CORS Coordinates
This page describes the current coordinates published at CORS sites by NGS as well as links to information about older coordinates.
A. Global Reference Frame Coordinates: IGS08 epoch 2005.00
B. National Spatial Reference System Coordinates: NAD83(2011,MA11,PA11) epoch 2010.00
C. Main Changes Compared to Previous Reference Frames
Change in reference epoch
Change in antenna calibrations
D. Multi-Year CORS(MYCS1) Solution: How CORS coordinates are computed
FAQ-MYCS: Numerous site and regional specific plots comparing previous and current coordinates
E. Criteria for Publishing Coordinates
F. Coordinate Monitoring
Short-Term Plots
Long-Term Plots
G. Composite Coordinate Tables and Reference Frame Transformation Parameters
A. IGS08 epoch 2005.00 Coordinates
Since April 17, 2011, the National Geodetic Survey (NGS) and the other Analysis Centers of the International GNSS Service (IGS) have been providing GPS satellite orbits (ephemerides) that are referred to a new terrestrial reference frame, called IGS08 and defined by the IGS. This new frame is based on GPS observations and was designed to be consistent with the International Terrestrial Reference Frame of 2008 (ITRF). ITRF2008 is the latest frame realization of the International Earth Rotation and Reference Systems Service (IERS) and is a multi space-based geodetic technique solution, combining Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) and GPS data. Although, the best fitting Helmert transformation between IGS08 and ITRF2008 for a set of well-established, international GNSS satellite tracking sites is the identity function, the transformed ITRF2008 positions have a site specific "correction" applied to them to create IGS08 positions (for additional details on IGS08 consult the following IGSMAILs 6354, 6355, 6356, 6374). Thus the IGS08 position for a particular site may differ from its corresponding ITRF2008 position; however, the velocities remain identical. By using IGS08 coordinates and the associated absolute antenna calibrations in combination with IGS orbits a consistent frame is realized. In addition, NGS has updated the IGS orbits from January 1, 1994 to April 16, 2011 in its online storage with the recently released IGS reprocessed (repro1) orbits that are all aligned consistently with IGS05. For most non-research applications, users can freely mix IGS05 and IGS08 orbits to compute coordinates for control points. Additional information is available in the following IGSMAIL 6475.
B. NAD83(2011,MA11,PA11) epoch 2010.00 Coordinates
On September 6, 2011, NGS updated the National Spatial Reference System NAD 83 (CORS96, MARP00, PACP00) positions and velocities for all CORS sites, to NAD 83 (2011, MA11, PA11). The NAD 83 (2011) frame, which is relative to the fixed North American plate, is used to define the coordinates for sites located in the CONterminous United States (CONUS), Alaska and US territories in the Caribbean. The NAD 83 (MA11) frame is realized with respect to the fixed Marianas plate and is used to define coordinates in the Marianas. The NAD 83 (PA11) is a Pacific plate fixed frame and is used to define coordinates in Hawaii, American Samoa, the Marshall Islands and other US territories residing on the Pacific Plate. For informative articles about NAD 83 see Snay and Soler, 2000, Snay, 2003. The new realization of NAD 83 involves no datum change, which means that, the origin, scale and orientation of NAD 83(2011) are identical to those of NAD 83(CORS96), and the same for the two other frames. The coordinates are not the same in the old and new realizations for multiple factors including the switch to absolute antenna calibrations, new/revised processing algorithms, improved discontinuity identification, several years of additional GPS data, change in reference epoch, and an improved definition of the global reference frame, IGS08. For a description of how NAD 83 is related to the global reference frame see Craymer et al., 1999, Snay and Soler, 1999. Users working in Canada should consult Craymer, 2006 for a review of how NAD 83 is implemented in Canada. Concisely, the two biggest changes are caused by the change in reference epoch and the move from relative to absolute antenna calibrations.
C. Main Changes Compared to Previous Reference Frames
Change in reference epoch
The new reference epoch has changed by 8 years from 2002.00 to 2010.00. The published NAD 83 coordinates correspond to the position of the site at January 1 2010 (or equivalently, epoch 2010.00), and if a position at a different time is required then the published velocity must be applied and a new position computed. By using a more recent epoch, systematic errors that occur when points are positioned relative to CORS without applying to them appropriate site velocities are reduced. This more current reference epoch date will especially benefit those involved in positioning activities in areas of active crustal motion, like western CONUS and Alaska.
Change in antenna calibrations
Users must take special note that CORS points defined in IGS08 and NAD83(2011, MA11, PA11) differ with respect to those defined in ITRF00 and NAD 83(CORS96, MARP00, PACP00) in the use of a different set of antenna calibrations. In the past users of CORS coordinates only needed to ensure they were using the calibration for the antenna and radome pair they collected their data with, as most calibrations were generated using relative techniques. With the introduction of ITRF2008 and IGS08 (actually IGS05 but this frame was never implemented by NGS), a completely new absolute calibration technique was used to generate antenna calibrations. Furthermore since the absolute antenna calibrations also include calibrations for satellite antennas as well as ground antennas they are revised periodically and are defined for a specific reference frame. NGS has updated its antenna calibration page to reflect the new absolute antenna calibrations that are associated with IGS08, NAD 83(2011, MA11, PA11), namely the IGS08 absolute antenna calibrations, however to continue providing access to older reference frames the set of relative antenna calibrations is also still available. The absolute antenna calibrations may differ from corresponding relative calibrations up to several centimeters so users should ensure they are using the appropriate set of calibration values associated with the particular reference frame that they are working in.
D. Multi-Year CORS (MYCS1) Solution
To obtain the new coordinates that were just described the CORS team completed a full reanalysis of all data from CORS and from a set of global sites with the goal of simultaneously computing a fully consistent set of coordinates, GPS satellite orbits and Earth Orientation Parameters (EOP). This initial Multi-Year CORS (MYCS1) effort is the first of a series of reprocessing projects that will occur periodically in the coming years. The last time a reanalysis of CORS data occurred was in 2002 and numerous inconsistencies and changes have occurred in our processing techniques since that date. The concern over the overall quality of the solutions was not limited to NGS, but also to other geodetic groups, in particular IGS. Thus IGS requested participation in a reanalysis of all data collected since 1994 to establish a new consistent set of GPS orbits, clocks and EOPs. This project was called IG1/repro1. NGS elected to contribute to this effort as an IGS Analysis Center and used this opportunity to simultaneously reprocess all its CORS data to provide a single consistent set of coordinates for all sites computed using the best available methods.
For regional and site specific plots and many details about the MYCS1 please consult theE. Criteria for Publishing Coordinates
Due to the fact that CORS positions vary over time with at least an annual and semi-annual variation, velocities cannot be rigorously estimated until 2.5 years of data or 130 weekly solutions are analyzed. Therefore the published positions and velocities for sites with more than 2.5 years of data and 130 weekly solutions are computed from our stacked solution (MYCS1). For the other sites, mostly new(er) sites, we compute a position, but assign a modeled velocity. The velocity is modeled and obtained using the Horizontal Time Dependent Positioning (HTDP) software. The current velocity models address only the 2-dimensional horizontal components and a 0 (zero) velocity is assigned for the vertical component. Since the modeled velocities may not reflect the actual long-term motion of a CORS and it may take some time to robustly estimate the velocity directly from the data, it is clearly identified that the source is either computed or modeled (see Tables of Composite Coordinates).
CORS coordinates are revised periodically using the following set of criteria (pdf document).
F. Coordinate Monitoring
Short-Term Plots
NGS monitors the quality of its published coordinates through daily solutions of the CORS network. After NGS acquires data for a particular day, processing of the data is delayed by ~18hrs to obtain the IGS rapid orbits. NGS compiles short-term time series plots of these solutions that show the variation of the CORS IGS08 coordinates with respect to the published values, corrected for the effect of the published velocity. The daily residuals positions are in a local geodetic topocentric frame, North, East, and Up (vertical). Conceptually, the plots indicate how consistently each daily solution fits its' average value, which, presumably represents the outcome of ideal standard conditions. Therefore participating CORS operators, CORS team members, and outside users of CORS data sets routinely examine these plots to ensure the quality of the data they provide, distribute and use.
To better illustrate the short-term plots consider the example time-series for station CORB (Corbin, MD). This plot shows the x-axis in day-of-year (ordinal days) for the previous 90-days and the y-axis in mm in the North, East, and Up components. The horizontal (red) broken line represents the published IGS08 values for the particular component, with a green shaded area corresponding to the accuracy that the published coordinates are given, 2cm in the North and East and 4cm in the Up component (For the criteria used to republishing CORS coordinates go here). At the top of the plot is the mean (bias) and 1 sigma standard deviation for each of the three components North, East, and Up (vertical) for the data that is plotted. Each individual point is shown with its corresponding 1 sigma error bar.
An abrupt change in a site's position often indicates an antenna change, most commonly due to an antenna being replaced or displaced by natural phenomena (e.g. earthquakes, hurricanes). However other equipment changes such as receiver upgrade, firmware update, or environment changes e.g. new construction may also result in changes. More gradual drifts may indicate an error in the published velocity. This is common for newer sites which have modeled velocities (see Publishing Criteria) especially in the Up component that is always set to zero, or environmental changes such as vegetation growth.
The magnitude of the vertical and horizontal errors are not necessarily correlated, although the day-to-day scatter in latitude is usually less than the scatter in longitude, which in turn is usally less than the scatter in height. Large residuals are generally correlated with less than 24hrs of data, but may also reflect unmodeled conditions at the observing sites, for example high humidity, passing storm fronts, ionospheric activity, local multipath, etc. The largest uncertainties are in the vertical (up) component and are consistent with the difficulty in modeling the atmospheric refraction (troposphere and ionosphere) and the physical impediment of collecting GPS observations below the horizon.
The complete list of short-term plots are available here.
Long-term Plots
Long-term plots created from our stacked/MYCS1 solution are also available. The plots show weekly residual positions (green dots) for all the data analyzed for a particular station. These long-term time-series show the common annual and semi-annual variation and indicate in magenta lines discontinuities that have been detected. The discontinuities may reflect equipment or environmental changes or a shift due to an earthquake. The amount of displacement/offset has been removed from the plots. The full list of long-term and short-term plots are available here.
G. Composite Coordinate Tables and Reference Frame Transformation Parameters
Cumulative coordinates tables in four reference frames are provided: IGS08 epoch 2005.00, NAD 83(2011), NAD 83(MA11) and NAD 83(PA11) and each is further split into sites that have computed velocities or modeled velocities (see discussion above Criteria for Publishing Coordinates)
There are a small number of sites that do not have updated coordinates. Most of these sites are decommisioned Cooperative CORS sites for which NGS has not archived the data. Therefore they could not be included in our reprocessing. For a list of CORS sites that do not have updated coordinates go here.
For composite tables of previous CORS coordinates and transformation parameters go here. Please note these tables are no longer updated.
Table 1. Composite coordinate files and transformation parameters are all given for the Antenna Reference Point (ARP).
| Composite Coordinate Tables Explain | Transformation Parameters Explain | ||
|---|---|---|---|
Computed
lat, lon, height; Vn, Ve, Vu Modeled IGS08 epoch 2005.00 x, y, z; Vx, Vy, Vz IGS08 epoch 2005.00 lat, lon, height; Vn, Ve, Vu |
Not applicable
| ||
Computed NAD 83 (2011) epoch 2010.00 x, y, z; Vx, Vy, Vz NAD 83 (2011) epoch 2010.00 lat, lon, height; Vn, Ve, Vu Modeled NAD 83 (2011) epoch 2010.00 x, y, z; Vx, Vy, Vz NAD 83 (2011) epoch 2010.00 lat, lon, height; Vn, Ve, Vu |
IGS08 --> NAD 83 (2011) [12 common points] t0 = 1997.0  x(t0) = 0.99343 m;
y(t0) = -1.90331m; z(t0) = -0.52655 m
 x(t0) = 25.91467 mas; y(t0) = 9.42645 mas; z(t0) = 11.59935 mass(t0) = 1.71504 · 10-9 (unitless)  x = 0.00079 m · year-1;
y = -0.00060 m · year-1;
z = -0.00134 m · year-1
 x = 0.06667 mas · year-1;
y = -0.75744 mas · year-1;
z = -0.05133 mas · year-1 = -0.10201 · 10-9 year-1
|
||
Computed NAD 83 (PA11) epoch 2010.00 x, y, z; Vx, Vy, Vz NAD 83 (PA11) epoch 2010.00 lat, lon, height; Vn, Ve, Vu Modeled NAD 83 (PA11) epoch 2010.00 x, y, z; Vx, Vy, Vz NAD 83 (PA11) epoch 2010.00 lat, lon, height; Vn, Ve, Vu |
IGS08 --> NAD 83 (PA11) [14 common points] t0 = 1997.0 x(t0) = 0.9080 m; y(t0) = -2.0161 m; z(t0) = -0.5653 mx(t0) = 27.741 mas;
y(t0) = 13.469 mas;
z(t0) = 2.712 mas
s(t0) = 1.10 · 10-9 (unitless) x = 0.0001 m · year-1;
y = 0.0001 m · year-1;
z = -0.0018 m · year-1
x = -0.384 mas · year-1;
y = 1.007 mas · year-1;
z = -2.186 mas · year-1 = 0.08 · 10-9 year-1
| ||
Computed NAD 83 (MA11) epoch 2010.00 x, y, z; Vx, Vy, Vz NAD 83 (MA11) epoch 2010.00 lat, lon, height; Vn, Ve, Vu Modeled NAD 83 (MA11) epoch 2010.00 x, y, z; Vx, Vy, Vz NAD 83 (MA11) epoch 2010.00 lat, lon, height; Vn, Ve, Vu |
IGS08 --> NAD 83 (MA11)
[14 common points] t0 = 1997.0 x(t0) = 0.9080 m;
y(t0) = -2.0161 m;
z(t0) = -0.5653 m
x(t0) = 28.971 mas;
y(t0) = 10.420 mas; z(t0) = 8.928 mass(t0) = 1.10 · 10-9 (unitless) x = 0.0001 m · year-1;
y = -0.0001 m · year-1;
z = -0.0018 m · year-1
x = -0.020 mas · year-1;
y = 0.105 mas · year-1;
z = -0.347 mas · year-1 = 0.08 · 10-9 year-1
|
Computed tables contain sites that have solutions in our MYCS1 was for >=2.5yrs and include at least 130 weekly solutions
Modeled tables contain sites that operated for <2.5yrs and/or include less than 130 weekly solutions in our MYCS1, new CORS sites are included in these tables
The composite tables include to the right of the coordinate information the NGS abbreviation for country and state/parish codes and the sites' status. The status can be: Operational-Is currently providing data; Non-Operational-Has not provided data for 30-days, Decommissioned-Has stopped operating permanently; IGS_not_CORS-These are IGS stations, for which NGS does not store data on its servers.
Transformation Parameters are applied using the equations shown below to transform from IGS08 to the various NAD 83
xNAD 83 = x(t) + [1+s(t)] · xIGS08 +
 z(t) · yIGS08 - y(t) · zIGS08
yNAD 83 = y(t) -
z(t) · xIGS08 + [1+s(t)] · yIGS08 +
x(t) · zIGS08
zNAD 83 = z(t) +
y(t) · xIGS08 - x(t) · yIGS08 + [1+s(t)] · zIGS08 |
|
|---|---|
| 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)] · mrz(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 xIGS08, yIGS08, and zIGS08 axes respectively.The sense of the rotations is counterclockwise (anticlockwise) positive. Note.- The equations given above serve to transform IGS08 positional coordinates (xIGS08, yIGS08, zIGS08) whose epoch date is at time t to NAD 83 positional coordinates (xNAD 83, yNAD 83, zNAD 83) 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 about the derivation of these equations consult the following article Soler and Snay, 2004. |
