TIME DELAY AND DISTANCE MEASUREMENT

• US 20100027602A1
• Filed: 07/31/2008
• Published: 02/04/2010
• Est. Priority Date: 07/31/2008
• Status: Active Grant

• Current Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Original Assignee: The United States of America As Represented By The Secretary of Agriculture
• Inventors: Sun, Xiaoli, Abshire, James B.
• Granted Patent: US 7,982,861 B2
• Time in Patent Office: Days
• US Class Current: 375/224
• CPC Class Codes: G01S 17/10 using transmission of interrupted, pulse-modulated waves determination of distance by phase measurements G01S17/32
• CPC Class Codes: H04L 25/4902 Pulse width modulation; Pulse position modulation

Abstract

A method for measuring time delay and distance may include providing an electromagnetic radiation carrier frequency and modulating one or more of amplitude, phase, frequency, polarization, and pointing angle of the carrier frequency with a return to zero (RZ) pseudo random noise (PN) code. The RZ PN code may have a constant bit period and a pulse duration that is less than the bit period. A receiver may detect the electromagnetic radiation and calculate the scattering profile versus time (or range) by computing a cross correlation function between the recorded received signal and a three-state RZ PN code kernel in the receiver. The method also may be used for pulse delay time (i.e., PPM) communications.

Claims

1. A method of measuring time delay, comprising:

providing an electromagnetic radiation carrier frequency;

modulating at least one of amplitude, phase, frequency, polarization, and pointing angle of the carrier frequency with a return to zero (RZ) pseudo random noise (PN) code, the RZ PN code having a constant bit period, and a pulse duration that is less than the bit period; and

transmitting the modulated electromagnetic radiation.

2. The method of claim 1, wherein providing an electromagnetic radiation carrier frequency includes providing an optical carrier frequency.

3. The method of claim 1, further comprising, after transmitting the modulated electromagnetic radiation,

detecting the modulated electromagnetic radiation;

extracting modulation from the detected electromagnetic radiation;

converting a time record of the detected electromagnetic radiation into a signal processing format;

providing an RZ PN code kernel; and

computing a cross correlation function between the time record and the RZ PN code kernel.

4. The method of claim 3, wherein providing an RZ PN code kernel includes providing a kernel having a pulse at a beginning of each bit period wherein a value of the pulse is +1 if the RZ PN code transmits a pulse, −

1 if the RZ PN code does not transmit a pulse, and zero otherwise.

5. The method of claim 3, further comprising, before receiving the modulated electromagnetic radiation, one of reflecting and scattering the electromagnetic radiation from an object.

6. The method of claim 4, wherein providing an RZ PN kernel includes providing an RZ PN kernel with a baseline of zero and pulse widths that are equal to pulse widths of the transmitted RZ PN code.

7. The method of claim 5, wherein computing the cross correlation function includes computing a cross correlation function wherein a value at a time delay corresponds to a signal'"'"'s backscatter strength at a range corresponding to the time delay.

8. The method of claim 6, wherein pulse widths of the transmitted RZ PN code are constant.

21. The method according to claim 1, wherein said return to zero (RZ) pseudo random noise (PN) code has a duty cycle less than 50%.

22. The method according to claim 21, wherein said return to zero (RZ) pseudo random noise (PN) code has a duty cycle of about 25%.

27. The method according to claim 21, further comprising the following step;

selectively choosing said duty cycle to minimize an amount of average power needed for said step of transmitting needed to achieve a given signal to noise ratio at the receiver.

28. The method according to claim 21, further comprising the following step;

selectively choosing said duty cycle to maximize a receiver signal to noise ratio for a given limits in average and peak power from the transmitter.

9. A transmitter, comprising:

an electromagnetic wave generator;

a modulator connected to the electromagnetic wave generator; and

a PN code generator and RZ pulse shaper connected to the modulator;

wherein the PN code generator and the RZ pulse shaper generate RZ PN code having a constant bit period and at least one pulse per bit period, the at least one pulse having a duration that is less than the bit period.

10. The transmitter of claim 9, wherein the modulator comprises an amplitude modulator.

11. The transmitter of claim 9, wherein the modulator comprises one of a phase, frequency, polarization, and pointing angle modulator.

12. The transmitter of claim 9, wherein the electromagnetic wave generator comprises a laser.

23. The transmitter according to claim 9, wherein said RZ PN code has a duty cycle less than 50%.

24. The time delay measurement device according to claim 23, wherein said RZ PN code has a duty cycle of about 25%.

26. The time delay measurement device according to claim 23, wherein said RZ PN code has a duty cycle of about 25%.

13. A receiver, comprising:

a detector;

a multi-channel integrator connected to the detector; and

a correlator connected to the multi-channel integrator, the correlator including an RZ PN code kernel having a constant bit period and at least one pulse per bit period, the at least one pulse having a duration that is less than the bit period.

14. The receiver of claim 13, wherein the detector comprises a photon counter.

15. The receiver of claim 14, wherein the multi-channel integrator comprises a histogrammer.

16. A time delay measurement device, comprising:

an electromagnetic wave generator;

a modulator connected to the electromagnetic wave generator;

a PN code generator and RZ pulse shaper connected to the modulator, wherein the PN code generator and the RZ pulse shaper generate RZ PN code having a constant bit period and at least one pulse per bit period, the at least one pulse having a duration that is less than the bit period;

a detector;

a multi-channel integrator connected to the detector; and

a correlator connected to the multi-channel integrator and to the modulator, the correlator including a kernel.

17. The device of claim 16, wherein the kernel includes a pulse at a beginning of each bit period wherein a value of the pulse is +1 if the RZ PN code transmits a pulse and −

1 if the RZ PN code does not transmit a pulse.

18. The device of claim 16, wherein the electromagnetic wave generator comprises an optical frequency generator.

19. The device of claim 16, wherein the kernel includes pulse widths that are equal to pulse widths of the transmitted RZ PN code.

20. The device of claim 16, wherein pulse widths of the RZ PN code are constant.

25. The time delay measurement device according to claim 16, wherein said RZ PN code has a duty cycle less than 50%.