Global navigation satellite system Receiver for Atmospheric Sounding Data AssessmentEntry ID: GRAS_ESA_01
Abstract: The entire GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) instrument weighs about 30 kg and comprises three antenna-receivers positioned separately on MetOp. One of the antennae, the GVA antenna, points in the velocity direction of the satellite, another, the GAVA antenna points in the anti-velocity direction, and the smaller GZA antenna points zenith.
The GVA and ... GAVA antennae are both rectangle-shaped, covered by a single layer of insulation. They carry 18 elements, each targeted to look tangentially through the Earth's atmosphere to receive radio signals from the constellation of navigation satellites. The GVA observes rising occultations and the GAVA observes setting ones. Together, they can connect to up to 12 satellites at any one time (two by the GVA chain, two by the GAVA chain and eight by the GZA for navigation. Therefore, four occultation measurements can be made in parallel, two on each side). Because of the positioning of the GAVA antenna on the MetOp satellite, it is deployed after launch to prevent its field of view being obscured by the ASCAT instrument.
The smaller GZA antenna is used to determine the precise position of MetOp by continuously tracking other GPS satellites.
Source: ESA - AOES Medialab
GRAS is a dual-frequency GPS high-performance instrument with semi-codeless operating capabilities. The codeless capability is mandatory in order to mitigate the anti-spoofing (encryption of the precise code), which prevents civilian users from benefiting from the P-code on the L2 frequency. The instrument provides carrier, code phase and signal amplitude measurements. The second GPS frequency (L2) is used for ionospheric correction of the signals. GRAS features twelve dual-frequency channels, four for rising and setting occultations and eight for navigation. The navigation channels provide the position of the Metop satellite in real-time. Carrier and code phase data are also provided for the processing of the precise orbit determination of Metop (position and velocity) necessary for the ground processing of the occultation data and in particular the effects on the Doppler caused by the GPS signal passing through the atmosphere and the troposphere. A ground network of GPS receiver tracking stations is used for determining the actual orbits of the GPS satellites. This ground network can operate in differential mode in order to cancel clock error variations. - The GRAS instrument consists of the following elements:
Access Constraints Access to Data during Commissioning
Users should note that until the entry into operational service, the priority is given to commissioning activities. The early access to EPS data is therefore provided on a best effort basis, without full service commitment. This implies that schedule changes might occur at any time and that dissemination might be interrupted due to other commissioning activities. Data delivered during commissioning are not considered to be validated data. The data disseminated via EUMETCast will be access controlled.
Source : EUMETSAT
Use Constraints Disclaimer - Status of instrument and processing quality (August 2007)
Data quality assessment is ongoing, so while the data are acceptable for this stage of commissioning no guarantee of quality will yet be given. Please note: Only the so-called key fields have been tested for acceptable values. These key fields are found in the MDR-1B (see the Products Guide) of the GRAS product.
Role: DIF AUTHOR
Phone: (301) 614-6898
Email: Tyler.B.Stevens at nasa.gov
NASA Goddard Space Flight Center Global Change Master Directory
Province or State: MD
Postal Code: 20771
Creation and Review Dates
DIF Creation Date: 2007-09-14
Last DIF Revision Date: 2011-03-15