Global Navigation Satellite System Receiver for Atmospheric Sounding Level 1B
Entry ID:
gov.noaa.class.GRAS
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Summary
Abstract:
ABSTRACT: The Global Navigation Satellite System (GNSS) Receiver for Atmospheric Sounding (GRAS) is a European GNSS receiver on board MetOp-A that operates as an atmospheric sounder. GRAS uses radio occultation to measure vertical profiles of atmospheric temperature and humidity by tracking signals received from a constellation of GNSS satellites while they are setting or rising behind the Earth's ... atmosphere. Radio occultation is based on the fact that when radio waves pass through the atmosphere, either during a rise event or during a set event as seen by the receiver, they are refracted along the atmospheric path. The degree of refraction depends on gradients of air density, which in turn depend on temperature and water vapor. Therefore, measurement of the refracted angle contains information about these atmospheric variables. As the measurements are made tangentially to the atmosphere, the profiles will be provided with a resolution within a few hundred meters to 1.5 kilometers, while horizontal coverage of each profile is in the order of a few hundred kilometers. With the nominal number of GNSS satellites, GRAS will provide 500 precise atmospheric profiles every day nearly equally distributed over the Earth's surface. GRAS can track up to eight satellites for navigation purposes, two additional satellites for rise and two others for set occultation measurements. GRAS has on-board GNSS satellite prediction for optimizing the navigation and occultation measurements. GRAS level 1b products contain information on a per occultation basis, along with auxiliary information.
CURRENTNESS REFERENCE: ground condition
Electronic download (FTP) of the data is free. There is a fee for copies of data on physical media. Optical disks are $25.00 per unit and magnetic tape is $100 per unit. Price does not include additional service and handling costs. 
Purpose:
PURPOSE: The main objectives are to provide stratospheric and tropospheric temperature and humidity profiles that will be assimilated into Numerical Weather Prediction (NWP) models. In addition GRAS provides navigation solutions of the MetOp satellite position along its orbit.
Related URL
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Description:
Download options for GRAS Level 1b data archived and distributed at CLASS.
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Geographic Coverage
(Click for Interactive Map)
Spatial coordinates
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N: 90.0
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S: -90.0
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E: 180.0
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W: -180.0
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Quality
ATTRIBUTE ACCURACY REPORT: The expected number of occultations per day is greater than 500. Expected bending angle error is less than 1.0 micro rad or 0.4% (whichever is larger). Expected sampling rate is 50 Hz (closed-loop) and 1 kHz (open loop).
LOGICAL CONSISTENCY REPORT: For GRAS Level 1b profiles, the core validation process is based on a one dimension ... variational assimilation (1DVar) approach. 1DVar provides bending angles along with robust statistics for the GRAS validation. Additionally, temperature, pressure, and humidity profiles (which present the main information that is extracted from radio occultation measurements by NWP center assimilation) are provided by the 1DVar. The availability of these profiles allows to further validate the GRAS measurements by comparing them to instruments providing these profiles, such as radiosondes, Lidar, etc. Hence, the validation is thus partly based on the variables which are of main interest to the NWP centers. Radio occultation instrument validation will be performed continuously by co-locating GRAS measurements with other available external radio occulation missions.
COMPLETENESS REPORT: TBD
VERTICAL POSITIONAL ACCURACY REPORT: Expected measurement ranges are 1 to 80 kilometers.
LINEAGE/PROCESS STEP:
PROCESS DESCRIPTION: 1) The beginning of the level 1b processing contains the selection of the best pivot GPS satellite and fiducial stations for clock correction by differencing, isolation of the auxiliary measurement data, number of corrections to remove various impact of the measurement system, and the retrieval of the bending angle profiles. The data corrections performed in the level 1b processing remove the geometrical path and clock errors from the measured phase values, and filter out noise outside the bandwidth containing the atmospheric information. Five parallel clock correction methods are implemented in the data processing. The final correction before the bending angle retrieval is the removal of the impact of the Earth oblateness by transforming the measurement geometry into a coordinate system defined by the local radius of curvature of the ellipsoid shape of the Earth. 2) The retrieval of the bending angle profile in level 1b processing is performed in parallel by three methods: Geometrical Optics (GO),Wave Optics (WO), and processing of the Raw Sampling (RS) mode data. 3) The Occultation table generation is another special functionality that is not directly part of the processing of the occultation data. An occultation table is a prediction of all occultation and navigation measurements to be performed by the GRAS instrument during the next 24 - 36 hours. It is based on the predicted GPS and MetOp satellite orbits, and on the measurement geometry. The Occultation table is used by the GRAS Ground Support Network (GSN) Service to select the Sounding Support Data (SSD) to be sent to the EPS Core Ground Segment for the GRAS level 1 processing. SSD contains the ground based measurements of the GPS carrier phase, pseudorange, and SNR for the clock correction by differencing. The Occultation table is also used by the occultation identification function in the GRAS level 1 processing.
SOURCE USED CITATION ABBREVIATION: GRAS_Level_1A
PROCESS DATE: Not complete
SOURCE PRODUCED CITATION ABBREVIATION: gov.noaa.class:GRAS
LINEAGE/SOURCE INFORMATION:
ORIGINATOR: EUMETSAT > European Meteorological Satellite Organization .
LINEAGE/SOURCE INFORMATION:
ORIGINATOR: EUMETSAT > European Meteorological Satellite Organization .
TITLE: GRAS Level 1A .
TITLE: Global Navigation Satellite System Receiver for Atmospheric Sounding Level 1B . Unpublished material .
PUBLICATION DATE: 2007-09-14.
ONLINE LINKAGE: http://www.class.noaa.gov/saa/products/search?sub_id=0&datatype_fam... .
ONLINE LINKAGE: http://www.eumetsat.int/Home/Main/What_We_Do/Satellites/EUMETSAT_Po... .
LINEAGE/SOURCE INFORMATION:
ORIGINATOR: EUMETSAT > European Meteorological Satellite Organization .
LINEAGE/SOURCE INFORMATION:
ORIGINATOR: EUMETSAT > European Meteorological Satellite Organization .
TITLE: GRAS Level 1A .
TITLE: Global Navigation Satellite System Receiver for Atmospheric Sounding Level 1B . Unpublished material .
PUBLICATION DATE: 2007-09-14.
ONLINE LINKAGE: http://www.class.noaa.gov/saa/products/search?sub_id=0&datatype_fam... .
ONLINE LINKAGE: http://www.eumetsat.int/Home/Main/What_We_Do/Satellites/EUMETSAT_Po... .
TYPE OF SOURCE MEDIA: digital satellite data .
SOURCE TIME PERIOD OF CONTENT/BEGINNING TIME: 20070914 .
SOURCE TIME PERIOD OF CONTENT/BEGINNING TIME: 20070914 .
SOURCE TIME PERIOD OF CONTENT/ENDING TIME: Present .
SOURCE TIME PERIOD OF CONTENT/ENDING TIME: Present .
SOURCE CURRENTNESS REFERENCE: ground condition .
SOURCE CITATION ABBREVIATION: GRAS_Level_1A .
SOURCE CONTRIBUTION: GRAS level 1a products contain reformatted raw instrument data along with all supplementary data needed for further processing, including geolocation and quality flags.
LINEAGE/SOURCE INFORMATION:
ORIGINATOR: EUMETSAT > European Meteorological Satellite Organization .
LINEAGE/SOURCE INFORMATION:
ORIGINATOR: EUMETSAT > European Meteorological Satellite Organization .
TITLE: GRAS Level 1A .
TITLE: Global Navigation Satellite System Receiver for Atmospheric Sounding Level 1B . Unpublished material .
PUBLICATION DATE: 2007-09-14.
ONLINE LINKAGE: http://www.class.noaa.gov/saa/products/search?sub_id=0&datatype_fam... .
ONLINE LINKAGE: http://www.eumetsat.int/Home/Main/What_We_Do/Satellites/EUMETSAT_Po... .
LINEAGE/SOURCE INFORMATION:
ORIGINATOR: EUMETSAT > European Meteorological Satellite Organization .
LINEAGE/SOURCE INFORMATION:
ORIGINATOR: EUMETSAT > European Meteorological Satellite Organization .
TITLE: GRAS Level 1A .
TITLE: Global Navigation Satellite System Receiver for Atmospheric Sounding Level 1B . Unpublished material .
PUBLICATION DATE: 2007-09-14.
ONLINE LINKAGE: http://www.class.noaa.gov/saa/products/search?sub_id=0&datatype_fam... .
ONLINE LINKAGE: http://www.eumetsat.int/Home/Main/What_We_Do/Satellites/EUMETSAT_Po... .
TYPE OF SOURCE MEDIA: digital satellite data .
SOURCE TIME PERIOD OF CONTENT/BEGINNING TIME: 20070914 .
SOURCE TIME PERIOD OF CONTENT/BEGINNING TIME: 20070914 .
SOURCE TIME PERIOD OF CONTENT/ENDING TIME: Present .
SOURCE TIME PERIOD OF CONTENT/ENDING TIME: Present .
SOURCE CURRENTNESS REFERENCE: ground condition .
SOURCE CITATION ABBREVIATION: gov.noaa.class:GRAS .
SOURCE CONTRIBUTION: GRAS level 1b products contain information on a per occultation basis, along with auxiliary information such as geolocation, quality flags, etc.
Access Constraints
ACCESS CONSTRAINTS: Pre-operational MetOp data is available upon request. To request access to this data, send an email to info@class.noaa.gov indicating your affiliation and need for this pre-operational data. CLASS personnel will contact you with access instructions.
DISTRIBUTION LIABILITY: Disclaimer - While every effort has been made to ensure that these data are accurate and reliable within the limits of the current state of the art, NOAA cannot assume liability for any damages caused by any errors or omissions in the data, nor as a result of the failure of the data to function on a particular system. NOAA makes no warranty, expressed or implied, nor does the fact of distribution constitute such a warranty.
Use Constraints
None
Data Set Progress
IN WORK
Distribution
Distribution Format:
EPS Native Format
Fees:
FTP is free. Optical disks = $25.00, magnetic tape = $100 + service and handling
Personnel
Role:
TECHNICAL CONTACT
Phone:
301-817-4418
Email:
james.g.yoe at noaa.gov
Contact Address:
NSOF Room 1235, E/OSD 4231 Suitland Road
City:
Suitland
Province or State:
MD
Postal Code:
20746
Country:
USA
Role:
DIF AUTHOR
Phone:
(303) 497-5099
Fax:
(303) 497-6513
Email:
Anna.Milan at noaa.gov
Contact Address:
325 Broadway, Mail Code E/GC4
City:
Boulder
Province or State:
CO
Postal Code:
80305-3328
Country:
USA
Creation and Review Dates
DIF Creation Date:
2009-09-01
Last DIF Revision Date:
2011-11-29
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