Introducing the
Nicola System
When complete, the Nicola System will provide an early warning system
and communication for rescue use in the Gouffre Berger. Graham Naylor
outlines the system and describes progress to date.
This article apearred in CREG
J 33
Introduction :
Development of the Nicola System in the Isère region of France
(which incorporates the Vercors and Chartreuse caving areas) is being undertaken
to develop underground communication systems. This work is being performed
with the help of the ADRASEC 38 (an association of radio amateurs who provide
emergency communication services – equivalent to the British RAYNET) and
several British cavers (Paul Mackrill, Paul Rice and myself). The development
of communications systems is motivated by the requirements of the department’s
cave rescue team the SSSI. The tragic flooding of the Gouffre Berger in
1996 led to the loss of two lives, Torda Istvan and Nicola Dollimore. Nick
Perrin (Nicola’s husband) set up a fund to finance research into the development
of communication systems for use in caves and in particular to allow a
warning message to be given in the Gouffre Berger.
The importance of prompt communication of the status of a victim in
the early stages of a rescue has long since been recognised by French rescue
teams – especially in the Isere where a return to the surface can take
up to 10 hours in certain of the deeper systems. Following very fruitful
input from others working in the same area in Switzerland, Britain, the
USA and Canada, we have produced four cave radios using the original John
Hey SSB LF transmitter and receiver boards (Hey, 1995). The first two were
prototype devices with which we used to demonstrate the principle. A further
two were produced to rather higher standards by F6EGY (these
latter devices are currently kept in the CRS Alpes headquarters ready
for use by the emergency services). A simple bridge amplifier using two
TDA2006s drives a transformer similar to the one Rob Gill described recently
(Gill, 1998). The efficiency of the earth current technique has been demonstrated
at 87kHz vertically through over 500m of rock in the Gouffre Berger and
horizontally through over 900m of rock in the Dent de Crolles cave system
as Jean-Jacques of the ADRASEC 38 reported in a previous issue of the CREG
Journal (Fauchez, 1998a). The requirement of the Nicola system as defined
by the SSSI was reliable communications through over 500m of rock. Recently
a new generation System Nicola radio has been developed called system
Nicola MK II.
Techniques:
There are many conditions which give rise to a variability in a
communication link over such a distance:
1 – Achievement of good ground
connections
2 – Power coupled into the output.
3 – Nature of the rock strata between
the transmitter and the receiver
4 – Background noise.
Being in control of all of these elements is necessary for a reliable
link and is the subject of development of the “Nicola System”. I would
like to outline some of the techniques we use.
Ground Connections
Achievement of good ground connections is essential to maximise the signal
picked up at the receiver. Without going into any detail of the mechanism
of the transfer of power, we can be fairly confident that if we increase
the current injected into the ground we increase the received signal. Thus
reducing the resistance to ground in-creases the cou-pled signal. There
have been several articles on the resistance be-tween electrodes in a supposed
uniform media. Unfortunately our media is rarely particularly ho-mogeneous.
In practice good earth connections (resistance be-tween electrodes less
than 500?) can be achieved if stout pegs are knocked into wet mud on the
surface or underground electrical braid is buried in some mud or thrown
into a pool of water. Braid stuffed into a crack or a rock anchor give
quite poor contacts.
The requirement of making good connections even in dry passages has
called for a rethink of how we should make the contact. By coupling the
current into the ground through the distributed capacitance of the wire
itself rather than a point-like electrode (the bottleneck of current lines
in the vicinity of the electrode increases dramatically the resistance)
we can achieve very low resistive impedances. The reactive component of
the capacitance can be cancelled at the radio frequency by using a series
inductor.
This technique has been demonstrated to give improved and clear communications
through 500m of rock with capacitive contact to very dry rock. This was
achieved by using a tuned inductor half way down one antenna wire, though
an inductor on each antenna wire should in principle give better results.
The inductor should be very carefully constructed as it should be a relatively
large value (several milli-Henrys) but should have a low parasitic capacitance
so as to remain inductive at the frequencies used. We constructed such
an inductor by switching an array of RF inductors (up to 10mH, 1.6mH +
up to 10, 840?H + up to 10, 120?H). This technique has not, however, been
adopted for operations as the tuning of inductances remote from the communication
set is an onerous and unacceptable complication. Furthermore the resonant
voltages especially when we try and increase the output power are likely
to be rather high (around 10kV!) and potentially quite lethal. Further
to the letter in a recent CREG Journal on this safety issue – in a wet
environment such as a bathroom or a river cave – the highest safe voltage
produce by an electrical appliance is 12V! If we have several hundred volts
on our antenna wires, we have wet hands (likely in a cave) and are standing
up to our knees in water (not beyond the bounds of possibility in a cave)
we have a recipe for passing rather more than 25mA through the human body
(good chance of death). If we have dry gloves on but are kneeling down
with a torn over suit (bad!) but the ground is fortunately dry, then we
will probably not feel too much. Our communication set must be safe in
all circumstances and is designed to save lives not to kill people. We
can therefore not accept high voltages.
The alternative is to use low voltages and dramatically drop the direct
contact resistance. This we have achieved by using a 10 – 20m length of
electrical fence tape connected to the end of the antenna wires. This gives
a distributed contact to overcome the bottleneck effect (it is equivalent
to having many small earth contacts). It is important to use the type of
electrical fence as tape as this lies flat much better than the string
type fence wire. This is important to get the most number of contact points.
The contact can be improved by putting stones on the fence wire at various
locations or better still by kicking it into some wet mud (if available).
Of course the most stupendous contacts are still made if the whole length
can be thrown into a large pool or several smaller pools.
Power Coupling
In principle we can couple more power in to the output stage to get a better
contact by simply using a high power amplifier. There are however several
words of caution.
We have only limited power available from our portable batteries and
so we would like the efficiency of the amplifier to be as good as possible
(Fortunately we have a complementary supply of 13Ah lithium cells which
give a good autonomy). Class A amplifiers give little distortion but are
very thirsty on current. The TDA type amplifiers, are class AB, are convenient
to use but are quite lossy in that they can not use the full voltage swing
at the output.
We have experimented with switch mode amplifiers and have demonstrated
that it is possible to encode an SSB signal with a PWM wave. The start
of the pulse encodes the phase of the SSB wave while the pulse width gives
the amplitude. A series LC filter before the output transformer allows
the PWM signal from a full bridge MOSFET switching circuit to be reconverted
to the SSB signal. Unfortunately the circuit I built was not too linear
between amplitude and pulse width (I used the SGS chip 3525) which led
to a distorted modulation. Also at high power levels the inductor used
in the output filter should be quite large so as not to saturate introducing
distortion. The lesson here was that we might gain 3dB in efficiency and
put out an extra 10dB but if we are not careful we can loose at least 10dB
in intelligibility of the modulation and use up 7dB (i.e. five times) more
power from the batteries with no overall gain. This as we say requires
further work!
Similarly as we drive up the power we start getting feedback problems
in the transmitter. Based on our experience I can give the following tips.
With large antennas the large RF field can be picked up by the microphone
and feed through the AF amplifiers (the radio frequency is fairly low remember!).
The AF power load in the power stage can lead to AM AF on the power lines
that again gets in to microphone circuit and into the mixer again (so good
filtering between stages for AF as well as RF is required). Using a voltage
regulator for the microphone and mixer might be a good idea. The half voltage
reference level derived from a potential divider and a unity gain op-amp
in the John Hey circuit can feedback this ripple very efficiently into
the microphone circuit if the potential divider doesn’t have a capacitor.
Though the circuit diagram includes this capacitor the printed circuit
boards don’t! So, in short, use sound analogue circuit practices and watch
out for coupling (careful of those wiring looms! I have to admit my wiring
practices are a sad mess!)
Nature of the Rock
The nature of the rock can be very variable so 1000m of rock in one place
may be less absorbing than 300m of rock in another place. Shale beds and
poor limestone block the signal but good solid limestone will pass the
signal well.
This effect is clearly demonstrated during our trial in the Berger
cave. We were keen to establish a link to the underground camp at the Hall
of the Thirteen. Last year we had tried to communicate from a point vertically
above the underground station. Due to the shale beds within the Senonian
limestone that accumulate moisture there was no contact at allberger3.gif
.
On the 13th of August this year (see Fauchez, 1998b) we achieved very good
results on both 87kHz with the Nicola system and on 137kHz in both directions
to point 4 (on the map and cross-section)through about 600m of rock. Contact
was also made from another underground station to the Hall of the Thirteen
(with the Nicola system) but not from underground to the surface. Here
the distance is nearly 1km and distant storms were perturbing the surface
reception.
The conclusion is that being vertically above or through the shortest
distance of rock does not always give the best contact. For operations
we use a loop type receiver (e.g. Lowe receiver with loop or a Molefone)
to search on the surface for the best signal. The best location can be
anticipated from a knowledge of the geological relief (see geological cross-section)
but local conditions may be quite profound (a cutting, rock exposure, a
stream, a hollow).
Background Noise
The background noise can be very strong, especially with earth contacts.
The background noise comes from atmospheric disturbances (particularly
bad later in the day, in the summer and during storms). The Loran navigational
transmitter gives rise to periodic noise as does any nearby motorised equipment.
The farmers’ electrical fences can also give wide band LF noise spikes.
The issue of the modulation frequency is an important issue and though
137kHz is slightly less troubled by atmospheric noise, 87kHz gives better
transmission through more absorbing rocks. The noise problem is particularly
acute on the surface and tends to be rather attenuated underground leading
to the frequent situation that the underground team can hear the surface
fine, but the surface can not hear the underground team.
To overcome this we intend to develop digital communications (sending
BPSK and using the ability of a phasing receiver to detect phase). This
should ensure return communication in conditions when the surface is troubled
by static, though in extreme cases the surface team may disconnect the
antenna wires and take cover.
Tests in the Gouffre Berger
On the 23rd of August this year we made a descent of the Gouffre Berger
with the Devon Speleological society to try and make contact from the lower
half of the cave. This is something that is of critical importance during
rescues from this part of the cave as well as being of great interest to
parties exploring the cave. A radio contact from the lower part of the
cave allows an updated report of the weather conditions before an underground
team engages itself in the more aquatic sections. Such a system, had it
been available in 1996 would have saved lives. The provision of such a
facility is a major goal of the Nicola foundation.
A remarkably clear (in both directions) voice link was established
from the Grand Canyon to a path just above the village of Fournel. The
excitement was intense as this is the first time such a link has been established,
and with a telephone-like quality. The different links established are
shown in the diagrams. During the establishment of the first link to the
surface from the Grand Canyon, a two way link was first made with a surface
Molefone. At nearly 500m this is probably a record for a Molefone! The
surface Molefone received clearly the strong signal from the earth current
Nicola system in the Grand Canyon due to its efficient power coupling.
The underground earth current receiver could just hear the Molefone (though
weakly) due to the relatively noise free underground environment and the
efficiency of signal collection of the earth coupling.
berger1.gif
When the surface Molefone was replaced by an earth current Nicola system,
the speaker practically leapt out of the underground set, so loud was the
signal. The success of this link must be due to a large extent the low
resistance of the earth electrodes made underground using electric-fence
tape. The placing of these electrodes, though easy, is very critical and
requires a little practice in order to achieve less than 300W
between terminals. At this level of earth resistance the capacitive impedance
of the wires is negligible. Unfortunately the day of the tests the cave
was not rigged to the bottom, but as can be seen from the vertical section,
the thickness of limestone is no thicker at the bottom of the cave and
we are quite confident that it will work at –1000m. The system was left
installed with one prototype set underground (which gave a poorer but still
quite acceptable performance)berger4.gif.
The DSS were able to use the system for the duration of their exploration
of the system. One member was even able to talk to his wife in the UK thanks
to a cellular phone on the surface. This communication from the lower part
of the Berger to the UK has to be a remarkable first! Though we do not
yet have enough working sets of the Nicola System to allow such a provision
to be performed on a regular basis, but is hoped to be made available in
the near future with the production of our next generation devices.
Operational Issues
The devices we currently use operate at 87.15kHz USB and are compatible
with the Molefones owned by the rescue team in France, despite the slight
shift in frequency. The shift gives a shift in the tone of the voice and
a slight loss in intelligibility. Molefone-type devices using a loop aerial
are useful on the surface to allow the location of the strongest signal.
Due to variable surface geology and surface topological features, the signal
to noise ratio can vary dramatically. Once a good location has been found,
the earth current radio can be installed there.
This technique was used in order to establish the link with the lower
part of the Berger and the surface though in fact the point used was, in
fact, close to that identified from the geological map as the theoretically
best location. We also use a Lowe 150HF receiver (which operates down to
30kHz) on the surface to get the best surface reception for cases where
the signal is weak. The Lowe gives a very clear and crisp reception and
works very strongly with earth electrodes. It is superior to all our home
built apparatus.
Loop based transceivers are also used underground as a mobile contact
point which can follow a stretcher or move with a search party to a stationary
underground base station up to about 200m away straight line through rock
distance. The underground base station must be an earth current type to
allow the long distance communication to the surface and also to give a
strong surface signal which must compete with the surface noise.
A 3-position beacon has been produced which plugs in to the microphone
socket (and supplies itself with a few microamps from the PTT pull up resistor)
using the sound generator chip UM3561. Three distinct sounds can give three
pre-agreed messages such as: alignment, party found rescue over, flood
coming take cover for cases where reception is week and voice can not be
understood. The circuit uses a 7555 which automatically switches between
emission and reception every 5 seconds. This is used during alignment etc.
to allow another party to intercept it. A more portable loop suitable for
underground use and allowing reception during progression in the cave has
been made using house wiring flexible conduit. The conduit when twisted
forms a double loop, which can be carried over the shoulder but by untwisting,
can form a large single loop for transmission.
Future Developments
The development of a small pager, that is active during normal progression
in the cave, is still required in order to pass simple messages such as
“please get your radio out we want to talk”, or “change to plan B”. We
have demonstrated the operation of a simple pager using an earth current
emitter on the surface and a Rugby receiver type detector (very small)
carried in the pocket underground. This device had a limit of reception
at about 500m but was quite erratic due to the fact it detects sudden drops
in the carrier. Unfortunately by moving the ferrite rod around, spurious
sudden drops in the carrier are frequently detected. A triple orthogonal
ferrite rod should be tried.
We hope to re-layout the complete transceiver system on a single surface
mount board with power amplifier and transformers included. The board should
also allow the emission of the carrier alone which can be used as a call
sign to activate a buzzer at the receiver in order to ‘wake up’ the other
party. To this end the following components have been identified
as interesting: AD607, TDA7396 and the Newport transformer 1015. The board
should have output IQ facility to allow BPSK detection and emission using
a microcontroller chip on a separate board which should also allow interfacing
to a further link (e.g. VHF set, cellular phone, SWT etc.).
There are plenty of ideas and most of these have long been discussed
within the CREG Journal. Unfortunately there are not enough hours in the
day. What is clear, however, is that there is a very high demand here in
France for good underground communication systems.
References
Fauchez, Jean-Jacques (1998a) Notes from our French Correspondent, CREGJ
32, pp19-32.
Fauchez, Jean-Jacques (1998b) The Berger on 137kHz, CREGJ 34, p12.
Gill, Rob (1998) A Transformer for Earth Current Communications, CREGJ
32, pp15,16.
Hey, John (1995) The G3TDZ Cave Radio, CREGJ 22, pp12-16.
