PE&RS January 2003

Direct Georeferencing

The Synergistic CORS Program

The tremendous diversity of Global Positioning System (GPS) applications has motivated numerous organizations to establish several (essentially independent) networks comprised of ground-based stations that collect dual-frequency GPS data on a continuous basis. The National Geodetic Survey (NGS), an agency of the National Oceanic and Atmospheric Administration, is collaborating with most of these organizations to make data from hundreds of such GPS base stations available to the public at no cost through the internet. The term CORS (Continuously Operating Reference Stations) identifies those stations participating in this integrated network. Regardless of the primary purpose of a CORS’ existence, its data may find use in positioning, navigation, geophysics, and/or meteorology. The success of the CORS program is due to contributions by people from more than 80 organizations, with each organization operating at least one CORS.

The CORS network currently contains more than 600 stations in the United States and its territories, plus several stations in Central America and the Caribbean. Today, the total land area in 49 states, plus much of Alaska, is located within 400 km of at least one CORS, and more than 55 percent of the land area in these 49 states is located within 100 km of at least one CORS.

To obtain 3D positional coordinates for a point with an accuracy of a few centimeters, at least two GPS receivers are necessary, one located at a point whose positional coordinates are already known and the other located at the point whose positional coordinates are to be determined. Because positional coordinates for the CORS are well known, CORS data enable GPS users to deploy only one GPS receiver for accurately positioning points of interest to them. Consequently, surveyors, GIS professionals, engineers, scientists, and others can apply CORS data to accurately and economically interrelate the locations of navigational aids, utility lines, important boundaries, and other map-worthy features. While centimeter-level accuracy usually requires a user to post-process several hours’ worth of GPS data collected at one location with corresponding CORS data, a user can easily achieve sub-meter accuracy by so post-processing only one minute’s worth GPS data.

Photogrammetrists, remote-sensing professionals, and others can apply CORS data to determine the travel path of a moving platform with an accuracy of a few decimeters, provided this platform is GPS-equipped. Such platforms include aircraft, watercraft, and land vehicles. This capability was applied with lidar aboard an aircraft to produce 3D maps of Manhattan within 24 hours of flight time to assist recovery operations after the terrorist attack on the World Trade Center in September 2001. To find out just how accurately the trajectory of an aircraft can be determined when using CORS data, investigators at Applanix Corporation conducted test flights in California and Texas. Their results imply a 3D RMS error of about 22 centimeters when using GPS data from a CORS located about 100 km from the centroid of the flight path. In their experiments, these investigators successfully interpolated CORS data that were collected with a 30-second sampling rate to compute the position of the aircraft every second for about 4 hours. These investigators also found that a more accurate trajectory is achievable when the CORS is located closer to the flight path. Moreover, better accuracy and reliability are achievable when using GPS data from several CORS. Although remote-sensing professionals may opt to establish a temporary GPS base station near the aircraft’s intended flight path to obtain a highly accurate trajectory, they can supplement this station’s GPS data with that from several CORS as a safeguard against the loss of data due to multipath, local obstructions to satellite visibility, equipment failures, and/or human errors.

The CORS program also contributes greatly to the fields of geophysics and meteorology. Earth scientists can apply several years of CORS data to determine the crustal velocities of these stations with an accuracy of a few millimeters per year. CORS data have been used to study plate tectonic motion, earthquake-related deformation, volcanic activity, land subsidence, glacial isostatic adjustment, tidal loading, and atmospheric loading. Meteorologists can apply CORS data to monitor the spatialtemporal distribution of water vapor in the Earth’s atmosphere. Knowledge of the water vapor distribution is critical for forecasting such severe weather as tornadoes, hurricanes, thunderstorms, and snowstorms. Atmospheric scientists studying “spaceweather” can apply CORS data to monitor the spatialtemporal distribution of free electrons in the ionosphere, a region of the upper atmosphere between 50 and 1,000 km. Solar and geomagnetic storms affect the distribution of these free electrons. Rapid changes in this distribution affect satellites, aircraft, certain radio communications, and even power-distribution grids on the Earth.

In conjunction with the CORS program, NGS has developed the web-based utility known as the Online Positioning User Service (OPUS), which will quickly and automatically calculate an accurate 3D position for a location corresponding to a user-supplied file of appropriate GPS data. Currently, OPUS requires the user to supply a file in the widely accepted receiver-independent-exchange (RINEX) format. This file must contain at least a two-hour span of dual-frequency GPS data (carrier-phase observations) collected at a single location. OPUS then automatically retrieves pertinent GPS data for three suitable CORS for use in calculating the positional coordinates associated with the user-supplied data. OPUS then emails the calculated coordinates to a user-specified email address.

The CORS program continues to evolve in many respects. The CORS network is currently growing at a rate of six new stations per month. The NGS staff continues to develop utilities to make CORS data more accessible. The NGS staff is also enhancing OPUS so that users may select one or more of the three CORS to be used in computing the positional coordinates associated with their GPS data.

For more information about CORS, visit www.ngs.noaa.gov/CORS/
Dr. Richard Snay, Manager, National CORS Program, National Geodetic Survey.

Edited by Dr. Mohamed M.R. Mostafa, Applanix Corporation.