Time shifted PN codes for CW LiDAR, radar, and sonar

• US 8,605,262 B2
• Filed: 06/23/2011
• Issued: 12/10/2013
• Est. Priority Date: 06/23/2010
• 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: Campbell, Joel F., Prasad, Narasimha S., Harrison, Fenton W., Flood, Michael A.
• Primary Examiner(s): Alsomiri, Isam
• Assistant Examiner(s): Abraham, Samantha K
• Application Number:
  US13/167,093
• Publication Number:
  US 20110317147A1
• Time in Patent Office: 901 Days
• Field of Search: 356/4.01, 356/5.09, 356/445, 356/337, 356/484, 356/401, 356/432
• US Class Current: 356/5.09
• CPC Class Codes: G01S 17/34 using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal

Abstract

A continuous wave Light Detection and Ranging (CW LiDAR) system utilizes two or more laser frequencies and time or range shifted pseudorandom noise (PN) codes to discriminate between the laser frequencies. The performance of these codes can be improved by subtracting out the bias before processing. The CW LiDAR system may be mounted to an artificial satellite orbiting the earth, and the relative strength of the return signal for each frequency can be utilized to determine the concentration of selected gases or other substances in the atmosphere.

Claims

1. A method of measuring absorption utilizing a transmitted optical signal, the method comprising:

providing a first laser that produces light having a first wavelength;

providing a second laser that produces light having a second wavelength that is not equal to the first wavelength;

generating at least one PN code;

modulating light produced by the first laser utilizing the PN code to provide a first outgoing optical signal comprising light at the first wavelength, the first outgoing optical signal including first PN code data;

modulating light produced by the second laser utilizing the PN code to provide a second outgoing optical signal comprising light at the second wavelength, the second outgoing optical signal including second PN code data that is time shifted relative to the first PN code data of the first outgoing optical signal;

directing the first and second outgoing optical signals towards a target to thereby cause at least a portion of the first optical signal and a portion of the second optical signal to be reflected from the target to provide return signal data;

detecting a return signal comprising at least a portion of the first signal that has been reflected from the target and a portion of the second signal that has been reflected from the target;

correlating the return signal data with the first and second PN code data to provide first and second distinct amplitude peaks corresponding to the portions of the first and second signals that have been reflected from the target, respectively;

determining a magnitude of the first amplitude peak;

determining a magnitude of the second amplitude peak;

utilizing the magnitudes of the first and second amplitude peaks to determine a relative absorption of the first and second signals.

2. The method of claim 1, including:

providing first and second continuous wave lasers that produce light having the first and second wavelengths, respectively; and

wherein;

the first outgoing optical signal is generated by the first laser; and

the second outgoing optical signal is generated by the second laser.

3. The method of claim 2, wherein:

the first and second PN codes are substantially identical to one another except that the second PN code is time-shifted relative to the first PN code.

4. The method of claim 1, wherein:

the PN code comprises a m-sequence code.

5. The method of claim 1, wherein:

the first and second outgoing optical signals comprise a single amplitude modulated carrier.

6. The method of claim 1, wherein:

the first and second outgoing optical signals comprise a single frequency modulated carrier.

7. The method of claim 1, wherein:

first and second continuous wave lasers are utilized to generate the first and second outgoing optical signals.

8. The method of claim 1, wherein:

the PN code defines a length in bits;

the second PN code data is shifted by one half of the length of the PN code relative to the first PN code data.

9. The method of claim 1, including:

utilizing a time delay in at least one of the first and second outgoing optical signals to determine a range from the target.

10. The method of claim 1, including:

generating a third outgoing optical signal comprising light at a third wavelength that is not equal to the first wavelength or the second wavelength, the third outgoing optical signal including third PN code data that is time shifted relative to the first and second PN code data.

11. A method of measuring absorption utilizing a transmitted signal, the method comprising:

generating a first PN code;

generating a second PN code that is substantially orthogonal to the first PN code;

generating a first signal at a first frequency, the first signal including the first PN code;

generating a second signal at a second frequency that is not equal to the first frequency, the second signal including the second PN code;

directing the first and second signals towards a target to thereby cause at least a portion of the first signal and a portion of the second signal to be reflected from the target;

detecting a return signal comprising at least a portion of the first signal that has been reflected from the target and a portion of the second signal that has been reflected from the target;

correlating the return signal utilizing the first and second PN codes to provide first and second distinct amplitude peaks corresponding to the first and second PN codes;

determining magnitude of the first amplitude peak;

determining a magnitude of the second amplitude peak;

utilizing a ratio of the magnitudes of the first and second amplitude peaks to determine a relative absorption of the first and second signals;

wherein the first signal comprises light having a first wavelength, and the second signal comprises light having a second wavelength that is not equal to the first frequency;

providing first and second continuous wave lasers;

wherein the first signal is generated by the first laser, and the second signal is generated by the second laser; and

further wherein the first and second PN codes are substantially identical to one another except that the second PN code is time-shifted relative to the first PN code;

determining a maximum possible distance to a target; and

wherein;

the delay generated for the second PN code corresponds to a distance that is greater than the maximum target distance, and wherein the maximum length for the first and second PN codes corresponds to a distance greater than twice the maximum possible distance to a target.

12. The method of claim 11, wherein:

the second PN code is shifted about one half of the maximum length of the first signal.

13. The method of claim 12, wherein:

the first and second lasers are mounted on an artificial satellite orbiting about a mass defining an outer surface;

the maximum possible distance comprises a maximum distance from the artificial satellite to the outer surface as the artificial satellite orbits about the mass.

14. A continuous wave laser ranging system, comprising:

a first laser that produces light having a first wavelength;

a second laser that produces light having a second wavelength that is not equal to the first wavelength;

a modulation device that modulates light from the first and second lasers whereby light originating from the first laser is modulated to provide a first optical signal comprising a first PN code, and light originating from the second laser is modulated to provide a second optical signal comprising a second PN code that is time shifted relative to the first PN code;

an optical transmitter that directs the time shifted first and second optical signals towards a target;

a receiver that receives a return signal including components of the first and second optical signals that have been reflected by the target;

a detector that converts the return signal into an electrical signal;

a processor that correlates the electrical signal and determines the magnitude of the first and second amplitude peaks corresponding to the first and second signals, respectively.

15. The continuous wave laser ranging system of claim 14, wherein:

the modulating device causes the first and second optical signals to comprise m-sequence PN codes.

16. The continuous wave laser ranging system of claim 15, wherein:

the PN codes of the first and second optical signals have substantially the same length.

17. The continuous wave laser ranging system of claim 16, wherein:

the PN code of the second optical signal is shifted relative to the PN code of the first optical signal by one-half the length of the first PN code optical signal.

18. The continuous wave laser range system of claim 14, wherein:

a single carrier is modulated by the first and second PN codes.