Time Shifted PN Codes for CW LIDAR, RADAR, and SONAR

• US 20110317147A1
• Filed: 06/23/2011
• Published: 12/29/2011
• Est. Priority Date: 06/23/2010
• Status: Active Grant

• Current Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Original Assignee: U.S.A. 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.
• Granted Patent: US 8,605,262 B2
• Time in Patent Office: Days
• 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 range 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 a 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.

2. The method of claim 1, 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.

3. The method of claim 2, including:

providing first and second continuous wave lasers; and

wherein;

the first signal is generated by the first laser; and

the second signal is generated by the second laser.

4. The method of claim 3, 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.

5. The method of claim 4, including:

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.

6. The method of claim 5, wherein:

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

7. The method of claim 6, 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.

8. The method of claim 1, wherein:

the first and second PN codes comprise m-sequence codes.

9. The method of claim 1, wherein:

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

10. The method of claim 1, wherein:

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

12. The method of claim 1, wherein:

the first and second PN codes comprise substantially the same sequences of binary values;

the first and second signals define a range corresponding to the first and second PN codes;

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

13. The method of claim 1, including:

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

14. The method of claim 1, including:

generating a third signal at a third frequency that is not equal to the first frequency or the second frequency, the third signal including a third PN code that is time shifted relative to the first and second PN codes.

11. The method of claim wherein:

first and second continuous wave lasers are utilized to generate the first and second signals comprising a carrier;

detecting a return signal includes converting an optical signal into an electrical signal; and

including;

utilizing a band pass filter to separate out the carrier.

15. A continuous wave laser ranging system, comprising:

a first laser that produces light having a first frequency;

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

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 substantially orthogonal to the first PN code;

an optical transmitter that directs the 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 first and second amplitude peaks corresponding to the first and second signals, respectively, and utilizes a ratio of the magnitudes of the amplitude peaks to determine a concentration of a species in an atmosphere.

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

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

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

the PN codes of the first and second optical signals are time-shifted relative to one another.

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

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

19. The continuous wave laser ranging system of claim 18, 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.

20. The continuous wave laser range system of claim 15, wherein:

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