National Ocean Service. National Geodetic Survey ... Nation

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CORS Industry Forum
NGS Products for Accu
rate GPS Positioning Tomás Sole
r CORS Team
March 2001
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Primary GPS Related Products
? Coordinates and Velocities

? Transformation software
? GPS Orbital Data ? GPS Antenna Phase Pattern
Models ? Geoid99 ? Total Electron Content (TEC
) Mapping
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ITRF Horizontal Velocities (mm/yr)
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Coordinates and Velocities (03-26-01)
? NGS provides accurate coordinates and
velocities for 216 CORS stations
? ITRF97, epoch 1997.0 soon
ITRF2000 ? NAD 83 (CORS96), epoch 1997.0
? These positions and velocity field form the
reference framework of the National Spatial
Reference System (NSRS) and are available
on-line from NGS Data Base
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CORS Network
? NGS computes and monitors the position of all
CORS stations on a daily basis
? Coordinates and velocities are updated
whenever a new ITRF frame is released
? NGS collaborates with the International Earth
Rotation Service (IERS) and provides CORS
positions and velocities that are integrated
into ITRF global solutions
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Position Time Series (last 60 days)
http//www.ngs.noaa.gov/CORS/download2
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Position Time Series (long-term)
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Horizontal Time-Dependent Positioning Software
(HTDP) http//www.ngs.noaa.gov/TOOLS/Htdp/Htdp.ht
ml
? Transforms positions and velocities between
spatial reference frames ITRFxx, WGS84 (G873),
NAD 83 ? Updates positions and/or observat
ions between dates (epochs)
? Predicts velocities due to crustal motion
? Predicts displacements between dates
? Transforms positions between the ITRF97
frame and the NAD 83 datum using 14 parameters
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14 Parameter Transformation http//www.ngs.noaa.g
ov/CORS/metada1/
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NGS Satellite Ephemerides http//www.ngs.noaa.gov
/GPS/GPS.html
? NGS is one
of the seven International GPS Service
(IGS) Analysis Centers (AC) participating in the
production of accurate GPS orbits
? Final Precise ( 13 days
latency) .2 nanoseconds ? Rapid (14 hours
latency) (supports real-time) te positions in SP3 format are given (once
every 15 minutes) in current ITRFxx frame

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GPS Antenna Phase Pattern Calibrations
http//www.grdl.noaa.gov/GRD/GPS/Projects/
ANTCAL
? Each antenna type has a unique phase pattern
? Each antenna type has a unique set of constan
ts ? GPS antennas must be calibrated ? Mixi
ng uncorrected antenna types can produce errors
of up to 10 cm in the vertical component
? NGS is at the forefront in relative field
calibration of GPS antennas
? NGS maintains a calibration database with
parameters for 88 antenna types (03-26-01)
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Antenna Calibration Facility in Corbin, Virginia
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Height Systems and the Geoid
http//www.grdl.noaa.gov
/GEOID/
? GPS determines geodetic heights
(geometric) h ? Leveling determines orthom
etric heights H ? The above heights are r
elated through the geoid height N ? h N H
? dH dh - dN ? Geoid The equipotential s
urface of the Earths gravity field which best
fits, in a least squares sense, global mean sea
level ? NGS produces GEOID99, a geoid cons
istent with NAD 83 and NAVD88
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GEOID99 the Link Between GPS Heights and Leveling
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GPS Derived Total Electron Content (TEC)
http//www.grdl.noaa.gov/GRD/GPS/Pr
ojects/TEC
? Sun light forms free electrons and positively
charged ions. Maximum disturbances occur at
about 300 km height (Ionosphere)
? Free electrons alter the speed that
electromagnetic waves travel through the
ionosphere. The effect is frequency dependent
and disrupt communications ? The differential ef
fect on the two GPS frequencies may be
applied to measure the distribution of free
electrons ? TEC is highly correlated with the l
ocation of the sun
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GPS Derived TEC (units 1016 electrons/m2)
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GPS Derived TEC (units 1016
electrons/m2)

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Conclusions
? NGS has extensive experience on GPS data
gathering, archiving, managing, and
interpretation of final processed results.
? NGS GPS products are monitored daily and
updated ? The information is free, availabl
e 24 hours a day, and reachable through a simple
link to our Internet server ? NGS is open to
suggestions from interested GPS users to make
available and/or improve our current line of
products
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Please join us now for the QA Session