Differential GPS - Understanding GPS: Principles and Applications - page 453

tocol (FTP) access though a Web browser at ftp://cors.ngs.noaa.gov/cors/rinex/
2004 (for example). Multiple years are stored under the rinex directory; multiple
days are stored under the year directory; the various site IDs are stored under the
day directories; and the RINEX files are stored under the site id directories. A sche-
matic of the FTP directory structure can be found at: ftp://cors.ngs.noaa.gov/cors/
readme.txt. Note that the FTP structure is most convenient for those software prod-
ucts that automatically download data for local processing.
In contrast to the standard access method, the UFCORS interface provides a cus-
tomized file collection that is automatically compressed and downloaded to a user's
computer (http://www.ngs.noaa.gov/CORS-Proxy/UFCORS/). The user fills in a
two-page menu, indicating the desired block of time and the CORS site. Other
options include receipt of the data sheet, IGS precise orbits, compression options, and
alternative data rates. When selecting an alternative data rate, the collected data will
be decimated or interpolated to accommodate the desired target rate. The UFCORS
interface frees the user from knowing specifics about the RINEX file storage system.
This section cannot be closed without addressing some of the support tools
available from the CORS and NGS site. Earlier in this section, HTDP was discussed
as a utility for conversion of coordinates between reference frames and epochs. A
dynamic map utility linked at the home page allows one to build customized views
of CORS coverage. Various RINEX file manipulation utilities (e.g., interpolate and
decimate) can be reached from http://www.ngs.noaa.gov/CORS/utilities.html. A
multifeatured Geodetic Toolkit at http://www.ngs.noaa.gov/TOOLS supports

numerous online computations and coordinate conversions.
Special remarks must be made about the Online Positioning User Service (OPUS)
tool found at http://www.ngs.noaa.gov/OPUS. This service allows users to upload 2
or more hours of dual-frequency RINEX data from a stationary antenna for auto-
mated, remote processing at NGS. Single baselines are computed and merged from
three nearby CORS stations, and the resultsaree-mailedbacktotheuser.Turn-
around is typically just a few minutes. At its heart, OPUS uses the CORS as a subsys-
tem in computation of the user's coordinates. This is suggestive of new directions and
roles that continuously operating reference stations can take in the future.

8.6.2.2 International GNSS Service (IGS)
The International Association of Geodesy established the IGS in 1993 to support
geodetic and geophysical research activities by providing GPS data and products.
The IGS serves a coordination role, sets standards and specifications, and encour-
ages international adherence to its conventions. The IGS operates through a govern-
ing board and a central bureau (its executive arm), and functions through the
cooperation of international groups of GPS satellite tracking networks, data
centers, analysis centers, and various working groups. The IGS Web site is
http://igscb.jpl.nasa.gov.
IGS is known foremost as the source of precise GPS orbits. However, IGS also
produces GPS satellite clock and ground receiver clock solutions, and Earth orienta-
tion parameter (EOP) products (polar motion, polar motion rate, and length of
day). The IGS solutions are combined products that integrate the solutions gener-
ated by the individual analysis centers.