] 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.

And then there is GNU Radio..

War in Iraq - How US military radio transmissions are intercepted