system; this can take place at individual remote stations or at the receiving centre;
(b) Preprocessing of raw data into integrated time or area values, or an estimated value, for exam- ple evapotranspiration from various climate variables;
(c) Generation of information in real time by combining data at multiple time points;
(d) Temporary storage of real-time information necessary for decision-making.
Information that will be stored for a long time and used to provide standards or reference statistics (means, extremes, ranges) should be stored as a database in an information-processing system that is separate from the HDTS, but accessible as part of the overall operational process.
6.1.2 Choice of communication channels
A number of choices are available for the means of transmitting data (telemetry), including wired lines, radio links, public telecommunication lines, mobile telephone networks and satellite communi- cation links. The type of communication link and method selected has to be decided by taking the following points into consideration:
(a) The communication facilities available in general within a country or region;
(b) The amount of information to be transmitted;
(c) The operating requirements in terms of delay time from field to centre, and transmission speed and reliability;
(d) The economics and cost of the system.
Figure 6.1. Conceptual configuration of an HDTS Sensor
Sensor
Sensor Recorder
Recorder
Recorder
Remote telemetry stations Receiving centre
Operating console Printer
Power supply
Power supply
Power supply
Power supply
Information- dissemination systems Information-processing
systems Communication
equipment Communication
equipment
Communication equipment
Monitoring equipment
Telemetry equipmentTelemetry equipmentTelemetry equipment
Communication equipment
Wired communication Wireless
communication
Out of the scope of hydrometric data transmission system
Note: There are hydrometric sensor(s) that measure precipitation, water level, and so on.
Note: Data transmission through a relay station.
Note: Data transmission through wire.
Relay station
CHAPTER 6. REAL-TIME DATA TRANSMISSION AND MANAGEMENT 6-3
The principal features, advantages and disadvan- tages of different communication methods are outlined in the following paragraphs.
Telephone lines
Telephone lines provide a switched telephone network with dial pulse or multi-frequency signal- ling. The cost is usually based on a communication time-dependent charge rate, although in some areas, service providers have a fixed rate, which may be negotiable with the company concerned.
Analog and digital lines may be available. The transmission standards will conform to interna- tional or domestic standards and the communication quality is guaranteed.
The transmission speed on an analog line using communication modems is over 50 kilobits per second (kbps) if the line is of good quality.
The transmission speed on a digital line using communication modems is usually over 100 kbps. The traffic on the lines may drastically increase during critical times, for example during a major flood event, and problems with delay and interruption may ensue due to congestion.
Mobile phone connections
Mobile phone links provide a switched mobile telephone network with multi-frequency signal- ling. For data communication, the cost may not be dependent on the communication time, but on the amount of data (that is, the number of pack- ets). There are examples of fixed cost but with limitation to data communications. Both analog and digital systems are available, although the analog system is classified as first generation, and is usually inappropriate for data communications.
The digital system of the second generation allows data communication at 9 600 bits per second (bps) or more. Generations 2.5 and 3 of mobile commu- nications allow high-speed data communication at several hundreds of kbps. The service specifica- tions usually conform to international or domestic standards, and the communication quality is guaranteed.
Mobile phone services use both wireless base stations on land and satellites. The latter services use a switching system, including those in the packet-cost or fixed-cost system. As with conven- tional telephone services, traffic on the lines may drastically increase at critical times and problems of communication may result. Mobile phone services can also use the short message service (SMS) provision for the transmission and recep- tion of words and characters between points in a network.
Common-carrier leased lines
Common-carrier leased lines provide dedicated line service to a specific user or group of users.
The cost is usually fixed by negotiation with a service provider, and as for public service tele- phone lines can carry analog and digital signals.
Usually, their transmission performances conform to international or domestic standards, and the communication quality is guaranteed.
The transmission speed on an analog line using communication modems is over 50 kbps if the line is of good quality. Transmission speed on a digital line can be 50 kbps or over. Some provid- ers may have high-density, high-speed services at speeds up to several hundreds of megabits per second (Mbps). Since lines under this type of arrangement are for exclusive use, there should be no congestion problems caused by increase in traffic.
Internet connection
A constantly connected high-speed line service is available for exclusive use of data communication through the Internet. Depending on the transmis- sion routes to be used, these services can be provided by optical fibre cables, digital subscriber line (DSL) and cable television (CATV). The lines are constantly connected and the cost is usually fixed. The available communication protocol is limited to Internet protocol (IP). Service on the Internet is based on the “best effort” of the Internet service provider (ISP), and there may be problems of congestion at busy times. Another source of problems could be random outage by the ISP, or a breakdown in their own facilities over which the end-user has no control.
Privately owned lines
In this situation, lines can be installed and oper- ated by users themselves, similar to railway signalling or emergency services. The initial investment may be high, but operating costs are very low. Various transmission media are availa- ble, such as copper twisted-pair cables, coaxial and optical-fibre cables. There are various types of line terminal equipment available from simple communication modems to multiplex terminal equipment. It is possible to select a suitable range of communication speeds from several tens of bps to several giga bits per second (Gbps) by combining the mentioned communication media and line terminal equipment. The quality of performance on private links is the responsibility of the users themselves, and thus there is a need for appropriate internal staff and logistic resources to manage the operation, especially when the
system comes under pressure during critical periods.
Very high frequency and ultra high frequency (VHF and UHF) radio communication links
The effective use of these frequencies is available worldwide, under the control of the International Telecommunication Union (ITU). Frequency ranges are globally specified as follows:
VHF: 30–300 megahertz (MHz);
UHF: 300–3 000 MHz.
The allotted frequency ranges are different from country to country. The number of central frequen- cies (the number of channels) that can be used in a region or country is usually limited, with one central frequency (one channel) jointly used by a number of remote telemetry stations. To avoid interference, neighbouring systems cannot adopt the same frequency group and the selected frequency for communication has to comply with local regula- tions and laws. Interference and loss of signal cannot be avoided, but risks of disruption can be minimized by careful allocation of frequencies. VHF and UHF can support analog and digital systems.
Radio communication equipment controls the speed and capacity of transmission and recep- tion of signals. The transmission speed is usually 200–1 200 bps on an analog system used for voice communication using frequency shift keying (FSK).
The transmission speed on a digital system or analog system by use of minimum shift keying (MSK) or Gaussian MSK (GMSK) is usually 1 200–9 600 bps.
Multiplex radio communication links
The use of frequencies for these systems is available worldwide under the control of ITU. The available frequency range extends from UHF to super high frequency (SHF1: 300 MHz to 30 gigahertz(GHz)).
The allotted frequency range varies from country to country. These links are used for multichannel tele- phone communications and high-speed data communications and can support analog and digital signals. If links do not pass through exchanges, the communications are always connected.
In the case that exclusive radio communication links are used, the operating frequencies and output powers are stipulated by international standards and national regulations. Radio communications can operate over distances of several tens of kilome- tres. Relay stations may be needed for longer distances or in hilly areas. As the quality of radio communication depends on the external condi- tions, propagation tests should be made after designing the communication links.
Satellite communication links
The very small aperture terminal (VSAT) and the International Maritime Satellite Organization (INMARSAT) are typical satellite communication facilities used for data transmissions. There are also other satellite communications available from serv- ices internal to individual countries, for example national communications satellites. The digital circuits via VSAT can also provide voice conversa- tion. The transmission speed via VSAT varies depending on the class of service, but it is over 9 600 bps. The transmission speed on INMARSAT also varies depending on the class of service, but is over 200 bps. Problems can ensue as transmission signals may attenuate during heavy rains, causing loss of communication.
As remote telemetry stations are located at selected hydrometric observation points, they are thus distributed over a wide geographical area. Remote telemetry stations cannot always be located at opti- mum hydrological sites. Therefore, depending on the communication medium, relay or transfer stations may need to be provided in the system.
These geographical constraints should be consid- ered not only at the time of designing the network but also with respect to plans for its future development.
The locations of remote telemetry stations are determined by considering the distances from the receiving centre and the topography of the sites at the remote stations. The availability of existing communication lines and radio links, the radio propagation conditions (if radio links are chosen), the supply facilities from power sources and the access roads should also be considered as important factors for determining the locations. Examples of the conditions required for locations for the instal- lation of radio communications equipment when VHF or UHF radio communication links are used are as follows:
(a) The degree of interference from one radio station to another should be low enough not to prevent reliable communications;
(b) If two or more antennas are to be installed close to each other within the same site, the degree of mutual interference should be low, so as not to hamper communications;
if there is unavoidable interference, this can be minimized by, for example, increasing the distance between the antennae or by insert- ing filters;
(c) Radio communication stations for point-to- point communication should be located at topographical elevations that are as low as possible through comprehensive evaluation of the radio paths, communication links, terrain profiles and conditions of the locations; they
CHAPTER 6. REAL-TIME DATA TRANSMISSION AND MANAGEMENT 6-5
should never be located at high points, such as the tops of mountains, with the exception of relay stations;
(d) The equipment components and the radio link design should be appropriate in terms of frequency, transmission method, radio paths and terrain profiles;
(e) The choice of the height of the antenna post should be based on the results of radio propaga- tion tests.
6.1.3 Interrogation methods and
frequency
A more detailed diagram of the functionality of an HDTS is shown in Figure 6.2. For a flood
forecasting and warning system, an HDTS is normally used on a real-time basis. The HDTS will time mark all data observations, but there will always be a delay between the delivery of the data and their processing. Usually, sensors at remote telemetry stations measure hydrological variables continuously or at short intervals, for example every 10 minutes for water levels. The receiving centre decodes the data and performs verification and processing. An information-processing system provided at this stage will convert the data into a suitable time series, for example an hourly data scan. Therefore, these time characteristics and their allowable error range should be determined for the purposes of operations. There are a number of different methods for reporting data from
Figure 6.2. Functional block diagram of an HDTS
Sensor Recorder
Input interface Input control
Control logic Encoding Communication control
Data verification Invalid data Communication control
System control Communicaton interface
Receive Transmit
Transmit Receive
Communicaton interface Simulation of a remote station Self diagnosis display control
Site data recording/
storage
Manual control Transmission medium (including relay stations)
Decoding, acceptance and message checking
presentationData Request repetition
storage/retrievalData Data processing
NOTE: Blocks enclosed with dashed
lines are options to the basic system. Information-processing system Information-dissemination systems
Receiving centreRemote telemetry station Communication equipmentCommunication equipmentTelemetry equipmentMonitoring equipment
Hydrometric data transmission system (HDTS)
remote sites, as described in the following paragraphs:
(a) Cyclic polling: This is used when a number of remote telemetry stations can use jointly a single communication line or waveband. The remote stations are polled one after another, and after the last telemetry station in the group has been polled, the first station is polled again, thus making the polling cyclic, as illustrated in Figure 6.3. In general, the entire system should be polled at intervals based on the highest demand standard, but the system may also be divided into several groups by the degree of demand for intervals, each of which is cycli- cally polled independently.
(b) On-demand polling: This is a variant of cyclic polling in which there is a set time inter- val, for example one hour, between separate
polling cycles (see Figure 6.4). This method is appropriate for systems that use a means of communication based on time-dependent charging, for example telephone lines. It is also effective for remote telemetry stations that need to minimize power consumption, such as those using battery power. The method may be less appropriate in the case that there are many remote telemetry stations to be polled.
(c) Batch polling and sequential reporting:
This is an improved method that combines on-demand polling with batch reporting (see (e) below). The arrangement is illustrated diagrammatically in Figure 6.5 and has the following characteristics:
(i) The receiving centre polls all remote telemetry stations at a fixed time interval, similar to on-demand polling;
Figure 6.4. Diagram of on-demand polling Figure 6.3. Diagram of cyclic polling Receiving
centre
Remote telemetry station 1 station 2 station 3 station 4 Key
measuring
Time
M
M M
M M
M M
M M
M M
M M
Figure 6.3
Receiving centre
Remote telemetry station 1 station 2 station 3 station 4 Key
measuring
Time
M
Observation Observation time
time
M M
M M
M M
M M
Figure 6.4
CHAPTER 6. REAL-TIME DATA TRANSMISSION AND MANAGEMENT 6-7
(ii) Each remote telemetry station performs measurement when it is polled, and stores the data in its local memory;
(iii) Each remote telemetry station transmits the measured data to the receiving centre after a specified waiting time (Wn); the Wn should be specified as a different time constant for each station to prevent jamming.
As mentioned previously, on-demand polling has the disadvantage that it may involve different measuring times at individual stations. The batch reporting method (see (e) below), however, needs a highly precise clock at each station. The method of batch polling and sequential reporting conducts measurements at all the stations at a set time when the receiving centre makes a simulta- neous call to all the stations. Therefore, the measuring timing is the same at all stations, and there is no need of a precise timing at each station.
(d) Continuous transmission: This can be carried out when a remote telemetry station has exclu- sive use of a communication line between the station and the receiving centre. This method allows the remote telemetry station to continu- ously and sequentially transmit the measured data to the receiving centre, this being in reality a sequence of transfers separated by a minimal time interval. The receiving centre, having access to all data, can then have a considerable degree of freedom in setting the interval for sampling the data, or conducting some preprocessing.
(e) Batch reporting: This is a method by which the data are stored at several remote telemetry
stations for a certain period of time and then transmitted as a batch of data to the receiving centre (see Figure 6.6). Transmission of data is made by polling from the receiving centre or by an automatic reporting function as part of the remote telemetry station functionality.
This method is effective for telemetry systems that cannot make communications over certain periods, as is the case with transmission via polar-orbit satellites.
Since the method involves the taking of measurements independently at each remote telemetry station, a highly precise clock is needed at each station to conduct punctual measurements and time transmissions. Each measuring station needs to be equipped with a storage facility device to retain measured data prior to transmission. This system is only viable on major river basins, where time of travel of flood peaks takes place over periods of days, rather than hours.
(f) Event reporting: This is a method involving the automatic transmission of data, by individ- ual remote telemetry stations to the receiving centre, when the stations’ sensors detect a precipitation or water level value exceeding the preset upper (or lower) threshold (see Figure 6.7). This method effectively allows the receiving centre to serve as an automatic alert, with the immediate detection if critical events.
However, when data are not transmitted to the receiving centre for a long period it is difficult to identify whether there is no event or if there is a system problem. Therefore, this method should be combined with a facility to auto- matically transmit a check signal at regular intervals to establish that the station and the transmission link are still working.
Figure 6.5. Diagram of the batch polling and sequential reporting method
Receiving centre
Remote telemetry station 1 station 2 station 3 station 4
Key
measuring waiting time
Time
M
M M M
M M
M M M Observation
time Observation
time
W1
W2 W3
W4
W1/W4
W1 W2
W3 W4
6.1.4 Reliability
The environmental conditions at remote telemetry stations can be quite harsh, especially in tropical, desert and mountainous areas. Special considera- tion needs to be given when using equipment outside the ranges of conditions for which they were designed. Therefore, the following conditions should be considered:
(a) Atmospheric temperature range and rate of change;
(b) Relative atmospheric humidity range;
(c) Wind velocity;
(d) Atmospheric salinity and dust;
(e) Environmental conditions inside equipment housing;
(f) Available power supply conditions (including protection against current surge due to lightning);
(g) Potential for flood damage and access during flooding;
(h) Seismic resistance.
Most equipment will include information on the manufacturer’s recommendations for operating ranges, and these should be carefully checked against expected field conditions. Manufacturers also provide information on the resilience of hous- ing to dust, moisture and salinity. The environmental conditions within the receiving centre also need consideration, as air conditioning and enhanced dust and moisture control may be needed. Other
Figure 6.7. Diagram of the event reporting method Receiving
centre
Remote telemetry station 1 station 2 station 3 station 4
Key
Time
Event
Event
Event Event Event
measuring
Event M
M M M
M M
M
Figure 6.7
Figure 6.6. Diagram of the batch reporting method Receiving
centre
Remote telemetry station 1
Observation
time Observation
time
station 2 station 3 station 4 Key
Time
measuring
M M
M M
M M
M M
M
Figure 3.2