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 fig1 . 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 ( fig2 ) of 100 ms followed by a 600 ms interval during which listening takes after which a second chirp 62 ( fig2 ) 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 fig2 , 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 fig2 , the altitude range covered by the use of chirp 60 is 16 - 116 m . fig2 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 fig3 provides and alternative way of depicting the ranges , times and dead zones associated with the two chirp set shown in fig2 . like fig2 , fig3 suggests that any desired range can be covered using chirp sets and , by way of second example , fig4 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 . fig5 - 8 illustrate some possible configurations using these transducers . fig5 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 fig6 . 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 fig7 . other configurations are also possible . for example , as shown in fig8 , 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 .