Regional ATOVS Retransmission Services (RARS)
RARS top level objective
The initial objective of the RARS project is to deliver ATOVS (Advanced TIROS (Television and Infrared Observational Satellite) Operational Vertical Sounder) data (Level 1c) from at least 90% of the globe within no more than 30 minutes from acquisition.
The Regional ATOVS Retransmission Services (RARS) are operational arrangements for the real-time acquisition of polar-orbiting satellite data through a worldwide network of local, Direct Readout stations and the rapid delivery of these data to the global user community after pre-processing in accordance with agreed standards. The name RARS is being replaced by "Direct Readout Acquisition and Relay System for Low Earth Orbit Satellite Data" (DRARS) to account for the expanded scope of this initiative which is global and includes more than ATOVS products.
The initial 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
RARS network and data sharing concept
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.
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.
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)