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The range is determined by the wavelength:
" Very long and long waves (3 kHz  300 kHz) propagate only via ground waves, because
space waves are not reflected. They have very short ranges.
" Medium waves (300 kHz  3 MHz) propagate via ground waves and at night also via
space waves. They are medium-range radio waves.
" Short waves (3 MHz  30 MHz) propagate primarily via ground waves; multiple
reflections make worldwide reception possible.
" Ultra-short waves (30 MHz  300 MHz) propagate only via ground waves, because space
waves are not reflected. They propagate in a relatively straight line, like light, with the
result that, because of the curvature of the earth, their range is determined by the height of
the transmitting and receiving antennae. Depending on power, they have ranges of up to
100 km (roughly 30 km in the case of mobile phones).
" Decimetre and centimetre waves (30 MHz  30 GHz) propagate in a manner even more
akin to light than ultra-short waves. They are easy to focus, clearing the way for low-
power, unidirectional transmissions (ground-based microwave radio links). They can only
be received by antennae situated almost or exactly in line-of-sight.
1
U. Freyer, Message transmission technology, Hanser Verlag, 2000.
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Long and medium waves are used only for radio transmitters, radio beacons, etc. Short wave
and above all, USW and decimetre/centimetre waves are used for military and civil radio
communications.
The details outlined above show that a global communications interception system can only
intercept short-wave radio transmissions. In the case of all other types of radio transmission,
the interception station must be situated within a 100 km radius (e.g. on a ship, in an
embassy).
The practical implication for the ECHELON States is that they can intercept only a very
limited proportion of radio communications.
3.3.1.3. Communications transmitted by geostationary telecommunications satellites1
As already referred to above, decimetre and centimetre waves can very easily be focused to
form microwave radio links. If a microwave radio link is set up transmitting to a
telecommunications satellite in a high, geostationary orbit and the satellite receives the
microwave signals, converts them and transmits them back to earth, large distances can be
covered without the use of cables. The range of such a link is essentially restricted only by the
fact that the satellite can receive and transmit only in a straight line. For that reason, several
satellites are employed to provide worldwide coverage (for more details, see Chapter 4). If
ECHELON States operate listening stations in the relevant regions of the earth, in principle
they can intercept all telephone, fax and data traffic transmitted via such satellites.
3.3.1.4. Scope for interception from aircraft and ships
It has long been known that special AWACS aircraft are used for the purpose of locating
other aircraft over long distances. The radar equipment in these aircraft works in conjunction
with a detection system, designed to identify specific objectives, which can locate forms of
electronic radiation, classify them and correlate them with radar sightings .They have no
separate SIGINT capability2. In contrast, the slow-flying EP-3 spy plane used by the US Navy
has the capability to intercept microwave, USW and short-wave transmissions. The signals
are analysed directly on board and the aircraft is used solely for military purposes3.
In addition, surface ships, and in coastal regions, submarines are used to intercept military
radio transmissions4.
1
Hans Dodel, Satellite communications, H�thig Verlag, 1999.
2
Letter from the Minister of State in the Federal Defence Ministry, Walter Kolbow, of 14 February 2001.
3
S�ddeutsche Zeitung No 80, 5.4.2001, p. 6.
4
Jeffrey T. Richelson, The U.S. Intelligence Community, Ballinger, New York, 1989, p. 188, p. 190.
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3.3.1.5. The scope for interception by spy satellites
Provided they are not focused through the use of appropriate antennae, radio waves radiate in
all directions, i.e. also into space. Low-orbit Signals Intelligence Satellites can only lock on to
the target transmitter for a few minutes in each orbit. In densely populated, highly [ Pobierz całość w formacie PDF ]