Regional ATOVS Retransmission Services (RARS)

RARS concept

Regional ATOVS Retransmission Services (RARS) are operational arrangements for the real-time acquisition of polar-orbiting satellite data over a wide region containing a network of direct readout stations and their rapid delivery to the global user community through regional Processing Centres.

A RARS involves several direct readout stations in a given region, each of which acquires local data from polar-orbiting satellites in real-time via the satellite's digital High Resolution Picture Transmission (HRPT) broadcast as the satellite passes within the range of the station. The data are then locally processed and passed to a regional Processing Centre that is responsible for overall coordination, for near-real time concentration of local data from the direct readout stations and for rapid delivery of consistent sets of data covering the region, for use throughout the region and worldwide. 

RARS system concept

RARS system concept (Click to enlarge)

Compared with the normal means of access to global data, this approach allows access by users to a regional sub-set of data without the delay associated with on-board data storage prior to a full orbit data dump to a Command and Data Acquisition station.

The WMO Space Programme has initiated this project to establish several RARS that, together, should cover most of the globe, with initial focus is on ATOVS data. It currently includes EARS (covering the European, Atlantic, North-American, Arctic and North-African areas), the Asia-Pacific RARS and the South-American RARS.

The way in which the global RARS network is built from individual regional RARS with well-defined links for inter-regional exchange is shown below.

RARS network concept

Motivation

ATOVS data are among the most important sources of observation used by Numerical Weather Prediction (NWP) models. A brief description of the benefits they bring is included later on this page. Like all NWP model inputs, their assimilation is constrained by their time of availability. While locally received data (via Direct Readout reception systems) can be available in a timely fashion for the NWP models, the availability of global data is delayed by the storage time on-board the spacecraft and the procedures involved for the data collection, processing and distribution. The RARS concept enables the delivery of ATOVS data collected from a regional network of HRPT stations within the timeliness constraints imposed by NWP models.

However, not all WMO regions are served by HRPT data collection networks and so the main objective of the RARS project is to facilitate the introduction of such regional networks, together with appropriate regional dissemination mechanisms, and inter-regional data exchange facilities, across the globe.

Background

The RARS project has its roots in the EUMETSAT ATOVS Re-transmission Service (EARS). EARS was initially established in 2002 in response to the requirements of Limited Area NWP modelling over Europe (HIRLAM then ALADIN) whose modelling area exceeded the coverage of a single HRPT station, and whose timeliness constraints were not compatible with the current scheme of global data collection. By merging the data sets from several HRPT stations EARS enabled the extended coverage and short timeliness requirements of the NWP operators to be met.

In recognition of the very positive impact that EARS data has made to NWP model performance, the Co-ordination Group for Meteorological Satellites (CGMS), at its thirty-second session in May 2004, asked whether the system could be expanded into other Northern Hemisphere regions, and then extended to cover the Southern Hemisphere.

With this aim in mind the CGMS encouraged the WMO Space Programme to contact CGMS members and potential regional participants with a view to forming local consortia to develop Regional ATOVS Re-transmission Services similar to EARS.

An important consideration in the establishment of regional collection networks is the capability to deliver homogeneous sets of data from different sources to NWP Centres so that they can merge them with confidence. Hence the RARS project also places considerable emphasis on the standards that RARS Operators should adhere to, in order to guarantee the global consistency of the various RARS data sets.

Benefits

It is a widely quoted fact that observations from satellites impact the performance of NWP models to an extent that exceeds the sum of all other observation types. Of the various types of satellite observations that are available it is the data that describe the temperature and humidity of the atmospheric column beneath the satellite, including those data from the well-established ATOVS instrument suite, that result in the greatest benefit. The advent of sophisticated so-called variational data analysis methods in recent years has enabled the full significance of these vast data sets to be realised. They play a particularly important role by virtue of their global coverage and consequently NWP model performance over otherwise data sparse areas (such as oceans) is markedly improved.

Typically NWP operators run their numerical models over a range of different domains, with the different variants having different spatial resolutions. For example, an NWP centre in Europe may run the forecast model for an area covering their local (national) region at the highest resolution, a version covering a continental domain at a slightly coarser resolution and also a version covering the whole globe at a somewhat lower resolution. The choice of resolution is governed by several factors including the ability of to model to represent the complex atmospheric processes at the chosen resolution, the availability of observational data at the model resolution and the available computer power. Generally the higher resolution, smaller domain, versions of the model are run with a deadline time for input observations much shorter than that for the global model. These arguments feed into the motivation and background for the RARS initiative described above.

(See for example: Impact of RARS data on the JMA global assimilation system in 2007)

RARS implementation status

The evolution of the RARS coverage is shown in the following diagrams :

The global RARS network in April 2010	The anticipated global RARS network by the end of 2010	The anticipated global RARS network by the end of 2011

The maps of "anticipated" networks reflect only what is firmly planned. A list of the current and planned participating HRPT stations is available (click here), with their geographical coordinates and BUFR identifiers. Further expansions are under consideration.

RARS Data quality monitoring

RARS data quality monitoring takes place at two levels:

• The Regional Processing Centres are responsible for monitoring the performance of the RARS in their region;
• The EUMETSAT Satellite Application Facility on Numerical Weather Prediction (NWP SAF) is monitoring the whole RARS network and routinely compares the data from all RARS with (full orbit) global data .
  
  Data quality flags and indicators are generated when the data are processed by the AAPP software. Embedded in the format of the retransmitted data, they alert users to any inherent quality problems that might be present in the data themselves.

Monitoring by the NWP SAF

The NWP SAF lead institute, the UK Met Office, is responsible for routinely and automatically performing comparisons of RARS data sets and the equivalent data in the global data sets received via another means. In this way the RARS data sets are examined for consistency with equivalent data that has not passed through the RARS processing chain. Such consistency is vital to enable the RARS data to be used alongside global data, for example, in NWP applications.

The NWP SAF publishes the results of the consistency checks on the NWP SAF web site and has the capability to generate alerts and send them via email to the appropriate centre. Data locally received in Exeter, UK are also compared with corresponding RARS data.

Monitoring by Regional Processing Centres

Service-related information is also gathered and published by the respective RARS Regional Processing Centres. They monitor the completeness of the data that they receive from direct readout stations by comparison with the data expected from the nominal regional configuration. Additional factors are monitored by regional centres such as end-to-end timeliness indicators, or consistency of the navigation.

How to access RARS data ?

Identifying the data of interest

The first step is to know what area you would like data from. The areas for which RARS data is available, and the corresponding centre(s), are graphically illustrated in section RARS status above.

If data is available from the considered area, the next step is to identify the particular reception stations from which data is required. The coordinates of each station and their associated processing centre are indicated in Station Locations and Plans.

The normal mechanism for receiving data is via the Global Telecommunications System (GTS). In some cases other mechanisms are available such as DVB-S broadcasts, or FTP over the Internet.

To receive data via the GTS

To receive data via the GTS, users should contact their National Meteorological or Hydrological Service (NHMS) indicating the nature of the data that is required. The detailed data descriptors for each reception station are given in RARS Coding Summary.

The point of contact within the NMHS is the Regional Telecommunication Hub (RTH). The list of RTH Focal Points per country is available in List of RTH Focal Points.

To receive data via other mechanisms

To receive data via other mechanisms (if supported) the User should contact the RARS Centre associated with the particular reception station (see Station Locations and Plans). The contact details for each RARS Centre are given below. The contact point will then inform the user about the relevant procedures to be followed for obtaining access to the data.

RARS Contacts and Links

RARS Contact Points

Links to Websites of RARS Operators

(for operational information concerning the provision of the RARS service)

Other Links

NWP SAF web site (Providing AAPP Software and global monitoring)

http://research.metoffice.gov.uk/research/interproj/nwpsaf/ears_report/monitor.html

Meeting Reports

Reports from RARS Implementation Group meetings and RARS workshops may be viewed on the Meeting Reports page

RARS Project Documents

• RARS Coding Summary
  
• RARS Stations Implementation Status and Plans
  
• RARS Implementation Group Terms of Reference
• RARS Operators Standards
• Project Plan for the Extension of RARS to Advanced Sounders
• Initial RARS Requirements Document

Communications and posters

A global network of Regional ATOVS Retransmission Services (RARS) by M.Dumont, J. Lafeuille, A.Soerensen. Paper presented at the 15th International TOVS Study Conference (ITSC XV, Maratea, October 2006).

Update on the RARS project, by D. Griersmith, paper presented at the 16th International TOVS Study Conference (ITSC XVI, Angra dos Reis, Brazil, May 2008)

"Evolution of the EUMETSAT ADvanced Retransmission Service (EARS)", by C. Ponsard, A. Soerensen, D. Lee, at NOAA's 2008 Satellite Direct Readout Conference (Miami, Florida, USA, 8-12 December 2008).

"RARS Global Network Status and Plans", by J. Lafeuille, poster presented at the 17th International TOVS Study Conference (ITSC XVII, Monterey, Ca, USA, 14-20 April 2010)

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