Global Navigation Satellite Systems (GNSS) such as the U.S. Global Positioning Service (GPS), the Russian GLONASS, and the European Galileo system provide autonomous geo-spatial positioning with global coverage. Ground (or space-based) receivers collect the signals from orbiting satellites to determine their location in three dimensions and calculate precise time. These receivers detect, decode, and process both pseudorange (code) and phase transmitted by the GNSS satellites.
The GNSS data consist of the receiver’s observation data, the broadcast orbit information of the tracked satellites, and supporting data, such as meteorological parameters, collected from co-located instruments. Currently, the IGS network consists of more than 500 globally distributed stations, equipped with geodetic-quality receivers that track the GPS satellite constellation; a subset of this network are capable of tracking both GPS and GLONASS satellite signals. Furthermore, some stations are upgraded with multi-GNSS receivers capable of obtaining signals from GPS, GLONASS, Galileo and other GNSS (BeiDou, QZS, etc.). The CDDIS archives GNSS data from additional receivers not part of the operational IGS network in support of IGS working groups and pilot projects as well as NASA programs.
GNSS data are archived in separate files by site, data type, and time span. The format for operational GNSS data files is Receiver Independent Exchange (RINEX) format, version 2 (2.10 or 2.11). As per IGS requirements, all data files are compressed with gzip as of December 1, 2020; data files received prior to this date are Unix compressed. GNSS observation data are also software compressed using algorithms to eliminate redundant information, recording only variations and reducing the digits of data by taking differences over adjacent epochs, using the “Hatanaka compression.” The resulting compacted observation file, when further compressed using UNIX compression, is reduced in size by a factor of eight, allowing for not only a decrease in storage requirements but also in network file transfer times. Users of these data must use freely available, specialized software to reverse the compaction process and retrieve the full observation file.
The CDDIS supports the user community by providing access to data from special programs, such as data from receivers tracking new signals (e.g., for GPS, the civil signal on L2, L2C and the new civil frequency, L5) or data in new formats (e.g., data in version 3 of the RINEX format, data generated from software receivers). The structure of the CDDIS archive is relatively simple to modify in order to accommodate these test data; the CDDIS strives to facilitate scientific research by providing access to new data types and formats.
Data from the global GNSS permanent receiver network supporting the IGS are organized in subdirectories by the following:
Products derived from Global Navigation Satellite System (GNSS) data archived in NASA's Crustal Dynamics Data Information System Distributed Active Archive Center (CDDIS DAAC) are generated and submitted by the International GNSS Service (IGS) analysis and associate analysis centers.
IGS products support scientific research in:
IGS operational products include precise GNSS satellite ephemerides, positions and velocities of stations in the global GNSS network, EOPs (polar motion and rates, length-of-day), and station and satellite clock solutions. Other products derived from the CDDIS GNSS data holdings include troposphere Zenith Path Delay (ZPD) estimates (both dry and wet components) and global ionosphere maps, both in support of IGS Working Groups or Pilot Projects.
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