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 nearly 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). All data files 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 support the IGS are organized in subdirectories by
- File content (daily 30-second sampled, hourly 30-second sampled, high-rate 1-second sampled)
- Year and day of year
- Data type (observation, broadcast navigation, etc.)
- List of GNSS sites (Tracking Network)
- Map of GNSS sites
- Daily 30-second sampled GNSS data from 1991 through present
- Hourly 30-second sampled GNSS data from 2005 through present
- High-rate 1-second sampled GNSS data from 2001 through present
- Daily 30-second sampled GNSS tracking data: ~2.5 Mbytes/site/day (compressed)
- High-rate 1-second sampled GNSS tracking data: ~20 Mbytes/site/day (compressed)