coordinates of ITRF and the displacement speed and meteoro-
logical data of its corresponding stations. NGS can also provide IGS precise and
broadcast GPS ephemerides as well as the coordinates and displacement rate of IGS
permanent continuously operating GPS stations all over the world. At present, the
relative point position accuracies of the endpoints of the 26-300-km baseline
calculated in America using CORS can reach 1.0 cm in the horizontal direction
and 3.7 cm in the vertical direction, both with a 95 % confidence measure. The
above case happens when users' observation at baseline endpoints lasts at least 4 h.
If the observation time increases to 12 h, although the horizontal accuracy remains
almost the same, the vertical relative accuracy can be improved to 2 cm.
Meanwhile, NGS also enables the transformation from NAD83 ellipsoidal
heights to NAVD88 (the North American Vertical Datum of 1988) orthometric
heights. It has primarily adopted the American geoid digital model to transform,
and the error is

2.5 cm. In addition, GPS manufacturers usually only provide users