About LRIT

Introduction.
The digital Low Rate Information Transmission (LRIT) is an international standard for data transmission that was developed by the Coordination Group for Meteorological Satellites (CGMS) in response to a recommendation on digital meteorological satellite broadcasts.  NOAA designed its LRIT system based on the CGMS standard.  The LRIT/HRIT Global Specification can be located at http://www.wmo.ch/pages/prog/sat/documents/CGMS-03_HRIT-LRIT_v2-6.pdf.  The NOAA LRIT system provides digital data, via a broadcast service, through its geostationary satellites.  NOAA operates an LRIT broadcast on its GOES East and GOES West satellites (I-M) and will continue an LRIT broadcast on the GOES N-P Series.  On the GOES-R series of satellites, the broadcast will be merged with the Emergency Managers Weather Information Network1 (EMWIN) service and will be a combined data rate of 400 Kilobits per Second.  The NOAA LRIT schedules are shown below in tables 1 and 2.

Table 1: NOAA GOES East LRIT Content Table (Updated 092109)

Table 2: NOAA GOES West LRIT Content Table (Updated 092109)

GOES Imagery Processing.
LRIT GOES imagery products are derived from the imager instrument on board the GOES spacecraft. The raw data stream is transmitted to the Command and Data Acquisition (CDA) station at Wallops, Virginia, where the data received, processed, earth located, and formatted in the GOES Variable (GVAR) format. The data (Level 1A data stream) is then retransmitted to the GOES spacecraft from which the raw data stream was generated. This part of GOES processing system which is common for all GOES data processing systems is not presented on the NOAA LRIT System Architecture chart (Figure 1).  Figure 1 begins with the reception of GVAR data at the NSOF and ends with the uplink of LRIT files from the CDA Wallops Island uplink processor.

Figure: 1. Architecture of the LRIT Processing and Uplink System

LRIT Domain 1: AFEP Level 1A ingestor.
The spacecraft transmits the GVAR formatted data that is then received at processing center at the NOAA Satellite Operations Facility (NSOF) in Suitland, Maryland. The Advanced Front-End processor (AFEP) ingests and sends the Level 1A data into the main processor. The main processor ingests the Level 1A data and divides the image into 5 segments. This processing can take up to 26 minutes for a full-disk product to be ingested. Segmentation of the data can lead to improvements in the latency of the data to the user community. The LRIT system has been designed so that LRIT-segment products can be generated and transmitted even as the GVAR data is still being ingested and processed.  The LRIT Domain 1 system architecture consists of a primary and back-up server.

LRIT Domain 2/3: LRIT products processor.
The LRIT Product Processor (LPP) creates product files that are inserted into the LRIT data stream. Designed for continuous operations, the system can reinitialize itself to change run-time parameters without shutting down. The system is stopped only for system maintenance, in which case a back-up processing system is used as a temporary processor.
The LPP performs the following functions:

• Obtains the input data from Domain 1.
• Converts the Level 1A data into the Level 1B data format.
• Creates metadata records describing the file contents.
• Packages the data into the LRIT format.
• Splits the data into East and West data streams.
• Sends the LRIT data to the LRIT Communication Processor.

The metadata describes the file size and structure, image structure and navigation, creation time, product classification, and various other attributes of the data, where applicable. Packaged into LRIT header records, the metadata, when combined with the raw data, forms the LRIT file.
Operationally, one machine is dedicated as the primary operational Domain 2/3 processor (Couch on fig.1). This machine accepts imagery from the GOES AFEP, DCS files from the CDA station at Wallops, Virginia, the Administrator text messages, NWS data (charts and texts) from NOAA web sites, Meteosat MTSAT global images from the web sites, and from any other accepted data source. This machine produces the LRIT files and forwards them to the designated LRIT Domain 4 Communication Processor. 

• Obtains the input LRIT imagery and DCS files coming from Domain 2/3.
• Obtains the input EMWIN data stream.
• Packages the EMWIN data into the LRIT format.
• Compresses all LRIT data files.
• Packages all LRIT data files into the data stream.
• Sends the LRIT data stream to the LRIT Domain 5 Uplink Processor at Wallops.

LRIT Domain 5: LRIT Uplink processor.

The LRIT Uplink processor (LUP) receives compressed LRIT data stream from the LRIT Domain 4 via a T1 line. After processing it sends the data stream to the antennas through GOES-East and GOES-West transmitters.
LUP performs the following:

• Obtains the input LRIT data stream.
• Buffers the LRIT data to provide the constant uplink data rate.
• Sends the LRIT data to the antennas.

LRIT Domain 6: LRIT User station.
The LRIT User station is depicted on LRIT Domain 6 on figure 1.  NOAA has two LRIT user stations at NSOF, Suitland, Maryland and another set at CDA Wallops Island.  These receive stations serve as monitoring and quality control tools for the NOAA LRIT system.  Two small antennas (1.6 m) for reception of GOES-East and GOES-West LRIT broadcasting are installed on NSOF building roof.  The NSOF LRIT user stations are equipped with standard software for LRIT data stream processing. Special software for monitoring and control LRIT transmission quality was additionally installed on these stations. The Help Desk Operators monitor the LRIT system availability on a 24/7 basis.