VLF radio position location system

• US 4,087,816 A
• Filed: 10/22/1976
• Issued: 05/02/1978
• Est. Priority Date: 12/19/1975
• Status: Expired due to Term

• Current Assignee: National Research Council Canada
• Original Assignee: Canadian Patents and Development Limited
• Inventors: Barszczewski, Andrzej, Hardwick, Charles D.
• Primary Examiner(s): Birmiel, Howard A.
• Assistant Examiner(s): Berger, Richard E.
• Application Number:
  US05/734,817
• Time in Patent Office: 557 Days
• Field of Search: 343/112 D, 343/103, 343/105 R
• US Class Current: 342/393
• CPC Class Codes: G01S 1/30 the synchronised signals being continuous waves or intermittent trains of continuous waves, the intermittency not being for the purpose of determining direction or position line and the transit times being compared by measuring the phase difference
• CPC Class Codes: G01S 11/02 using radio waves G01S19/00 takes precedence
• CPC Class Codes: G01S 5/14 Determining absolute distances from a plurality of spaced points of known location

Abstract

A low frequency radio position location system for aircraft or other vehicles for operating in conjunction with ground stations transmitting at a first VLF radio frequency which is shifted in frequency by a small amount to a second frequency at fixed intervals with the shifting being carried out coherently with a defined transition point at the frequency signal is made up of an RF receiver and amplifier, a phase-locked loop for providing a continuous output frequency signal precisely locked to the first frequency, a detector for deriving a signal at the modulation frequency, a phase comparator for comparing the phase relationship of the derived modulation frequency signal to the phase relationship of the output frequency signal and the second frequency signal. The derived modulation frequency signal and its phase information is then available for providing distance and position information in relation to the ground stations.

Claims

1. A low frequency radio position location system for aircraft or other vehicles and adapted for operating in conjunction with ground stations transmitting at a first radio frequency which is shifted in frequency by a small amount to a second radio frequency at fixed intervals, the shifting being carried out coherently with a defined transition point at the shifted time, said shifting frequency constituting a modulation frequency signal, comprising:

(a) an RF receiver and amplifier,

(b) limiting means connected to the amplifier for providing a square waveform output,

(c) a phase-locked loop connected to the limiting means for providing a continuous output frequency signal precisely locked to the first frequency,

(d) means for deriving a signal at the modulation frequency,

(e) phase comparison means for comparing the phase relationship of the derived modulation frequency signal to the phase relationship of the said output frequency signal and the second frequency signal, the said derived modulation frequency signal and its phase information then available for providing distance information in relation to said ground stations.

2. A low frequency radio position location system as in claim 1 further comprising frequency conversion means connected between RF receiver and amplifier and the limiting means for reducing the frequencies of the said first and second radio signals to much lower intermediate frequencies and wherein the phase-locked loop and the phase comparison means operate in conjunction with these IF signals.

3. A low frequency radio position location system as in claim 1 wherein the signal at the modulation frequency is derived from the said output frequency signal by means of a frequency divider network connected to the output of the phase-locked loop.

4. A low frequency radio position location system as in claim 1 wherein the phase-locked loop comprises a phase detector for providing a phase error voltage signal, low pass filter means, and a voltage controlled crystal oscillator for providing the phase-locked frequency signal.

5. A low frequency radio position location system as in claim 1 further comprising a second phase-locked loop connected to the limiting means for providing a second continuous output frequency signal locked to the second frequency and wherein the phase comparison means compares the phase relationship of the derived modulation frequency signal to the phase relationship of the said output frequency signal and the said second continuous output frequency signal.

6. A low frequency radio position location system as in claim 1 wherein the phase comparison means comprises means for measuring the phase delay between the said continuous output frequency signal and the said second frequency signal, said measurements being taken at a rate equal to the said modulation frequency and means for comprising the phase relation of the said derived modulation frequency signal with the said phase delay and thus with the transition point.