Obtaining CORS Information Using Anonymous FTP DATE ==== August 2, 2005 INTRODUCTION ============ The National Geodetic Survey (NGS), an agency of the National Oceanic and Atmospheric Administration, manages the National CORS (Continuously Operating Reference Station) network. This network currently contains more than 650 GPS data-collection sites in the United States and its territories, with a few in foreign countries. The following text describes how to obtain GPS data from these sites via the Internet using the anonymous file transfer protocol (FTP) utility. Users may also obtain these GPS data via the World Wide Web at http://www.ngs.noaa.gov/CORS. While most users should find the Web more convenient for downloading small amounts of GPS data from the CORS network, NGS recommends that people use anonymous ftp to download large amounts of GPS data such as data for more than five days and/or data for more than ten sites. See http://www.ngs.noaa.gov/CORS/newsletter2/ for a list of the sites currently contained in the National CORS network. NGS also manages the Cooperative CORS network which contains more than 140 additional GPS data-collection sites in the United States. Users need to download GPS data from these sites directly from the organization that is responsible for the site. Appropriate Web links are maintained at http://www.ngs.noaa.gov/CORS/Coop . CORS sites are operated by a collection of more than 155 organizations, representing various federal, state, local and foreign government agencies, as well as various academic and commercial institutions. The CORS network enables GPS users to determine centimeter-level positional coordinates for user-selected locations. Additionally, CORS data serve to monitor crustal motion, the distribution of moisture in the atmosphere, and the distribution of free electrons in the ionosphere. DATA FORMATS ============== NGS uses four types of files to store the GPS data. The following table provides a summary of these file types. All stored files have been compressed using the UNIX-Z compression algorithm. Also, all files are either in the Receiver Independent Exchange (RINEX) format, version 2(1) ( http://www.ngs.noaa.gov/CORS/Rinex2.html ), or the HATANAKA-compacted RINEX format ( http://sopac.ucsd.edu/dataArchive/hatanaka.html ) ____________________________________________________________________________________ | Day 0 | Days 1-30 | ____________________________________________________________________________________ RINEX FORMAT | | Type D1: Daily file | | | at original | | | sampling rate | ____________________________________________________________________________________ HATANAKA | Type H: Hourly | Type D2: Daily | Type D2: Daily COMPACTED | files at original | file, decimated to | file, decimated to RINEX FORMAT | sampling rate | a sampling rate of | a sampling rate of | | 30 seconds | 30 seconds ____________________________________________________________________________________ Each Type H file contains one hour's worth of GPS data in the Hatanaka-compacted RINEX format for an individual CORS. These files contain the data at its original sampling rate (or sampling interval), that is, at the sampling rate that the GPS receiver recorded the code and carrier-phase signals from the satellites. Popular sampling rates include 1 second, 5 seconds, 10 seconds, 15 seconds, and 30- seconds. These files are created only for those CORS whose data are available to NGS on an hourly or more rapid basis. These files are created within a few minutes after the data have become available to NGS (usually within a few minutes after the turn of the hour). Each Type D1 file contains 24-hours worth (from UTC midnight to UTC midnight) of GPS data in the RINEX format for an individual CORS at the data's original sampling rate. These files are created after all 24-hours of data have become available to NGS (usually within a few hours after EDT/EST midnight). These files are kept for 30 days. Each Type D2 file contains 24-hours worth (from UTC midnight to UTC midnight) of GPS data in the Hatanaka-compacted RINEX format for an individual CORS at a 30-second sampling rate. These files are created after all 24-hours of data have become available to NGS (usually within a few hours after EDT/EST midnight) . These files are kept indefinitely. FILE NAMING CONVENTION ====================== The RINEX file naming convention is as follows: {ssss}{ddd}{h}.{yy}{t}; where ssss is the four character site identifier, "ddd" is the day of year, "h" is a letter which corresponds to an hour long UTC time block, "yy" is the year and "t" is the file type. For daily files, the format would be {ssss}{ddd}0.{yy}{t}. Hour long UTC time block identifier (h): 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 a b c d e f g h i j k l m n o p q r s t u v w x Ending (t) File Type _________________________________________________ o Observation d Hatanaka-compacted observation n Navigation (satellite orbit) m Meteorological s Summary __________________________________________________ FILE TRANSFER PROTOCOL (FTP) ============================ FTP is a user interface to the File Transfer Protocol. FTP copies files over a network connection between the local "client" (user) computer and a remote "server" computer. FTP runs on the client computer. The user's system must have access to the INTERNET and support the File Transfer Protocol (FTP). Some useful ftp commands are given below. ascii set ascii transfer type binary set binary transfer type bye terminate ftp session and exit cd change remote working directory dir list contents of remote directory get retrieve one file help print local help information mget retrieve multiple files mput send multiple files prompt force interactive prompting on multiple commands put send one file quit terminate ftp session and exit show display the contents of an ASCII file * Actual commands may vary among operating systems. TO ACCESS CORS FTP DIRECTORIES ============================== To access the CORS FTP directories, follow the steps below. Type the "ftp" command followed by the INTERNET address as follows: ftp cors.ngs.noaa.gov Respond to the following: Login: anonymous Password: your complete e-mail address You will arrive at the ftp command level indicated by the prompt "ftp>". Then use the command " cd cors " If you have trouble, type "help" to print local help information or review the section FILE TRANSFER PROTOCOL for help with additional commands. The following sub-directories contain additional files and information. coord NAD83 and ITRF positional information for the CORS sites. rinex Rinex data files. station_log Station information, antenna specifications, and site contacts. utilities Programs for manipulating the RINEX files. Plots 60-day Time Series plots of CORS sites. ACCESS VIA WWW ============== NGS has developed a home page on the Internet World Wide Web (WWW) for online access to information on its products, services, and program activities. The CORS products and data are available through NGS' home page which is accessible through either NOAA's home page or by typing in the URL (Uniform Resource Locator): http://www.ngs.noaa.gov/CORS when using a browsing tool such as Internet Explorer (IE) or Netscape. Browse to find the information you need. Users may also access the CORS FTP directories via the WWW by typing in the URL ftp://www.ngs.noaa.gov/cors/ . DIRECTORY STRUCTURE (FTP) ========================= | | | | |-- yyyy --|-- ddd --|--site id--|-- GPS data |-- rinex --| | | | | | | --cors-| |-- global nav file | | | |- igs orbits (igs, igr, igp) |-- select.txt | |-- UFCORS.txt | |-- Plots | |-- README.txt | |-- RINEX-2.txt | |-- news.let (newsletter) | |-- coord | |-- station_log --|-- *.log | |-- utilities |-- Unix--|-- HPUX -- programs | | |-- SunOS | | |-- Solaris | | |-- SGI | | | |-- NT -- |-- programs | | | |-- Dos-- |-- interpo.exe | |-- join24pc.exe |-- join24.txt |-- gpscal.exe |-- gpswk.exe |-- cato.exe |-- cato.txt |-- decimate.exe |-- decimate.inp |-- decimate.txt |-- gzip386.exe |-- uudecode.exe |-- uuencode.exe |-- tar4dos.zip FILE COMPRESSION ================ All GPS data files are stored in the UNIX compressed format. The compressed files have the extension ".Z" appended. You may retrieve the compressed files as is by using the "get or mget" command and the complete file name, including file name extensions. If you transfer the file in compressed mode, be sure to set the transfer mode to "binary". Executable program files should be transferred in "binary" mode, text files should be transferred in "ascii" mode. UTILITIES ========= Several DOS based utility programs are available to manipulate the RINEX data files. Versions also exist for other platforms such as Silicon Graphics (sgi), Sun Microsystems (sun), and Hewlett Packard (hp). gzip386.exe Executable file which contains the utility "gzip.exe". join24pc.exe Utility program to join two or more hourly RINEX observation or navigation files. decimate.exe Utility program to decimate 5 second data to a user specified rate. interpo.exe Utility program to interpolate between data epochs. Please read the documentation for this utility for more details. E-MAIL ====== If you would like to add your name to the NGS CORS e-mail list, please email your request to ngs.cors@noaa.gov. You will receive the weekly NGS CORS Newsletter and other special notifications. ADDITIONAL INFORMATION OR REQUESTS ================================== For additional information please email ngs.cors@noaa.gov Or call us at (301) 713-3563.