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International GNSS Service (IGS) Working Groups supported by the CDDIS

The CDDIS has supported the IGS as a global data center since its inception in 1992, including active participation in several IGS Working Groups and Pilot Projects. These working groups include:

Ionosphere Working Group (1998-present)

The Ionosphere Working group began routine generation of the combine Ionosphere Vertical Total Electron Content (TEC) maps in June 1998. Today, contributing groups generate three types of ionospheric products: final, rapid and predicted. Four IGS Ionosphere Associate Analysis Centers (IAACs) produce these products on a routine basis: CODE (Center for Orbit Determination in Europe, Astronomical Institute, University of Berne, Switzerland), ESA/ESOC (European Space Operations Center of ESA, Darmstadt, Germany), JPL (Jet Propulsion Laboratory, Pasadena CA, USA), and UPC (Technical University of Catalonia, Barcelona, Spain). The Ionosphere Product Coordinator computes the official IGS combined products from individual products provided by these contributing IAACs. Since January 2008, this coordination is carried out by the GRL/UWM (Geodynamics Research Laboratory of the University of Warmia and Mazury in Olsztyn, Poland). The IGS global ionosphere maps (GIMs) are provided in Ionosphere Exchange (IONEX) format with a spatial resolution of 5.0 degrees in longitude and 2.5 degrees in latitude, and a temporal resolution of 2 hours. Latency of the final and rapid GIMs is 10 days and 1 day, respectively. In late 2009, the Working Group began providing predicted ionospheric products 1 and 2 days in advance. These new IGS products are currently based on predicted ionosphere maps prepared by UPC and ESA.

(Source: IGS Ionosphere WG website, July 2013)

Troposphere Working Group (1998-present)

The goal of the Troposphere Working Group (TWG) is to improve the accuracy and usability of IGS troposphere estimates. Coordination of IGS troposphere activities, including computation of IGS Final Troposphere Estimates and chairpersonship of the IGS TWG, was transferred in 2011 from the NASA Jet Propulsion Laboratory (Pasadena, California, USA) to the United States Naval Observatory (Washington, DC, USA). The IGS troposphere product has transitioned from pilot phase to an official IGS product in 2012. The IGS Tropopshere Coordinator generates daily zenith path delay estimates based on Precise Point Positioning techniques with an approximate three-week latency for all active IGS sites. IGS Final Troposphere estimates are used by scientists worldwide to support climate-change and meteorological studies.

(Source: IGS Troposphere WG website, July 2013)

GPS Tide Gauge Benchmark Monitoring – Working Group (TIGA-WG) (2011-present)

Sea level changes are of great concern. For decades to centuries the sea level has been measured using tide gauges. However, they only constitute a local datum, affected not only by climate but also by local land height changes. Space geodetic techniques are the only viable tool to establish the necessary global reference frame. Providing this frame facilitates the distinction between the relative and geocentric referenced sea level changes by accounting for the vertical uplift of the station, and is, therefore, an important contribution to climate changes studies.

GPS Tide Gauge Benchmark Monitoring – Working Group was formed in late December, 2010, as an extension of the TIGA Pilot Project. The goal of the TIGA-WG is to analyze GNSS data from stations directly at or near tide gauges (TG) on a preferably continuous basis and provide information specifically for the vertical rates. TIGA is recognized by the Global Sea Level Observing System (GLOSS) to provide position and vertical rates for GLOSS tide gauges and is an important contribution of the IGS to the goals of the Global Geodetic Observing System (GGOS), the Global Climate Observing System (GCOS) and the World Climate Research Programme (WCRP).

The primary product of TIGA is daily sets of coordinates for analyzing vertical motions of Tide Gauge Benchmarks (TGBM). When sufficient solutions (3 or more) are available, a combination product of all available TIGA solutions will be generated. The products are made public to support and encourage other applications, e.g. sea level studies. The service may further contribute to the calibration of satellite altimeters, other oceanographic applications or the establishment of a World Height System.

The goals of the TIGA-WG are identified as follows:

  1. Maintain a global virtual cGNSS @ TG network
    • Select a set of tide gauges equipped with GNSS, with a long and reliable history, useful for both sea level change studies, and e.g. satellite altimeter calibrations. IGS network operation standards should be applied.
    • Promote the establishment of more continuous operating GNSS stations, in particular in the southern hemisphere.
    • Provide meta information, e.g. on levelling between benchmarks or data access
    • Provide training to tide gauge operators through workshops, encourage station operators to provide necessary metadata. Through GLOSS, advise station operators about the operation of cGNSS @ TG stations.
  2. Compute precise coordinates and velocities of GNSS stations at or near tide gauges with a delay of a few weeks to months, to allow as many as possible stations to participate. Provide a combined solution as the TIGA official product.
  3. Study the impacts of corrections and new models on the GNSS processing of the vertical. Encourage other groups to establish, e.g. nearby absolute gravity sites.
  4. Provide advice to new applications.

(Source: IGS TIGA WG website, February 2014)

Low Earth Orbiters (LEO) Working Group (2000-2010)

Recognizing the compelling climate and geodetic science possible with, and the rapid growth of, satellites carrying precise, geodetic-quality GPS receivers, the IGS Governing Board created the Low-Earth Orbit (LEO) Working Group with the charter of exploring the role of LEOs in the IGS, and suggesting possible activities for the IGS and its components in the world of spaceborne GPS. The activities of the working group led to the release of the IGS Call for Participation in support of Low Earth Orbiting Missions.

The establishment of a space network of orbiting GPS receivers could be developed as an extension of the ground network while utilizing many of the resources which the IGS currently has in place. Therefore with a Call for Participation the IGS solicited support in the establishment of an enhanced subset of IGS infrastructure targeted at supporting Low Earth Orbiting missions.

The IGS has a role in the development and applications for orbiting GPS receivers and the stage is set for the IGS to now play a significant role in the development of spacebased GPS receiver applications. With this development, the IGS will serve the broader geoscience community as well as potentially provide services for commercial interests.

(Source: LEO WG web site, January 2000)