BACKGROUND TO THE INVENTION

1 Field of the Invention

This invention relates to sodar methods, systems and apparatus for atmospheric sounding in the marine environment and, in particular, for use in detecting the boundary of an evaporative duct lying near the sea surface. While not limited to ship-board use, this is expected to be the principal use of the invention and most attention herein will be directed to such use.

2. Description of Related Art

Prior art pertaining to sodar methods for detecting evaporative duct boundaries over water, including any shipboard sodar system, could not be located. However, there are many known sodar systems capable of atmospheric sounding. Such known systems include my prior inventions disclosed in earlier patent applications, such as international applications PCT/AU01/00247, PCT/AU02/01129, PCT/AU04/00242 which relate to the use of long chirps and 'listen while sending' long-pulse techniques. And, such known systems, of course, include the conventional 'send then listen' short-pulse techniques that are well known in the prior art. These systems are reviewed by Coulter & Kallistratova in a 1999 review article "The Role Acoustic Sounding in a High-Technology Era" [Meteorol. Atmos. Phys. 71 , 3-19]. This article points out significant advantages associated with the use of upward-looking sodar for sounding the lower atmosphere generally, but notes that the potential of sodar has not been achieved because of the problem of atmospheric and man-made acoustic noise. No mention is made of the possible use of sodar in the marine environment. However, those skilled in the art will certainly appreciate that significant problems exist in using upward looking sodar on board a ship because of the ship's motion in normal seas and because information about the state of the atmosphere cannot be obtained from below the sodar. This problem together with the transmission/reception 'dead zone' associated with conventional sodars means that it very difficult to obtain any information at all about the lower 40 or so meters of the marine environment close to a ship.

A serious threat to ships in modern warfare is surface-skimming cruise missiles that fly within what is termed the 'evaporative duct', which renders them almost invisible to horizontally-looking defensive radar. What may be worse is that attempts to either detect the presence of an evaporative duct or detect on-coming surface-skimming missiles using low-mounted radar may well result in the probing radar beams being guided by the duct and providing powerful homing beacons for the missiles. Furthermore, the use of radar to identify poorly reflective / scattering atmospheric anomalies at very close ranges is difficult, requiring low-aiming of high-power millimeter beams that constitute a serious danger to the crew of the ship.

The evaporative duct is a cool layer of air, usually less than 10 m thick, located above open water such that the interface between this cool layer and the warmer air above it forms an atmospheric 'thermocline' that reflects or refracts most radar beams and sometimes visible light. The air in such ducts is cooled by the evaporation of wind-raised water droplets. While such ducts can be characterized using fixed or floating towers fitted with conventional meteorological instruments at various levels to determine the depth of the duct and the associated temperature differential, I am unaware of any operational shipboard system for doing so, as it is not useful to instrument the mast of a ship in a similar manner to such meteorological towers because of the air disturbance caused by the ship's movement, structure and heat emissions.

It will be appreciated that it is of great value to a ship's commander to know whether a significant evaporative duct is present in the vicinity of the ship and, if so, how thick it is and how reflective its upper surface is likely to be to ship-borne radar. This knowledge would greatly advantage a commander wishing to launch surface-skimming cruise missiles against an enemy ship. A defensive commander with this knowledge would also be advantaged, whether or not an in-coming missile could be identified in time to take defensive action. Of great and unique advantage would be a system that could continuously probe for the presence and

characteristics of an evaporative duct without providing a beacon for incoming missiles to follow.

BRIEF SUMMARY OF THE INVENTION In broad terms, the invention comprises the use of sodar to sound the atmosphere in the vicinity of the surface of a body of water such as the sea to detect the boundary of an evaporative duct (if present) lying above the water surface. The sodar system is preferably mounted so that it can 'look down' onto the water surface and detect acoustic echoes returned from the duct boundary, if present, and from the water surface so that the height of the boundary from water level can be determined. The system can be mounted on a fixture in the sea, such as an oil rig, lighthouse or signal tower but it will be more ^~ commonly be a shipboard system. In view of the likely presence of strong acoustic noise, the system preferably employs a pulsed acoustic beam in which the pulses are encoded so that the receiver can employ matched filter techniques to detect returned echoes. The invention therefore includes methods, systems and apparatus for detecting evaporative duct boundaries.

In shipboard applications the sodar transmitter and receiver elements can be mounted high on the ship so that there is a good chance that a duct boundary will be below them. While it is preferable that at least portion of the transmitted sodar beam is directed downwards and that the receiver is adapted to receive echoes that are returned upwards, this is likely to be achieved by the rolling or pitching motion of the ship. On the other hand, the sodar system may have an 'upward- looking' capability to allow detection of duct boundaries that are above the level at which the transmitter and/or receiver are mounted. This would be necessary on small ships that maybe completely contained within the duct or where the sodar system is mounted on the superstructure of the ship rather than the mast. While the sodar system is preferably of the send-then-listen type, systems of the listen- while-sending type are also envisaged in this invention. Finally, this specification should be read in conjunction with our co-pending international patent applications entitled "Staged Sodar Sounding", which teaches sodar techniques wherein a sets of long chirps are employed in a 'send-then-listen' mode in which

the echoes generated by the pulses are extracted using matched filter methods, "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.

An sodar transmitter and receiver (separately or as a combined unit) are preferably mounted so as look outwards from each side of the ship. Either or both the transmitter and receiver can be mounted on a stabilized platform so that the angle of illumination/reception (with respect to the horizontal) does not vary significantly with the rolling and/or pitching movements of the ship. In addition or alternatively, the sodar transmitter may be of the steerable phased-array type and may be electronically controlled so that axis of the downwardly-directed illuminating beam is at a substantially constant angle with respect to the horizontal. One arrangement is for a mast-mounted side-looking phased-array transmitter antenna to be servo-stabilized in the fore and aft direction to compensate for pitching motion of the ship and for the illuminating beam to be electronically steered in elevation to compensate for the rolling motion of the ship.

From another aspect, the transmitter may illuminate a strip of sea that is substantially parallel with the ship's axis on one side of the ship. The receiver on the same side may have an antenna lobe that subtends a large angle in elevation but a small angle in azimuth. Thus, the receiver interrogates an arcuate slice of space that extends substantially orthogonal to the ship and always includes portion of the narrow elongate strip of sea illuminated by the transmitter during any normal rolling of the ship and without the need for roll-stabilization. In one implementation of this feature, the transmitter may comprise a vertical array of transmitting transducers while the receiver may comprise a horizontal array of receiving transducers, the two arrays preferably being mounted along side one another on the mast. It will be appreciated that beam-steering (eg., using phased array techniques) can be readily implemented by those skilled in the art when transmitting single tones (as in a linear chirp) but will be difficult to implement in a receiver where multiple tones are received simultaneously from multiple echoes.

Where electronic receiver steering is desired, a form of aperture synthesis may be used and, if desired, in association with the use of a sequence of differently aimed transmitter beams. Thus, the arrangement outlined above has many advantages.

From another aspect, 'system' error may be mitigated or essentially removed by making a reference dataset indicative of the pattern of echo returns generated in conditions when it is known that no evaporative (or other surface) duct is present, making a similar interim dataset under operational conditions and differencing the reference and interim datasets as part of the process of generating a system output. The reference pattern will include components due to echoes from the ship's structure and components (eg, phase-shift patterns) due to the characteristic of the electronic transmitter and receiver systems employed. The reference pattern allows these constants and variables to be substantially removed under operational conditions. If desired, a series of reference signals may be generated for each of a series of angles of heel (both to port and starboard) so that the appropriate reference signal may be selected when an active reading is taken at sea with the ship at a given angle of heel.

To facilitate the use of a steerable phased-array transmitting antenna, it is desirable to use a chirped acoustic pulse that, at each instant, consists of a single frequency or pure tone - though the frequency may change from instant to instant. As disclosed in the patent applications relating to my earlier inventions, a chirp in which the tone rises or falls in linear manner is generally preferred because the echoes generated by such chirps can be efficiently processed using matched filter techniques that operate in the frequency domain.

While my earlier inventions envisaged the use of chirps that were tens of seconds in duration and, for example, extended from between 1 kHz and 5kHz, shorter and higher frequency chirps will be desirable for shipboard sodars in recognition of the much shorter ranges involved and of the need for more rapid up-dating. Thus, chirps of between a few tens of milliseconds a few seconds in duration with tonal ranges between 10kHz and 15kHz are desirable (though not essential) in this

context. For example, for relatively small ships - such as destroyers or frigates - that move a great deal with the sea, linear chirps with tonal ranges between 11kHz and 13kHz and with durations of between 50 and about 500 ms are suitable, though longer chirps will have advantages with larger ships and, therefore, longer ranges. Chirps in this tonal range also have the advantage that they are barely audible to most people in a normally noisy shipboard environment. Besides being more audible, chirps with longer wavelengths tend to result in lack of precision when determining the location of a duct boundary over small distances.

From another aspect, the sodar system may involve the use of software-related techniques to compensate for or to mitigate the effects of the roll or pitch of the ship on the signals received from ship-mounted sodars. For example, the output of a gyroscope may be used to gate the transmitter so that interrogating acoustic signals are only transmitted when the ship's mast is approximately vertical and/or to gate the received signals for the same purpose. Additionally or alternatively, the gating may be effected by examining the Doppler components of the received signal to determine when the ship is approximately vertical.

From another aspect, the sodar system may include a first sodar transmitter and receiver looking in a first direction (say to port) and a second sodar transmitter and receiver system looking in the opposite direction (say to starboard). Preferably, the transmitter beams are narrow (highly focused or collimated) and the same interrogating chirp signal is transmitted from each transmitter at the same time (though that is not essential). The received signals can then be processed and their amplitudes and/or Doppler components can be differenced to select the portion of the signals to be used or to highlight differences from one side of the ship to the other. The tones of echoes received by a fixed side-looking port-side receiver will be up-shifted as the ship rolls to port, while the tones of echoes will be down-shifted in the starboard side receiver. There will be a crossover point as the ship approaches vertical when the tones of the port and starboard echoes will be at a maximum shift with opposite signs before the ship rolls to starboard and the positive Doppler echoes received by the starboard

receiver start to slow down while the negative Doppler echoes at the port receiver start to slow down. This cross-over point can be used to gate or otherwise process the received signals to extract more information therefrom. Second, with strong wind coming from one side of the ship, there will be interest in determining to what extent the evaporative duct is disrupted on the lee side with respect to the windward side. Comparison of the amplitude returns from the two sides should be of value here.

It will be appreciated that the last-mentioned method of received signal processing (using two oppositely facing sodars and the differencing or comparison of the two sets of received signals) offers greater sensitivity than the use of a system in which separate sodars on each side of the mast or ship operate independently and their received signals are not compared or manipulated in combination. However, the use of a single system gated to operate (or select receiver inputs) only when the ship is at a selected angle of heel is also envisaged.

It will also be appreciated that this invention envisages gating the received signal(s) at times when the ship is not substantially vertical. This may be done to select or vary the range from which echoes are returned. Indeed, in the rare event that the upper boundary of the duct is above the level of the sodar transmitter and receiver, such gating can be used to select echoes returned from above the level of the receiver so as to determine the height of the upper boundary of the duct. In that case, gating would be effected when the ship is at a significant angle of heel so that the gated sodar is looking upward to detect a duct boundary that is higher than the ship.

The gated or selected receiver signal generally will be processed to extract echo amplitude and turbulence information indicative of the sea-air boundary and of the upper duct boundary and turbulence information from Doppler signals due to wave motion and air movement or turbulence within the duct. This emphasis or selection is greatly assisted where two signal streams have been generated from substantially identical transmitter signals, as where port and starboard side

transmitters each transmit the same interrogation chips and where substantially identical port and starboard receivers are employed.

Since multiple frequencies or tones are returned to the sodar receiver (from multiple echoes) at any given instant, it is not practicable at the present time to employ an electronically steerable phased array for the receiving antenna to compensate for the motion of the ship, though this may be made possible by future advances in signal processing techniques. The mounting of the receiving antenna on a stabilized platform is therefore of greater benefit for the receiver than for the transmitter antenna. However, according to another aspect of this invention the receiving antenna lobe can be configured as a vertical fan that is wide in elevation but narrow in azimuth and the transmitting antenna lobe or beam can be configured as a downwardly angled horizontal fan that is narrow in elevation but wide in azimuth, the transmitted beam falling centrally within the receiver lobe so as to define a small area of the sea surface or boundary duct from which echoes are generated. Thus, electronic aiming of the transmitted beam by phased-array methods or platform stabilization maintains the point at which the horizontal fan of the transmitter beam strikes the water substantially fixed during rolling of the ship, while the fan-like lobe of the fixed receiving antenna rocks up and down over the area illuminated by the transmitted beam so that the illuminated spot on the water is substantially always within the receiving lobe, despite rolling of the ship.

Details of the encoding of the 'chirp' (herein used for any pulse-compression waveform), and processing of the returned echoes using matched filter and/or wavelet methods are disclosed in the patent applications relating to my aforementioned inventions. By using those techniques, very large system and processing gains with high resolution can be obtained, sufficient to detect and process refracted or reflected echoes to generate meaningful data - in graphic form if desired - indicative of the location and strength of the top of the evaporation duct. From this information, it will be possible to determine in realtime whether the duct is strong enough to be impenetrable by radar or whether it is likely that radar can reliably detect incoming surface-skimming missiles.

The angle of declination at which the transmitter beam is directed (when the ship is upright) is not critical, but will normally be greater than 15° - and preferably greater than 25° - because lesser (more horizontal) inclinations tend to result in insufficiently strong returned echoes and smearing out of the return signals from beam-spreading, depending upon the character of the interface between the evaporation duct and the warmer air. Angles of declination much greater that 80° are also undesirable because the illuminated portion of the sea surface is so close to the ship that any duct boundary is likely to be disturbed by the presence of the ship by wind shadowing or turbulence. Such large declinations will also result in excessive echoes from the hard surfaces of the ship.

As in my preceding inventions mentioned above, multiple receivers are preferably employed in association with a single transmitter and phase data from two opposed receivers can be compared to mitigate common undesired signals.

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: Figure 1 is a diagrammatic end elevation of the bow of a ship with a stabilized mast-mounted sodar transmitter and an unstabilized receiver, the area illuminated by the narrow transmitter beam and the broad area monitored by the receiver being shown along with echoes returned from the sea and two different duct interfaces. Figure 1A is a pair of graphs (a) and (b) indicating the type of amplitude signal generated by a sodar system of Figure 1 operating in either of two different modes.

Figure 2 is a plan view of the ship of Figure 1 showing the area illuminated by the transmitter beam and the area monitored by the receiver, viewed from above.

Figure 3 is a similar view to Figure 1 showing the ship heeling and the locations of the transmitter beam and the area monitored by the receiver.

Figure 4 is a block diagram of a signal processing system that may be employed in the first example.

Figures 5A and 5B are displays of phase and amplitude (respectively) illustrative of the results generated using the system of the first example, Figure 6 illustrates a second example where both transmitter and receiver have broad beams in elevation and where neither is stabilized with respect to the ship.

Figure 6A is a series of graphs indicating phase shifts caused by rolling of the ship and one why in which the opposite shifts from the two sodar systems maybe neutralized in the second example.

DETAILED DESCRIPTION OF THE INVENTION

Turning to Figures 1 , 2 and 3, the marine sodar system 10 of the first example basically comprises port and starboard transmitters 12p and 12s and respectively associated port and starboard receivers 14p and 14s mounted on the mast 16 of a ship 18 shown facing the viewer. Transmitters 12p and 12s generate beams 2Op and 20s of acoustic energy which are narrow in elevation (see Figure 1) and wide in azimuth (see Figure 2), while receivers 14p and 14s have antenna lobes 22p and 22s that are wide in elevation (see Figure 1) and narrow in azimuth (see Figure 2). Thus, as shown in Figure 2, starboard transmitter 12s illuminates an elongate strip 24s of the sea surface extending in parallel spaced relation to ship 18 while receiver 14s interrogates an elongate strip 26s of the sea surface extending outward orthogonal to the ship. Strips 24s and 26s overlap only in a relatively small area 28s (shown shaded in Figure 2) so that echoes of the transmitted signal arising from this area are selectively detected by receiver 14s. The same situation exists on the port side of the ship 18.

In this first example, the transmitter beams 2Op and 20s are stabilized in elevation so that they are unaffected by normal rolling motion of ship 18. As previously indicated, this may be done by mounting the transmitters 12p and 12s on a mechanically stabilized platform (as is well known in the art of marine and aircraft instrumentation), or by employing an electronically steerable phased array transmitter antenna assembly (as is well know in the radar art). The latter option is assumed in this example. Since the antenna lobes 22p and 22s of receivers 14p and 14s are a wide angle arc in elevation, there is no need to stabilize the receivers in this first example. The situation for the port transmitter 12p and port receiver 14p when the ship 18 rolls to starboard are illustrated in Figure 3. Since transmitter 12p is stabilized, the rolling of ship 18 to starboard does not affect the angle that its beam 2Op subtends to the sea surface 30 and, since the arc of the antenna lobe 22p of receiver 14p is so wide that beam 2Op still falls within it at or near sea surface 30 (despite the lean of the ship), echoes from sea surface 30 will be scattered back to receiver 14p. The corresponding situation exists on the starboard side where stabilized transmitter beam 20s is adjusted to subtend the same angle with respect to the horizontal but receiver antenna lobe 22s swings down with the roll of the ship but, nevertheless, continues to encompass beam 20s at water level and considerably above.

For the sake of illustration, three possible alternative upper boundaries for the evaporation duct near ship 18 are shown in Figures 1 and 3. For a ship at least as big as a Destroyer in gentle to moderate breezes (Force 3 - 4 on the Beaufort Scale), the evaporation duct surface would normally be around 10m high - as indicated at 32 - in ambient temperatures around 3O ⁰ C. Under fresh and strong breeze conditions (Force 5 - 6) at these ambient temperatures, the evaporation duct surface could be at least 20m high - as indicated at 34. For smaller ships and more extreme conditions of ambient temperature and wind, the entire ship could be within the duct - as indicated by duct surface 36. As clear from the drawings, interrogating beam 2Op will intersect all normal evaporation duct levels and echoes 42 or 44 from duct surfaces 32 or 34 will be back-scattered to receiver 14p along with echoes 40 generated from the sea surface 30.

Though the signal processing system of the first example is similar to those described in our prior patents, there are important differences and the system of the present example will be described below with reference to Figure 4. While either a send-then-listen or a listen-while-sending strategy may be employed, the former was selected for this example because the close proximity of the transmitter and receiver and the limited space available for acoustic shielding mean that the direct signal will be very powerful at the receiver. Another consideration is that the target anomalies are relatively close-range - normally 50 - 500 m depending on the declination angle so that short pulses must be used with the result that relatively high audio frequencies (i.e., above 10 kHz) are desirable for the chirp in order to maximize sensitivity and discrimination. Such frequencies also have the advantage that they will be nearly inaudible to the crew of the ship. In this example, a linear chirp ranging from 11 - 12 kHz for a duration of 40 ms, followed by a 45 ms delay before reception is enabled, was selected. This results in a range dead-zone of about 35 m from which echoes cannot be detected, it being noted that strong echoes from much of the ship's super structure will be returned and excluded during this dead-zone. At 11 kHz, a 40 ms pulse includes total of 440 cycles, which is adequate for the matched filter discrimination techniques employed.

Figure 4 shows port sodar transmitter 12p as a phased array of four transducers 50, each of which receives the 11 - 12 kHz chirp generated by a PC 52 via PC output line 54 and a respective variable delay line / driver unit 0 - 4 that is phase-adjusted by PC 52 via output 55, by reference to a roll angle input 56 from the ship's guidance system, to set the phase delays such that the center of transmitter beam 2Op (Figures 1 , 2 and 3) is maintained at a constant angle to the horizontal. The receiver 14p is shown as having four acoustic detection transducers 58 selected to have maximum sensitivity between 10 and 14 kHz which are essentially arranged and connected in parallel to provide the desired receiver antenna lobe 22p, the combined output being shown at 60. [It will be appreciated that realistic phased arrays will have many more active elements than the four selected for the purpose of illustration.]

Receiver output 60 is sampled at 96.1 kHz in analog-digital converter 62, then subjected to fast-Fourier transformation in unit 64 before being band-pass limited by unit 66 in the frequency domain to between 10.5 and 12.5 kHz and then reconverted into the time domain by an inverse-fast-Fourier-transformation in unit 68. The resultant train of clipped digital sample pulses appears on output 70 which is fed into two mixers 72 and 74 where each sample is effectively multiplied by a correspondingly sampled stream of sine and cosine 13 kHz signals derived from PC 52 to give phase-displaced high and low side-band products. The output of mixer 72 is subjected to Fourier transformation in unit 76 and then clipped by a low-pass digital filter 78 to remove the upper side-band and, similarly, the output of mixer 74 is Fourier-transformed in unit 80 and clipped in low-pass filter 82 to yield respective outputs 84 and 86 comprising digitized signals of less than 3 kHz including echo frequencies in the range 2 - 1 kHz. In order to extract echo signals from these outputs, they are mixed with a 1 - 2kHz chirp signal generated by PC 52 in mixers 88 and 90, the outputs of which are fed to a complex inverse- fast-Fourier-transform unit 92 from which two output sample streams emerge on lines 94 and 96 that are respectively indicative of the amplitude and incremental phase of the extracted echo signals.

A predetermined (largely arbitrary) number of adjacent amplitude samples are averaged and stored in a 'range bin' in unit 98, each bin representing a range increment. The sample-by-sampie phase-increments on output 96 are converted by an 'unwind function ¹ in unit 100 to a cumulative phase shift indicative of the Doppler shift at a given range. If desired, a batch of chirps is transmitted for a short period of time (up to a few seconds) at a repetition rate of about four chirps per second and each chirp of the batch is processed as described and the amplitude and phase outputs are averaged over the batch. Batch lengths of more than a few seconds will be undesirable if the ship is rolling because of the resultant 'smearing inaccuracies' introduced by averaging.

In this example, a reference signal from each transmitter-receiver pair representative of 'ambient conditions' (i.e., where there is no appreciable evaporative duct) is derived by using a batch of chirps as described above and is

stored in PC 52. These reference signals will include echoes from portions of the ship's superstructure and be colored by system phase-shift and amplitude spectrum.

Thus, the final signal processing function needed in the process of Figure 4 is performed under 'active' conditions (where evaporation ducts need to be characterized). The stored reference 'binned' samples in PC 52 and the active binned samples in unit 98 are differenced to remove the ambient signal components in differencing unit 102 and the result is fed to a display device 104. Similarly, the stored reference cumulative phase shift is differenced with the active cumulative phase shift signal from unit 100 in differencing unit 106 before being fed to display 104. Display unit 104 can be programmed to make the simple adjustment from angular down-range distance to height above sea-level by using the known stabilized transmitter angle, the known height of the receiver and the known angle of heel of the ship.

Graph (a) of Figure 1 A illustrates what the derived amplitude signal from a listen- while-sending system might look like, taking down-range echo returns, while graph (b) indicates what an amplitude signal from a 'send-then-listen' system might look like, also taking down-range echo returns. Early peaks 110 indicate residual echoes from the ship's superstructure, peaks around range / time-line 112 indicate a low lying duct boundary such as 32 in Figures 1A and 3, peaks around range / time-line 114 indicates a higher duct boundary such as 34 in Figures 1 and 3, while the strongest and most distant peaks around time-line 116 indicate the sea level 30.

Figures 5A and 5B show the type of displays that can be produced after correcting for sighting angle and after repeated soundings. Figure 5A is a color display chart (here reproduced in grey-scale) in which the color of a 'pixel' indicates Doppler or phase-component as proxy for local wind speed or turbulence, the vertical location of the pixel indicates the altitude above sea level where those local wind conditions apply and the horizontal position of the pixel indicates the time of day when the data was collected. In this chart, lighter color

pixels generally indicate higher wind speeds but much information in the chart is lost through its reproduction in grey-scale. As will be seen from the horizontal scale, the sodar system was used to continuously scan for an evaporative duct from 11 :00 to 16:00 on the day of the test and a duct boundary appears likely between 10 m and 25 m above sea level from 11 :00 and 13:00, after which it appears that the duct boundary fell to between 5 and 20 m before the duct broke up. The situation is not entirely clear from the phase data of the chart of Figure 5A and needs to be supplemented by the amplitude data of the chart of Figure 5B.

Figure 5B is also a color chart in which the color and position of a pixel is indicative of the amplitude of echoes returned at a given time and from a given altitude above sea level. Again, much information is lost in the grey-scale reproduction of Figure 5B but, in very crude terms, darker pixels indicate higher amplitudes. The amplitude data was collected at the same time as the phase data by the system described above with respect to Figure 4. As before, the chart of Figure 5B plots computed height above sea level on the vertical axis and time of day on the horizontal. Here the initial duct boundary at about 25 m is more clearly indicated, as is its fall and break-up. Moreover, it is evident that the heavy vertical dark lines are indicative of rain, initially a rain shower at 13:00 did not penetrate to sea level but showers at 14:00 started to do so and clearly did so at 15:30. Clearly, the break up of the duct was caused by the rain showers.

The second example will now be described with reference to Figure 6 where a small ship 200 such as a Frigate or mine-sweeper is assumed to be rolling heavily in rough conditions. The ship is assumed to be too small to justify the stabilization of either the transmitter beam or the receiver antenna lobe, both of which in this case subtend wide angles in elevation and relative narrow angles in azimuth. As before, port and starboard transmitters 202p and 202s generate port and starboard interrogation beams, indicated by boundary lines 204p and 204s, while the boundaries of the antenna lobes of the port and starboard receivers 206p and 206s are indicated at 208p and 208s. Echoes from the sea 210 on the port and starboard sides are indicated at 212p and 212s while those generated from the surface 214 of a duct are indicated at 216p and 216s. An alternative and higher

location for the duct boundary positioned above the ship is shown 220 and it will be noted that echoes 222 from such a boundary will be returned to port receiver 206p when the ship heels significantly to starboard, as shown in Figure 6.

A problem resulting from a rolling ship without any form of stabilization of the sodar is that the angle and distance between the sodar transmitter-receiver and the sea or duct boundary will vary widely. However, there are a number of ways of addressing this problem. First, by implementing the digital signal processing techniques of Figure 4, PC 52 can take account of the changing roll-angle (on input line 56) and make the appropriate computations, but this may be so computationally intensive that the processing must be done 'off line' (not in realtime). Second, the collection of echo data may be gated to occur only at a pre- decided roll-angle, but this has the disadvantage that a lot of echo data must be thrown away. The awkward fact is that the times when the roll-velocity of the ship is the least - viz. at the extremes of a roll - the data is of least value because of the beam angles, and at the time that the beam angles are best - viz. when the mast is vertical - the roll-velocity is highest and the gating period is likely to be the shortest.

It thus would be desirable to extend the sample period during the time that the mast is near-vertical (and including vertical) by comparing the data received by the port and starboard systems as the ship rolls through upright. Consider, for example, that ship 200 shown in Figure 6 is righting itself by rolling to port through upright as indicated by arrow 230. In that case, the rapid decrease in the distance between port receiver 206p and the origin of echoes 212p and 216p results in an up-shift in the phase component of these echoes, while the rapid increase in the distance between starboard receiver 206s and the origin of echoes 212s and 216s will result in a down-shift of the phase component of these echoes. These opposite phase or Doppler shifts decrease in magnitude as the ship rights until they are zero when the ship is vertical and then reverse as the shift leans to port. Figure 6A illustrates this, the graph (p) being the Doppler or phase shift in the port receiver system due to the ship's motion, the graph (s) being the Doppler or phase shift in the starboard receiver system due to the ship's motion and the

graph (+) being the addition of graphs (p) and (s). Such differencing or other comparative manipulation of the echoes from each side of the ship can offer considerable value in processing both phase and amplitude data when the ship is rolling.

While two examples have been provided that offer significant advantages over the radar prior art, it will be appreciated that many variations and additions can be made without departing from the principles of the invention as set out in the following claims.