Satellite data transmission as an aid to hydrological telemetry

Transcription

1 Hydrologie Applications of Space Technology (Proceedings of the Cocoa Beach Workshop, Florida, August 1985). IAHS Publ. no. 160, Satellite data transmission as an aid to hydrological telemetry R. W. HERSCHY CNS Scientific & Engineering Services, 20, Eldon Road, Reading, UK Abstract The paper describes the European Space Agency (ESA) METEOSAT system for the transmission and reception of hydrological data. The data collection platforms (DCPs) transmit, via METEOSAT 1, hydrological data such as river level, river flow, rainfall and evaporation information. These data are received at the European Space Operations Centre (ESOC) in Darmstadt, West Germany and retransmitted via METEOSAT 2 to the user. The user receives the data by means of a small 1.5m diameter receiving dish. If weather imagery is required this can be accomplished by the addition of a standard display and processing equipment which presents an image every half hour. Weather images and DCP data can be viewed together. The paper also describes briefly the technical details of the DCPs, the receiver unit, the antenna unit and the mechanical configuration of the system. This development of fast, cheap hydrological telemetry has made data collection, transmission and reception by satellite a cost effective alternative to existing terrestrial methods. Introduction At most of the world's stream gauging stations hydrometric data are collected by one of three methods. 1. By an observer reading the staff gauge and either telephoning the reading to base, or filling up a weekly or monthly postcard of staff gauge readings taken once or more times per week. The postcard, when completed, is mailed by means of the normal postal service. 2. By weekly or monthly autographic chart. The chart is then either mailed or manually transported to base. 3. By monthly digital punched paper tape. The tape is then either mailed or manually transported to base as in the case of charts. Data collected by the above methods are therefore essentially historical. Indeed in the case of monthly charts or tapes, some of the data may be about 6 weeks old before being processed. Such data may therefore be of use mainly for planning or design purposes. When data are required for operational purposes, however, such as flood control or other water management control purposes, in relative real-time, transmission by means of telemetry is normally employed. However it is quite possible that if most of the data, now classed as historical because of their late arrival at base, were available in relative real-time much more use could be made of the data for management control purposes. 369

2 370 R.W.Herschy Telemetry usually takes the form of transmission by terrestrial means namely telephone land line or radio. Although such systems are quite common in telemetering hydrometric data, because of the usually harsh environment surrounding streamflow stations, terrestrial methods may suffer from several disadvantages. They may be expensive to install and maintain, they may be prone to interference and installation can be a fairly lengthy process depending on terrain. Radio frequencies are becoming extremely difficult to obtain and both systems are liable to failure during floods when reliability could be crucial. The introduction of communication satellites now offers a reliable and cost effective alternative to terrestrial systems. Satellite Telemetry The satellites GOES (US), GMS (Japan), GOMS (USSR) and METEOSAT (European Space Agency) form a system of geostationary satellites with both a remote sensing and telecommunications facility. These satellites are placed in orbit coincident with the earth's equatorial plane at a height of about 36,000km and since they rotate at the same speed as the earth, appear to be stationary. METEOSAT 1 was launched in 1977 followed in 1981 by METEOSAT 2. Generally the telecommunications mode of the satellite is designed for a life of about five years but-meteosat 1 is still operating in the telecommunications mode. Polar orbiting satellites, on the other hand, orbit the earth several times a day generally at a height of less than 1000km, and although providing a higher resolution in the remote sensing mode, only provide a mutual view of the transmitting and receiving stations for a small percentage -Of the time - namely a few minutes every 2 hours or so. It is expected that METEOSAT 1 will be replaced by GOES 4 in late This arrangement between NOAA and ESA will continue until the launch of METEOSAT P2 in The satellite itself is composed of a main cylindrical body on top of which a screen-shaped section and two cyclinders are stacked concentrically. The satellite is 2.1m in diameter and 3.2m long. The weight at the beginning of life in orbit was 293kg; this will gradually fall to 245kg as the hydrazine propellant is used during its lifetime. In orbit the whole satellite spins at 100 revolutions per minute about its main axis which is closely aligned to the earth's north-south axis. The main cylindrical body contains most of the satellite subsystems, including the radiometer. The cylindrical surface is covered with solar cells for power supply. The spacecraft has four main (25 N) thruster motors and two vernier (2.5 N) thrusters. All six are fed by hydrazine popellant contained in three interconnected spherical tanks having sufficient capacity for a five-years lifetime. This system is used to control METEOSAT's attitude in space and to make small changes in its orbit, principally to move the satellite in its orbital plane to make fine adjustments to the longitude over which METEOSAT is stationed (METEOSAT 1 longitude 10, METEOSAT 2 longitude 0 ). The motors are operated by telecommands from the ground station and normally several months elapse between manoeuvring operations. METEOSAT together with other geostationary satellites perform three principal functions or missions.

3 Satellite data transmission S telemetry 371 a) A microprocessor which starts the transmitter at a preset time, formats and spectral bands (infra-red and visible). b) Retransmission (dissemination) of computer processed image-derived data by landline and via the satellite to users' ground stations. c) Data collection via the satellite the environmental in situ measurements using land based, ship or airborne DCPs. Data Collection Platforms Operational studies on hydrometric data collection by geostationary satellites have been successfuly carried out in recent years, notably in the USA, Canada and Europe. These studies have confirmed that a satellite telemetry system is reliable and cost effective. The device which transmits the data to the satellite for retransmission to the ground receiving stations is known in satellite terminology as a data collection platform. The satellite used for hydrometric data transmission in North America is GOES and METEOSAT is used in Europe and the African continent. Many hundreds of hydrometric DCPs are now in operation in North America and many hundreds more are proposed. The first METEOSAT DCP was installed some 5 years ago in the UK to record river water level at a streamflow station. The number of DCPs in Europe using METEOSAT however still falls short of 100. Indeed Europe has been slow to take advantage of the system. A DCP is essentially an electronic device containing a small UHF low power 6W transmitter operating at 402MHz. There are three types of DCP offering a range of possible applications. a) Self-timed DCPs where the data are transmitted in buffered blocks of data at fixed intervals. b) DCPs which provide an immediate alert transmission when a critical data value has been reached. c) Interrogable DCPs where the DCP receives a command message from the control centre via the satellite. The DCPs intended for use with meteorological geosynchronous satellites have been designed to an international specification and are adaptable to all of the satellites listed above. Whatever the form of the DCP, it has to be approved and certified by the satellite owner, in the case of METEOSAT, by the European Space Agency (ESA). Self-timed alert DCP The DCP used in the UK and other European countries is a combined selftimed alert DCP and includes the following features. a) A microprocessor which starts the transmitter at a preset time, formats the data, and adds the preamble and end of transmission code around the data; a shutdown timer is included to turn off the transmitter in the event of controller failure. b) A highly stable crystal-controlled clock and oscillator which

4 372 R.W.Herschy provides the timing signals to initiate transmission and a stable signal defining the transmission radio frequency. c) A radio frequency modulator (phase modulator) and low power transmitter (6W at 12V). d) An alert facility. e) A narrow beam aerial (antenna) directed to the satellite. f) A source of power; battery or mains. The most common DCPs are the self-timed variety which transmits the previously collected data, usually of 3 to 24 hours duration, at a preset time. By allocating to each transmitter a specific time slot, as well as one of about 60 radio channels on METEOSAT, the satellite can relay up to 40,000 messages per day. Each DCP message starts with a unique code to allow identification of its source and its destination. An alert DCP is used where a hazardous situation may arise which requires data to be sent immediately a measured value falls outside a preset limit or rate of increase. It would be unusual for a DCP to be exclusively alert so an alert DCP is arranged to operate normally as a self-timed DCP, the alert procedure only, operating when necessary. The DCP may be fixed, as in the case of a hydrometric DCP, or mobile. A fixed unit has a small directional antenna which points towards the satellite, in the case of METEOSAT 1, at 10 longitude and zero latitude. A mobile DCP, normally used on buoys or ships, has the same configuration if the antenna can be mounted on a stabilized structure to ensure that it points to the satellite, or a broad beam antenna to eliminate the need for stabilization. The latter DCP requires a higher transmitter power of 50W to compensate for the lower antenna gain. Both forms may be operated either by battery or mains electrical power, the former having about six months' life. All except the aerial and the source of power are housed within a single compact box which may be free-standing or mounted on a rack. There is sufficient space available within the DCP to fit a set of printed circuit boards (PCBs) for a microprocessor-controlled data store and interfacing may be made with up to six sensors simultaneously. The DCPs used in the UK have 8-bit or 16-bit parallel data interfaces and use either two or four inputs. The data format is in the international WMO (SX) code. At the preset time of transmission, sensor data are passed to the DCP where they are modulated on to a radio carrier and transmitted. To prevent interference between transmissions from different DCPs, each DCP in the network is arranged to transmit in a particular radio channel at a particular time. Typically one satellite DCP transponder provides about 4,800 time slots of about one minute within 30 discrete radio channels during one day. Each DCP transmits a preamble code to identify itself, the data and an end of transmission code to complete the message. Hydrometric Sensors for DCP Transmission By various combinations of plug-in modules, data from up to 24 sensors (or 120 with an extension unit) can be collected, processed, formatted and transmitted to METEOSAT.

5 Satellite data transmission & telemetry 373 Sensors may be of many different types, having either analog or digital outputs. In the UK the hydrometric sensors most commonly used include digital punched paper tape recorders for river level measurement, tippingbucket rain gauges for rainfall measurement, sensors for the measurement of potential evaporation, ultrasonic and electromagnetic streamflow gauges and water quality sensors. Data can be input in any of three ways. a) The DCP transmitter can request data from the sensors at regular time intervals. b) The DCP transmitter accepts data from the sensors when available. c) By an interactive mode, using a visual display unit, whereby the DCP transmitter requests data from the observer who types in the data manually. A mixture of the above data entries is also possible. Prior to transmission, the raw data can be processed and formatted by the microprocessor. This can include various calculations, notably the processing of stream discharge from stage by entering the stage-discharge relation into the microprocessor; or potential evaporation from an array of sensors from an automatic climate station which measures windspeed, wind direction, temperature, wet bulb depression, solar radiation, net radiation and rainfall. In this case readings are taken every five minutes and these are used to produce average or total values for a three-hour period. Using five of these measurements, the potential evaporation is calculated from the Penman equation. The resulting data are transmitted once per day, complete with station name, headings etc. The time intervals and microprocessor software can easily be changed and additional sensors can be added if required. The DCP transmitter is designed to include self-monitoring and test facilities, thus simplifying commissioning and fault diagnosis. A compact hand-held unit is available to synchronize the real-time clock in the DCP transmitter, and also to perform on-site testing of the DCP, sensors and batteries. The self-monitoring facility also allows measurement of battery voltage, clock accuracy etc from the user's base via the satellite. The reliable transmission of data by DCPs enables the detection of faults at the hydrometric station. Faults such as blocked intakes, fouling or sticking float tape may be diagnosed by inspection of each received message. This fault-finding capacity enables quick action to be taken which would not normally be taken until the regular site visit. The receiving system The receiver is designed on a modular basis giving a variety of possible user options in display and storage and contains two major assemblies, the antenna unit and receiver unit. The DCP data are transmitted via METEOSAT 1 to ESA at the Darmstadt ground station. After minor processing at ESA the data are retransmitted to METEOSAT 2 for relay to the user's own ground station receiver. Two satellites are necessary in this case since METEOSAT 2 does not have a DCP facility and METEOSAT 1 does not have a WEFAX facility. In a fully opera-

6 374 R.W.Herschy tional system one satellite would be adequate. The antenna unit consists of a 1.5m diameter dish and associated down converter. The latter amplifies the received signals, filters and down converts to VHF (from 1961 MHz to 133-9MHz). Cross-sdte transmission to the receiver unit is carried out at this frequency. The receiver unit consists of the following parts. a) The second down converter introduces a further conversion to an intermediate frequency of 10.7MHz. This section also provides automatic frequency control and signal strength indication. b) The demodulator and bit conditioner recover the baseband signal and process it to produce digital data and clock, signals. c) The format decoder detects the presence of retransmitted data in the incoming bit stream, strips of all format coding and forwards the derived DCP data for storage and processing. The DCP data are also routed to a high speed interface. d) The storage and processing section performs the essential processing and storage required to interface the incoming DCP data with a variety of output options. Buffer storage is provided to ensure that, with the worst case data rate, the output options function without loss of data. User facilities Certain options are available in modular subunits such as a mini-floppy disc unit which provides a mass storage medium for DCP data for up to 1,500 DÇP messages, a high speed printer which provides hard copy of received DCP data and a user terminal which enables full control of the receiver facilities and interfacing via an RS232 port. If METEOSAT imagery is required this can be accomplished by the addition of standard display and processing equipment. Weather images and DCP data can be viewed together. Mechanical configuration The antenna and down converter assembly are mounted on a 120mm OD rod with the down converter enclosure below it. The location is such that a 'line of sight' is available to METEOSAT 2. The dish antenna may, however, be up to 100m from the receiver unit. The receiver unit is housed in a small case which is located indoors in a position convenient to a power socket and to the printer or computer or other terminals. The front panel contains various status indicators for signal reception, a channel selection switch for use with other geostationary satellites, an alert indicator, a DCP data indicator and stored data and image data reception. The rear panel holds all the interface connectors for VHF input, high speed interface, printer output, terminal interface, floppy disc unit, video output, power input and alert message. Comparison with Existing Telemetry Systems A variety of telemetry networks is springing up in Europe based on landlines, line of sight radio or a mixture of both. This has inevitably led to local independent systems which often prove expensive because of the high installation and maintenance costs of relay stations, cables and hardware redundancy. It is expected that by the end of the decade these systems will become even more expensive to maintain with high redundancy. Such systems have often suffered from lack of reliability during severe weather conditions when the need for the data is greatest. Satellite systems on the other hand do not necessarily suffer from any of these disadvantages and because of the

7 Satellite data transmission S telemetry 375 largest potential international market are expected to become more cost effective with time. Conclusions The development by the European Space Agency of the DCP telemetry system now offers the hydrometric user a low cost method of hydrometric telemetry. The data can be received by the user within a period of two to six minutes of transmission by installing a 1.5m dish antenna receiver. A variety of options are available for recording, displaying or archiving the data. The system offers the following facilities: a) compact, low cost electronics package; b) direct interface to a wide variety of hydrometric sensors; c) versatile data acquisition and processing capability ideally suited to remote locations ; d) simple installation taking a few hours; e) wide operating temperature range; f) low power requirements - battery, solar power etc; g) built-in self monitoring; h) transmission over any distance, needs no repeaters and can transmit from bottom of valleys, between buildings etc; i) electronics package needing no routine maintenance; j) data transmission in WMO code, ASCII text, or 8-bit binary data formatting, with DCP name, heading etc; k) direct data reception by user's 1.5m diameter receiving antenna; 1) data return typically 95 to 100%; m) no licence required. The system will operate anywhere in the area covered by the European Space Agency's METEOSAT satellites and offers a cost effective alternative to terrestrial telemetry. Bibliography European Space Agency 19 81, Introduction to the METEOSAT System (Paris: ESA). Herschy, R.W., 1980, Hydrological data collection by satellite. Civil Eng. 68(1) pp Proc. Inst. Herschy, R.W., 1982, Towards a satellite-based hydrometric data collection system. In: Advances in Hydrometry (Proc. Exeter Symp., July 1982), pp Herschy, R.W., 1985, Collection of data using the METEOSAT DCP retransmission system. In: Hydrological Applications of Remote Sensing and Remote Data Transmission (Proc. Hamburg Symp., August 1983), IAHS Publ. no. 145.

8 376 R.W.Herschy Herschy, R.W., 1986, New Technology in Hydrometry, (Ed.). Bristol, UK. Adam Hilger,