How to intercept Mil Radio Communications

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Posted by andreas from ( on Sunday, March 23, 2003 at 4:38PM :

In Reply to: RussIntelNews: Iraq 03-03-22 posted by andreas from ( on Sunday, March 23, 2003 at 4:27PM :

How military radio communications are intercepted?

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 Ás with the front-end receiver 4 MHz frequency setting time, the processing speed of this interceptor is 4 MHz/200 Ás 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.

-- andreas
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