Abstract:
A monostatic sodar system ( 10 ) for atmospheric sounding includes a processor and display unit ( 12 ) that generates a set of acoustic chirps for transmission by transmitter  20 . Discontinuities ( 22, 24  and  26 ) result in echoes ( 28, 30  and  32 ) being returned to a receiver ( 38 ) of system ( 10 ). Receiver ( 38 ) outputs extracted echo signals on line to the processor ( 12 ) for analysis. Detector ( 36 ) implements a Fourier domain matched-filter to extract echo signals from noise. By using a set of multiple chirps of increasing length with increasing intervals between them, substantially any feasible range can be accommodated using send-then-listen techniques with the benefit of high s/n performance.

Description:
This is a national phase application in the United States of International Application PCT/AU2006/000247 filed 28 Feb. 2006, claiming priority from Australian application number 2005900901 filed 28 Feb. 2005, the disclosures of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to sodar methods and apparatus for atmospheric sounding using encoded pulses in the transmit-then-listen mode. The techniques of the invention have particular application to so-called ‘monostatic’ sodar configurations in which the receiver(s) is located ‘up-range’ near the transmitter and backscattered echoes are detected by the receiver. However, the methods of the invention are also applicable to so-called ‘bistatic’ sodar configurations where the receiver(s) is located down-range and forward-scattered echoes are detected. 
     2. Description of Related Art 
     Transmit-then-listen sodar sounding techniques, in which the transmission of a short high-intensity acoustic pulse is followed by a longer listening period in which echoes from the pulse are detected, are standard in the prior art and have been known for nearly 100 years. The basic problem with such techniques is that the pulse must be very short—preferably a few milliseconds—so that the transmit pulse ends before the echoes from short-range anomalies are returned and detected, otherwise the energy of the transmitted pulse will swamp the receiver. This is a particularly important consideration in monostatic systems where the receiver is located close to the transmitter. Two problems associated with the use of short pulses: they must have very high energy to achieve an acceptable signal-to-noise ratio [s/n], and they are difficult to encode in any effective way so that pulse-compression techniques are difficult to apply. 
     In our prior international applications PCT/AU01/00247, PCT/AU02/01129 and PCT/AU04/00242 we disclosed sodar techniques using long pulse-compression-encoded transmitted pulses—called ‘chirps’ for short—that allowed ‘listening while sending’, so long as the received echoes were digitally processed using matched filters tailored to the encoding used in the chirps. Matched filtering is a well know technique practiced in radar and may be effected in the frequency domain (sometimes called the Fourier domain) or in the time domain by correlation methods. In the work associated with our prior patent applications, Chirp lengths of more than 40 seconds were demonstrated, the length being dependent upon the processing power devoted to the DSP (digital signal processing) techniques that implement the matched filtering. Very high s/n was achieved relative to the conventional transmit-then-listen methods as the total energy of a long chirped pulse can often be 1000 times that of a very short high-intensity pulse used in send-then-listen techniques. 
     However, we believe that there are situations where there may be advantage in using a hybrid system; that is, one which operates on a send-then-listen basis but has at least some of the s/n advantages of a chirped, long-pulse listen-while-sending system of the type disclosed in our above-mentioned prior patents. 
     BRIEF SUMMARY OF THE INVENTION 
     From one aspect, the invention relates to sodar systems and methods that employ a set of encoded transmit pulses, each successive pulse of the set having a duration that is longer than the preceding pulse and successive pulses being time-spaced by increasing intervals during which echoes from the immediately preceding pulse can be detected. The number of pulses in a set will be determined by the desired range of the sodar system, the minimum number of pulses being two. 
     Though not essential, the coding of the successive pulses is preferably substantially the same and the coding method is preferably adapted for matched filter processing. In that case, such pulses can be called chirps within the meaning employed in our prior patent applications. In one preferred option, each pulse may be frequency modulated so that the frequency increases or decreases in a linear manner for the duration of the chirp. Preferably each pulse has the same starting frequency or tone and the same finishing frequency or tone. The rate at which the tone increases (or decreases) will then, of course, decline as the pulses get longer, but each pulse will have effectively the same bandwidth. This allows greater simplicity in the design of the receiver system because the bandwidth of each pulse is the same and the same matched filter, together with and much of the same DSP processes, can be used to extract echo data from the received signals following the end of the transmission of each pulse. 
     Though there will be a ‘dead zone’ for each chirp during which returned signals cannot be received because transmission of the respective pulse is still continuing, there need be no gaps in the range if the dead zone of one pulse is shorter than the range band covered by the preceding pulse, except of course for the dead zone of the first pulse which cannot be covered. However, by making the first pulse short, the dead zone associated with that pulse can be minimal if not negligible. The portion a pulse that is not dead zone is the ‘range band’ because that portion will generate return signals that can be received and that will contribute a section of the system range. The range band will generally be most of the total length or duration of the pulse. 
     According to another aspect, the range dead-zone of a system can be further minimized by alternately employing two (or more) different sets of chirps, one set being used to ‘fill in’ the dead-zones of the preceding set. 
     Finally, this specification should be read in conjunction with our co-pending Australian patent applications entitled “Narrow Chirp Sodar” which teaches the use of chirped sodar signals with a bandwidth/chirp centre frequency ratio of between 0.1 and 0.2 and “Sodar Methods and Apparatus” which teaches the use of time domain matched filtering of received sodar signals in a listen-while-sending sodar. 
     Having portrayed the nature of the present invention, a particular example will now be described with reference to the accompanying drawings. However, those skilled in the art will appreciate that many variations and modifications can be made to the chosen example while conforming to the scope of the invention as defined in the following claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a diagrammatic side elevation of the sodar system of the example in operation. 
         FIG. 2  is a graph of frequency vs. time graph showing the timing of transmission and listening operations for a two-pulse set of the first example, including data relating to range distances. 
         FIG. 3  is an alternate chart-like representation of the timing of transmission and listening operations of the two-pulse set indicated in  FIG. 2 . 
         FIG. 4  is a tabulation setting out transmission, listening and range data applicable to a four-pulse set comprising the second example in which the first two pulses are the same as those of the first example. 
         FIG. 5  is a diagrammatic sectional side elevation showing two combined transducer units that may be used in the systems of the first or second example. 
         FIG. 6  is a diagrammatic plan view of a set of four transducer units, arranged on the cardinal points of the compass, that may be used in the systems of the first and second examples. 
         FIG. 7  is a similar view to that of  FIG. 6  showing a set of three transducer units that may be used in the systems of the first and second examples. 
         FIG. 8  is a diagrammatic sectional side elevation of a double transducer unit that may be used in the systems of the first and second examples. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before describing the chosen examples, it should be noted that the layout of transmitters and receivers and the processing of the received echoes can conveniently follow the teachings of our prior patent applications mentioned above. In those patent applications we showed how atmospheric anomalies could be graphically indicated, how the speed and bearing of airflow at any desired range (altitude) could be determined and depicted and how other variables such as temperature, humidity, wind-shear and total energy could be estimated or approximated. Accordingly, the description of received signal manipulation using DSP techniques (typically employing Fourier transformations) contained in our prior patent applications is incorporated herein and should be read in conjunction with the following description. 
     However, as will be described below, recent developments in acoustic transducers allow significant economies and simplifications with respect to the systems described in our above mentioned patent applications. 
     The basic components and operation of known apparatus suitable for use in the chosen examples are illustrated in  FIG. 1 . A monostatic sodar system  10  consists of a processor and display unit  12  that generates the signal for each chirp to be transmitted and outputs it on line  14  to an audio driver circuit  16  which drives a loudspeaker  18 . Driver  16  and loudspeaker  18  serve as the sodar transmitter  20  of system  10 . Loudspeaker  18  delivers the chirp in the form of an acoustic beam  21  upward into the atmosphere. Discontinuities such as those indicated at  22 ,  24  and  26  result in echoes  28 ,  30  and  32  being returned to a microphone  34  and associated signal conditioner and detector  36  that serve as sodar receiver  38  of system  10 . Receiver  38  outputs extracted echo signals on line  46  to processor  12  for analysis. Detector  36  implements a Fourier domain matched-filter (not shown) to extract echo signals from noise and for that purpose receives a reference input on line  48  representing each transmitted chirp (though such a reference signal can equally well be supplied by processor  12 ). 
     Finally, it will be appreciated that in a monostatic system where transmitter and receiver are located close together, such a strong direct acoustic signal, indicated by arrow  50 , will be received at microphone  34  during the transmission of a chirp that it will totally swamp any returned echo signal and may, indeed, damage detector circuit  36 . Accordingly receiver  38  is switched off during transmission. 
     The first example of the implementation of at least one aspect of the present invention is a two-chirp sodar signal comprising an initial short chirp  60  ( FIG. 2 ) of 100 ms followed by a 600 ms interval during which listening takes after which a second chirp  62  ( FIG. 2 ) of 600 ms duration is transmitted that is followed by a listening period of 1800 ms. Each chirp is encoded as a linear variation in tone (frequency) from 800 to 1500 Hz, the glissandi being accomplished in a shorter time in the first chirp than in the second. The encoding, timing, spacing and repetition of the sodar signal are determined by processor unit  12 . As already noted, chirps with other time/frequency/amplitude/phase encoding may be employed but are preferably those suited to matched-filter processing techniques. 
     With reference to the graph-like diagram of  FIG. 2 , the first chirp  60  commences transmission at time zero with a frequency of 800 Hz and ends after 100 ms with a frequency of 1500 Hz, the frequency of the chirp (in this example) increasing linearly with time. For the sake of illustration it is assumed that, at 100 ms when the transmission of chirp  50  ends, the head of first chirp  60  will have reached atmospheric anomaly  24  just as the tail leaves loudspeaker  18 , which (assuming the speed of sound to be 330 m/s) will be at an altitude of about 30 m. This means that the first echo that can be detected will be echo  28  that will have commenced its return from an altitude of 15 m and will start arriving at receiver  38  as soon as listening commences at 100 ms. Echo  30  will be received 50 ms later, the altitude of anomaly  24  being 30 m. Since echoes from below 16 m cannot be detected, the range 0-16 m is referred to as the ‘dead zone’ of the first chirp. 
     Receiver  38  accepts echoes for 600 ms after the end of chirp  60  before switching off at 700 ms to allow the second chirp  62  to be transmitted. It is assumed that, at that time, the tail of echo  32  will have just been received, so that the round trip for chirp  10  and complete echo  32  has taken 700 ms making the incremental range zone of chirp  60  100 m and the altitude of anomaly  26  about 116 m. Thus, as indicated in  FIG. 2 , the altitude range covered by the use of chirp  60  is 16-116 m. 
       FIG. 2  also depicts the situation for second chirp  62  that commences transmission at 700 ms for a period of 600 ms and terminates at 1300 ms after which it is followed by a listening time of 1800 ms that ends at 3100 ms. Applying the same reasoning as for the first chirp  60 , it can be seen that the 600 ms duration of second chirp  62  creates a range dead-zone of about 100 m; that is, no echoes can be detected from the second chirp from atmospheric anomalies below 100 m. However, the listening time of 1800 ms represents a range increment of about 300 m so that echoes can be detected from atmospheric anomalies between 100 m and 400 m, providing an overlap of about 16 m with the 16-116 range of first chirp  60 . In practice, however, some buffer period or guard time must be been allowed for the transition between the send and listen modes and this will substantially reduce range overlap. 
     The self-explanatory chart of  FIG. 3  provides and alternative way of depicting the ranges, times and dead zones associated with the two chirp set shown in  FIG. 2 . Like  FIG. 2 ,  FIG. 3  suggests that any desired range can be covered using chirp sets and, by way of second example,  FIG. 4  provides a tabulation showing how third and fourth chirps can be used in a set to cover a range of 16.5 m to 3567 m. The general is that the dead zone associated with a chirp (other than the first) should be no greater than the effective range covered by the preceding chirp. 
     There may be exceptional signal processing or other constraints (eg resonance) in some systems that result in significant time gaps between the transitions from chirp transmission to echo reception/processing and/or from reception/processing to chirp transmission. Such gaps may then result in the dead zone associated with one chirp being greater than the range covered by the preceding chirp. Though the most desirable remedy is to remove the constraints that cause the problem, the whole range can be covered without any gaps if two different sets of chirps are used in succession, such that the spacing and timing of the chirps or the second set cover the gaps created by the first set, and vice versa. 
     As already noted above, suitable arrangements of transmitters (loud speakers) and receivers (microphones), along with suitable DSP techniques for processing receiver signals have been disclosed in our prior patent applications. However, it has now been found that certain types of commercially available loudspeaker driver and horn combinations will function surprisingly well as microphones. Using such transducers can simplify and minimize the physical structure of the transmitter receiver assembly. Examples of suitable transducers that will function in this way are paging horn speakers SC-610/SC, SC-615/SC and SC-630M manufactured by TOA (www.toa.jp/) which are particularly sensitive as microphones to audio signals in the range 800 to 1500 Hz.  FIGS. 5-8  illustrate some possible configurations using these transducers. 
       FIG. 5  shows two such horn transducers  80  and  82  facing downwards above respective parabolic acoustic reflecting dishes  84  and  86 , horn  80  and its dish  84  being housed within a tubular acoustic insulating baffle  88  and horn  82  and its dish  86  being housed within a similar baffle  90 . Dish  84  is slightly angled toward the West while dish  86  is slightly angled toward the East. Horn  80 , dish  84  and baffle  88  form a West horn assembly  92  while horn  82 , dish  86  and baffle  90  form an East horn assembly  94 . Similar North and South horn assemblies  96  and  98  complete the set up, the four horn assemblies being shown in plan view in  FIG. 6 . Our prior patent applications disclose how the received signals derived from such a collection of receivers can be processed to reveal important characteristics of the atmosphere within range. Though computationally more challenging, the same results can be achieved by the use of three horn assemblies  100 ,  102  and  104 , as shown in plan view in  FIG. 7 . Other configurations are also possible. For example, as shown in  FIG. 8 , two or more horns  106  and  108  may be mounted over a common dish  110  and within a common baffle  112 , the horns being slightly angled toward or away from one another to monitor opposite portions of the atmosphere. This arrangement was also suggested in our prior patent applications. What is different here is that the horns or tranducers function both as loudspeakers and microphones so that separate dishes are not required for transmission and reception. 
     While various examples of the implementation of aspects of the present invention have been described, it will be appreciated by those skilled in the art that many variations and of the described examples are possible and that many other examples can be devised or postulated without departing from the scope of the invention as set out in the following claims.