Global Positioning System Reference
In-Depth Information
The NGS makes use of stations established by other groups rather than building
an independent network of reference stations. The backbone of the CORS network is
the MDGPS/NDGPS and WAAS stations. A listing of CORS contributors can be
found at http://www.ngs.noaa.gov/CORS/Organizations/Organizations.html. Figure
8.40 shows a breakdown of the CORS network partners affiliations.
The diverse character of the CORS partners has lead to a grouping of CORS sta-
tions into two major categories, National CORS and Cooperative CORS. The funda-
mental distinction is that NGS archives and disseminates the data from National
CORS, and does not archive and disseminate data from Cooperative CORS.
National CORS are expected to operate 24 hours/day, 7 days/week. The site,
http://www.ngs.noaa.gov/CORS/Becoming_a_CORS.html, shows other distinguish-
ing characteristics. Cooperative CORS provide a supplement to the National CORS
by providing more local base stations, shorter baselines, and a variety of data rates.
As of August 2005, over 660 National CORS sites and over 140 Cooperative CORS
sites were operating.
The fundamental data of National CORS are receiver independent exchange
(RINEX) format (version 2.10) files containing dual-frequency carrier-phase and
pseudorange measurements. For many sites, Doppler data are also available. If
supported by a receiver, the L1 pseudoranges derived from both C/A code (the C1
pseudorange) and the P(Y) code (the P1 pseudorange) are provided. The principal
translation package that converts the varieties of manufacturers' binary data into
RINEX is the program TEQC, maintained by UNAVCO. TECQ is documented at
http://www.unavco.org/facility/software/teqc/teqc.html.
CORS positional coordinates and velocities are key values needed to use CORS
as base stations for carrier-based differential GPS applications. CORS coordinates
and velocities are provided in two distinct reference frames, NAD 83 and ITRF00.
The formal datum label for the National CORS is NAD83 (CORS96), and they are
realized with an epoch of 2002.00. Stations in the Pacific are an exception, since
CORS partners
State government
24%
USDOT and
USACE surface
navigation 20%
USN/timing
1%
NASA/satellite
tracking 3%
University-based
crustal
motion 14%
Academic 4%
NOAA
meteorolgy
11%
NOAA and
Foreign/Geodesy
5%
FAA/Air
navigation
6%
Local
government
6%
Commercial
6%
Figure 8.40
Partners in the CORS system. ( Source: National Geodetic Survey, NOAA.)
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