March 21, 2003
www.aeronautics.ru
The
US military is using SINCGARS (Single Channel Ground
And Airborne Radio System) frequency-hopping
radios in the field. These radio sets are categorized
as Low Probability of Intercept (LPI) Frequency Hopping
Spread Spectrum (FHSS) transceivers. The FHSS method
is not new: it originated from the Second World War and,
simply stated, it employs a narrow band carrier, shifting
frequency in a pattern known only to the transmitter
and the receiver. The frequency can be changed several
hundred times per second.
The FHSS military radios are synchronized daily to use
the same frequency modulation algorithm. The synchronization
process occurs either through a direct physical connection
of the radio sets to each other or to a special device
known as the programmer. Some radios can also synchronize
frequency modulation algorithms via an encrypted transmission
of the frequency modulation algorithm in a non-frequency-hopping
mode, although this method is generally considered to be
less secure.
The military radios in the US armed forces commonly use
encryption and the frequency hopping methods provide an
additional layer of security during transmission of the
encrypted signal. An example of a frequency-hopping field
VHF/FM transceiver used by US Special Forces would be the
Caracal RPM 4740 manufactured by Thales Communications
of France.
The Caracal covers the 30 to 87.975 MHz frequency range.
It also has 10 programmable simplex or half-duplex channels
out of its repertoire of 2,320. Hopping in narrowband (6.4
MHz) and wideband (30 to 87.975 MHz) orthogonal modes,
Caracal contains high-grade internal digital encryption
and has an output of 1 W. Insertion of frequency and security
codes is accomplished using the MA 4073B programmer or
MA 4083B fill gun. A reset switch on each radio is used
to erase codes rapidly. The synchronization function is
broadcast, requiring about 6 seconds. Other features include
receive-only selective calling, frequency barring and `hailing'
by fixed-frequency radios when in the hopping mode.
However, security afforded by frequency-hopping methods
is very dependant on the strict adherence to protocols
for operating such radios. The US troops and other operators
of frequency-hopping radio sets frequently disregard these
protocols. An example would be an artillery unit passing
digital traffic in the frequency-hopping mode, which would
enable an unauthorized listener to determine the frequency-hopping
algorithm and eavesdrop on the transmission.
Even when proper protocols for using frequency-hopping
radios are being adhered to interception and decryption
of these signals is still possible. The frequency-hopping
interceptors are special advanced reconnaissance wideband
receivers capable of simultaneously tracking a large number
of frequency-hopping encrypted transmissions even in high
background noise environments.
An
example of such a reconnaissance device would be the
FH-1 frequency-hopping interceptor
manufactured by VIDEOTON-MECHLABOR Manufacturing and
Development Ltd of Hungary. The FH-1 frequency-hopping
interceptor
is a modern reconnaissance system based on parallel signal
processing technology. The equipment
has 160 independent receiving channels covering a 4 MHz
wide IF band with 25 kHz channel spacing, 60 dB
channel selection and 60 dB intermodulation suppression.
The 4 MHz wide IF band is the IF output of a special high-speed
front-end receiver which has a 20 to 1,000 MHz frequency
range.
The
digitized output signals of the channels are multiplexed
and fed as 1 Mbits/s data to a fast dedicated
signal-processing computer. As the processing time of
the 160 channels is 200 µs with the front-end receiver
4 MHz frequency setting time, the processing speed of this
interceptor is 4 MHz/200 µs or 20 GHz/s. This high
speed makes it possible to process the complete 30 to 80
MHz ground-to-ground VHF band within a 2.5 ms time slot. The system's processing algorithm filters out noise spikes
and stationary transmissions and in this way hopping transmissions
can be classified either in the traditional frequency versus
amplitude mode or in a waterfall-like frequency versus
time display mode. Optional software modules are available
for direction-finding the FH transmission and for controlling
a remote follower/jammer.
Venik

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