THE TRANSITION OF THE WEATHER FACSIMILE (WEFAX) SERVICE TO THE LOW-RATE INFORMATION TRANSMISSION (LRIT) SYSTEM

INTRODUCTION

The National Oceanic and Atmospheric Administration (NOAA) Low-Rate Information Transmission (LRIT) System began its transition to operations in August 2003 and completed the transition by October 1, 2005. The LRIT processing system was developed and implemented by NOAA to provide a replacement for the current Weather Facsimile (WEFAX) Processing System.

The transition to operations was done in two phases for each of the satellites in the Geostationary Operational Environmental Satellite (GOES) constellation (GOES-EAST and GOES-WEST). The first phase was a timesharing, alternate transmission mode with WEFAX. The second phase is full replacement of WEFAX. WEFAX is an analog communications transmission broadcast service in operation since the early 1970's. LRIT is a digital communications transmission design to provide more data and additional services. The transition from current WEFAX service to total LRIT service occurred over a two-year period for the GOES constellation. Parallel service, albeit in timesharing mode, facilitated the ease of transition to LRIT for existing WEFAX users.

The NOAA has completed the transition from the WEFAX system to the LRIT system, on GOES I-M spacecraft. The transition to full time LRIT on GOES-East and GOES -West was respectfully implemented March 1, 2005, and October 3, 2005. This activity completes the NOAA transition plan from WEFAX to LRIT, ending the successful mission of the WEFAX system and beginning the new mission of LRIT.

OVERVIEW OF LRIT

The digital LRIT is an international standard for data transmission that was developed by the Coordination Group for Meteorological Satellites (CGMS) in response to the recommendation for digital meteorological satellite broadcasts. The CGMS Global Specification (Reference 1) provides the standard that is supported by all operational geostationary meteorological satellites to be flown by the United States, European agencies, Japan, China, and Russia. NOAA and other world meteorological agencies have developed subsequent system specifications, designs, and implementations of their specific LRIT systems.

The LRIT service is a combination of several information sources onto a single higher capacity broadcast (64 kbps through 256 kbps) that contains significantly more meteorological data, imagery, charts and other environmental information. The LRIT system provides NOAA the opportunity to transmit more data, of various types, and with more flexibility than the WEFAX system allows.

The Initial Operational Capability (IOC) and Final Operational Capability (FOC) requirements were driven and constrained by the IOC data rate of 64 kilobits per second (kbps) and the FOC data rate of 256 kbps. These two data rates were determined by the CGMS LRIT standards and their planned implementation by the two other major meteorological broadcast regions, Europe and Japan. The 64 kbps IOC data rate is considered an interim or transition data rate to: 1) facilitate the ease of transition from WEFAX to LRIT services for the user community and 2) enable earlier LRIT service implementation on the existing GOES I-M series. The IOC requirements focus on four major objectives:

• LRIT formatting of the current WEFAX service,
• additional products and services with improvements on latency and product
flexibility,
• the inclusion of additional National Weather Service (NWS) products, information and
services, possibly including all or part of the Emergency Managers Weather and Information Network (EMWIN) data and
• the inclusion of GOES Data Collection System (DCS) information that is currently broadcast by commercial satellite to the continental U.S.

The first area has highest priority whereas support for the other improvements will depend on available data transport capacities after the highest priority LRIT needs are addressed.

WEFAX vs. LRIT

Using the LRIT service has several benefits over that of the WEFAX service. The LRIT system, for instance, uses digital data rather than analog data. The benefits include how much data can be transmitted, how consistent the data stays in transmission, and what type of data the signal can carry. For the same amount of bandwidth, much more information can be packed into a digital signal than an analog signal and maintain a more
accurate value per pixel than that produced using an analog signal. Figures 1, 2, and 3 demonstrate the value added in using digital data over analog data. Figure 1 is a GOES-East full disk image as viewed on the LRIT user station. Figure 2 is the WEFAX product for the same full disk image. Comparing figures 1 and 2 doesn’t show much difference visually, but in actuality the LRIT image is at 4km resolution and the WEFAX image is at 16km resolution. Figure 3 demonstrates one of the benefits of digital data. It shows a zoomed in portion of the full disk image. With the WEFAX image, figure 2, what you see is what you get. With the LRIT data, one can take advantage of the higher resolution when using the data.

Although digital representations approximate analog events, they are useful because they are relatively easy to store and can be manipulated electronically. Manipulation of the data can lead to improvements in the latency of the data to the user community, a feature not available with the WEFAX service. This reduced latency is realized in two ways. First, WEFAX products are generated at NOAA’s Information Processing Division (IPD) only after the GOES VARiable (GVAR) length data have been fully processed

Figure 1. GOES-East Full Disk Image Viewed Using LRIT

by the Front-End Processor (FEP). This processing can take up to 26 minutes for a full-disk product to be ingested. The LRIT system has been designed so that LRIT-segmented products can be generated and transmitted even as the GVAR data is still being ingested and processed.

These LRIT segments can be transmitted and received prior to completion of the ingested GVAR data at IPD. Secondly, digital data lends itself to compression. LRIT image data can be compressed, in accordance with the CGMS LRIT/HRIT Global Specifications. Compression has a dual purpose in the LRIT system. Not only does compression allow data to be transmitted to the user more quickly, it also allows for larger volume products to be included in the LRIT product suite that could not otherwise be included in the WEFAX service.

Figure 2. GOES-East WEFAX Full Disk Image

Figure 3. Zoomed In Portion of the LRIT Full Disk Image.

Table 1 compares the daily volume of data for both WEFAX and LRIT.

Table 1. Volume Comparison

Both the GOES-East and GOES-West WEFAX volumes are computed based on product types. For the LRIT volumes in Table 1, the byte counts received at the user terminal were recorded for several 29-minute periods and averaged. It should be noted that the byte counts for LRIT consist of a count of bytes before decompression of the data. This yields more product data bytes after decompression.

Additionally, the manipulation of the digital data allows for LRIT products to be encrypted, as specified in the CGMS Global Specification. NOAA has no current plans to encrypt any of the LRIT data.

Another benefit of LRIT over WEFAX is the flexibility gained by not transmitting according to a very rigid schedule. Figure 4 is an excerpt of the GOES-East WEFAX schedule before timesharing.

In studying the WEFAX schedule presented, it can be seen that the GOES-8 data that was ingested beginning at 20:45 was not transmitted to the users until beginning at 22:30; that is an hour and a half after receipt began. The last of the 20:45 data was not transmitted until 23:14, which is almost 2 and1/2 hours after receipt. With LRIT, since the data is processed in near-real-time and not relying on a schedule, the 20:45 data, which consisted of three full-disk images, one each visible, water vapor, and infrared, were processed and transmitted to the users beginning at 20:55 and completed at approximately 21:20. With LRIT, the users have the earliest data at least an
hour sooner and almost 2 hours sooner for the last data.

Figure 4. Excerpt From GOES-East WEFAX Schedule

Unlike WEFAX, which is directly tied to a schedule, LRIT has the ability to prioritize data as it enters the processing system, enabling data that is deemed higher in priority to supersede other data being processed. LRIT allows data to be transmitted to the user community without waiting for its slot in a rigid schedule. It adds flexibility to the processing and transmission of LRIT products.

CURRENT LRIT CAPABILITIES

The processing of data and the subsequent transmission of the LRIT products was designed to be more robust and flexible than the WEFAX service. This flexibility extends to the ingestors, the processors, and the transmission and reception portions of the system.

Able to process GVAR data, the LRIT system ingestors are configurable. These ingestors provide the operator the ability to specify the number of segments each product will be divided into for processing and transmission. They also permit the configuration of such parameters as the region of coverage, the data resolution, and the channel(s), to be processed. Additionally, Rapid Scan Operations (RSO) data are ingested and can be made available to the LRIT Product Processor (LPP) system. The flexibility of the ingestors enhances the overall robustness of the LRIT system.

The CGMS LRIT Global Specification defines a reference model with multiple layers. The model is generally consistent with both the Consultative Committee for Space Data System (CCSDS) and the International Standards Organization’s (ISO) Open Systems Interconnect (OSI) reference model. The LRIT system is essentially a unidirectional flow of data from a transmission side (uplink) to a reception side (downlink). The model used for LRIT implementation includes seven layers consisting of:

1. Application Layer
2. Presentation Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data Link Layer
7. Physical Layer

On the transmission side, data is processed into LRIT files in the Application and Presentation Layers, which correspond to the LRIT Product Processor. Performed in layers 3 through 7, LRIT Communications Processing is concerned with sending LRIT files from the transmission side to the reception side.

The LRIT Product Processor (LPP) system adds flexibility and robustness to the LRIT system. A configurable system, the LPP’s primary purpose is to create LRIT product files from various input sources, such as the GOES data from the ingestors. It is in the LPP that product priority is determined and assigned. Not only is the LRIT data assigned a transmission priority, it is also given a processing priority. The distinction between the two relates to where the priorities are implemented. The processing priority’s significance is in the LPP; whereas the transmission priority is significant in the communications processing. Processing priority represents the amount of central processing unit (CPU) a given process receives relative to the other processes concurrently being performed. Transmission priority represents the priority an LRIT product file receives in the communications processing system. This priority allows for higher priority data to supersede lower priority data currently being transmitted. Such priority processing permits data that is deemed more urgent the opportunity to be transmitted quickly, thus reducing the latency for higher priority data.

The configurable design of the LPP lends itself to setting other features that improve the overall performance of the LRIT system. One of the most important of these parameters is the compression flag. The compression flag, currently applicable only to image data, gives the flexibility to perform compression of data prior to transmission. There are three compression options:

• No compression
• Lossless compression
• Lossy compression

Currently, the NOAA LRIT implementation of lossless compression is the Rice compression algorithm and is implemented in the Transport Layer. Lossy compression currently employs Joint Photographic Experts Group (JPEG) compression and is implemented in the Session Layer. The benefit of Rice compression in the transport layer is that the compression is performed on packets rather than on a file. The significance of this is realized in the reduced risk of losing data in transmission. File compression risks losing the whole file if any data is lost or corrupted in transmission. The decompression software on the reception side does not have the necessary information to reconstitute the file. Packet compression, however, limits loss or corruption of transmitted data to the affected packet rather than losing the whole file. The reception software can reconstitute the file with missing data for the lost or corrupted packets.

In conjunction with the Initial Operational Capabilities (IOC) of LRIT, is the LRIT receiver software. NOAA is making available a limited amount of user station software that provides basic receiver functionality and product display capabilities.

TESTING

Testing the LRIT system, as compared to the WEFAX service, was very favorable. The results demonstrated a reduction in product latency, an increase in product volume, and a broadening of product content. Table 2 reflects the improvements in product latency. For an actual GOES full disk image, the data shows that the user received all of the data within 32 minutes of ingest (rightmost column). However, the first segment (1/5 of the full disk) was received within 10 minutes of ingest. With LRIT, the user is getting the data sooner.

Table 2: Sample Product Latency

In Table 2, there is an unaccounted-for time period between the beginning of a segment processing time and the time of receipt of the previous segment at the user station. This would be considered dead time, if LRIT were processing only one set of input at a time. In general however, LRIT is processing other products such as, other GOES, DCS, etc. In this case, LRIT was ingesting three full-disk products and within that 32 minutes had transmitted all three products to the users.

Latency was reduced substantially, as compared to WEFAX. This reduction in latency permits the LRIT system to be more near-real-time than was possible with the WEFAX service.

The increase in product volume, as shown in Table 1, is realized by using lossless compression. An additional increase in product volume can be gained by using lossy compression. The gain in product volume, using lossy compression, is offset by a loss in data accuracy. This loss in data accuracy can be weighed against the increase in product volume to determine whether the exchange is detrimental to the products being transmitted. Initial results indicate that lossy compression is useful for some products but not worth the loss in accuracy for other products. Another benefit gained through the LRIT system is an increase in the product content.

System integration testing began in April 2003 with the installation of hardware at NOAA’s Wallops Command and Data Acquisition (CDA) facility. Successful integration testing of NOAA’s LRIT system included transmission of data to and from the GOES-East satellite and ensured the availability of test data files in May of 2003 to all users on NOAA’s LRIT web site.

The contents of the data files published for review and testing by the LRIT user community are as follows:

• 3 GMS
• 2 GOES-10 full disk (one segment missing)
• 1 GOES-10 northern hemisphere
• 1 GOES-10 southern hemisphere
• 1 GOES-10 United States
• 1 GOES-12 full disk
• 1 GOES-12 northern hemisphere
• 1 GOES-12 southern hemisphere
• 1 GOES-12 United States
• 1 Alphanumeric message

On 21 August 2003, NOAA began transmitting LRIT test data during two daily "vacant" time slots in the GOES East WEFAX schedule. The LRIT test data was transmitted Monday through Friday, excluding U.S. Holidays, during these vacant times throughout the months of August and September. The first daily test transmission period was from 16:50 to 17:06 UTC, and the second daily test transmission period was from 17:50 to 17:58 UTC. A one-minute buffer at the start and end times was used to avoid interference with WEFAX.

All of the testing concluded successfully.

The recently completed plans for LRIT implementation and transition were as follows:

Initial ground testing of simultaneous LRIT and existing EMWIN transmissions were positive (i.e., acceptable performance) at the 128 kbps data rate.
Testing continues whenever a non-operational GOES spacecraft is brought out of storage of health and safety validation.
Timesharing on GOES-East began October 2003.
Timesharing on GOES-West began October 2004.
Full LRIT on GOES-East began March 01, 2005.

TRANSITION

The LRIT/WEFAX timeshare was active on GOES-west from 0000Z on 9 October 2004, until 03 October 2005. The transition phase lasted approximately 12 months, at which point WEFAX was eliminated from the GOES-west and LRIT began to operate continuously. Figure 1 below represents the timeline for transition.

The transition on the GOES-West satellite commenced October 03, 2005. Full LRIT service is operational on both GOES-East and GOES-West spacecrafts. NOAA will no longer support WEFAX operations through it environmental satellites. Our current planning will focus on improving data dissemination in the future and providing low cost system to acquire the NOAA broadcasts.

Current LRIT Capabilities

This LRIT broadcast is a consolidated mode of data dissemination that includes the combination of Weather Facsimile (WEFAX) products, a portion of the US National Weather Service’s Emergency Manager’s Weather Information Network (EMWIN), and information from the GOES Data Collection System (DCS). The NOAA LRIT development focused on five major objectives:

LRIT formatting of the current WEFAX service,
additional products and services with improvements on latency and product flexibility,
the inclusion of additional National Weather Service (NWS) products and information,
the Emergency Management Weather Information Network (EMWIN) data, and
the inclusion of GOES Data Collection System (DCS) information that is currently broadcast by commercial satellite to the continental U.S.

The first area has highest priority whereas support for the other improvements depends on available data transport capacities after the highest priority LRIT needs are addressed. The product suite of the WEFAX service transmitted through the LRIT system is the baseline of the LRIT data content. The baseline products include the following:

GOES Infrared full disk
GOES Water vapor full disk
GOES Visible full disk
GOES Infrared northern hemisphere
GOES Water vapor northern hemisphere
GOES Visible northern hemisphere
GOES Infrared continental U. S.
GOES Water vapor continental U. S
GOES Visible continental U. S.
GOES Infrared southern hemisphere
GOES Water vapor southern hemisphere
GOES Visible southern hemisphere
GOES Rapid Scan
GOES-9 Infrared full disk (GMS replacement)
GOES-9 Visible full disk (GMS replacement)
Various National Weather Service charts
Various administrative ASCII messages

The EMWIN (Emergency Managers Weather Information) data is now included in the LRIT data stream. The basic, or native, data formats of the LRIT system and the EMWIN system are not immediately compatible. The former is a file based system and the latter is serial data.

The EMWIN data, in its current implementation, is a 9600 baud, serial, RS-232 data stream. To include the EMWIN data in the LRIT data stream, one second chunks of the serial EMWIN data are converted to small files. The EMWIN serial data contains start and stop bits which are necessary for the serial protocol but are simply added overhead for the LRIT file. These are removed leaving only the 8 bit ASCII characters to be transmitted in the EMWIN file within LRIT. These files are identified uniquely within the LRIT data stream as are all other files in LRIT

At the LRIT receiver end, these files are received and stored as are all LRIT files. Software within the receiver reads the ASCII character data from the EMWIN files, adds the start and stop bits back in and makes the newly serialized data available to any of the EMWIN data display programs or to an outgoing serial port on the receiver.

NOAA has recently completed a project that will integrate the GOES Data Collection System (DCS) data into the LRIT broadcast. The GOES DCS data has been enhanced so that all of the data files being transmitted to the SES Americom spacecraft (i.e., the US domestic satellite service, DOMSAT) is ported to a computer at the Wallops Command and Data Acquisition (WCDA) Facility. Software has been designed to build a file of the GOES DCS data based on three criteria. First, there can be a parameter based on time; second, their size of the file can be based on the number of messages; third, the number of bytes can be the determining factor. The criteria are selectable.

When the file is built at the WCDA, it is tagged with a “date/time group” name. This is useful in determining the confirmation of the file transfer. The DCS files are transmitted to the Environmental Satellite Processing Center (ESPC) facilities in Suitland, Md. At this point the files are incorporated in the Low Resolution Information Transfer (LRIT) data stream.

This completed stream is then re-transmitted to the WCDA station where it is up-linked to the GOES spacecraft. When it reaches the spacecraft it is transmitted to the entire hemisphere through a transponder. This service allows any DCS customer who can transmit to the spacecraft – to receive from the spacecraft.

The WCDA facility has the capability to receive the LRIT data. Here a comparison is made with the data that was originally transmitted. This is where the naming convention becomes a real use in the quality monitoring of the GOES DCS data in the LI broadcast. If a file is transmitted and not received, then an automatic retransmission is generated.

In addition, the WCDA has commissioned software that is open-source and Government-owned that will select the GOES DCS data from the total LRIT stream to allow users to display the DCS data in its original format. This software is available to all manufacturers that would like to use it. To obtain a copy of this software, please visit our website at www.noaasis.gov/LRIT/.

Summary

Now that NOAA’s LRIT is operational on both the GOES-East and GOES-West spacecrafts, our immediate plans include continuing the transition from the 128 kbps to 256 kbps during the GOES-N era.

Other items being considered for future implementation include expanding the product suite, adding higher resolution imagery (e.g. 1km), adding other compression algorithms (e.g. JPEG2000, zip), improving navigation information, and adding a web server for LRIT files. Though these are some of the ideas for future enhancements, by no means is LRIT limited to just these. As LRIT becomes utilized by the user community and matures, so shall the capabilities.

Top of page