A frequency memory system having a smart noise component is receptive of an RF burst signal of a given frequency and of relatively short duration for determining the frequency and storing a signal of amplitude indicative of the given frequency and for producing by means of a voltage controlled oscillator (VCO) a signal at the given frequency of relatively long duration. The stored signal and a signal for amplitude corresponding to the actual frequency produced by VCO are utilized to produce an error signal to correct any error in the frequency of the signal produced by the VCO relative to the given frequency. Since the VCO is not capable of immediately accurately tracking the input frequency, a smart noise generator produces a signal of random amplitude which is added to the error signal to cause the VCO to produce at least a frequency component of its output signal which is equal to that of the input RF burst signal.

1. A method for jamming frequency-shift-keyed radio teletype signals which omprises: receiving said frequency-shift keyed signals; converting said signals to teletype current pulses; modification of selected ones of said teletype pulses; converting said modified teletype current pulses to frequency shifted signals; and transmitting said converted modified pulses.

During a distress call, a distress location transmitter 10 generates a high frequency carrier signal 40 that is modulated by a predetermined distress waveform characteristic 29. The classification of user associated with the distress call is identified by periodically interrupting modulation 42; user classification is determined by the repetition rate of the interruptions, the interruption periods, or both.

A jammer transmitter having a comb spectrum generator which includes a pulse generator adapted to produce a pulse train at a frequency corresponding to the channel spacing of signals to be jammed, and a bandpass filter which has a passband including a bandwidth occupied by the signals to be jammed and which is responsive to the pulse train for providing within this bandwidth comb generator output signals at harmonics of the pulse train frequency, a power amplifier operative to amplify the comb generator output signals so as to produce jamming pulses and an aerial for radiating the jamming pulses, the width of the pulses of the pulse train being short enough to ensure that the harmonics of the pulse train frequency occur throughout the bandwidth.

The object of the call masking system for mobile telephones is to prevent the operation of mobile telephones which are present in a closed environment without interfering with the normal operation of other telephones which are outside the closed environment. To this effect, the system generates interference signals which will prevent the mobile telephones from receiving the signaling channels sent by the base stations (for example, GSM call signaling channel). The system monitors the power with which the carriers of the neighboring base stations are received inside the room and will adjust the interference power radiating in the signaling channels so that the carrier to interference signal ratio will make the operation of the mobile telephones impossible. The system can be applied to closed environments wherein mobile telephones should not ring when receiving a call.