Method and apparatus for measuring distance

• US 4,757,315 A
• Filed: 02/20/1986
• Issued: 07/12/1988
• Est. Priority Date: 02/20/1986
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Kobayashi, Herbert S., Lichtenberg, Christopher L., Shores, Paul W.
• Primary Examiner(s): Blum, Theodore M.
• Assistant Examiner(s): GREGORY, BERNARR E
• Application Number:
  US06/831,193
• Time in Patent Office: 873 Days
• Field of Search: 342/5, 342/6, 342/30, 342/42-51, 342/125, 342/127, 342/32, 342/187
• US Class Current: 342/125
• CPC Class Codes: G01S 13/753 using frequency selective elements, e.g. resonator
• CPC Class Codes: G01S 13/84 for distance determination by phase measurement

Abstract

A signal is transmitted from a first location to a remote second location where a target carrying a transponder is positioned. The transponder re-radiates the signal to the first location where it is received. A phase comparator generates from the transmitted and received signals a measurement of their phase difference which is functionally related to the distance between the first and second locations. In one embodiment, the transponder is a passive parametric oscillator, being powered by energy received from the transmitted signal, which generates and transmits a subharmonic of the transmitted signal.

Claims

1. A method for determining distance "d" between first and second locations, comprising:

generating at said first location a first electromagnetic wave S₁ at a frequency f₁ ;

transmitting said S₁ from said first location to said second location;

receiving said S₁ at said second location;

passively generating from said received S₁ using energy solely from said received S₁ at said second location a second electromagnetic wave S₂ phase-coherent with said received S₁ and at a subharmonic frequency f₂ of said f₁ frequency;

transmitting said S₂ from said second location to said first location;

receiving said transmitted S₂ at said first location; and

deriving a measurement signal corresponding to the magnitude of phase difference between said transmitted S₁ and said received S₂ signals.

2. The method of claim 1, wherein f₂ =1/2 f₁.

3. The method of claim 1, wherein

said transmitted S₁ is of the form

4. The method of claim 1, wherein said transmitted and received signals S₁ and S₂, respectively, are continuous waves.

5. The method of claim 4, wherein said transmitted and received signals S₁ and S₂, respectively, are sinusoidal waves.

6. Apparatus for determining distance "d" between first and second locations, comprising:

first generator means for generating at said first location a first electromagnetic wave S₁ at a frequency f₁ ;

first transmitter means for transmitting said S₁ from said first location to said second location;

first receiving means for receiving said S₁ at said second location;

second generator means for passively generating from said received S₁ using energy solely from said received S₁ at said second location a second electromagnetic wave S₂ phase-coherent with said received S₁ and at a subharmonic frequency f₂ of said f₁ frequency;

second transmitter means for transmitting said S₂ from said second location to said first location;

second receiving means for receiving said transmitted S₂ at said first location; and

phase detector means for deriving a measurement signal corresponding to the magnitude of phase difference between said transmitted S₁ and said received S₂ signals.

7. The apparatus of claim 6, wherein said second generator means generates said S₂ at said subharmonic frequency f₂ =1/2 f₁.

8. The apparatus of claim 6, wherein

said transmitted S₁ is of the form

9. The apparatus of claim 6, wherein said first and second generator means generate continuous electromagnetic waves at said f₁ and said f₂ frequencies, respectively.

10. The apparatus of claim 9, wherein said elecromagnetic waves are sinusoidal.

11. Apparatus for determining distance, comprising:

a base system means for transmitting signal S₁ at frequency f₁ and for receiving signal S₂ at frequency f₂ respectively, where said f₂ is a subharmonic of f₁ and for measuring phase difference between said transmitted S₁ and received S₂ signals; and

a transponder system comprising;

a capacitor;

a first inductor,

said capacitor and said first inductor being electrically interconnected in parallel to define first and second electrical junctions;

a varactor diode having first and second ends;

a second inductor having first and second ends,

said first end of said diode being electrically interconnected to said first junction,

said second end of said diode being electrically interconnected to said first end of said second inductor,

said second end of said inductor being electrically connected to said second junction;

a first length of wire electrically interconnected at one end of said first junction;

a second length of wire electrically interconnected at one end of said second junction; and

a third length of wire electrically interconnected at one end of said second end of said diode and said first end of said second inductor.

12. The apparatus of claim 11, wherein said capacitor is a variable tuning capacitor.

13. Apparatus for determining distance, comprising:

a base system means for transmitting signal S₁ at frequency f₁ and for receiving signal S₂ at frequency f₂, respectively, where said f₂ frequency is a subharmonic of f₁ and for measuring phase difference between said transmitted S₁ and received S₂ signals; and

a transponder system comprising;

a receiving antenna means for receiving said transmitted S₁ ;

a transmitting antenna means for transmitting said S₂ 2 ;

a first resonant circuit means resonating at said f₁ frequency electrically interconnected in series between said transmitting and said receiving antennas;

a second resonant circuit means resonating at said f₂ frequency;

a varactor diode,

said diode and said second resonant circuit means being electrically interconnected in series terminating at first and second ends, said first end of said series circuit being interconnected to one of said transmitting or receiving antennas and said second end of said series circuit being interconnected to the remaining one of said transmitting or receiving antennas.

14. A method for determining the distance "d" between first and second locations, comprising:

generating a signal S_a of the form ##EQU23## transforming said S_a into a signal S_b of the form

space="preserve" listing-type="equation">S.sub.b =Cos (ω

+Δ

ω

)t;

transmitting said S_b from said first location toward said second location;

receiving at said second location in response to said transmitted S_b a signal S_c of the form ##EQU24## transforming said S_c to a signal S_d of the form ##EQU25## transmitting said S_d from said second location toward said first location;

receiving at said first location in response to said transmitted S_d a signal S_e of the form ##EQU26## transforming said S_e into a signal S_f of the form ##EQU27## mixing said S_a and S_f to derive a first mixer output S_g ;

transforming said S_g into a signal S_h of the form ##EQU28## transforming said S_h into a signal S_i of the form

space="preserve" listing-type="equation">S.sub.i =Cos (ω

t);

mixing said S_i and said S_b to derive a second mixer output S_j ;

transforming said S_j into a signal S_k of the form ##EQU29## generating a phase difference signal corresponding to the magnitude of the difference between the phases of said S_h and S_j.

15. The method of claim 14, further including the steps of:

deriving a correction constant factor for correlating said phase difference signal to said distance "d";

multiplying said phase difference signal by said correction factor;

generating a distance signal in response to said multiplication; and

displaying a visual indication of the magnitude of said distance signal as an indication of said distance "d".

16. The method of claim 15, wherein said step of transforming said S_e into S_f comprises filtering said S_e.

17. The method of claim 16, wherein said step of transforming said S_g into S_h comprises filtering said S_g.

18. The method of claim 14, wherein said step of generating a phase difference signal comprises:

generating a plurality of reference time pulses;

detecting a 0 crossing of said S_h ;

detecting a 0 crossing of said S_j ; and

counting a portion of said timing pulses occurring between said detected 0 crossings.