Acoustic sounding

• US 20030056591A1
• Filed: 09/03/2002
• Published: 03/27/2003
• Est. Priority Date: 03/09/2000
• Status: Active Grant

• Current Assignee: Windbidco Pty Ltd
• Original Assignee: Tele-Ip Limited
• Inventors: Martin, Andrew Louis
• Granted Patent: US 6,755,080 B2
• Time in Patent Office: Days
• US Class Current: 73/602
• CPC Class Codes: G01S 15/003 Bistatic sonar systems; Multistatic sonar systems
• CPC Class Codes: G01S 15/34 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
• CPC Class Codes: G01S 15/588 measuring the velocity vector
• CPC Class Codes: G01S 15/87 Combinations of sonar systems
• CPC Class Codes: G01S 15/885 Meteorological systems
• CPC Class Codes: G01W 2001/003 Clear air turbulence detection or forecasting, e.g. for aircrafts

Abstract

Methods and apparatus for atmospheric sounding using acoustic chirps are disclosed, the transmitted and echo chirps being compared in a mixer that yields frequency sums and differences. Preferably, the mixing is performed as a complex multiplication in the Fourier domain. In one system (1) a signal generator (5) such as a PC sound card drives a loudspeaker (3) that serves as a transmitter and echo pulses are detedcted by a microphone (4) that serves as a receiver. Chirps transmitted by the loudspeaker (3) travel by different paths (7a and 7b) due to reflection from TILS or thermal inversion layers (2a and 2b) at different altitudes. The transmitted and echo chirp signals are compared in a mixer (6) from which various outputs (8 and 9) can be generated. One output (8) might be the magnitude of the difference between the transmitted and echo chirp tones, instant by instant, which is indicative of the altitudes of the respective TILs. The other output (9) might be the phase shift in an echo tone with transit time, which is indicative of vertical wind velocity at different altitudes. Such sounding methods and apparatus are useful for weather research, prediction of plume dispersal and the design of microwave links to minimize multipath fading.

Claims

1. A method of locating temperature and/or velocity discontinuities in the lower atmosphere, comprising the steps of:

generating an input signal comprising a chirp including a sequence of different audio tones,

employing said input signal to drive an acoustic transmitter to effect the transmission of an acoustic chirp into the atmosphere,

receiving and detecting an acoustic echo from said transmitted acoustic chirp and generating an echo signal comprising at least a portion of said sequence of tones,

comparing the sequence of tones in the input signal with the time-shifted sequence of tones in the echo signal and deriving a first output signal indicative of the time delay between transmission of the transmitted chirp and detection of said echo, said time delay also being indicative of the location of a discontinuity in the atmosphere.

2. A method according to claim 1 including the step of:

comparing tones in the input signal with corresponding tones in the echo signal and deriving a second output signal indicative of the phase shift between a tone in the input signal and the corresponding tone in the echo signal, said phase shift also being indicative of the velocity at which said discontinuity is moving in the direction of travel of the acoustic chirp.

3. A method according to claim 1 including the steps of:

mixing the tones of the input signal with the tones of the echo signal to obtain, as said first and/or second output signal, the sum and/or difference frequencies of the mixed tones.

4. A method according to claim 1 including the steps of:

subjecting the tones of the input signal to a Fourier transformation to form a Fourier input signal,

subjecting the tones of the echo signal to a Fourier transformation to form a Fourier echo signal, and

combining said Fourier input and said Fourier echo signals to obtain said first and/or second output signals or a precursor thereof.

5. A method according to claim 1 including the steps of:

sampling the input signal at intervals to generate a series of digital samples of the input signal,

sampling the echo signal at the same intervals to generate a series of digital samples of the echo signal,

deriving the Fourier transform of the series of digital samples of the input signal to generate a Fourier input signal,

deriving the Fourier transform of the series of digital samples of the echo signal to generate a Fourier echo signal, and

combining said Fourier input and said Fourier echo signals to obtain a Fourier output signal indicative of the time and phase shifts in the echo signal with respect to the input signal.

6. A method according to claim 5 including the steps of:

subjecting said Fourier output signal to an inverse Fourier transformation to obtain a time sequence of amplitudes indicative of the magnitude of the respective discontinuities in the atmosphere traversed by said acoustic chirp.

7. A method according to claim 1 including the steps of:

modulating the input signal to generate an acoustic chirp in which the component tones increase or decrease in frequency uniformly with respect to time,

for each one of a series of time intervals during the transmission of the acoustic chirp, comparing the tone of the input signal during that interval with the tone of the echo signal during the same instant to obtain said output signal.

8. A method according to claim 1 including the steps of:

modulating the input signal to generate an acoustic chirp in which the component tones increase or decrease in frequency uniformly with respect to time,

storing or recording the input signal,

after a predetermined time following initiation of the transmission of the acoustic chirp, playing the stored or recorded input signal as a delayed input signal, and

for each one of a series of time intervals during the playing of said delayed input signal, comparing the tone of the delayed input signal during that interval with the tone of the echo signal during the same instant to obtain said output signal.

9. A method according to claim 1 including the steps of:

generating the acoustic chirp from a stored input signal,

recording the echo signal,

replaying the stored input and recorded echo signals in a predetermined time relationship,

for each one of a series of time intervals during the replaying of said stored and recorded echo signals, comparing the tone of the stored input signal during that interval with the tone of the replayed echo signal during the same instant to obtain said output signal.

10. Apparatus for use in atmospheric sounding comprising:

means for generating and input signal comprising a chirp that includes a sequence of different audio tones,

an acoustic transmitter connected to said generating means adapted to effect the transmission of an acoustic chirp into the atmosphere upon receiving said input signal,

means for receiving and detecting an acoustic echo from an acoustic chirp transmitted into the atmosphere by said transmitter, said receiving means being adapted to generate an echo signal comprising at least portion of said sequence of tones, and

comparator means connected to said generator means for receiving said input signal and connected to said receiving means for receiving said echo signal, said comparator means being adapted to compare the tones in said input signal with the tones in said echo signal as they are received and to derive a first output signal indicative of the frequency difference there between.

11. Apparatus according to claim 10 wherein said comparator means is adapted to compare tones in the echo signal with the corresponding tones in the input signal and to generate a second output signal indicative of the phase difference between said compared tones.

12. Apparatus according to claim 10 wherein said comparator includes a mixer connected to receive the input signal and the echo signal, said mixer being adapted to generate said first and/or second output signals.

13. Apparatus according to claim 10 wherein:

first Fourier transform means are provided for effecting the Fourier transformation of the input signal, thereby generating a transformed input signal,

second Fourier transform means are provided for effecting the Fourier transformation of the echo signal, thereby generating a transformed echo signal,

said comparator comprises a complex multiplier adapted to multiply said transformed input signal and said transformed echo signal to generate a complex output from which said first and/or second outputs can be derived.

14. Apparatus according to claim 13 comprising inverse Fourier transform means connected to receive said complex output and to generate said first and/or second output.