Radio navigation system

A line-of-position navigation system for determining the position of a mobile station, comprising:

• a first transmitter located at a fixed station and responsive to a first local oscillator for propagating a continuous wave at a first frequency;
• a second local oscillator and a first receiver located at the mobile station, said second oscillator operating at a second frequency differing from that of the first oscillator by a frequency in the audio range, said receiver being responsive to the continuous wave and the output of the second oscillator to produce a first signal at said audio frequency;
• synthesizer means joined to said second oscillator for producing a second audio frequency signal;
• means including a phase shifter for connecting said second audio frequency signal as a first input to a phase meter;
• means for connecting said first audio frequency signal as a second input to the phase meter, said meter being operative to provide line-of-position information in response to the phase relationship between said first and second audio frequency signals;
• a second transmitter located at said mobile station;
• means for intermittently operating said second transmitter to propagate a signal at the frequency of said second oscillator;
• a second receiver located at said fixed station, said receiver being responsive to receipt of the output of said first oscillator and the signal propagated by the second transmitter to produce an audio frequency signal;
• means for connecting said audio frequency signal produced by the second receiver to said first transmitter to produce a modulated signal having a carrier at the frequency of said first oscillator, said transmitter being operative to propagate, together with said continuous wave, a sideband of the modulated signal, said receiver means at the mobile station being responsive to said sideband to produce a third audio frequency signal at said mobile station;
• means for connecting said third audio frequency signal to said phase shifter, said phase shifter being operative during the intermittent periods of operation of the second transmitter to compare the phase relationships of said second and third audio frequency signals and to adjust the phase of the first input to the phase meter in response to any change in phase relationship occurring between said periods.