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Article written by Vasant Dave & Nitin Deodhar published in the TELEMATICS INDIA, May 1998.

Tail Circuits in Satellite Telemetry

Many remote sites do not have adequate information-load to justify investment on their own satellite terminals. If they happen to be within 30 KM radius of the nodal office where a satellite terminal exists, they can be cost-effectively inducted into the satellite-based telemetry network using a tail circuit.

Tail circuits have proven to be the proverbial ‘elephant’s tail’. That is because the bulk of the satellite-based telemetry network is installed successfully in time. However we cannot commission it due to ‘last mile’ problems that arise in the final connectivity to the distant RTU.

Wireless tail circuits

Normally, we can establish wireless communication upto a distance of 30 KM, e.g. if the radio antennae at the remote site and the nodal office can virtually ‘see’ each other without any obstruction between them.

Data communication over wireless is possible in higher VHF (140-175 MHz), UHF (400-500 MHz, 920-960 MHz) and spread spectrum (2.4 GHz, 5.7 GHz). VHF / UHF bands can accommodate data speeds upto 9600 bps which are adequate for telemetry applications. Often the radio and the modem are specially designed for each other, integrated, tuned and housed in a common chassis. They admirably suit the tail circuit requirements of satellite-based telemetry.

VHF / UHF Radiomodems

The ability of the satellite-based telemetry network to monitor and control its various elements is vital to ensure their proper working and to locate faults as and when they occur. The VHF / UHF radiomodems having individual as well as group addressability are able to respond to specific interrogation from the satellite hub station. The provision of an auxiliary port in the radiomodem helps to monitor and control (e.g. switch ON or OFF) remotely located site-equipments from the satellite hub station. It also enables to exert local control and monitor health and status of RTUs and sensors.

Automatic remote control calls for extremely high fidelity in data communication. In power and steel projects, the radio environment contains heavy magnetic disturbances. These cause frequent corruption of data transmitted over wireless. VHF / UHF radiomodems which fight this inevitability are designed with three specific features:

• The radio portion works on Differential Gaussian Minimum Shift Keying frequency modulation. It resists impulse and electrical noise at reasonably high signal levels.

• The modem portion works on packet-switching principle. Data is arranged in packets and transmitted. Hence a sporadic disturbance only affects few packets rather than upsetting the entire train of data.

• Subsequently the receiving radiomodem checks each packet for any error using advanced CRC-based technique. An ACK signal is returned for each packet that is received without any error. The transmitting radiomodem retransmits selectively those packets for which it does not receive an ACK. Such packets get inserted in their original sequence by the receiving radiomodem.

In the event of radio signal strength fading to low levels, those VHF / UHF radiomodems having Forward Error Correction feature can well maintain the integrity of data.

Normally, a point-to-point link comes to mind when a tail circuit is discussed. If the satellite terminal at the nodal office has several remote sites dispersed within line-of-sight, the network designer might prefer to make it the hub of a point-to-multipoint radio network. If VHF / UHF radiomodems can function on carrier sense multiple access, it becomes simple to configure point-to-multipoint radio system independent of the user hardware.

While satellite terminal-to-terminal communication takes care of data encryption, we need to incorporate it separately for the tail circuit. To meet this requirement, VHF / UHF radiomodems with built-in data privacy using say a 128 bit key may be selected.

In certain telemetry applications, several DTEs such as PC, a measuring instrument, and a GPS all located at the remote site may be required to be connected to the radiomodems. Integrated radiomodems with multiple facilities are available with RS 232C interface. The inputs from upto 5 DTEs get identified, packetised, multiplexed and transmitted on a single radio channel to a similar radiomodem located at the distant satellite station. The receiving radiomodem demultiplexes and routes the data to the related DTEs.

In a rare case where telemetry data exceeds 9600 bps speed, there should be a buffer memory within the radiomodem. What it does is to store the additional information upto say 30,000 bits and then requests DTE e.g. the RTU connected to it to hold. As the radiomodem goes on transmitting the stored information, it instructs the DTE to release further data. Automatic flow control ensures smooth data transmission during heavy traffic.

Normally for telemetry applications, the satellite terminals would be TDM-TDMA type where the outgoing traffic from the satellite terminal is transmitted in TDMA mode (in the form of packets separated in the time domain). These satellite terminals have indoor electronics that can support multiple data communication ports. The terminal in the above application can be equipped with as many data ports as there are tail-end circuits. Appropriate protocol spoofing and flow control would be required to connect VHF / UHF radiomodems with data ports of the satellite terminals.

© Vasant Davé

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