Fixed monitoring facilities

The primary role of the Terrestrial Monitoring Section (TMS) is to provide a service for the clearance of interference to UK licensed users of the radio spectrum. Within this work, a significant proportion of calls for assistance concern the safety of life services. The section also makes a major contribution to the International Monitoring System of the ITU by offering reciprocal monitoring information to overseas administrations, particularly for the investigation and resolution of interference to international radio circuits at high frequencies (HF).

The HF part of the radio spectrum has a number of advantages for users. It permits propagation over short or extremely long distances; equipment is relatively cheap and the services provided are flexible. On the other hand, the propagation medium is extremely variable and the band of frequencies available for use for a particular communication circuit varies with the time of day, season and the epoch of the sunspot cycle. Signals that propagate well during daylight hours will not be heard at night and vice versa. In order to be able to resolve interference complaints the emission source has to be identified as quickly as possible and TMS is therefore manned 24 hours a day, 365 days a year.

The section provides the Agency's 24-hour focal point of contact for out-of-hours interference reports to all other UK radio services and, depending on the priority, the shift engineer will pass on the safety of life reports to staff throughout the United Kingdom around the clock.

Clearance of international interference will also involve liaison with other government departments, foreign administrations and monitoring stations. The work of the section therefore is international. Routine monitoring of the HF frequency bands is carried out in support of the Agency's spectrum reviews and also to provide occupancy information for planning purposes and to meet the Agency's obligations to the ITU and other international fora.

Although the section was originally set up to provide monitoring for HF frequencies, over the years VHF and UHF monitoring facilities have been added and over the next five years a comprehensive network of remote monitoring and direction finding sites will be installed throughout the United Kingdom.

These new facilities will enable the rapid elimination of interference at V/UHF as well as providing a means for assessment of the loading of channels to aid the accurate and efficient allocation of spectrum to users.

Frequency coverage

The frequency range covered by work in TMS is currently 9 kHz to 1 GHz; the frequency measurement range on amplitude modulated and plain carrier signals, to an accuracy of ±1 Hz, is from 9 kHz to 30 MHz. For frequency modulated signals the measured results are given to the nearest 10 Hz over the same frequency range. Measurements between 30 MHz and 1 GHz can also be made with an accuracy dependant on the modulation mode of the emission.

Field strength measurements

Field strength measurements using inverted cone antennas can also be made on any type of signal in the range 9 kHz to 30 MHz. The measured accuracy is ±1dB in the VLF and LF frequency ranges and ±2dB for frequencies up to 30 MHz. The antennas used for these measurements have been built to an ITU-R design, providing predictable characteristics. Correction curves are available to enable true field strength measurements to be made.

Direction-finding

Direction-finding for the lower frequency bands is also available in two ranges; 100 kHz to 2 MHz using a microprocessor-controlled Adcock array and from 2 MHz to 30 MHz a wide aperture monopole antenna array in conjunction with digital signal processing techniques and "music" algorithms. For the future, the Agency nation-wide V/UHF direction-finding and monitoring system will also be available. This comprehensive facility will enable the shift engineers to access a number of sites around the country to more closely pinpoint the origin of interfering signals up to 3 GHz.

Automatic spectrum occupancy

Spectrum occupancy information in the frequency range 100 kHz to 1.5 GHz is produced automatically using three dimensional "waterfall" plots in either A3 or A4 format and printed in four colours. Signals can be displayed as max-held to show all emissions occurring during the scan period - essentially aimed at emphasising short term events. Alternatively, the display can be programmed to show video average, which will emphasise the long-term signals. This latter facility is particularly useful in analysing signals over relatively long periods of up to seven days. The spectrum analysers are fully computer controlled and the occupancy plots can be individually tailored to the requirements of the signal environment and other emission parameters.

Demodulation

Demodulation facilities available cover almost all types of emissions found in the HF bands; specialist equipment is used, together with associated software. Modes covered include AM, FM, PM, CW, and SSB/DSB/ISB. Systems covered include RTTY (FEC, ARQ, etc.), Facsimile, Morse, Piccolo and numerous data modems.