Over the years, I have collected various anecdotal information concerning the reception of long-wave broadcast signal underground. Cavers have reported that, whilst their induction radios have failed to work in deep caves, they have been able to pick up BBC Radio 4 (on 198 kHz) at underground camps. This highlights the fact that direct through-the-earth communications can be extremely limited in scope and usually has to use a low frequency and a large antenna system. Even mine communications systems using kilometre-wide loops and tens of kilowatts of power fail (in accordance with theory) to achieve good communications depths and, of course, are not straightforward to deploy in an emergency.
Although penetration of the earth at 198 kHz should be, in theory, well below the level required for communications, there are various possible physical mechanisms to explain the phenomenon reported by cavers besides the more straightforward (and unfeasible) one of penetration from the surface. In addition to favourable propagation along sub-surface strata there is also the point that a very powerful radiating transmitter at a long distance may well give rise to a stronger signal than a less powerful non-radiating local induction loop. In mining it has been shown that propagation is also favoured by the metallic infrastructure of the mine.
The question therefore arises of how a 1 MW broadcast transmitter could be utilised for sub-surface communications - one-way communications, of course. The infrastructure to do this is already in place because, although it is not well-known, long-wave broadcast transmissions already have the capability to carry 'radio data system' (RDS) signals. On VHF, this is the information that tells you which FM radio station you are tuned to. A similar system for long-wave broadcasts was originally developed by the BBC to carry, amongst other data, control signals for industrial use, e.g. for switching your electricity meter to the night-time 'Economy 7' tariff; a service now known as radio tele-switching [1, 2]. I noted, some time ago, that this system had possibilities for sub-surface communications [3], and if a rescue organisation could lease data packets on this system it could provide a simple method of providing emergency communications from the surface to trapped cavers or, more likely, miners.
This signalling system has been in existence for a long time, and at least two articles appeared in the hobbyist electronics press some 20 years ago. [4, 5]. The system is known, these days, as AMSS (AM signalling system) and was standardised in 2006 by ETSI as an extension to the Digital Radio Mondiale (DRM) system. Essentially, the data is bi-phase modulated onto a 25 Hz sub-carrier carrier, which then phase-modulates the 198 kHz carrier at +/- 22.5 degrees. The combination of low data-rate and low angle of modulation ensures that there is no interference with the audio signal. There are 30 blocks of 50 bits of data that are repeated every minute. The last block before the minute contains date and time information and all blocks include a CRC word. Because the carrier is at only 198 kHz, the demodulation and decoding task can easily be carried out in a microprocessor. A summary of AMSS is given in Wikipedia [6] and includes links to some useful reference material [7, 8]. Additionally, the Wikipedia page on the BBC Droitwich transmitter [9] includes a link to a specification [10].
I currently work for the UK's Mines Rescue Service Ltd (MRSL), in its research and consultancy division. MRSL was set up, by statute, to provide emergency services to the mines in the UK and we are working on a project funded by the European Commission's Research Fund for Coal and Steel to study 'Mine Emergency Support Technologies'. One aspect of the project covers communications and, in particular, how to provide resilient communications that will survive a major incident in the mine. I intend to make some further (and more accurate) measurements of the BBC Radio 4 signal strength underground, although there is a difficulty in getting the necessary equipment approved for use underground in a hazardous atmosphere. If the signal strength is favourable, AMSS could be a useful facility, provided there is spare capacity and the commercial structure is in place to allow the leasing of this data capacity.
David Gibson, 1 September 2010. (Minor editing subsequently)
- http://en.wikipedia.org/wiki/Radio_teleswitch
- http://www.energynetworks.org/rts
- Gibson, D. (1996). An Introduction to Data Transmission - Part 2. CREG Journal 26, pp22-23,27. http://creg.org.uk
- GEC Plessey Semiconductors, Low-cost 198kHz Radio Data Receiver, Application note AN86. Reviewed in Electronics World 99 (1692), pp960-1. November 1993
- Parnall, S.J. (1989), Decoding RDS, Electronics & Wireless World 95(1636), pp148-152, February 1989.
- http://en.wikipedia.org/wiki/Amplitude_ ... ing_system
- Murphy, A., Poole, R. (2006). The AM Signalling System: AMSS - does your radio know what it's listening to? EBU technical review. http://www.ebu.ch/en/technical/trev/trev_305-murphy.pdf
- Cornell, L. (2007). The AM Signalling System (AMSS). Broadcast Papers. January 29, 2007. http://www.broadcastpapers.com/whitepap ... ss2006.pdf
- http://en.wikipedia.org/wiki/Droitwich_AM_transmitter
- Specification of BBC phase-modulated transmissions on long-wave. http://downloads.bbc.co.uk/rd/pubs/reports/1984-19.pdf