Range determining system

• US 4,278,977 A
• Filed: 05/04/1979
• Issued: 07/14/1981
• Est. Priority Date: 05/04/1979
• Status: Expired due to Term

• Current Assignee: L-3 Communications Corporation (L3Harris Technologies, Inc.)
• Original Assignee: RCA Corporation
• Inventors: Nossen, Edward J.
• Primary Examiner(s): Tubbesing, T. H.
• Application Number:
  US06/036,012
• Time in Patent Office: 802 Days
• Field of Search: 343/6.5 R, 343/6.5 LC
• US Class Current: 342/42
• CPC Class Codes: G01S 13/79 Systems using random coded signals or random pulse repetition frequencies , e.g. "Separation and Control of Aircraft using Non synchronous Techniques" [SECANT]

Abstract

A range finding method and system comprising an interrogator and a transponder. The interrogator generates a signal with a first portion comprising a carrier signal amplitude modulated by a first two-level iterative pseudo random sequence (PRS) whose level changes coincide with level changes of a first two-level tone of frequency f_T, which phase modulates said first portion. A second portion of the signal is phase modulated by the product of the PRS and said first tone. The transponder tracks the received interrogation signal and then generates and transmits a responsive signal having first and second portions comprised of a carrier signal phase modulated by a second two-level tone phase synchronized with the received first tone. The first portion is further amplitude modulated by a second iterative PRS phase synchronized with the received PRS signal, and the second portion is further phase modulated by the second, phase synchronized PRS. In response to the responsive signal, the interrogator generates a tracking tone of frequency f_T, precisely phase locked by means including an arithmetic synthesizer, with the received second tone. The phases of the received second PRS and the originally generated first PRS are compared to determine the round-trip signal propagation time and thus the range.

Claims

1. A range finding system comprising:

interrogation means for generating and transmitting an interrogation signal having at least one portion comprising a first carrier signal modulated by a signal which is a concurrent function of a first two-level tone of frequency f_T and a first iterative two-level pseudo random sequence whose level transitions are coincident with level transitions of said first two-level tone;

responding means for receiving said interrogation signal and for generating a first tracking signal which identifies the propagation delayed phases of said first tone and said first pseudo random sequence;

said responding means further comprising means for generating a responsive signal comprising a second carrier signal modulated by signal which is a concurrent function of a second two-level tone of frequency f_T and a second iterative psuedo random sequence whose phases are the same as the propagation delayed phases of said first tone and said first pseudo random sequence;

said interrogation means further comprising;

logic means for receiving said responsive signal, for generating a second tracking signal which identifies the propagation delayed phases of said second two-level tone and said second pseudo random sequence,

and for determining the phase difference between said interrogation signal before transmission thereof and said second tracking signal; and

means for computing the distance between said interrogation means and said responding means from said phase difference.

2. A range determining means in accordance with claim 1 in which said logic means comprises:

first counting means for continuing to generate said first tone with its phase unchanged after transmission of said interrogation signal;

arithmetic frequency synthesizer means for generating an overflow output signal at a rate substantially equal to f_T ;

means for phase locking the overflow output signal of said arithmetic frequency synthesizer with the phase of said received second tone;

second and third counting means responsive, respectively, to said continued first tone and to said second tracking signal to determine phase relation between said first iterative pseudo random sequence and said propagation delayed second iterative pseudo random sequence; and

means for taking the difference between the count of said first counting means and the count of said arithmetic frequency synthesizer means at a given time which occurs at a change of count of said first counting means and

further for taking the difference between the counts of said second and third counting means at said given time.

3. A system as in claim 1 in which said interrogation means and said responding means each further comprises:

antenna means; and

radio transceiver means having a nominal intermediate center frequency f_i and a nominal intermediate frequency pass band of sufficient width to pass therethrough received carrier signals amplitude modulated by a received pseudo random sequence.

4. A range determining means in accordance with claim 3 in which said logic means comprises:

first counting means for continuing to generate said first tone with its phase unchanged after transmission of said interrogation signal;

arithmetic frequency synthesizer means for generating an overflow output signal at a rate substantially equal to f_T ;

means for phase locking the overflow output signal of said arithmetic frequency synthesizer means with the phase of said received second tone;

second and third counting means responsive, respectively, to said continued first tone and to said second tracking signal to determine the phase relation between said first iterative pseudo random sequence and said propagation delayed second iterative pseudo random sequence;

means for taking the difference between the count of said first counting means and the count of said arithmetic frequency synthesizer means at a given time which occurs at a change of count of said first counting means and

further for taking the difference between the counts of said second and third counting means at said given time.

5. A system as in claim 1 in which said responding means further comprises:

correlation means responsive to said first iterative pseudo random sequences of said interrogation signal to produce correlation pulses; and

control means comprising first signal generating means responsive to said correlation pulses to produce a signal having a repetition rate equal to the repetition rate of said first iterative pseudo random sequences and in phase with the period thereof and to identify the beginning of a second portion of said received interrogation signal;

said control means further comprising a second signal generating means responsive to the second portion of said received interrogation signal to generate a signal which is phase synchronous with said first two-level tone component of said interrogation signal.

6. A system as in claim 1 in which said interrogation means further comprises:

correlation means responsive to said second iterative pseudo random sequences of said received responsive signal to produce correlation pulses; and

control means comprising first signal generating means responsive to said correlation pulses to produce a signal having a repetition rate equal to the repetition rate of said second iterative pseudo random sequences and in phase with the period thereof and to identify the beginning of a second portion of said received responsive signal;

said control means further comprising a second signal generating means responsive to the second portion of said received responsive signal to generate a signal which is phase synchronous with said second two-level tone component of said received responsive signal.

7. A method for finding the distance between an interrogation means and a transponder means comprising the steps of:

generating and transmitting an interrogation signal comprising a carrier signal modulated by a signal which is a concurrent function of a first two-level tone of frequency f_T and a first iterative two-level pseudo random sequence whose level transitions are coincident with level transitions of said first tone;

responding to said interrogation signal at said transponder means to generate a first tracking signal which identifies the times of occurrence of the propagation delayed phases of said first tone and said first pseudo random sequence;

generating and transmitting a responsive signal comprising a carrier signal modulated by a signal which is a concurrent function of a second two-level signal of frequency f_T and a second iterative pseudo random sequence whose phases are the same as the propagation delayed phases of said first tone and said first pseudo random sequence;

receiving said responsive signal at said interrogation means;

generating a second tracking signal at said interrogation means which identifies the times of occurrence of the propagation delayed phases of said second tone and said second pseudo random sequence;

determining the phase difference of said first pseudo random sequence before transmission thereof and the propagation delayed second pseudo random sequence; and

computing the distance between said interrogation means and said transponder means from said phase difference.

8. A method for finding the distance between an interrogation means and a transponder means comprising the steps of:

generating and transmitting an interrogation signal of at least two successive portions;

first and second portions comprising a carrier signal amplitude modulated by a first iterative two-level pseudo random sequence whose level transitions are coincident with level transitions of a first two-level tone of frequency f_T ;

said second portion being further phase shift modulated by said first two-level tone;

said third portion being phase shift modulated by the product of said first tone and said first pseudo random sequence;

responding to said interrogation signal at a transponder to generate a first tracking signal having first and second signal components;

phase locking the phases of said first and second signal components with the propagation delayed phases of said first tone and said first pseudo random sequence;

generating and transmitting a responsive signal comprised of two successive responsive portions;

the first responsive portion being a carrier signal amplitude modulated by a second iterative pseudo random sequence and phase shift modulated by a second two-level tone of frequency f_T ;

the second portion being phase shift modulated by the product of said second two-level tone and said second pseudo random sequence;

phase locking said second two-level tone and said second pseudo random sequence with the propagation delayed phases of said first tone and said first pseudo random sequence;

receiving said responsive signal at said interrogation means;

generating a second tracking signal at said interrogation means;

phase locking a first and second signal component of said second tracking signal with the propagation delayed phases of said second two-level tone and said second pseudo random sequence;

determining the phase difference of said first pseudo random sequence before transmission thereof and the propagation delayed second pseudo random sequence; and

computing the distance between said interrogation and said transponder means from said phase difference.

9. A radio for use in a system of the type in which a first radio station transmits a first carrier modulated by a first code sequence to a second radio station, said first code sequence including a plurality of successive cycles of a given pseudo random code which occur in time synchronism with a time reference at said first station;

in which said second station receives said modulated first carrier after a certain propagation time delay which is short relative to a cycle period of said given pseudo random code sequence;

in which said second station transmits a second carrier modulated by a second code sequence to said first station, said second code sequence including a plurality of successive cycles of said given pseudo random code which are phase locked with said pseudo random code of said first code sequence received by said second station; and

in which said first station receives said modulated second carrier after a certain propagation time delay and then determines said range between said stations in accordance with the phase delay between said pseudo random code received by said first station and said pseudo random code transmitted from said first station;

the improvement wherein a radio station for use in said ranging system comprises;

a tone source generating a tone at a given frequency which is low relative to the frequency of the carrier of that station;

a pseudo random code generating means responsive to said tone for repetitively deriving a pseudo random code consisting of a predetermined number of successive binary code chips with the code chip period of said pseudo random code being equal to a period of said tone; and

modulation means for modulating the carrier of that station during at least one certain portion of said code sequence of that station with a modulating signal that is a preselected concurrent function of both said tone and said pseudo random code.