Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 111

with the ARP (antenna reference point), which refers to the distance between the
antenna pedestal and the antenna phase center when the GPS receiver receives L1
channel. Due to the difference in the GPS signal's direction of entrance and
surrounding electronic field, the ARP positions will change with the electronic
materials used by different GPS antennae manufacturers. Therefore, in order to
improve the vertical positioning accuracy, NGS provides users with measured data
concerning changes in antenna phase centers of various instruments for users'
reference.
EUREF Permanent Network
Based on permanent satellite datum station networks established by European
countries and some organizations (academic groups and universities), the EUREF
Permanent Network (EPN) was established by the Regional Reference Frame
Sub-Commission for Europe (EUREF) of the International Association of Geodesy
(IAG) as a cooperative regional network of continuously operating stations. It is
now composed of 122 permanent datum stations, among which 42 are IGS datum
stations. Its working process is as follows: several permanent datum stations of
different countries or organizations form subnets with their own operation centers;
several operation centers constitute regional data centers whose data are gathered in
the European regional center, which then transmits the product data back to IGS
data centers, regional data centers, and various users.
At present, the assignment for EPN is to maintain the European Spatial Regional
Reference Framework. The main service is to provide weekly computational results
of the station coordinates throughout the network, the analysis of existing problems
with their relative accuracy, and the analysis of time series weekly solutions. EPN
also provides the original observed values of various stations to meet the require-
ments of precise positioning for different organizations in Europe. Apart from being
the national spatial reference framework, CORS in European countries like
Germany, Britain, and Switzerland still provides post-processed and real-time
precise positioning based on differential and real-time kinematic (RTK) techniques.
COGRS System in the UK
The UK National Network of Continuously Operating GPS Reference Stations
(COGRS) is similar to the above-mentioned CORS in terms of function and
objectives. However, according to the situation in Britain, this COGRS system
also monitors the relative and absolute changes in the sea level surrounding the
British Isles. The UK COGRS is under the administration of its Ordnance Survey,
Environment Agency, Meteorological Office, Ministry of Agriculture, and National
Oceanography Centre. There are now nearly 60 GPS continuously operating active
(reference) stations and over 900 GPS passive stations (points). At present, the UK
Ordnance Survey has set up a GPS online service center, enabling transmission,
supply, archiving, processing, and analysis of data from all GPS active and passive
stations. This center also provides an online service of coordinate transformation.