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TAGGING your opponent’s weapons certainly takes some of the glamour
out of espionage. But this military equivalent of putting a bell on the
cat could make lighter work of verifying treaties that limit fire power.
Tagging means marking an object – a missile or canister, a tank, a plane,
an armoured vehicle, even a soldier – in order to follow its movements.

The military reductions being discussed by the Warsaw Pact and NATO
would require a massive accounting effort to enforce. They would cut troops
on each side to 275 000, tanks to 20 000, armoured vehicles to 28 000, artillery
pieces to either 16 500 or 24 000 (depending on whose definition of an artillery
piece is accepted), and aircraft and helicopters to 15 per cent below current
levels. The reduction on each side varies because the size of the arsenals
differs as well as definitions of equipment. The agreement would apply to
forces located between the Atlantic and the Urals. But negotiators may also
set tighter geographical limits on where soldiers and material can be at
particular times, which would make verification much more complicated.

To succeed, tags must possess three ingredients, says Steve Fetter,
a tagging expert at the University of Maryland near Washington DC. First,
the number of tags should equal the number of allowed weapons set by treaty.
Secondly, an individual tag should be matched to a unique weapon, like a
serial number on a car’s engine block. Finally, tags should not be able
to be copied by those who are being monitored.

Tagging techniques range from the exotic to the mundane. There are biological
tags, such as an antibody created by a rabbit to a toxin. Each antibody
would be unique and could be attached to a weapon to label it. Another method
being studied would place a piece of DNA on a tag. The application of a
chemical would turn the tag a certain colour.

More likely is a method of spreading a sparkling epoxy glue over a weapon’s
identifying mark: the epoxy sparkles because it contains metallic flakes.
Each ‘painting’ would possess a unique pattern of flakes that could be photographed
from several angles. This key file of transparencies would be kept by treaty
enforcers, perhaps in a computer’s memory. Each time a field inspection
is made, a new transparency of the tag on a vehicle or plane (soldiers,
presumably, would not be spray-painted) is compared with the key file.

The simplest tag involves recording surface roughness of a patch of
a weapon. Roughness can be measured with a diamond stylus to record a unique
signature of undulations with a resolution of one micrometre. For extra
protection, the area to be measured could be the bottom of a hole drilled
into a metallic surface and covered with a plate. ¿ìè¶ÌÊÓƵs at government
laboratories are also studying the use of a scanning electron microscope
to identify the unique structure of a tiny portion of the surface of a missile
or vehicle.

Although dozens of ideas have been tested, says Fetter, the political
realities of a treaty will finally determine what kinds of tags could be
used. For example, electronic tags that emit continuous signals detectable
by satellites would provide ‘real-time’ tagging. These would broadcast the
whereabouts of every tagged object all the time.

A less intrusive, and thus more palatable, alternative might consist
of a receiver and programmable chip that would intermittently receive and
record signals from navigational satellites, such as the US Navy’s NAVSTAR
satellites. For example, the armed forces and civilian sailors use signals
from NAVSTAR to pinpoint their locations anywhere on the globe. In a tag,
the receiver fixes its geographical position using a signal from NAVSTAR
and records it on the microchip at various times. A subsequent inspection
would reveal where a tank has been recently. Receivers now on the drawing
boards could fit in a shirt pocket. ‘You could also check on your kids with
one of these,’ noted Fetter.

Monitoring tags remotely, from satellites or planes, saves effort but
will be more difficult to sell to the admirals, air marshals and generals,
who don’t want the location of their equipment known during a military crisis.
A compromise would consist of a tag similar to the infrared transponders
in television sets that respond to remote controls. The inspector holds
a transceiver that emits a signal that the tag can receive and respond to
without requiring a power source in the tag. Moreover, inspectors need not
get closer to the weapons than a few metres, which pleases the military
chiefs, says Fetter.

Taggers get especially excited about designing ‘black aspects’. These
are devices in tags that will indicate whether they have been tampered with
or reproduced. To resist tampering, for example, two tags on a tank could
communicate electronically with each other; if one is moved, the contact
is broken and the break recorded. For example, a pair of tags could measure
capacitance, essentially the voltage potential between the two when the
capacitor discharges. Or an ultrasonic tag could regularly monitor the resonant
frequency of the object it is attached to. Other methods rely on unstable
elements in glue that will deteriorate if the tag is tampered with.

To catch a fake tag, designers have considered marking a microscopic
part of each tag with a unique isotope. Lead, for example, possesses several
isotopes so close in atomic mass that a microscopic piece of metal with
an isotopic label would go unnoticed unless you knew what to look for. Ultrasonic
scanning could also catch an attempt to cut the tag from its mounting and
transfer it to another missile or vehicle. A scan would reveal the cuts
and marks made by the removal.

The negotiators of a conventional arms treaty will also wrestle with
how to monitor stored weapons. One possibility under study would place delicate
seismic, magnetic and infrared sensors near these outposts, or at ‘choke
points’, such as major roads or railroad junctions near storage depots.
Developed for the war in Vietnam, these can detect the difference between
a truck full of vegetables and a tank, and could switch on cameras should
they sense unusual traffic. Christopher Joyce