CORS Coordinates

On March, 2002, NGS upgraded NAD 83 positions and velocities for all CORS sites, except those located on Pacific islands, so that they equal the transformed values of recently computed ITRF00 positions and velocities. This upgrade will remove inconsistencies among previously published NAD 83 positions and velocities which are detectable with modern high accuracy GPS surveys. These inconsistencies would have corrupted results obtained with the OPUS (Online Positioning User Service) utility that uses CORS positions and velocities to compute accurate positions of other points. Moreover, for each CORS site, NGS will now provide NAD 83 coordinates that are referred to an epoch date of 2002.00. That is, the published coordinates for a site's position will correspond to this site's location on January 1, 2002. The site's NAD 83 velocity needs to be applied to compute the site's position on any other date. Previously, NAD 83 positions for the CORS sites were published for an epoch date of 1997.00 (January 1, 1997). The use of the more current epoch date will reduce those systematic errors occurring when points are positioned relative to CORS sites without applying appropriate site velocities. This more current epoch date will especially benefit those involved in positioning activities in areas of active crustal motion, like western CONUS and Alaska.

On October, 2002, NGS updated the NAD 83 positions and velocities for all continuously operating reference stations (CORS) located on Pacific islands to epoch 2002.00. Stations on Hawaii, American Samoa, and in the Marshall Islands refer now to the frame NAD 83(PACP00). Stations on the Mariana Islands (GUAM and CNMI) refer to the frame NAD 83 (MARP00). The extensions PACP00 and MARP00 (see NAD 83 (PACP00)) indicate that the published positions and velocities were transformed from the ITRF00 to the fixed-Pacific-plate and fixed-Mariana-plate NAD realizations, respectively.

Since March 2003, and due to significant displacements at the Alaskan CORS sites caused by the Denali earthquake (November 3, 2002), the epoch of all NAD 83 (CORS96) coordinates for sites in Alaska was changed to 2003.00 (JAN 1 2003).

Published ITRF00 positional coordinates and velocities will continue to be referenced to an epoch and date of 1997.00 to remain consistent with international convention

ITRF00 Coordinates Monitoring

NGS monitors through daily solutions of the GPS CORS network (24h of observed data) the quality of the computed coordinates. As a byproduct, NGS compiles time series plots of the variation of the CORS ITRF00 coordinates with respect to the published values in the local horizon reference frame (n, e, u), spanning a period of 60 days. It should be emphasized that all plots were corrected for the effect of station velocity. For example, to see the differences between the published and daily computed coordinates during the last 60 days at station GAIT (Gaithersburg, Maryland) click below:

ftp://www.ngs.noaa.gov/cors/Plots/gait.gif.

Notice that in this particular example the behavior of the determined coordinates agrees well with the horizontal (red) broken line that represents the adopted (published) ITRF00 values. On the top of the graph are given the mean (bias) and rms in each of the three components North, East, and Up (vertical), during the 60 day period. Each individual point is plotted with its corresponding error bar. The scrupulous checking of the 60-day time series often detects unreported antenna changes, whenever an antenna was replaced or displaced by natural phenomena (earthquakes, hurricanes, subsidence, etc.). Geophysical processes may produce significant station displacements that should be computed and corrected. This information is critical to the CORS users if they want to determine accurate relative positions from CORS sites.

The full list of 60 day plots given alphabetically by site name is available at:

ftp://www.ngs.noaa.gov/cors/Plots/plots.html.

Vertical uncertainties are consistent with the difficulty in modeling the atmospheric refraction (troposphere and ionosphere). The magnitude of the vertical and horizontal errors are not necessarily correlated, although a systematic degradation in the order latitude, longitude, and height is prevalent and has been known since the introduction of GPS in geodetic operations. Large residuals are generally correlated with unmodeled conditions at the observing sites, e.g., high humidity, passing storm fronts, ionospheric activity, etc. Conceptually, they indicate how consistently each daily solution fits its average value which, presumably, represents the outcome of ideal standard conditions. Daily solutions are obtained using all available data through a least squares adjustment that constraints to the published values 3 CORS sites (GODE, MDO1, NLIB) and 2 IGS sites (ALGO, DRAO).

NGS also produces a so-called multiyear solution, where data available in our CORS archives is combined into a single solution using every third day since the collection of data at the station started until the last complete available year on record. Click below to see the results for the same GAIT station:

ftp://www.ngs.noaa.gov/cors/Plots/Longterm/gait.gif.

This multiyear solution determines the final estimates of the adopted and published ITRF00 coordinates and velocities. Further statistical analysis of these data may resolve unknown annual, semiannual, etc. periodic variations at the sites.

Thus, every few years, NGS uses all GPS data in its archives since 1994 to compute provisional positions and velocities for all CORS relative to the current ITRF realization: call it ITRFxx. If, for any station, these provisional ITRFxx positional coordinates differ from the currently adopted ITRFxx positional coordinates by more than 1 cm in the north-south/east-west component and by more than 2 cm in the vertical component, then NGS adopts the provisional position and velocity to supersede the previously adopted ITRFxx position and velocity.

Similar analysis is done with respect to the adopted NAD 83 (CORS96) coordinates. When the daily provisional transformed coordinates referred to the NAD 83 frame differ by more than 2 cm in the north-south/east-west component or by more than 4 cm in the vertical component, then NGS adopts the provisional NAD 83 positions and velocity to supersede the previously adopted NAD 83 values. As a result of these less stringent tolerances, adopted NAD 83 (CORS96) positions and velocities are less likely to be updated than their ITRF counterparts.

Criteria for recomputing CORS coordinates

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CORS displacements in Alaska caused by Denali earthquake (November 3, 2002)

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Get the most recent Coordinates for a particular site.

The following coordinate files contain coordinates for all CORS sites together in a single text table.

Reference Frame	Station Reference Point	To Be Used With Precise Orbits From ...	Orbit Frame
ITRF93 x,y,z	ARP	0:00 UTC January 1, 1995 - 24:00 UTC June 29, 1996	ITRF93
ITRF93 lat,lon,height
ITRF94 x,y,z	ARP	0:00 UTC June 30, 1996 - 24:00 UTC February 28, 1998	ITRF94
ITRF94 lat,lon,height
ITRF96 x,y,z	ARP	0:00 UTC March 1, 1998 - 24:00 UTC July 31, 1999	ITRF96
ITRF96 lat,lon,height
ITRF97 x,y,z	ARP	0:00 UTC August 1, 1999 - 24:00 UTC June 3, 2000	ITRF97
ITRF97 lat,lon,height
		0:00 UTC June 4, 2000 - 24:00 UTC December 1, 2001	IGS97
ITRF00 x,y,z	ARP	0:00 UTC December 2, 2001 - 24:00 UTC January 10, 2004	IGS00
ITRF00 lat,lon,height
		0:00 UTC January 11, 2004 - 24:00 UTC November 4, 2006	IGb00
		0:00 UTC November 5, 2006 - present	IGS05
;	;	Transformation parameters to be used with equations given below
	L1 Phase Center	ITRF93 --> NAD83 (CORS93) [9 common points] t0 = 1995.0 x(t0) = 0.9769 m; y(t0) = -1.9392 m; z(t0) = -0.5461 m x(t0) = 26.40 mas; y(t0) = 10.10 mas; z(t0) = 10.30 mas s(t0) = 0.0 (unitless) x = y = z = x = y = z = = 0.0
	L1 Phase Center	ITRF94 --> NAD83 (CORS94) [8 common points] t0 = 1996.0 x(t0) = 0.9738 m; y(t0) = -1.9353 m; z(t0) = -0.5486 m x(t0) = 27.55 mas; y(t0) = 10.05 mas; z(t0) = 11.36 mas s(t0) = 0.0 (unitless) x = y = z = 0.0 x = 0.09 mas · year-1; y = -0.77 mas · year-1; z = 0.02 mas · year-1 = 0.0
	ARP	ITRF96 --> NAD83 (CORS96) [12 common points] t0 = 1997.0 x(t0) = 0.9910 m; y(t0) = -1.9072 m; z(t0) = -0.5129 m x(t0) = 25.79 mas; y(t0) = 9.65 mas; z(t0) = 11.66 mas s(t0) = 0.0 (unitless) x = y = z = 0.0 x = 0.0532 mas · year-1;y = -0.7423 mas · year-1;z = -0.0316 mas · year-1 = 0.0
	ARP	ITRF97 --> NAD83 (CORS96) [12 common points] t0 = 1997.0 x(t0) = 0.9889 m; y(t0)= -1.9074 m; z(t0) = -0.5030 m x(t0) = 25.915 mas; y(t0) = 9.426 mas; z(t0) = 11.599 mas s(t0) = -0.93 · 10-9 (unitless) x = 0.0007 m · year-1; y = -0.0001 m · year-1; z = 0.0019 m · year-1 x = 0.067 mas · year-1; y = -0.757 mas · year-1; z = -0.031 mas · year-1 = -0.19 · 10-9 year-1
NAD 83 (CORS96) x,y,z NAD 83 (CORS96) lat,lon,height	ARP	ITRF00 --> NAD83 (CORS96) [12 common points] t0 = 1997.0 x(t0) = 0.9956 m; y(t0) = -1.9013 m; z(t0) = -0.5215 m x(t0) = 25.915 mas; y(t0) = 9.426 mas; z(t0) = 11.599 mas s(t0) = 0.62 · 10-9 (unitless) x = 0.0007 m · year-1; y = -0.0007 m · year-1; z = 0.0005 m · year-1 x = 0.067 mas · year-1; y = -0.757 mas · year-1; z = -0.051 mas · year-1 = -0.18 · 10-9 year-1
GPS users in Canada should notice that NAD83 (CSRS98) =NAD83 (CORS96)
xNAD83 = x(t) + [1+s(t)] · xITRF + z(t) · yITRF - y(t) · zITRF yNAD83 = y(t) - z(t) · xITRF + [1+s(t)] · yITRF + x(t) · zITRF zNAD83 = z(t) + y(t) · xITRF - x(t) · yITRF + [1+s(t)] · zITRF
x(t) = x(t0) + x · (t-t0) y(t) = y(t0) + y · (t-t0) z(t) = z(t0) + z · (t-t0) x(t) = [x(t0) + x · (t-t0)] · mr y(t) = [y(t0) + y · (t-t0)] · mr z(t) = [z (t0) + z · (t-t0)] · mr s(t) = s(t0) + · (t-t0) mr = 4.84813681×10-9 , conversion factor from milli-arcseconds (mas) to radians x(t0),y(t0), and z(t0) are differential rotations about the xITRF, yITRF, and zITRF axes respectively. The sense of the rotations is counterclockwise (anticlockwise) positive. Note.- The equations given above serve to transform ITRF positional coordinates (xITRF, yITRF, zITRF) whose epoch date is at time t to NAD 83 positional coordinates (xNAD83, yNAD83, zNAD83) for this same epoch date. Positional coordinates for a different epoch date (and for a particular reference frame) can only be obtained by knowing the velocity of the point in this reference frame. For more information consult the following paper

Last Modified: November 13, 2007
Tomás Soler