EUM TD 03. Meteosat WEFAX Dissemination. Technical Description

Transcription

1 EUM TD 03 Meteosat WEFAX Dissemination Technical Description

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3 EUM TD 03 Technical Documentation Meteosat WEFAX Dissemination Technical Description Revision 6 March 2001

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5 Table of Contents 1 INTRODUCTION Dissemination of Images Dissemination of Meteorological Information Status of Meteosat Satellites METEOSAT DATA USER STATIONS OPERATIONAL SPECIFICATIONS OF WEFAX TRANSMISSIONS Analogue Dissemination Format Line Start Signal WEFAX Digital Header Annotation WEFAX Formats originating in Darmstadt C Formats D Formats E Formats CnD Formats xtot Formats Administrative Messages CTH Formats Test Pattern Enhancement Grid WEFAX Formats originating in CMS Lannion L Formats GMS Formats GOMS Format Meteosat Dissemination Schedule Operational Constraints DESCRIPTION OF A "TYPICAL" SDUS Input/Output Specifications Satellite Output Data Typical SDUS Front-end Characteristics...27 Appendix A Meteosat Dissemination Schedule... A-1 A.1 The Key to the Dissemination Schedule...A-1 A.2 Typical Meteosat Dissemination Schedule S9712M01...A-2 Appendix B WEFAX Format Definition Parameters...B-1 Appendix C Link Budget - Meteosat Satellite to SDUS...C-1 Appendix D Acronyms and Abbreviations... D-1 Appendix E References...E-1 Technical Description Rev. 6 i

6 List of Figures Figure 1.1 Meteosat System Schematic...Error! Bookmark not defined. Figure 3.1 Structure of a WEFAX Format...7 Figure 3.2 Line Start Signal...8 Figure 3.3 C Formats (visible)...12 Figure 3.4 Coverage of the CnD, D and E Formats...14 Figure 3.5 ADMN Format...15 Figure 3.6 CTH Format...16 Figure 3.7 TEST Format...17 Figure 3.8 L Formats...20 Figure 3.9 The GMS WEFAX Formats...21 Figure 3.10 Coverage area of the GOMS WEFAX Format...22 Figure 4.1 Block Diagram of Typical SDUS...25 List of Tables Table 4.1 Satellite Transponder Characteristics Table 4.2 WEFAX Modulation Characteristics Table 4.3 Typical Specifications for an SDUS Front-end ii Technical Description Rev. 6

7 Preface The aim of this document is to provide the information enabling operators of Secondary Data User Stations (SDUS) to receive and process Meteosat WEFAX dissemination formats. This version of the document has been prepared by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), Darmstadt, Germany, which is the body responsible for the Meteosat satellite system. The WEFAX standard is based on the Automatic Picture Transmission (APT) service which was first used on board American ESSA satellites in the early 1960s. The APT standard was developed for direct broadcast from polar orbiting satellites. WEFAX is an extension of this standard for use with images from geostationary meteorological satellites. The original APT format was extended by start, stop and phasing signals. Fixed length WEFAX formats were defined and internationally adopted. An annotation was superimposed on the image information and, in more recent years, a digital header has been added. With only minor technical variations the WEFAX standard is presently used by all operators of geostationary meteorological satellite systems. A new definition of a digital Low Rate Information Transmission (LRIT) format has been defined and agreed by the Coordination Group for Meteorological Satellites (CGMS). LRIT will become the standard low rate image format provided by EUMETSAT's Meteosat Second Generation programme satellites, as a replacement for the present WEFAX format. Periodic test transmissions of this new format can be expected before this date; however, users will be kept informed about progress in newsletters, Internet pages and by electronic mail. To obtain a complete list of available EUMETSAT documentation and for further information about the Meteosat missions please contact: The User Service EUMETSAT Am Kavalleriesand 31 D Darmstadt Germany Phone: +49 (0) Fax: +49 (0) Telex: metsat d Internet: Technical Description Rev. 6 iii

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9 1 INTRODUCTION The purpose of this document is the provision of information for users planning to receive and exploit Meteosat WEFAX imagery. It provides details necessary for the operation of a Secondary Data User Station (SDUS). It is assumed that the reader is familiar with the document The Meteosat System, which is also available from EUMETSAT (see Ref. 1 in Appendix E). Meteosat dissemination is the process whereby image data and other meteorological information are relayed via the satellite to the user community. 1.1 Dissemination of Images The satellite has two dedicated dissemination channels, operating in the L-band at MHz (channel A1) and MHz (channel A2), which are both used to distribute a wide variety of image data. Two forms of image transmissions are utilised: Analogue transmissions (WEFAX) compatible with the Automatic Picture Transmission (APT) standard for users interested in picture recording. The WEFAX transmission and format characteristics are described in this document. Digital High Resolution Image (HRI) transmissions, with a specific Meteosat data design, for users requiring full resolution data in a form suitable for local computer processing. Information about digital transmissions can be obtained from the technical document Meteosat High Resolution Image Dissemination (see Ref. 2 in Appendix E). For both image dissemination missions, a user guide has also been published (see Ref. 3 in Appendix E). 1.2 Dissemination of Meteorological Information Two Meteosat missions distribute digital meteorological information: DRS Meteorological and environmental data from data collection platforms (DCP) are collected through the Meteosat satellite. The DCP Retransmission System (DRS) automatically retransmits a selection of these data to small user stations. This mission uses the gaps between WEFAX dissemination on channel A1. Information about this mission can be obtained in documents Meteosat Data Collection and Retransmission System (Technical Description) (see Ref. 4 in Appendix E) Data Collection System (User Guide) (see Ref. 5 in Appendix E). Technical Description Rev. 6 1

10 MDD The Meteosat Meteorological Data Distribution (MDD) mission provides meteorological messages (GTS format) and charts (T.4 coded) on up to four channels. The MDD mission is primarily designed to provide Meteorological Services located in regions with poor conventional meteorological telecommunications with the means to receive data that would normally be available through the Global Telecommunication System (GTS) of the World Meteorological Organization (WMO). Further information about the MDD mission can be obtained in documents Meteorological Data Distribution (Technical Description) (see Ref. 6 in Appendix E) Meteorological Data Distribution (User Guide) (see Ref. 7 in Appendix E). 1.3 Status of Meteosat Satellites EUMETSAT s primary missions are based mainly on a Meteosat satellite located over the equator at a position of 0 longitude. An in-orbit stand-by satellite is normally located near 10 W. Figure 1.1 provides an overview of the Meteosat system. The status of the satellites is usually published via the EUMETSAT Internet pages at and operational changes are frequently announced via the satellite broadcast administrative messages. 2 Technical Description Rev. 6

11 Figure 1.1 Meteosat System Schematic Technical Description Rev. 6 3

12 2 METEOSAT DATA USER STATIONS The two types of user stations are distinguished both by the type of transmission and by the complexity of the receiving station, although there is such a variety of possibilities that the more sophisticated SDUS can be almost as complex as the simpler forms of Primary Data User Station (PDUS) which are required for the acquisition of HRI data. Originally the WEFAX standard was developed for user stations equipped with hard copy units since WEFAX products were mainly used for visual applications. The SDUS in its simplest form consists of an antenna, down-converter, receiver and a recording device. The latter could be either a recorder producing pictures by a variety of processes, including the use of electrolytic paper, or electrostatic, photographic or laser beam recording techniques, or a TV screen. The possibility also exists for adding tape recorders to store images. In recent years small computer systems have become relatively cheap and powerful and can provide image processing, display and storage capabilities by the addition of an analogue to digital converter. The role of the hard copy recorder as the main output instrument has been taken over by black and white or colour video screens and the storage media have changed from analogue tape recorders to floppy and hard disks. However, local image processing is more generally associated with the use of a PDUS, which receives digital images together with calibration data and is specifically designed for computer processing. Here the basic station consists of the antenna plus receiver and a frame synchroniser followed by at least a small computer. The user must decide which additional devices are necessary to meet image processing needs. The choice is open-ended and could include high resolution colour displays for presentation of images in the original grey levels or with added false colours as single images or animations, tape recorders or disk drives to store images and facilities to manipulate the images or to superimpose other data (e.g. locally produced meteorological charts). Given the flexibility of a computer based system, each PDUS could be unique and its precise specification depends far more on user requirements than in the case of the SDUS, where simple standard stations are readily available as a complete package from a variety of manufacturers, or can be assembled from available units (including the modification of APT stations). Two independent and interchangeable satellite image dissemination channels are available, each of them being used for either type of data. WEFAX transmissions are formats compatible with the APT system, which has been used by most meteorological satellites (e.g. ESSA, METEOR, NOAA). The WEFAX mission therefore provides all APT operators with the opportunity to modify their station to an SDUS, at low cost, and receive Meteosat images. The descriptions of all existing WEFAX formats are contained in Chapter 3. The main characteristics of the satellite transponder and details related to the modulation can be found in Chapter 4. 4 Technical Description Rev. 6

13 3 OPERATIONAL SPECIFICATIONS OF WEFAX TRANSMISSIONS The WEFAX dissemination service supports: the relay of image data from Meteosat at the nominal position (0 longitude); the relay of image data from the American GOES-E (75 W); the relay of image data from the Japanese GMS (140 E); the relay of image data from the Russian GOMS (76 E); meteorological parameters extracted from the basic image data (e.g. cloud top height maps); administrative messages sent in WEFAX image format; test pattern transmissions to check and adjust receiving stations. Since December 1995 the majority of WEFAX transmissions have been provided by the EUMETSAT Mission Control Centre (MCC) in Darmstadt. The remainder originate in the Centre de Météorologie Spatiale (CMS) in Lannion, France. The CMS contribution is the relay of foreign satellite image data. The data acquisition and processing of foreign satellite data through Meteosat may be subject to modifications, dependent upon future developments of other satellite operators. NOAA has launched the first two satellites of a new family of three-axis stabilised geostationary weather satellites (GOES I-M). GOES-I, the first in the series, was launched on 13 April It was renamed GOES-8 after achieving orbit and, by May 1995, became operational at 75 W. The second satellite, GOES-9, was launched on 23 May 1995 and is now located at 135 W. In these operational positions the satellites are referred to as GOES-E and GOES-W respectively. These GOES satellites broadcast the image data in a GOES Variable (GVAR) format. The GOES-E image data are directly received at CMS Lannion. GOES-W data are received in Canada and routed via the UK Met. Office (Bracknell, England) to CMS Lannion. Image data are reprocessed to standard Meteosat formats before being rebroadcast every three hours through the operational Meteosat satellite at 0 to the users. Currently only data from GOES-E are relayed through Meteosat as WEFAX formats; GOES-W data are relayed only as High Resolution Images. The GOES-E relay provides a three-hourly broadcast of IR data covering North and South America, and occasional VIS data covering the North American east coast. Data from GMS are also received and reprocessed on a three-hourly basis. The satellite images are initially received by the Bureau of Meteorology (Melbourne, Australia) and routed via the UK Met. Office to CMS Lannion. Currently only IR images are rebroadcast by Meteosat. Data from GOMS are acquired by CMS Lannion via a dial-up link from the Russian Hydrometeorological Service (Roshydromet) facilities operated by RPO Planeta in Moscow. Reprocessed full earth disc IR images have been relayed through Meteosat on a three-hourly basis since June Technical Description Rev. 6 5

14 3.1 Analogue Dissemination Format As previously mentioned, WEFAX formats are compatible with APT, each format consisting of 800 image lines transmitted at a rate of 4 lines per second, which is identical to the standard 240 lines per minute format used by all other geostationary weather satellites. In addition to the image data, standard start and stop signals, a phasing signal, digital header and a suitable annotation are superimposed on each picture. The total dissemination time of one WEFAX format is 3 min 33.5 sec (213.5 sec) whether it originates from the EUMETSAT MCC Darmstadt or CMS Lannion. Disseminations are scheduled in 4-minute slots, therefore, there is a pause of 26.5 sec between the end of the stop signal and the start of the next dissemination. In the case of dissemination channel A1, the gap between two WEFAX formats is used for the retransmission of DCP messages. The gap between images on channel A2 is not used. A complete WEFAX format is illustrated in Figure 3.1 and consists of: a start signal (300 Hz tone) for 3 sec, a phasing signal (12.5 msec of black level followed by msec of white level) transmitted for 5 sec, two lines of WEFAX digital header, 800 lines of image data, a stop signal (450 Hz tone) for 5 sec. The amplitude/frequency modulation scheme of the WEFAX transmissions using the above data as a video input is described in further detail in Chapter 4. The digital header and the image lines include a line start signal (refer to Section 3.1.1). The line start is followed by msec of either digital header information (refer to Section 3.1.2) or image data. The image data part of the image line is produced by digital to analogue conversion of 800 elements (pixels) of the Meteosat image. The image data are disseminated according to the scan direction of the radiometer. The image lines run from East to West and from South to North. The descriptions in relation to the format names used in the Meteosat dissemination schedule are contained in Sections 3.2 and 3.3. More detailed information concerning the boundaries of each format in relation to the nominal rectified Meteosat frame is contained in Appendix B. 6 Technical Description Rev. 6

15 Figure 3.1 Structure of a WEFAX Format Technical Description Rev. 6 7

16 3.1.1 Line Start Signal The digital header and the image data lines of the WEFAX formats are preceded by a line start signal which enables SDUS users to operate an AGC (Automatic Gain Control) or a synchronisation activated for each image line. This line start signal corresponds to the one used by the NOAA-N satellite (APT SYNC B). At the beginning of each line a pattern of 2 white pixels followed by 2 black pixels is modulated in a sequence of seven cycles. The alternating pattern simulates a frequency of 840 Hz (NOAA-N = 832 Hz) wave form (see Figure 3.2). Figure 3.2 Line Start Signal 8 Technical Description Rev. 6

17 3.1.2 WEFAX Digital Header In the past, one of the main problems affecting SDUS use was the method of selection of formats from a continuous stream of transmitted information. The only available selection criterion was the time of transmission of the individual format. During periods of changes in satellite operations when there might be transmission of additional administrative messages to report interruptions of the regular operations or the repetitive transmission of image data from previous image slots, systems tended to process and display the wrong information. A digital header was therefore devised, and internationally agreed, to identify the content of the analogue format automatically. In order to maintain compatibility between WEFAX formats of the various satellite operators it was proposed to implement identical codes on all geostationary meteorological satellites. The eighteenth meeting of the Coordination Group for Meteorological Satellites (CGMS-XVIII) adopted the following standard for a digital header in analogue WEFAX transmissions. This header has been inserted in all WEFAX transmissions containing Meteosat data, as well as CTH, TEST and ADMN formats, from 1 August 1990 onwards. Similar header information was introduced in WEFAX images containing foreign satellite data (e.g. LY, LR, LZ or GMS formats). The digital header is in addition to the annotation in image format superimposed on the actual image. Content of Digital Header The digital header contains, as a minimum, the following information: - satellite name (METn) - spectral band (IR,VIS,WV) - date - year (YY) - month (MM) - day of the month (DD) - time - hour and minute of end of image acquisition (HHmm) - sector (format name) (FFFF) - non-standardised information (see note below) The information is grouped as follows: Technical Description Rev. 6 9

18 SAT-ID Spectral Date Time Sector Non-standardised Band information YYMMDD HHmm FFFF 8 char. 3 char. 6 char. 4 char. 4 char. 25 characters The number of characters dedicated to the different data fields is the maximum allowed; however, not all characters have to be used. Information starts at the far left hand position of the field. Unused fields are filled with space characters. Note: For Meteosat applications, the non-standardised information (25 characters) was initially a repetition of the first 25 characters of information. From 10 November 1992 the non-standardised field has been used to specify the nominal satellite longitude in degrees and direction, e.g. 075W. During a satellite's drift period this information will contain the rectification position of the disseminated images (this applied to Meteosat-3 during its drift to a new position in early 1993 when images were disseminated throughout the drift manoeuvre). Four characters are required for this information. The remaining 21 characters are set to the ASCII character space. This unused capacity can be used for additional information in future. Example: MET4^^^^VIS C2D^000W^^^^^^^^^^^^^^^^^^^^^ The reader should note that other satellite operators could use the non-standardised field in different ways. Method of insertion The digital heading information is inserted between the phasing signal and the image lines as shown in Figure 3.1. One line of digital header contains: - line start signal image pixels = 400 bits = 50 characters. The information is transmitted twice (2 lines) (2 x 50 characters). The characters and figures are represented by black and white image pixels as described below (black = 0, white =1). Coding One information bit is represented by 2 image pixels. Each character or figure is represented by 8 bits = 16 pixels. Therefore, bit zero is transmitted as two black image pixels, bit one as two white image 10 Technical Description Rev. 6

19 pixels. Characters are coded into ASCII, most significant bits first. Only the approved characters from the International Alphabet No. 5 are used Annotation To indicate the content of the format visually an annotation is superimposed. Each character is a 7 x 5 dot matrix, each dot is 2 x 2 pixels. There are two pixels between each character and the equivalent of one unfilled character space between words (= 12 pixels). Example of annotation (maximum length): AAAAAA DD MMM YYYY HHMM FFFFFFFFFF LLLL where: AAAAAA = Origin of data (e.g. MET3 = Meteosat-3 MET4 = Meteosat-4 MET5 = Meteosat-5 MET6 = Meteosat-6 MET7 = Meteosat-7 GOES-E = GOES-East GOES-W = GOES-West GMS-5 = GMS-JMA GOMS = GOMS) DD = Day of month date/time MMM = Month (JAN, FEB, etc.) of end YYYY = Year of image HHMM = Hour and minute acquisition FFFFFFFFFF = Format letter and number (max. 10) (For example IR D2, VIS 1+2, C02 etc.) LLLL = Spare The hour and minute of the end of the image acquisition are used as explained in Appendix A. If parts of the annotation are shorter than the above specified maximum length (for example AAAA = MET4), the remaining information is shifted to keep the annotation as short as possible. The annotation of foreign satellite data up-linked by CMS Lannion (as outlined in Section 3.3) may vary from that described above. Technical Description Rev. 6 11

20 3.2 WEFAX Formats originating in Darmstadt C Formats The C formats cover approximately 90% of the earth disc as observed by the visible (VIS) detectors ( µm) of Meteosat. The visible image is divided into 24 sub-formats (C01 to C24) as shown in Figure 3.3. At the present time only two of these formats (C02 and C03) covering the European area are disseminated at half-hourly intervals during daylight hours. Two VIS detectors provide a line resolution of 5000 pixels. Therefore, after 2500 scans of the radiometer a full resolution image of 5000 x 5000 is created by making use of both detector data sets. If data from both VIS detectors are used in a C format the resolution is 2.5 km at the sub-satellite point; if only one channel is used, the resolution in the East-West direction reduces to 5 km. The respective mode of operation is indicated in the annotation (e.g. VIS 1 or VIS 1+2). In the case where only one VlS channel is used, each second line is a duplicate of the previous one. Figure 3.3 C Formats (visible) 12 Technical Description Rev. 6

21 3.2.2 D Formats The D formats cover nearly the whole earth disc as observed by the infrared (IR) detector ( µm) of Meteosat. The image is divided into nine sub-formats (D1 to D9) as shown in Figure 3.4. The processed IR digital counts are inverted (subtracted from 255) before formatting, so that the pictures will look similar to the VIS pictures, with white clouds and dark land surfaces. Cold high clouds with low IR radiance therefore appear white, whereas warmer low level clouds appear grey, while land and sea, with high IR radiances, will look relatively dark E Formats The E formats cover the same area as the D formats but provide data from the water vapour (WV) detector ( µm) of Meteosat. The formats are referred to as E1 E9. The processed WV data for the E formats are inverted in a similar way to the D formats but in this case dark areas correspond to dry regions of the upper troposphere CnD Formats The CnD formats are visible formats whose coverage is identical to that of D and E formats. These formats contain data at single VIS resolution (5 km). The variable n within the format name identifies the sector numbering (1 9) according to Figure 3.4. Since mid-1993 full VIS rectification has been applied in both line and column directions, therefore, the CnD formats which are identified as a single VIS detector contain data from either detector xtot Formats These formats cover the full earth disc. That means that image data of the relevant spectral channel are a subsample (by factor 3 or 6) of the resolution of a WEFAX format. The formats are: CTOT - visible spectral band DTOT - infrared spectral band ETOT - water vapour spectral band. Technical Description Rev. 6 13

22 Figure 3.4 Coverage of the CnD, D and E Formats 14 Technical Description Rev. 6

23 3.2.6 Administrative Messages Administrative messages, coded as WEFAX formats, are sent to inform users about changes in operations of the Meteosat system. The messages are formatted as follows: - up to 44 lines of text, each containing up to 60 characters - each character is a 7 x 5 dot matrix as described in Section on annotation. The messages are marked by an Administrative Message Number, which consists of a running number of the messages and the month of issue (e.g. 06/08 is the eighth ADMN message in June). All scheduled interruptions for the following week are announced in advance in Friday ADMN messages. Note: Information concerning the previous seven day dissemination, scheduled interruptions and general dissemination news can be found on the EUMETSAT World Wide Web pages Figure 3.5 ADMN Format Technical Description Rev. 6 15

24 3.2.7 CTH Formats Cloud Top Height formats are a product of the Meteosat Meteorological Products Extraction Facility and present the altitude of the highest cloud on a resolution of about 20 x 20 km. The altitude is indicated by eight grey levels in steps of 1500 m. Black indicates the absence of cloud tops above 3000 m, white indicates cloud tops above m. Figure 3.6 CTH Format 16 Technical Description Rev. 6

25 3.2.8 Test Pattern A standard test pattern (as shown in Figure 3.7) is transmitted in WEFAX format. It is marked in the dissemination schedule by TEST. It can be used to assess the SDUS quality (e.g. resolution, linearity, signal-to-noise ratio). Figure 3.7 TEST Format Technical Description Rev. 6 17

26 3.2.9 Enhancement The image data from each of Meteosat's spectral channels consist of grey levels in the range 0 255, although the actual dynamic range of the data will often be less. The dynamic range of the image data can be expanded by software enhancement, i.e. any grey level in the original image can be allocated to another level in the nominal range. Such an enhancement is applied on a continuous basis to all WEFAX images processed in Darmstadt and CMS Lannion. It should be noted that since this implies a modification to the original data content and calibration of the image, these WEFAX formats are not suitable for quantitative analysis Grid All Meteosat raw data are rectified to the nominal Meteosat image coordinate system. A grid is superimposed on the rectified data which is finally disseminated via the satellite transponder. A missing grid is usually a sign that the disseminated image does not fully meet all the usual quality requirements. The processing of foreign satellite data relayed by CMS Lannion (see Section 3.3) may be different. For example the grid for the GMS formats is adjusted at source to the coordinates of the underlying image data and may, therefore, result in jumping overlays when images are viewed in an animated sequence. 18 Technical Description Rev. 6

27 3.3 WEFAX Formats originating in CMS Lannion All the formats described in this section are acquired by CMS Lannion. The foreign satellite data are processed and converted, following Meteosat format specifications, before they are retransmitted via the satellite. Most formats may be subject to some modification depending upon the operational status of satellites located at other than the nominal Meteosat position L Formats The formats LR and LY contain image data as observed by the infrared sensors on board the American GOES-E satellite (nominal position 75 W). The LR format covers South America, while the LY format shows North America (see Figure 3.8). An area of 1300 x 1300 IR pixels is subsampled down to 800 x 800 pixels to create the LY and LR formats (resulting resolution 8 km at the sub-satellite point). The LZ format represents a sector of the image observed by the visible sensor on board the American GOES-E satellite. The format contains 800 x 800 pixels of a single VIS detector (resolution 5 km at the sub-satellite point). More information concerning the format definition can be found in Appendix B - WEFAX Format Definition Parameters. Technical Description Rev. 6 19

28 Figure 3.8 L Formats 20 Technical Description Rev. 6

29 3.3.2 GMS Formats The GMS formats are received via the UK Met. Office from the Bureau of Meteorology (Melbourne) as digital 1250 x 1250 pixel images (HRI resolution formats). To avoid over-sampling of the acquired image three new GMS WEFAX formats (GMSN, GMSS and GMST) have been defined for use within the Meteosat dissemination schedule. GMSN and GMSS are based on a 700 x 700 area of the original 1250 x GMST is currently not disseminated. The images are received every three hours. They are converted according to the Meteosat format specifications. The formats are inserted into the dissemination schedule with lowest priority using the existing gaps in the dissemination schedule on channel A2. GMSN GMST GMSS Figure 3.9 The GMS WEFAX Formats Technical Description Rev. 6 21

30 3.3.3 GOMS Format GOMS IR data are acquired directly from the Roshydromet facilities in Moscow via a dial-up link. The WEFAX format GOMS provides a full earth disc view in the IR channel from the geostationary position of 76 E. The coverage is shown in Figure 3.10 below. Figure 3.10 Coverage area of the GOMS WEFAX Format 22 Technical Description Rev. 6

31 3.4 Meteosat Dissemination Schedule The following philosophy is used to determine the Meteosat dissemination schedule: Channel A MHz Analogue transmissions from Meteosat at 0, comprising Meteosat image data and Cloud Top Height maps in WEFAX format. In the 26.5 second gap between two WEFAX transmissions the DCP Retransmission System (DRS) is active; DCP messages are then retransmitted at 12.5 kbit/s. Channel A MHz All digital data (A, B and X formats transmitted to PDUS), full earth disc WEFAX formats (DTOT, CTOT, ETOT) plus all WEFAX transmissions up-linked from CMS Lannion (identified by "L", "GMS" or "GOMS" in the schedule). In addition, administrative messages and WEFAX test formats are transmitted on both channels. This philosophy may be subject to change for technical or other reasons. User stations should, therefore, always be capable of receiving both Meteosat dissemination channels, either simultaneously or by switching between channels. To standardise the start times of individual formats, four-minute transmission slots are used. Formats commence at the nominal times H +02', +06', +10' etc., with actual transmissions scheduled to start a few seconds after these nominal times. All WEFAX transmissions can be completed within one four-minute slot. Meteosat Dissemination Schedules listing PDUS and SDUS formats are produced by EUMETSAT and can be mailed to interested persons or organisations upon request. A typical schedule is shown in Appendix A Operational Constraints Scheduled interruptions Interruptions in the regular service which are known in advance (e.g. ground segment maintenance, satellite eclipse periods) will be announced by ADMN messages. As a rule, all interruptions to the normal service for the following week will be announced in the Friday ADMN message. Replacement of missing formats Each Meteosat format refers to a specific image slot, covering each half-hour period, and numbered 1 48 each day. In situations where the nominal image for slot n is missing or corrupted, the latest available image, in the sequence slot n-1, n-2, will be disseminated in its place, e.g. instead of the nominal D2 18, the alternatives D2 17 or D2 16 might be transmitted. Obviously, these changes of schedule cannot be announced to users in advance, but the correct image time is placed in the annotation which is superimposed on the disseminated format (e.g or 0800 in the above example). Technical Description Rev. 6 23

32 Radiometer decontamination Once or twice a year there is a build up of ice or other contaminants on the cold optics part of the radiometer. Heating up the radiometer assembly over two or three days usually successfully removes all contamination. Thus, during periods of radiometer decontamination, the IR and WV detectors cannot be used and only VIS data would be retransmitted during daylight hours in these periods. Split-mission scenario The nominal satellite plus a stand-by may occasionally be used in a so-called split-mission scenario. This means that the imagery is provided by the stand-by but the retransmission is performed via the transponders of the nominal satellite located at 0, in order that all the users do not have to repoint their antennas. This scenario may be used either during radiometer decontamination or in other restricted periods of satellite maintenance (e.g. battery reconditioning), or following the failure of a satellite subsystem affecting one or more missions. In this case the rectification process transforms the acquired raw data from a satellite with a different orbital location to the nominal Meteosat coordinate system. Therefore, some areas in the image formats may suffer from missing data, and horizons may show some irregularities. Eclipse An eclipse of the sun occurs when the satellite passes into the conical shadow region of the earth or of the moon. A geostationary satellite will pass through the earth s shadow at the time of the equinoxes, i.e.: - end of February to beginning of April - end of August to beginning of October. The precise occurrence and duration of these eclipses depends upon the characteristics of the satellite orbit and are announced via ADMN messages. During these eclipse seasons the transponders of a Meteosat satellite will normally be switched off to conserve battery power. Depending on the raw data acquisition, up to four dissemination slots may be lost. For the nominal position (0 longitude) these times occur around midnight. The total number of days affected within one eclipse season is around 42. The length of the eclipse varies and reaches a maximum of more than 70 minutes at the date of the equinox itself. Sun - satellite - ground station collinearity Sun-satellite collinearity occurs when the sun is aligned with the satellite as seen at the ground station. This situation leads to a sudden increase in the station antenna noise temperature. The ground station will be affected by this phenomenon under the following conditions: - the latitude of the satellite is equal to the declination of the sun, - the longitude of the satellite is equal to the hour angle of the sun. Due to this effect the raw data acquisition at the Primary Ground Station is also interrupted around noon near the beginning of March and the second week of October. The date and time of dissemination reception interruption at the user station may differ from that at the Primary Ground Station because of differences in geographical location, therefore a longer period of signal drop-out might be experienced. 24 Technical Description Rev. 6

33 4 DESCRIPTION OF A TYPICAL SDUS The typical SDUS will comprise the main subsystems shown in Figure 4.1. The image receiving equipment must be specified for the WEFAX signal characteristics (e.g. signal level, modulation characteristics, bandwidth), as described in Section A more detailed description of the SDUS front-end (comprising antenna, LNA, down-converter and receiver) is contained in Section For long duration storage of images, a disk drive unit, hard or floppy, is normally used. RAM image memories permit the storage of several WEFAX formats to produce animated loop sequences. High quality hard copies of the received WEFAX formats can be obtained through the use of, for example, a laser fax recorder or a video printer. Further advice and information concerning the construction or purchase of an SDUS may be obtained from the address given in the Preface of this document. Figure 4.1 Block Diagram of Typical SDUS Technical Description Rev. 6 25

34 4.1 Input/Output Specifications Satellite Output Data Table 4.1 provides the specification for the satellite output data. Down-link Dissemination Channel A1 Dissemination Channel A2 RF carrier frequency MHz MHz Long-term frequency stability (in 3 years); worst case ±1.2 x 10-5 Short-term frequency stability; worst case Satellite antenna polarisation Worst case EIRP towards earth stations with an elevation of 5 ±7 x 10-9 /sec. Linear (horizontal on the longitude of the SSP) 20.5 dbw 19.0 dbw Table 4.1 Satellite Transponder Characteristics The WEFAX transmissions consist of an AM modulated sub-carrier, which is frequency modulated onto the RF carrier. The peak FM frequency deviation is 9 khz, the peak AM modulation of the 2400 Hz sub-carrier is 80%. The maximum corresponds to white. Table 4.2 summarises all relevant WEFAX modulation characteristics. RF carrier modulation FM Max. FM deviation 9 khz FM modulation non-linearity <±5% Sub-carrier modulation AM Sub-carrier frequency 2400 Hz ±0.25Hz Sub-carrier modulation index 80 % ±2% AM modulation non-linearity <±0.2% Video frequency khz Table 4.2 WEFAX Modulation Characteristics 26 Technical Description Rev. 6

35 Satellite station keeping The Meteosat satellites normally remain stationary with respect to the earth. However, the combined effect of oscillations over a 24h period due to orbit inclination and eccentricity together with the longterm slow drift of the mean longitude leads to an apparent movement of the satellite from its nominal position. The station-keeping window or box represents maximum permitted values of the excursions of satellite longitude and latitude. A typical specification for the Meteosat station-keeping box during its operational lifetime is ±1.0 longitude and ±0.3 latitude. At the end of life, in terms of station-keeping fuel, the orbit inclination will slowly increase at the rate of about 0.9 /year. For that reason, additional pointing losses may have to be taken into account depending on the user station location and chosen antenna size Typical SDUS Front-end Characteristics An SDUS front-end comprises an antenna, a low noise amplifier (LNA), a down-converter and an FM receiver. The system specification, used for the SDUS design of the Meteosat satellite, requires a G/T of 2.5 db/k. Depending on the location, however, a relaxation of this requirement is possible. The SDUS front-end characteristics provided in Table 4.3 are typical. G/T (system specification) LNA gain Antenna gain Polarisation Front-end bandwidth Front-end centre frequency 2.5 db/k db db i Linear, adjustable over 180 deg 6 MHz 1693 MHz Table 4.3 Typical Specifications for an SDUS Front-end The antenna must be capable of operating in the 1.7 GHz band. A parabolic dish is suitable, although other configurations (e.g. Yagi) may be used. The required antenna gain can be provided by parabolic reflectors of 1.50 m or 5' (25 dbi) and 1.80 m or 6' (27 dbi). The relevant -3dB beamwidths of such antennas, assuming an efficiency of η = 55%, are 8.3 and 6.9, respectively. Normally, no provisions are required for frequent readjustment of the pointing distortion; a semi-fixed installation will be adequate. Where the situation regarding the station-keeping window is appreciably worse, a more stringent specification may be required. An example of a worst case link budget calculation (5 elevation) is provided in Appendix C. Technical Description Rev. 6 27

36 The following may lead to a reduction in antenna size: - use of better LNA reducing the system noise temperature - use of an antenna system with higher efficiency - taking into account the gain increase of the satellite electronically despun antenna (EDA) from 11.5 dbi (for 5 elevation), 13 dbi (for 30 elevation) to a maximum of 14 dbi at the sub-satellite point. However, a more directive antenna provides better protection against man-made noise and interference from nearby satellites (such a situation may arise when simultaneous transmissions are made from two Meteosat satellites separated by only 8 longitude). Since antennas of these dimensions are relatively cheap, the selection of a 1.8 m reflector will be suitable for most urban environments. The low noise amplifier must be mounted onto the antenna as near to the feed as possible. Normally, the (first) down-converter will be integrated with the LNA. For the demodulator, an already available VHF FM (APT) receiver may be used. In this case, down-conversion to about 137 MHz will be required. Otherwise more freedom exists in the selection of intermediate frequencies. The RF signal bandwidth is about 26 khz. Allowing for some filter drift, the receiver bandwidth should therefore be about 30 khz. A wider bandwidth would increase antenna requirements. For the link budget example it is assumed that the receiver uses a simple frequency discriminator with a threshold of 12 db S/N. At this signal-to-noise ratio, noise will be practically invisible in the image. However, some margin must be provided for non-optimum performance of equipment, slight misadjustments, drift etc. For this reason, at least 2 db of margin are recommended. After FM demodulation, the output signal will be a double side-band amplitude-modulated sub-carrier which will normally be forwarded to the following A/D converter and a small computer system which enables the operator to store a number of WEFAX formats, to animate sequences of images and to use basic functions of image processing. 28 Technical Description Rev. 6

37 Appendix A Meteosat Dissemination Schedule A.1 The Key to the Dissemination Schedule Schedule Number The schedule number is generated with the following design: YYMMCNN where: YY = Year of introduction MM = Month of introduction C = Character which indicates the satellite (M=Meteosat) NN = Version number The schedule number is important since changes to schedules will be announced by ADMN messages. The information valid from is important as schedules are normally distributed in advance. Column and Row Headings CH A1 is the Meteosat dissemination channel at MHz CH A2 is the Meteosat dissemination channel at MHz HH is the time (hour) during which the transmission starts. MM gives the minute of the hour for the start of transmission. All times are in UTC. Table Entries These give the type of format followed by the slot number of the image used. The formats are indicated with the characters used in the format description. The numbers/characters following the indicating characters define the area of the format (as shown in Figures 3.3, 3.4, 3.8, 3.9 and 3.10) e.g. D2 18 = LY 24 = infrared image of Europe generated for slot 18 (image completion time = 0900 UTC) infrared image of North America as seen by GOES-E, relay transmission from CMS Lannion, slot 24 (image completion time = 1200 UTC) The slot number is the time to the nearest half hour of completion of the image. e.g. image ending by 1200 UTC = slot 24 image ending by 1930 UTC = slot 39 The actual end time of the image is shown on the transmitted format. Technical Description Rev. 4 A-1

38 A.2 Typical Meteosat Dissemination Schedule S9712M01 Valid from 15 December 1997, 0000 UTC Meteosat WEFAX Dissemination HH HH MM CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 MM 2 D1 48 AIW 48 D1 06 AIW 06 C02 12 AIVH 12 C02 18 AIVH 18 C02 24 AIVH 24 C02 30 AIVH 30 D1 36 AIVH 36 D1 42 AIVH D3 48 AIW 48 D3 06 AIW 06 C03 12 AIVH 12 C03 18 AIVH 18 C03 24 AIVH 24 C03 30 AIVH 30 D3 36 AIVH 36 D3 42 AIVH D4 48 AIW 48 D4 06 AIW 06 D1 12 AIVH 12 D1 18 AIVH 18 D1 24 AIVH 24 D1 30 AIVH 30 D4 36 AIVH 36 D4 42 AIVH D5 48 DTOT 48 D5 06 DTOT 06 D3 12 BW 12 D3 18 BW 18 D3 24 BW 24 D3 30 BW 30 D5 36 BW 36 D5 42 DTOT D6 48 ETOT 48 D6 06 ETOT 06 D4 12 DTOT 12 D4 18 DTOT 18 D4 24 DTOT 24 D4 30 DTOT 30 D6 36 DTOT 36 D6 42 ETOT D7 48 D7 06 D5 12 ETOT 12 D5 18 ATEST 01 D5 24 CTOT 24 D5 30 CTOT 30 D7 36 ETOT 36 D7 42 ATEST D8 48 D8 06 D6 12 D6 18 ATEST 01 D6 24 D6 30 D8 36 D8 42 ATEST D2 01 BIW 01 D2 07 BIW 07 D2 13 BIV 13 D2 19 BIV 19 D2 25 BIV 25 D2 31 BIV 31 D2 37 BIV 37 D2 43 BIW D9 01 AIW 01 D9 07 AIW 07 C02 13 AIVH 13 C02 19 AIVH 19 C02 25 AIVH 25 C02 31 AIVH 31 D9 36 AIW 37 D9 43 AIW D1 01 AIW 01 D1 07 AIW 07 C03 13 AIVH 13 C03 19 AIVH 19 C03 25 AIVH 25 C03 31 AIVH 31 D1 37 AIW 37 D1 43 AIW D3 01 AIW 01 D3 07 AIW 07 C3 D 13 AIVH 13 C8 D 19 AIVH 19 C3 D 25 AIVH 25 C8 D 31 AIVH 31 D3 37 AIW 37 D3 43 AIW C2 D 13 AW 13 C9 D 19 AW 19 C2 D 25 AW 25 C9 D 31 AW D3 13 AW 13 C2 D 19 AW 19 C1D 25 AW 25 D3 31 AW E_XI 48 E_XI 06 D1 13 E_XI 12 D1 19 E_XI 18 D1 25 E_XI/XVH 24 D1 31 E_XI/XVH 30 E_XI/XVH 36 E_XI/XVH D2 02 BIW 02 D2 08 BIW 08 D2 14 BIV 14 D2 20 BIV 20 D2 26 BIV 26 D2 32 BIV 32 D2 38 BIV 38 D2 44 BIW HH HH MM CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 MM 2 D1 02 AIW 02 D1 08 AIW 08 C02 14 AIVH 14 C02 20 AIVH 20 C02 26 AIVH 26 C02 32 AIVH 32 D1 38 AIVH 38 D1 44 AIW D3 02 AIW 02 D3 08 AIW 08 C03 14 AIVH 14 C03 20 AIVH 20 C03 26 AIVH 26 C03 32 AIVH 32 D3 38 AIVH 38 D3 44 AIW AIW 02 E1 08 AIW 08 D7 14 AIVH 14 D7 20 AIVH 20 D7 26 AIVH 26 D7 32 AIVH 32 E1 38 AIVH 38 AIW LY 48 E2 08 LY 06 D8 14 BW 14 D8 20 BW 20 D8 26 BW 26 D8 32 BW 32 E2 38 BW 38 LY LR 48 E3 08 LR 06 D9 14 LY 12 D9 20 LY 18 D9 26 LY 24 D9 32 LY 30 E3 38 LY 36 LR GOMS 01 E4 08 GOMS 07 D3 14 LR 12 D3 20 LR 18 D3 26 LR 24 D3 32 LR 30 E4 38 LR 36 GOMS E5 08 GOMS 13 GOMS 19 LZ 24 LZ 30 E5 38 LZ D2 03 BIW 03 D2 09 BIW 09 D2 15 BIV 15 D2 21 BIV 21 D2 27 BIV 27 D2 33 BIV 33 D2 39 BIV 39 D2 45 BIW D1 03 AIW 03 D1 09 AIW 09 C02 15 AIVH 15 C02 21 AIVH 21 C02 27 AIVH 27 C02 33 AIVH 33 D1 39 AIVH 39 D1 45 AIW D3 03 AIW 03 D3 09 AIW 09 C03 15 AIVH 15 C03 21 AIVH 21 C03 27 AIVH 27 C03 33 AIVH 33 D3 39 AIVH 39 D3 45 AIW AIW 03 E6 09 AIW 09 D1 15 AIVH 15 D1 21 AIVH 21 D1 27 AIVH 27 D1 33 AIVH 33 E6 39 AIVH 39 AIW J_XI/XVH 48 E7 09 J_XI/XVH 06 D3 15 J_XI/XVH 12 D3 21 J_XI/XVH 18 D3 27 GOMS 25 D3 33 GOMS 31 E7 39 GOMS 37 J_XI/XVH W_XI/XVH 01 E8 09 W_XI/XVH 07 W_XI 13 C1 D 21 W_XI 19 C1 D 27 J_XI 24 C1 D 33 J_XI 30 E8 39 J_XI 36 W_XI/XVH GMSN 48 E9 09 GMSN 06 C2 D 15 GMSN 12 C2 D 21 GMSN 18 C2 D 27 W_XI 25 C2 D 33 W_XI/XVH 31 E9 39 W_XI/XVH 37 GMSN D2 04 BIW 04 D2 10 BIV 10 D2 16 BIV 16 D2 22 BIV 22 D2 28 BIV 28 D2 34 BIV 34 D2 40 BIW 40 D2 46 BIW HH HH MM CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 CH A1 CH A2 MM 2 D1 04 AIW 04 D1 10 AIW 10 C02 16 AIVH 16 C02 22 AIVH 22 C02 28 AIVH 28 C02 34 AIVH 34 D1 40 AIW 40 D1 46 AIW D3 04 AIW 04 D3 10 AIW 10 C03 16 AIVH 16 C03 22 AIVH 22 C03 28 AIVH 28 C03 34 AIVH 34 D3 40 AIW 40 D3 46 AIW TEST AIW 04 ADMN AIW 10 C3 D 16 AIVH 16 C3 D 22 AIVH 22 C3 D 28 AIVH 28 C1 D 34 AIVH 34 TEST AIW 40 ADMN AIW GMSS 48 GMSS 06 TEST BW 16 C4 D 22 BW 22 C4 D 28 BW 28 C4 D 34 BW 34 GMSN 36 GMSS ADMN BW 10 GMSS 12 ADMN GMSS 18 TEST GMSN 24 ADMN GMSN 30 GMSS 36 TEST D2 05 BIW 05 D2 11 BIV 11 D2 17 BIV 17 D2 23 BIV 23 D2 29 BIV 29 D2 35 BIV 35 D2 41 BIW 41 D2 47 BIW D1 05 AIW 05 D1 11 AIVH 11 C02 17 AIVH 17 C02 23 AV 23 C02 29 AIVH 29 D1 35 AIVH 35 D1 41 AIVH 41 D1 47 AIW D3 05 AIW 05 D3 11 AIVH 11 C03 17 AIVH 17 C03 23 AV 23 C03 29 AIVH 29 D3 35 AIVH 35 D3 41 AIVH 41 D3 47 AIW AIW 05 E1 11 AIVH 11 C5 D 17 AIVH 17 E1 23 AV 23 C5 D 29 AIVH 29 E1 35 AIVH 35 AIVH 41 E1 47 AIW ATEST 02 E2 11 AW 11 C6 D 17 ATEST 02 E2 23 AV 23 C6 D 29 GMSS 24 E2 35 GMSS 30 ATEST 02 E ATEST 02 E3 11 AW 11 C7 D 17 ATEST 02 E3 23 AV 23 C7 D 29 ADMN E3 35 TEST ATEST 02 E E3 11 BW 11 C7 D 17 E3 23 AV 23 C7 D 29 ADMN E3 35 TEST E CTH 05 CTH 16 ADMN AV 23 CTH 28 CTH 40 ADMN D2 06 BIW 06 D2 12 BIV 12 D2 18 BIV 18 D2 24 BIV 24 D2 30 BIV 30 D2 36 BIV 36 D2 42 BIW 42 D2 48 BIW A-2 Technical Description Rev. 4

39 The following format abbreviations are used in the Meteosat dissemination schedule: Meteosat HRI Data AIVH Full Disc IR & Half Res VIS AIW Full Disc IR & WV AW Full Disc WV AV Full Disc Full Res VIS BIW European Sector IR & WV BIV European Sector IR & Full Res VIS BIVW European Sector IR, WV & Half Res VIS BW European Sector WV ATEST01 Test Pattern (identified as AW 01) Meteosat WEFAX Images C nn Sector nn VIS Full Res C n D Sector n VIS Half Res D n Sector n IR E n Sector n WV CTOT Full Disc VIS DTOT Full Disc IR ETOT Full Disc WV CTH Cloud Top Height (product of the MPEF) ADMN Administrative Message TEST Test Pattern Foreign Satellite Data Relay (up-linked by CMS Lannion) XI HRI Full Disc IR XVH HRI Full Disc Half Res VIS HRI prefixes used in the schedule as satellite indicators: E_ GOES-E W_ GOES-W J_ GMS LY GOES-E WEFAX Sector N. America IR LR GOES-E WEFAX Sector S. America IR LZ GOES-E WEFAX Sector N. America East Coast VIS GMS x GMS WEFAX Sector x IR GOMS GOMS WEFAX Full Disc IR ATEST02 HRI Test Pattern (identified as AW 02) x substitutes for a character n substitutes for a number Ranging gap Encrypted data in bold Note: 1. Formats starting with A, B, J_X, E_X or W_X indicate high resolution digital disseminations and can be received only with PDUS equipment. 2. Some VIS formats are subject to suppression due to seasonal under-illumination. Technical Description Rev. 4 A-3