A constellation of GPS RO sensors: from research to long-term sustained operational capabilities

In April 2006, the COSMIC mission (a joint project between the United States and Taiwan) launched six low-earth orbit (LEO) satellites into a circular polar orbit. Each of the LEO satellites carries a GPS Radio Occultation (RO) receiver to measure time delays of the GPS signals traveling from the GPS to the LEO satellites. After several processing steps, very accurate information (< 0.5 Kelvin temperature accuracy) on the thermodynamic state of the atmosphere can be retrieved from the rawer measurements. The major objective of the COSMIC mission is to demonstrate the significant benefits of incorporating GPS RO data into NWP models to improve forecast skill. Although COSMIC was not designed as an operational mission, COSMIC data is being assimilated operationally at many weather centers, including NCEP and ECMWF.

A significant amount of work was necessary to get ready for the use of COSMIC in NWP models. New assimilation algorithms, quality control procedures, observation errors estimates, data flow architecture, etc, were developed, tested and implemented in the data assimilation systems. The assimilation of GPS RO observations from the COSMIC mission became operational at NCEP on 1 May 2007. The assimilation of this new type of data has resulted in significant benefits in terms of model skill, indicating that GPS RO observations provide a unique and valuable piece of information to the assimilation system, not contained in other type of satellite observations. Since the initial use of COSMIC data, NCEP has incorporated GPS RO data from additional missions into its operational suite, and it is currently evaluating additional sensors to enable assimilation in the near future.

The high accuracy and vertical resolution of the GPS RO profiles have been proven to enhance the use of other satellite observations. The fact that GPS RO observations are accurate under all-weather conditions, minimally affected by clouds and precipitation, provide equal accuracy over land than over ocean; and do not require bias correction schemes, are some of the many characteristics of this technique. The significant improvement in model skill achieved with COSMIC has encouraged NOAA to work on the design and planning for a COSMIC-2 operational mission.

During this presentation, pre-operational and operational impact of the COSMIC observations at NCEP will be presented. In addition, an overview on how the data is being used in the assimilation schemes will be discussed. Finally, plans for a COSMIC-2 mission will be introduced, emphasizing the major differences between the current and future constellations.