Global Positioning System Reference
In-Depth Information
they are on differing tectonic plates. These CORS will be in the NAD 83 (PACP00)
or the NAD 83 (MARP00) frames. Also, CORS in Alaska are computed in NAD83
(CORS96), but with an epoch date of 2003.00, to accommodate the Denali earth-
quake activity. All published ITRF00 positions and velocities carry a common
datum tag ITRF00 (1997.00) and are realized with the epoch date of 1997.00. Of
course, the price of such ITRF uniformity is that tectonic plate motion, as well as
local motion, is expressed in the velocity values and is seldom negligible for preci-
sion applications. More detail on the computation of CORS coordinates can be
found at http://www.ngs.noaa.gov/CORS/Coords.html. A conversion utility (hori-
zontal time-dependent positioning) between these reference frames is located at
http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.html. It should be noted that, while
the International GNSS Service (IGS) Analysis Centers provide GPS orbits in a refer-
ence frame denoted IGb00, users may treat IGb00 and ITRF00 as equivalent.
Coordinate locations for a CORS antenna are referred to two different station
reference points, the ARP and the L1 phase center (L1 PC). The ARP is defined as the
center of the bottom-most, permanently attached surface of the antenna. The L1 PC
is a notional, electrical location for receipt of the L1 signal. Under most antenna
designs, the L1 (and L2) phase center varies with the elevation angle to a given GPS
satellite. Thus, establishment of an L1 PC origin is done in conjunction with a com-
panion model of the L1 PC variation (http://www.ngs.noaa.gov/ANTCAL). Due to
the abstract character of the L1 PC and its dependence on specific calibration mod-
els, NGS considers the ARP the definitive location for a CORS site.
Extensive metadata are also available for stations in National CORS. This
includes availability profiles, detailed data sheets and site logs, maps, photos, and
time series of daily coordinate solutions. These metadata answer many questions on
stability and reliability.
Other data that are not RINEX receiver data or metadata are available at the
CORS site. In particular, both broadcast and precise GPS orbits can be obtained.
The broadcast orbits are collected from the IGS global tracking network and do not
show the satellite dropouts common to single-site collections. Both IGS and NGS
precise orbits are available. The IGS orbits are combined products and include the
NGS contributions. Further discussion on IGS orbits is continued in the next sec-
tion. For CORS sites that have a weather sensor, RINEX meteorological files are
produced. And, as described earlier, users may obtain files and diagrams describing
receiver antenna phase center offsets and phase center variation.
CORS RINEX data are stored in standardized directory locations. Users may
access these files though the “Standard” method, or through “User Friendly
CORS—UFCORS.” Standard access is most readily obtained by clicking on the cover-
age map found on the CORS home page (http://www.ngs.noaa.gov/CORS). Successive
clicks on a location of interest will enable a user to zoom down to a specific site with a
unique 4-character ID (e.g., GAIT). A menu to the left will enable a user to select
RINEX data or other metadata. A request for RINEX data will transfer the user from
the map interface to the standard download interface. One must then again select the
station of interest, the request for RINEX data, and then add the year, month, and day
of interest. This will lead the user to the directory holding the RINEX data.
The standard method is convenient for access of the various metadata. But, if
one is interested solely in RINEX data, it can be obtained by direct file transfer pro-
Search WWH ::




Custom Search