The GPS Observer
After years of preparing data, the U.S. National Geodetic Survey (NGS) finally adopted the North American Datum of 1983, which is denoted NAD 83 (1986) in 1986. At that time, NGS published an internally consistent set of geodetic latitudes and longitudes for more than 200,000 reference stations located in the United States and its territories. Observations from Canada, Mexico and Central America were also included in the adjustment. The ellipsoid used for NAD 83 is the Geodetic Reference System of 1980 (GRS 80), with the origin at the center of mass of the earth (geocentric), and the orientation is that of the Bureau International de l'Heure (BIH) Terrestrial System of 1984. NAD 83 is similar to other modern global reference systems, such as the World Geodetic System of 1984 (WGS 84) used by the U.S. Department of Defense.
At the time of this major geodetic adjustment, GPS observations were not included in the data. Greenland, Hawaii and the Caribbean islands were connected to the datum through Doppler satellite and Very Long Baseline Interferometry (VLBI) observations. With the introduction of GPS to the geodetic community, NGS has published several newer realizations of NAD 83 to keep pace with positioning technology. One of the first new realizations was the establishment and adjustment of individual state High Accuracy Reference Networks (HARNs). NGS provided an adjustment for the individual states or a combination of states. (For example, I live in New Mexico and the adjustment for our state also includes Colorado.) The geodetic control for both Colorado and New Mexico is denoted NAD 83 (1992). Arizona, an adjoining state, is also denoted NAD 83 (1992), but that was a different adjustment. I have discussed this in earlier articles; HARNs are not to be used when crossing state lines not included in the same adjustment.
After the establishment of the state HARNs, the advancement in GPS receiver technology allowed NGS to establish the National CORS network. Each CORS is a permanent ground-based station that collects dual-frequency GPS data 24 hours each day, seven days a week. A CORS is denoted as NAD 83 (CORS96), although all data sheets I see do not include "96.'
Plate Tectonics and the International Terrestrial Reference Frame (ITRF)Most North American surveyors have heard of plate tectonics (continental drift), but because of the "fixed" control (latitudes and longitudes) established by U.S., Canadian and Mexican geodetic control agencies, they only see these words in surveying journals. We are located on the North American plate, but the United States has a state and territories that are located on other plates. Islands located in the Pacific Ocean move horizontally by as much as several centimeters per year relative to the North American plate. Worldwide, there are many tectonic plates. Because of this, the International Earth Rotation and Reference Systems Service (IERS) was established in 1988 to provide the worldwide scientific and technical community reference values for earth rotation parameters. The IERS is in charge of promoting the International Terrestrial Reference System (ITRS). What it has done is produce reference frames called "International Terrestrial Reference Frames" (ITRF). These ITRFs are three-dimensional coordinate systems (X,Y,Z), and because of plate tectonics, a new ITRF was introduced every year from 1989 to 1997; the latest is ITRF 2000.
CORS and ITRFAll CORS are referenced to an ITRF. When a CORS first becomes operational, NGS computes its positional coordinates relative to the current realization of the ITRF. The current realization, as of December 2001, is denoted ITRF2000, or ITRF00. NGS verifies these ITRF positional coordinates daily using a 24-hour span of recent GPS data to ensure that these coordinates are consistent with corresponding ITRF coordinates from other reference stations around the world. The ITRF coordinates of a CORS are updated if they are found to be inconsistent by more than 1 cm horizontally or 2 cm vertically.
For each CORS there are two different coordinate listings. To show this, I chose the CORS at Penn State University, named PSU1. I got this information from the NGS website, www.ngs.noaa.gov, clicking on "CORS/GPS data" to display the CORS map and pointing at the Mid-Atlantic Region of United States (see Figure 1), then zooming in on Pennsylvania (see Figure 2), and finally clicking on PSU1. This displays a photograph of the antenna, shown in Figure 3 above. Notice in Figure 3, on the left side, there is the following listing:
- Data Availability
- Data Sheet
- Log file
- RINEX2 Data
- Time Series (60-day)
- Time Series (long-term).
So that NAD 83 users won't have to cope with ever-changing coordinates, NGS incorporated numerical parameters to compensate for this movement. Points located within the interior of the North American plate experienced little or no movement. (This is not the case for states like California and the islands in the Pacific Ocean. That will be discussed in Part 2 to this article.)