Abstract:
A bi-static sodar system and method are used to measure and monitor the wake vortices of aircraft in the flight path of an airport runway. A loudspeaker ( 16 ) is arranged on one side of the flight path ( 12 ) and transmits a series of acoustic pulses to illuminate portion of the flight path. Multiple microphones ( 18, 20  and  22 ) are arranged on the opposite side of the flight path ( 12 ) so as to receive direct signals from the loudspeaker ( 16 ) and forward-scattered echo signals from an echo source ( 26 ) within the illuminated portion of the flight path. The microphones ( 18, 20  and  22 ) are arranged at different distances from the loudspeaker so that the time intervals between the receipt of the direct and echo signals from each pulse will vary because of the different locations of the microphones. This variation is used to assist in identifying the location and other characteristics of the echo signals and in generating an output indicative of a wake vortex ( 28 ).

Description:
This is a national phase application in the United States of International Application PCT/AU2006/000245 filed 28 Feb. 2006, claiming priority from Australian application number 2005900899 filed 28 Feb. 2005, the disclosures of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to sodar apparatus, methods and systems for use in detecting, characterising, recording and/or displaying the wake vortices shed from large aircraft on the approach to or departure flight path of an airport runway. It is more particularly concerned with sodar systems of the bi-static type where the acoustic receiver is down-range of the atmospheric echo source of interest. Thus, the echoes of interest are forward scattered, reflected and/or refracted having regard to the direction of the interrogating acoustic beam. This is to be contrasted with a mono-static system in which the transmitter and receiver tend to be co-located and the echoes of interest are those that are backward scattered, reflected and/or refracted. 
   2. Description of Related Art 
   Bi-static sodar systems have long been proposed and used for the sounding of the lower atmosphere where atmospheric discontinuities tend to be horizontal, slow moving and wide spread, except in storm conditions. This allows the use of a single transmitter and a single receiver located down-range from the transmitter by a known distance and the use of simple triangulation to determine the height of a source of ‘echoes’ from forward scattered, refracted and/or reflected transmitted signals. It is well known in such systems to track atmospheric anomalies over long periods of time by using a pulse transmitter and listening for echoes between pulses. Sufficiently strong echo signals allow Doppler components to be extracted that provide an indication of the velocity of movement of echo sources either vertically or down-range. 
   However, conventional bi-static sodar systems of the type indicated are unsuited to the characterization of wake vortices which are typically close to the ground and capable of rapid movement in three dimensions over their relatively short durations. The simple triangulation techniques of bi-static sodar are inadequate for such an application. 
   A serious and more general problem with the use of sodar techniques for atmospheric sounding arises from their inherently poor signal-to-noise ratio [s/n] due to (i) the limited power of acoustic transmitters (ii) the strong attenuation of acoustic waves in the atmosphere and (iii) the prevalence of acoustic noise. The latter problem is of particular importance in a noisy airport environment, especially when attempting to detect wake vortices of aircraft as they pass down (or up) the flight path. 
   The above problems have been addressed in our prior international applications PCT/AU01/00247, PCT/AU02/01129, PCT/AU04/00242 that disclosed sodar systems with exceptionally high s/n ratios. Our prior systems have variously employed long duration transmitted pulses encoded in a ‘pulse compression’ manner, over-sampling of received echoes for good resolution and processing gain, and the use of matched filter tailored to the pulse-compression code to provide low-power, long range sodars capable of extracting excellent Doppler signals from the received echoes. The pulses—generically called ‘chirps’—employed in our prior inventions preferably had durations in the order of tens of seconds. The pulse-compression technique employed was preferably a linear increase or decrease in phase (tone) over the duration of the pulse; for example, a steady increase in tone from 500 to 1500 Hz, or a steady decrease in tone from 1500 to 500 Hz. The methods disclosed involved ‘listening while sending’; that is, echoes are received and processed while transmission of the chirp is still under way. This technique not only allows very high system and processing gains that result in exceptionally good s/n (signal to noise ratio), but it also enables atmospheric discontinuities that occur close to the ground to be detected. 
   The present specification should also be read in conjunction with our international patent application PCT/AU2004/001075 entitled “Detection of Wake Vortices and the Like in the Lower Atmosphere” which taught the technique of sounding the atmosphere near an airport runway when a wake vortex is not present to generate a reference dataset, then sounding the atmosphere when it is suspected that an wake vortex might be present to generate an active dataset and differencing the two datasets to highlight a vortex, if present. Finally, this specification should be read in conjunction with our co-pending Australian patent application entitled “Staged Sodar Sounding” (to be published), which teaches sodar techniques wherein a set of long chirps is employed in a ‘send-then-listen’ mode in which the echoes generated by the pulses are extracted using matched filter methods. While the sodar systems disclosed in our prior applications were capable of detecting wake vortices and of monitoring wind conditions in the vicinity of airports with much greater sensitivity and precision than was previously possible, they still left something to be desired in tracking an intense wake vortex as it moves in three dimensions. 
   For brevity, the disclosures in our published applications are regarded as being incorporated herein, including the extensive discussion of the prior art contained in the specifications of those applications. In addition, some of the terminology that is used herein is explained or defined in those specifications. 
   In the following, the term ‘flight path’ will be used to designate the volume of the lower atmosphere near either end of an airstrip or runway through which aircraft pass on approach to or take-off from an airport. It is here where persisting wake vortices are caused by large aircraft and can be dangerous for other smaller aircraft using the flight path even minutes later. 
   BRIEF SUMMARY OF THE INVENTION 
   From one aspect, the present invention comprises a bi-static sodar system for measuring and monitoring the wake vortices of aircraft in the flight path of an airport runway. The system includes sodar transmitter means arranged on one side of the flight path and adapted to transmit a series of acoustic pulses to illuminate portion of the flight path. The system also includes sodar receiver means arranged opposite the transmitter means on the other side of the flight path for receiving echoes of transmitted sodar signals from atmospheric disturbances within the illuminated portion of the flight path. The receiver means includes acoustic sensors or microphones located at different locations which are at different distances from the transmitter and the receiver means is adapted to determine the time interval between a direct signal received from the transmitter means at each location and subsequent echo signals received at each location. The receiver means includes processor means adapted to correlate the time intervals determined at each location and to compute the position within the wake vortex from which the respective echo signals are generated. 
   The transmitter means may comprise a linear array of multiple loudspeakers aligned with the flight path and the receiver means may comprise multiple parallel linear arrays of microphones also aligned with the flight path, each microphone array being located at a different distance from the loudspeaker array. The transmitter means is preferably adapted to transmit a series of acoustic pulses in a pulse-compression format and the receiver means preferably includes matched-filter detectors for processing the received signals and extracting the echoes therefrom. A loudspeaker driver means is preferably employed to drive the loudspeakers in parallel so that each loudspeaker generates the same acoustic pulse at the same time and in the same phase as each other speaker. Preferably, the loudspeakers are baffled and configured so as to facilitate or enhance such ‘iso-phase’ operation. 
   We have found that three arrays of microphones are generally adequate for effective echo detection, discrimination and location, but we have also found that the performance of the system can be significantly improved if each microphone array is mounted at a different height to the others—and most preferably so that the height of the arrays increase with distance from the loudspeaker array. 
   From another aspect, the invention comprises a method of monitoring and measuring aircraft wake vortices in the flight path of an airport by illuminating portion of the flight path with acoustic pulses from transmitter means located one side of the flight path and using receiver means on the other side of the flight path opposite the transmitter means to (i) detect both direct and echo signals from each transmitted pulse at each one of a plurality of locations that are spaced at different distances from the flight path (ii) determine the time interval between receipt of the direct and echo signals at each location and (iii) employ the determined time intervals to compute parameters or characteristics of the detected echoes such as location within the flight path of the source of the echoes and phase, frequency [Doppler] and amplitude components. The receiver means will therefore also preferably be used to analyze the echo signals and to generate outputs indicative of the amplitude and Doppler or phase components in the detected echo signals using techniques known in the art and, in particular, as taught by our prior patent applications. 
   It will be appreciated that there will normally be multiple anomalies (ie, echo sources) associated with the aircraft wake vortices that will result in multiple overlapping echoes being detected by the receiver. These echoes need to be identified, extracted and analyzed. It is therefore desirable to apply the chirped transmit signals and matched filter processing of received signals that have been disclosed in our associated published patent applications, whether ‘listen-while-sending’, ‘send-then-listen’, ‘narrow chirp’ or ‘time domain’ methods are employed. The matched filtering may be performed in the frequency domain or in the time domain, with the former being preferred because of cost and efficiency. Again, the reader is referred to our associated patent applications for a detailed description of such techniques. 
   However, we have found that, in the present application, linear (pulse-compression) chirps in the frequency range of 1.5 kHz to 3.5 kHz are suitable with the range of 2 kHz to 3 kHz being preferred. We have also found that, for the present application, transmit pulse lengths of between 0.2 s and 0.6 s and pulse intervals of between 0.75 s and 2 s are suitable for send-then-listen systems of the type indicated herein, with pulse lengths of 0.25 s to 0.35 s being optimum for normal size runways at international airports. On the other hand, we have found that transmit pulse lengths of between 0.5 s and 2.5 s and listening times of between 1 s and 3 s are suitable for send-while-listening systems, with pulse lengths of 0.75 s to 2.25 s being optimal for normal size runways at international airports. 
   In operation, the array of loudspeakers will illuminate a volume of air along and across the aircraft flight path with acoustic signals and the microphones of each array will detect both the direct signals from the array of loudspeakers and the resultant forward-scattered echoes. 
   Having portrayed the nature of the present invention, particular examples 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 outlined above and as defined in the following claims. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a diagrammatic end elevation of an airport runway showing a landing aircraft within the portion of the flight path that is illuminated by an array of transmitters and interrogated by three arrays of receivers. 
       FIG. 2  is a simplified version of  FIG. 1  in which illustrative dimensions are shown. 
       FIG. 3  is a set of graphs indicating the delay between receipt of the direct and echo signals at each microphone of  FIGS. 1 and 2 . 
       FIG. 4  is a similar view to those of  FIGS. 1 and 2  intended to illustrate the benefit of using three microphones. 
       FIG. 5  is a plan view of a modified system of the type shown in  FIG. 1 . 
       FIG. 6  is a chart of the results of a practical test using the system of  FIG. 1  and with one transmitter and three microphones. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 ,  2  and  3  are intended to illustrate the general principles of the invention. In  FIG. 1 , an aircraft  10  is shown approaching an airport runway  12  and located within an approach flight path indicated at  14 . Aircraft  10  is assumed to have just passed between a loudspeaker  16  (constituting the acoustic transmitter means) located on the left side of runway  12  and an array of three microphones  18 ,  20  and  22  (that constitutes the acoustic receiver means) located on the other side of runway  12  opposite loudspeaker  16 . Microphones  18 ,  20  and  22  are located at successive greater distances from loudspeaker  16 . The portion of flight path  14  that is effectively illuminated by loudspeaker  16  is shown in section and shaded at  24 . The sections of flight path  14  interrogated by microphones  18 ,  20  and  22  are indicated by shapes  18   a ,  20   a  and  22   a . That is, these are the areas in which echo sources can be detected by the respective microphones. 
     FIG. 2  depicts an illustrative simple two-dimensional geometry applicable to  FIG. 1  in order to illustrate the different path lengths that the direct signals from loudspeaker  16  must travel to microphones  18 ,  20  and  22  and the different path lengths that echo signals from an echo source  26  in a vortex  28  must travel to each microphone  18 ,  20  and  22 . It is assumed that echo source  26 , loudspeaker  14  and microphones  18 ,  20  and  22  are all in the same vertical plane, that echo source  26  is at an elevation of 30 m centrally above runway  14 , that loudspeaker  16  is located 50 m on the left of the center of runway  14 , microphones  18 ,  20  and  22  are located opposite at distances of 50 m, 65 m and 80 m respectively from center of runway  14 . 
   It can be easily calculated that the distances which echo signals must travel from echo source  26  to microphones  18 ,  20  and  22  are respectively 58.3 m, 71.6 m and 85.4 m. In other words, the echo signals from echo source  26  in vortex  28  have to travel 8.3 m, 6.6 m and 5.4 m to microphones  18 ,  20  and  22  respectively more than the corresponding direct signals indicated by broken-line arrows  30 ,  32  and  34 . These differences in path lengths for direct and echo signals are diagrammatically illustrated in  FIG. 3  and directly translate (by using the known velocity of sound in air) into respective time intervals between receipt of direct and echo signals at each microphone in the array. 
   While the simple triangulation method of conventional bi-static sodar using a loudspeaker  16  and any one of microphones  18 ,  20  an  22  would allow variations in the height of echo source  26  to be tracked effectively so long is it remained substantially in the same vertical plane as the speaker and microphone, this is of little value for tracking vortex anomalies that move in three dimensions because many echo sources that do not lie in the vertical plane passing through transmitter and receiver would yield the same time interval between direct signal and echo signal reception. We have found that, by combining the time intervals measured at multiple locations that vary in distance from the transmitter, it is possible to accurately determine the location of the echo source in three dimensions and in time. Furthermore, we have found that the accuracy of echo location is significantly enhanced if the multiple microphones are located at different heights and, preferably, if the height increases with distance from the runway or transmitter. These experimental findings have now been mathematically confirmed and publication in the scientific literature is planned. 
   Though it is difficult to depict in a two dimensional diagram, the manner in which the time intervals between reception of a direct signal and an echo signal by each microphone can be used to locate the echo source in three dimensions is indicated schematically in  FIG. 4 . Here, microphones  18 ,  20  and  22  are mounted at successively increasing heights. The broken-line ellipses  18   b ,  20   b  and  22   b  respectively indicate (necessarily in two dimensions) the three dimensional contour of positions where echo source  26  will result in the same time intervals between direct and echo signals at microphones  18 ,  20  and  22  respectively. This would lead to serious ambiguity in echo location if any one of the microphones was use alone. However, for any particular location of echo source  26  (in range) there will be a unique pattern of time intervals from the three microphones that will uniquely signify that location. This is diagrammatically indicated by the intersection of the ellipses  18   b ,  20   b  and  22   b  at echo source  26  (and approximately at loudspeaker  16 ). 
   Since the position along a flight path at which a wake vortex might occur cannot be determined with any certainly, it is preferable to arrange multiple loudspeakers and microphones in elongate parallel arrays on their respective sides of the flight path. This is illustrated in  FIG. 5  in plan view where a transmitter array  40  of nine loudspeakers  16  is arranged parallel with flight path  14  on the approach end of runway  12  and is opposed by a receiver array  42  comprising three rows  44 ,  46  and  48  of nine microphones  18 ,  20  and  22  (respectively) in each row. In practice there are likely to be at least thirty loudspeakers in the transmitter array and 3×30 microphones in the microphone array. The spacing of transmitter array  40  and each row of microphones can be assumed to be as in  FIG. 2 ; that is, the distances indicated by arrows (a), (b), (c) and (d) are respectively 15 m, 15 m, 50 m and 50 m. The loudspeakers  16  in array  40  are preferably all mounted at the same height, and the microphones  18  in row  44  are also preferably mounted at the same height, the microphones  20  in row  46  are also preferably mounted at the same height, and the microphones  22  in row  48  are also preferably mounted at the same height (though, as already noted, the microphones of different rows will be at different heights). 
   In this example, loudspeakers  16  are connected and driven in parallel by a common driver circuit  34  that delivers a common chirp signal simultaneously to each. We have found that, with the short ranges indicated in  FIGS. 2 and 3 , it is preferable to use the send-then-listen technique using chirped transmit pulses that increase in frequency from about 2.2 kHz to about 2.8 KHz in a strictly linear fashion over a period of about 0.3 s (pulse length) and to allow a time interval (listening time) of about 1.2 s between pulses. This gives a repetition (update) rate of about 1.5 s. Longer ranges suggest the use of send-while-listening techniques but satisfactory results can be obtained in the system of the example using transmit pulses of about 1 s in duration and (overlapping) listening times of about 1.9 s so that the repetition rate can be about 2 s. In either case, however, it is preferred to use the frequency [Fourier] disclosed in our prior patent applications to ensure adequate s/n despite the noisy airport environment. 
   Also, care needs to be taken in the design or choice of loudspeaker horns to ensure that the chirps emerging from all loudspeaker units will be in-phase. It is convenient to select loudspeaker horn assemblies that will serve as both transmitters and microphones. Suitable horns are model SC-630 of the TOA brand manufactured in the USA, which are capable of delivering a three Watt 2 kHz-2 Khz chirp without distortion. Having folded horns, these transducers are well suited for mounting facing upwards as would be required for service as microphones. However, when each microphone horn needs to be separately and thoroughly baffled at all points except the open top. It is also advantageous to baffle each transmitter horn in the same manner to minimize resonance caused by adjacent speakers. 
   In this example, and as shown in  FIG. 5 , each microphone  18 ,  20  and  22  is shown as a circle in a box to indicate that the microphone is built into an associated receiver indicated at  18   c ,  20   c  and  22   c  respectively. Each receiver includes at least a filter and pre-amplifier but, preferably, also includes a PC-based matched filter system of the type disclosed in one or more of our prior patent applications that is adapted to output amplitude and Doppler signals derived from the echoes received. The output of each receiver is independently connected for storage, consolidation, analysis and display in receiver processor  36  in the manner disclosed in one or more of our prior patent applications. The manner in which receiver outputs are combined depends on the linear spacing of the receivers in their arrays or rows. If compact, outputs can be combined; if spread out over a considerable distance (eg, of the order of 1 km), the outputs will best be treated separately to allow echo sources to be tracked along a flight path. 
   As already indicated, a variety of signal formats and signal processing methods can be used. A send-then-listen system can be used that takes into account the spacing of the receivers from the transmitter. This would give a maximum pulse length of 0.3 s and a minimum listen time (ie, interval between pulses) of about 1.2 seconds for an echo path length of up to about 300 m. Alternatively, if a listen-while-sending system was adopted, transmit pulses of about 1 s could be used with listening times of about 1.9 s for a 300 m echo path length. Details of both such systems have been provided in our above-referenced prior patent applications. 
   Finally, the graph shown  FIG. 6  has been included to indicate the type of results that can be obtained in the field using the system disclosed herein where only one loudspeaker and three opposing microphones are employed. This graph shows data relating to a wake vortex shed by a Boeing 737 during an approach to the main runway at Melbourne airport using a send-while-listening system. The phase response shows a spacing of 8 m between peaks of the Doppler signal, the peaks being indicative of the maximum core edge velocities (as noted in the graph). Assuming that the circulation strength of the wake vortex of a Boeing 737 was 70 m/s 2  (in line with published data) at the time as the sodar measurements were taken and that the relationship between circulation strength and averaged sodar-measured maximum core edge velocity is linear, then the constant of proportionality can be calculated to be 16.6, giving:
 
Circulation strength=16.6 Measured Peak Velocity
 
   This simple relationship needs to be calibrated for each sodar setting. Calibration can be achieved by first using the theory below to estimate the scaling factor and then calibrating the scaling constant against other know measurement techniques. 
   The theory for spatial averaging for short pulse sodars is given in http://wwwe.onecert.fr/projets/WakeNet2-Europe/fichiers/publications/publi2005/Bradley%20et%20al%20SODAR.pdf, 22 Oct. 2005. This theory of spatial averaging indicates that the relationship between the velocity measured by a sodar and the circulation strength is probably linear. 
   While an example of the implementation of aspects of the present invention has been described, it will be appreciated by those skilled in the art that many variations and of the described example 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. For example, because of the proximity of the receiver to the transmitter, the direct transmitter signal may be formed by an electronic wire or wireless signal to provide a reference for the time delays associated with echo reception at the respective microphone arrays. However, there is little advantage in this as the direct acoustic signal will be received at each microphone whether or not an electronic direct signal reference is employed.