Reasons for telemetering data

Telemetry is the transmission of data from one point to another. If data are needed in real time they must be telemetered, for example for weather forecasting and flood warning. Telemetry also has two significant advantages over in situ data logging, even if the measurements are not required in real time: the cost of visiting field sites to collect data is saved and the failure of field stations can be detected — months of data could be lost if a logging station failed soon after a visit. Logging is best suited to applications where stations are within relatively easy access or where the loss of some data is not a serious problem.

The general process of telemetering data is sometimes referred to generically as system control and data acquisition (SCADA), although the term applies more strictly to management applications — where not only are data acquired from a remote location but remote control is also exercised back. A dam managed from a distant control-room, for example, is a more appropriate use of the term SCADA than is the one-way collection of environmental data.

The structure of a telemetry system

Figure 12.1 is a schematic of a telemetry system, showing its main subdivisions into sensors, logger, modem, communications link and a PC at the base station. This basic arrangement is similar for all telemetry systems although it will differ in detail, mostly depending on the communications link used. A telemetry system is in effect a logging station with a communications link appended and with a remote base station to receive the transmitted data. The front end of such a field station is composed of sensors, identical to those used at a logging station, and a unit that performs the same functions as a logger, even if it is not called such – interfacing, multiplexing, analogue-to-digital conversion and memory. Previous chapters have been concerned with the sensors and with logging; all that needs to be addressed in this chapter is the communications link.