AU2011100797B4 - System and method for detecting adverse atmospheric conditions ahead of an aircraft - Google Patents

Abstract

-17 Abstract System and method for detecting adverse atmospheric conditions ahead of an aircraft. The system has multiple, infrared cameras 8 adjusted to spatially detect 5 infrared radiance in different bands of infrared light, wherein each camera is connected to an image processing computer that processes and combines the images, and generates video display signals for producing a video display which indicates the position of the adverse atmospheric conditions relative to the aircraft. Each of the cameras is provided with a respective filter adjusted to filter infrared light with a 10 bandwidth corresponding to infrared bandwidth characteristics of an adverse atmospheric condition from a set of adverse atmospheric conditions. The image processing computer is adapted to identify adverse atmospheric conditions, said identifying being based on threshold conditions and using the detected infrared radiance, data from a look-up table and measured parameters including information 15 on the position and/or attitude of the aircraft. The image processing computer is further adapted to display the identified adverse atmospheric conditions as a spatial image on a display.

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR AN INNOVATION PATENT Name of Applicant: Norsk Institutt For Luftforskning Actual Inventors: Alfredo Jose PRATA and Cirilo BERNADO Address for Service: DAVIES COLLISON CAVE, Patent Attorneys, of 1 Nicholson Street, Melbourne, Victoria 3000, Australia Invention Title: "System and method for detecting adverse atmospheric conditions ahead of an aircraft" The following statement is a full description of this invention, including the best method of performing it known to us: - 1- System and method for detecting adverse atmospheric conditions ahead of an aircraft The present invention relates to a system and method for detecting adverse 5 atmospheric conditions ahead of an aircraft. The system has a plurality of infrared cameras that may detect, for example, sulphur dioxide and particles such as volcanic ash, wind-blown dust and ice particles. It also comprises a computer that processes the images and a display to show the crew of the aircraft the adverse conditions. There are a number of adverse atmospheric conditions that are desirable to 10 detect. These include volcanic ash, toxic gases such as sulphur dioxide gas, wind blown dust and ice particles. Volcanic clouds contain silicate ash and gases that are hazardous to aviation. Several encounters between jet aircraft and volcanic ash have resulted in significant damage due to ingestion of ash into the hot parts of the engine, subsequent melting 15 and fusing onto the turbine blades. Ash can also block the pitot static tubes and affect sensitive aircraft instruments, as well as abrade the leading edges of parts of the airframe structure. Volcanic gases, principally S02 are less dangerous to aircraft, but detection of S02 can be used as an indicator of volcanic ash as these substances are often collocated and are transported together by atmospheric winds. 20 Another important gas in volcanic clouds is water vapour (H20 gas). Water vapour occurs in copious amounts in volcanic clouds either through entrainment of ambient air or from water from the volcanic source (e.g. sea water is a common source for volcanoes on islands or in coastal regions). Once in the atmosphere, the water vapour can condense on ash nuclei rapidly forming ice with a much smaller radius 25 than ice in normal meteorological clouds. These abundant, small-sized ice particles are hazardous to aircraft because the rapid melting of the ice when in contact with the hot engines, releases the ash nuclei which then fuses onto the turbine blades, affecting the engine performance and potentially causing the engine to stall. Damage to aircraft can be counted in the millions of dollars. Most serious 30 aircraft encounters with ash clouds have been at cruise altitudes, but there is also a hazard to aircraft at airports affected by volcanic ash. These airports are usually -2 close to an active volcano but they can also be at some distance from the source of the eruption due to atmospheric transport that brings ash into the region. The cost of ash hazards to airport operations is not known, but must be significant if the costs include those due to delays to landings and take-offs as well 5 as re-routing costs incurred by airline operators. The recent (14 April, 2010) eruption of Eyjafjallajoekull in Iceland is estimated to have cost the airline industry approximately US$2 bn. Currently there are no regulatory requirements for airport operators to provide warnings of ash hazards. Warnings are issued based on information from volcano observatories, meteorological advisories and, in some 10 cases, radar observations of eruption columns. Radar information is generally only reliable at the start of an eruption when the ash cloud is thick and usually such information is only available at airports in close proximity to an erupting volcano. For airports distant from the source of ash there are few direct observations available. Some observations come from satellite systems and other sources of 15 information come from trajectory forecasts based on wind data and cloud height information. Much of this information is sporadic and untimely and there is a need for better detection systems. Other adverse atmospheric conditions include the toxic gases emitted by volcanoes and industrial plants. Of particular importance and abundance is sulphur 20 dioxide (SO 2 ) gas.

SO

2 clouds from volcanoes will react with water vapour in the atmosphere to produce sulphuric acid which can damage aircraft. It will be appreciated that the sulphur dioxide may be found in areas separate from the volcanic ash. An aircraft can fly through sulphur dioxide without passing through ash. Post encounter 25 treatment of the engine in the case of sulphur dioxide encounter would be different to and considerably cheaper than the equivalent treatment required of an engine during an ash encounter. Accordingly, it would be desirable to be able to warn aircraft of SO 2 clouds. Ash and other particles can under the right conditions initiate ice particle 30 formation when water freezes around these cores. Accordingly, wind-blown dust and ice particles can be a significant hazard to aircraft, vehicles and the like.

-3 Jet aircraft at cruise altitudes (above 15,000 feet), travel rapidly (>500 km hr 1) and currently do not have a means for detecting volcanic cloud hazards ahead. Because of the high speed, a detection method must be able to gather information rapidly and provide an automated alert and species identification algorithm, capable 5 of distinguishing volcanic substances from other substances in the atmosphere (e.g. meteorological clouds of water and ice). W02005031321A1, W02005068977AI and W02005031323A1 teach methods and apparatus for monitoring of sulphur dioxide, volcanic ash and wind blown dust, using at least two wavelengths of infrared radiation corresponding to an 10 adverse atmospheric condition. US3931462 teaches the use of an UV video system for measuring S02 in plume from a smokestack. US4965572 teaches methods and apparatus for detecting low level wind shear type turbulence remotely, such as by an infrared temperature detector. 15 US5140416 discloses a system and method for fusing or merging video imagery from multiple sources such that the resultant image has improved information content. The sensors are responsive to different types of spectral content in the scene being scanned, such as short and long wavelength infrared. US5654700 and US 5602543 teach an adverse atmospheric condition 20 detection system for aircraft that monitors conditions ahead of aircraft using infrared detectors, displays the position, warns and reroutes aircraft. According to a first aspect, the present invention provides a system for detecting adverse atmospheric conditions ahead of an aircraft, including a plurality of infrared cameras mounted on the aircraft, wherein: the infrared cameras are 25 adjusted to spatially detect infrared radiance in different bands of infrared light, each camera is connected to an image processing computer that processes and combines the images, and generates video display signals for producing a video display which indicates the position of the adverse atmospheric conditions relative to the aircraft; each of the cameras is provided with a respective filter adjusted to 30 filter infrared light with a bandwidth corresponding to infrared bandwidth characteristics of an adverse atmospheric condition from a set of adverse atmospheric conditions; the image processing computer is adapted to identify -4 adverse atmospheric conditions, said identifying being based on threshold conditions and using the detected infrared radiance, data from a look-up table and measured parameters including information on the position and/or attitude of the aircraft; and the image processing computer is further adapted to display the identified adverse 5 atmospheric conditions as a spatial image on a display. The present invention is advantageous in that it provides an apparatus suited for aircraft that detects adverse atmospheric conditions, in particular caused by volcanoes, and visualizes them for the crew of the aircraft. The invention is particularly useful for detecting volcanic clouds. For example, the present invention 10 can enable the rapid detection of volcanic substances ahead of a jet aircraft at cruise altitudes and the simultaneous detection and discrimination of volcanic ash, SO 2 gas and ice-coated ash particles. Preferably, the invention provides algorithms and processes for converting raw camera data to identify ash, SO 2 gas and ice coated ash. 15 The system preferably monitors the field of view of the aircraft. The cameras of the invention may be uncooled microbolometer collocated cameras. In one embodiment of the system the pitch angle and ambient temperature are accounted for in the look-up table. 20 The adverse atmospheric conditions preferably include volcanic ash, ice coated ash, water vapour and sulphur dioxide. The measured parameters can include pitch angle and ambient temperature. Preferably, the threshold conditions are pre-computed using a radiative transfer model of the atmosphere. 25 Preferably, the image processing computer is arranged to determine brightness temperatures from the detected infrared radiance, and said identifying includes determining whether values related to the brightness temperatures meet the threshold conditions. The system may also include one or more external blackened shutters against 30 which the imaging cameras are pre-calibrated for providing in-flight calibration values.

-5 Most preferably, the system provides a statistical alert based on analysis of images determined to show an adverse condition of ash, sulphur dioxide or ice coated ash. The statistical alert uses spatial and temporal information and can be tuned according to in-flight tests to reduce false-alarms and ensure robustness. 5 For these embodiments there can be a computer program loadable into the internal memory of a processing unit in a computer based system, comprising software code portions for performing the said steps. For these embodiments there can be a computer program product stored on a computer readable medium, comprising a readable program for causing a processing 10 unit in a computer based system, to control an execution according to the said steps. Preferably the system is arranged to detect at least the three volcanic substances (ash, SO 2 and ash coated ice particles) in the air ahead of the aircraft by a remote method, and in addition be capable of discriminating these from other meteorological clouds of water droplets and ice. 15 The invention also more generally provides a system for detecting adverse atmospheric conditions ahead of an aircraft, including a plurality of infrared cameras mounted on the aircraft, wherein: the infrared cameras are adjusted to spatially detect infrared radiance in different bands of infrared light; each camera is connected to an image processing computer that processes and combines the images, 20 wherein each of the cameras is provided with a respective filter adjusted to filter infrared light with a bandwidth corresponding to infrared bandwidth characteristics of an adverse atmospheric condition from a set of adverse atmospheric conditions; and the image processing computer is adapted to identify and display adverse atmospheric conditions, said identifying being based on threshold conditions and 25 using the detected infrared radiance and measured parameters including information on the position and/or attitude of the aircraft. According to another aspect, the present invention provides a method for detecting adverse atmospheric conditions ahead of an aircraft and displaying said adverse atmospheric conditions, comprising spatially detecting infrared radiance in 30 different bands of infrared light using a plurality of infrared cameras; and, for each camera: i) Filtering the infrared radiation with a filter adjusted to filter infrared light with a bandwidth corresponding to infrared bandwidth characteristics of an adverse -6 atmospheric condition in a set of adverse atmospheric conditions; ii) identifying likely occurrences of adverse atmospheric conditions based on threshold conditions and using the detected infrared radiance, data from a look-up table and measured parameters including information on the position and/or attitude of the aircraft; and 5 iii)processing the identified likely occurrences of adverse atmospheric conditions to create a spatial image. In one embodiment, the method further comprises the step of iv) combining the image with images from other cameras and information on the aircraft flight path. 10 The adverse atmospheric conditions preferably include volcanic ash, ice coated ash, water vapour and sulphur dioxide. The measured parameters can include pitch angle and ambient temperature. In a further aspect, the invention provides a system for detecting volcanic clouds ahead of an aircraft, including one or more infrared cameras mounted on the 15 aircraft, the infrared cameras are adjusted to spatially detect infrared radiance in different bands of infrared light, each camera is connected to an image processing computer that process and combines the images, combining them with flight path information from the aircraft and generates video display signals for producing a video display which indicates the position of the adverse conditions relative to the 20 aircraft; characterized in that each of the cameras is provided with a respective filter adjusted to filter infrared light with a bandwidth corresponding to infrared bandwidth characteristics of one of the volcanic species in a set of volcanic species, and that the image processing computer is adapted to identify and display species as a spatial image on a display by means of threshold look-up tables for the respective 25 species mapping thresholds for the infrared radiance, above which species are likely to occur, with measured parameters. In a still further aspect, the invention provides a method for detecting a volcanic cloud ahead of an aircraft and displaying said cloud, processing information from one or more infrared cameras spatially detecting infrared radiance 30 in different bands of infrared light, combining the information with flight path information from the aircraft characterized in the steps of for each camera: i) Filtering the infrared radiation with a filter adjusted to filter infrared light with a -7 bandwidth corresponding to infrared bandwidth characteristics of one of the volcanic species in a set of volcanic species; ii) identifying likely occurrences of species by looking up detected infrared radiance values in a threshold look-up table mapping thresholds for the infrared radiance, above which species are likely to 5 occur, with measured parameters; iii)processing the identified likely occurrences of species to create a spatial image. Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic of a single camera with filter, lens, shutter and 10 protective window; Figure 2 is an example configuration for the multiple camera system; Figure 3 shows an ash cloud on the display; Figure 4 shows a diagram of radiative transfer calculation for a horizontal path in a clear atmosphere for three different flight altitudes; and 15 Figure 5 shows a diagram of line strengths for the two bands of SO 2 at 8.6 im and 7.3 pm. The response functions for the filters of the system are also shown. The basic principle of detection of volcanic substances ahead of the aircraft relies on the use of filtered infrared radiation in the region of 6-13 im. Within this region, narrow (0.5-1.0 pm) bands are selected for detection of ash, water vapour, 20 SO 2 gas and ice coated ash. The preferred detection method is to use wide-field-of view, rapid sampling, imaging, uncooled microbolometer cameras. A microbolometer is used as a detector in thermal cameras. Infrared radiation strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into temperatures which can be 25 used to create an image. Unlike other types of infrared detecting equipment, microbolometers do not require cooling. Typically this camera may contain 640x512 pixels x lines, have a noise equivalent temperature difference of 50 mK (or better) at 300 K in the 10-12 pim region, and provide sampling rates up to 60 Hz. Five collocated cameras are 30 envisaged for the simultaneous detection of ash, SO 2 gas, H 2 0 gas, and ice-coated ash. Each camera has a detector that is sensitive to infrared radiation within the region 6-13 jim. Narrowband filters are placed over each camera to restrict the -8 spectral content of the radiation for the purpose of species identification. The cameras share the same field of view ahead of the aircraft and therefore, in principle, multiple, simultaneous narrowband infrared images can be acquired in real-time at sampling rates of up to 60 Hz. These collocated images can be rapidly processed 5 using special algorithms to identify each of the four target species specified earlier. One embodiment of the system has 5 collocated imaging cameras, but this number could be more or less depending on the requirements of the user. A generic example of the camera in the system is shown in Figure 1. Infrared radiation from ahead of the aircraft enters the filter 1 of each camera and is focused through the 10 camera lens 2 and falls on the detector array 3. The shutter 4 is used for calibration (see below). The signals are transferred via a standard high-speed communication protocol 5 to a computer for further processing. To protect the filter and lens while the system is viewing ahead of the aircraft, an IR transparent window 7 (e.g. Germanium glass) is attached between the shutter and filter. The shutter is 15 temperature controlled 6 and blackened on the side facing the optics. An example configuration for the multiple camera system is shown in Figure 2 with five cameras 8. The protective shutter 4 may be mechanically driven in front of the assembly 9 and withdrawn when the system is in use. The germanium glass window 7 provides protection from debris, while in viewing mode. The signal 5 and 20 power 10 lines are at the back of the assembly housing 9, which houses electronics, frame grabber and computer hardware. Five cameras are shown, but the configuration could consist of more or less cameras depending on the number of hazards to be identified. For example a system with two cameras would permit identification of volcanic ash and ice-coated ash. 25 The cameras are pre-calibrated prior to installation on the aircraft so that each camera registers the same digital signal when exposed to the same amount of infrared radiation. This can be achieved by pointing each camera, without its filter, at a known source of infrared radiation (known constant temperature) and recording the digital signal from each pixel of each camera. A look-up table can be 30 determined by varying the source temperature through the range 210 to 300 K, in steps of 10 K (for example) for each camera, giving a table of 640x512x 10x2 values, assuming a linear calibration. This process can be repeated for each narrowband -9 filter used. Once on board the aircraft, intermittent re-calibrations can be performed by inserting a heated and blackened shutter in front of the filter and recording the digital counts corresponding to the known (controlled) temperature of the shutter. The shutter also serves the dual purpose of providing protection against debris and 5 dirt directed toward the camera during take-off and landing, when the system of the present invention is deactivated. It will be understood that, optionally, a second shutter could be used to provide a second calibration point in a linear calibration equation. The use of a second shutter is simply a matter of practical convenience and does not alter the main operating principle of the invention. 10 The system is activated once the aircraft has reached cruise altitude and whenever an airborne hazard is detected and the aircraft conducts evasive manoeuvres by altering direction-flight altitude and heading. In deactivated mode the shutter is closed. Before activation a pre-calibration cycle for the system (all 5 cameras) is conducted. The shutter is opened and the system begins to collect 15 images. Commercial cameras can sample as fast as 60 Hz and this is the preferred sampling rate (or higher). However, some export restrictions apply to some cameras and this means lower sampling rates may apply. In the discussion that follows we assume a sampling rate of 8 Hz, as at this frequency there are no export restrictions. The basic principle is unchanged when using a higher sampling frequency. 20 Each camera provides 8 images of size N pixels by M lines every second. The look-up table is used together with the on board calibration data to convert the digital signals to a brightness temperature (BTij,k), where k represents the camera number and k=1, 2, 3, 4 or 5, in the current system, and i and j are pixel and line numbers, respectively. The brightness temperature is determined from: 25 C 3 CVi Ri,j,k 2 vk/BTjk 1 e Where: Rij,k is the radiance at pixel i, line j and filter k vk is the central wavenumber for camera filter k -10 BTij,k is the brightness temperature cl and c 2 are the Einstein radiation constants The radiance Rij,k is determined from the pre- and post-calibration 5 procedures and is assumed to be a linear function of the digital signal counts. Camera images may be averaged in order to reduce noise and improve the signal-to noise ratio of the system. For illustration purposes only, we shall concentrate on one image pixel and assume that all other pixels can be treated in the same manner, noting that the 10 calibration look-up table is different for every pixel. Then, the data for one pixel consists of the measurements: BTl, BT2, BT3, BT4 and BT5, where these represent brightness temperatures from each of the five cameras (e.g BTI is the brightness temperature for that pixel in camera 1 which has filter 1). The system of the present invention is linked into the aircraft instrument data 15 stream so that GPS coordinates, altitude (z), longitude (1), latitude (q), heading (h), direction (d), roll (r), yaw (y), pitch (x), time (t), speed over the ground (v), wind speed (w) and ambient temperature (Ta) are available at a sampling rate of at least 1 s and preferably faster. In an embodiment the system uses filters at the following central 20 wavenumbers (in cm~'): Filter Central Bandwidth (cm~ NEDT (mK) Purpose wavenumber I) (cm~') 1 1410 100 200 H20 2 1363 100 200 S02 3 1155 100 200 S02/ash 4 929 60 100 Ash/ice 5 830 60 100 Ash/ice Table 1. Filter specifications for an embodiment of the present invention.

- 11 Ash Detection Algorithm A pixel is declared to be ash if the following conditions are met at each instance: DTlAh = (BT4-BT5)/Ta > TIAsh(Ta, r, y, x)/Ta (1) DT2Ash = (BT3-BT5)/Ta > T2Ash(Ta, r, y, x)/Ta (2) 5 Where TI Ah and T 2 Ah are temperature differences determined from pre-computed radiative transfer calculations for a set of parameters, including ambient temperature (Ta) and realistic aircraft roll, pitch and yaw values. Note that DTI Ash and DT 2 Ash are non-dimensional quantities and are strictly indices. 10 An alert is sounded if a sequence of 8 consecutive occurrences of condition (1) and (2) happen for a pre-defined fraction of the total image. A value of 5% of the total number of pixels in the difference image is used, but this can be tuned as necessary - a lower value set if the aircraft is operating in airspace declared, or 15 likely to be influenced by volcanic ash; a higher value in unaffected areas.

H

2 0 Detection Algorithm A pixel is declared to be water vapour affected if the following conditions are met at each instance: 20 DT, = BT I - Ta > T,(Ta, r, y, x) (3) Where T, is a temperature difference determined from pre-computed radiative transfer calculations for a set of parameters, including ambient temperature (Ta), and realistic aircraft roll, pitch and yaw values. 25 No alert is sounded, but T, is used with the ice algorithm if that alert is sounded. Ice-Coated Ash (ICA) Detection Algorithm 30 A pixel is declared to be ICA if the following conditions are met at each instance, DTcA = (BT4-BT5)/Ta < TIcA(Ta, r, y, x)/Ta (4) -12 Where TICA is a temperature difference determined from pre-computed radiative transfer calculations for a set of parameters, including ambient temperature (Ta), and realistic aircraft roll, pitch and yaw values. 5 An alert is sounded if a sequence of 8 consecutive occurrences of condition (4) happen for a pre-defined fraction of the total image. A value of 5% of the total number of pixels in the difference image is used, but this can be tuned as necessary - a lower value set if the aircraft is operating in airspace declared or likely to be influenced by volcanic ash; a higher value in unaffected areas. When the alert is 10 sounded condition (3) is checked and if this condition is met, the pixel is confirmed to be ICA. The use of the water vapour condition is entirely novel and reduces the false alarm rate for detecting hazardous small-sized ice-coated ash particles.

SO

2 Detection Algorithm 15 A pixel is declared to be SO 2 if the following conditions are met at each instance, DTlso 2 = (BTI-BT2)/Ta < Tlso 2 (Ta, r, y, x)/Ta (5) DT2so 2 = (BT3-BT5)/Ta < T2so 2 (Ta, r, y, x)/Ta (6) Where T1 S02 and T2so 2 are temperatures determined from pre-computed radiative 20 transfer calculations for a set of parameters including ambient temperature (Ta), and realistic aircraft roll, pitch and yaw values. An alert is sounded if a sequence of 8 consecutive occurrences of conditions (5) and (6) happen for a pre-defined fraction of the total image. A value of 5% of 25 the total number of pixels in the difference image is used, but this can be tuned as necessary - a lower value set if the aircraft is operating in airspace declared to be or likely to be influenced by volcanic ash; a higher value is used in unaffected areas. An example of the display shown to the crew for the detection of an ash 30 cloud is shown in Figure 3. This is based on an ash cloud composed of silicate material and shows the DTlAsh signal for 6 frames separated by a constant short time difference from two cameras imaging ahead of the aircraft. Highest concentrations -13 of ash are indicated in red (or dark in Figure 3 that is in black and white); the background sky is shown in light purple (or light grey in Figure 3). As the aircraft approaches the hazard, the pilot can alter the heading of the aircraft to avoid it. An important part of this invention is the use of pre-computed threshold 5 values from a detailed radiative transfer model of the atmosphere, with and without a volcanic cloud and utilizing geometrical considerations appropriate for viewing in the infrared region (6-13 ptm) from an aircraft. Figure 4 shows a horizontal path simulation of the radiance of the clear atmosphere from 700-1600 cm' at three different flight altitudes. At 9.5 km the atmosphere appears very cold-the 10 equivalent blackbody temperature of the horizontal path is about 227 K. Any volcanic cloud placed between the aircraft and the cold background will alter the radiance received by the system in a known way. The spectral content of the radiation contains signatures of ash, SO 2 , H 2 0 and ice-coated ash particles. These signatures can also be simulated by the radiative transfer model and the results 15 stored in a large look-up table. Notice that the radiance curves change with altitude and hence with ambient temperature-the ambient temperature is determined by the on board aircraft instrumentation and used by the detection algorithm. It could equally use the height (flight altitude) instead of the temperature, but the temperature is a more robust measure. 20 The ash signal in these spectra is characterised by a higher brightness temperature in filter 4 (BT4) than in F5 (BT5), when viewing a cold background. The threshold values are determined by using refractive index data for silicates and scattering calculations are based on measured particle size distribution for particle with radii in the range 1-20 pm, according to the art. Generally, the instrument 25 would look in the horizontal or slightly upwards (aircraft usually have a 3* pitch angle upwards). However, the aircraft may pitch downwards, in which case the background temperature might change from a cold background to a warm background. In this case, the ash signature is identified by BT4<BT5. The look-up table is constructed in such a way that the pitch angle and ambient temperature are 30 accounted for. Additionally, the roll and yaw angles are compensated for, although these have only a minor influence on the detection algorithm. Extra fail-safe -14 thresholds are also incorporated into the detection algorithm by utilizing a filter near 8.6 pm that has sensitivity to volcanic ash. The operation of the ice-coated ash algorithm is similar to the ash algorithm, except the threshold look-up table is now determined using data for ice (refractive 5 indices and scattering data for small particles, radii < 30 pm). In the case of small ice particles, BT4<BT5 for viewing into a cold background (the opposite to ash without an ice coating). Background conditions are accounted for in a similar way to that used for the ash detection. Normalisation of the temperature differences is done to provide some 10 robustness and to make the detection independent of the ambient air temperature. S02 and H20 look-up tables are also used. SO 2 has very strong absorptions near to 8.6 pim and 7.3 pm as Figure 5 illustrates. The principle of detecting S02 has been described earlier and is based on radiative transfer calculations assuming the line strengths and transmissions applicable to the case of an atmosphere loaded 15 with S02. Under normal conditions SO 2 has an extremely low abundance (<10 ppm), and so detection of SO 2 using these absorptions features is very effective in the case of volcanic clouds ahead of an aircraft.

Claims (5)

1. A system for detecting adverse atmospheric conditions ahead of an aircraft, including a plurality of infrared cameras mounted on the aircraft, wherein: 5 the infrared cameras are adjusted to spatially detect infrared radiance in different bands of infrared light, each camera is connected to an image processing computer that processes and combines the images, and generates video display signals for producing a video display which indicates the position of the adverse atmospheric conditions relative 10 to the aircraft; each of the cameras is provided with a respective filter adjusted to filter infrared light with a bandwidth corresponding to infrared bandwidth charadteristics of an adverse atmospheric condition from a set of adverse atmospheric conditions; the image processing computer is adapted to identify adverse atmospheric 15 conditions, said identifying being based on threshold conditions and using the detected infrared radiance, data from a look-up table and measured parameters including information on the position and/or attitude of the aircraft; and the image processing computer is further adapted to display the identified adverse atmospheric conditions as a spatial image on a display. 20

2. A system as claimed in claim 1, wherein the set of adverse atmospheric conditions includes volcanic ash, ice coated ash, water vapour and sulphur dioxide; and/or wherein the measured parameters include pitch angle and ambient temperature; and/or wherein the threshold conditions are pre-computed using a radiative transfer model of the atmosphere; and/or wherein the image processing 25 computer is arranged to determine brightness temperatures from the detected infrared radiance and said identifying includes determining whether values related to the brightness temperatures meet the threshold conditions; and/or wherein the system further includes one or more external blackened shutters against which said imaging cameras are pre-calibrated for providing in-flight calibration values. 30

3. A method for detecting adverse atmospheric conditions ahead of an aircraft -16 and displaying said adverse atmospheric conditions, comprising spatially detecting infrared radiance in different bands of infrared light using a plurality of infrared cameras; and, for each camera: i) Filtering the infrared radiation with a filter adjusted to filter infrared light with a 5 bandwidth corresponding to infrared bandwidth characteristics of an adverse atmospheric condition in a set of adverse atmospheric conditions; ii) identifying likely occurrences of adverse atmospheric conditions based on threshold conditions and using the detected infrared radiance, data from a look-up table and measured parameters including information on the position and/or attitude 10 of the aircraft; and iii)processing the identified likely occurrences of adverse atmospheric conditions to create a spatial image.

4. A method as claimed in claim 3, including the additional step of: iv) combining the image with images from other cameras and information on the 15 aircraft flight path; and/or wherein the set of adverse atmospheric conditions includes volcanic ash, ice coated ash, water vapour and sulphur dioxide; and/or wherein the threshold conditions are pre-computed using a radiative transfer model of the atmosphere; and/or further comprising, for each camera, determining brightness temperatures from the detected infrared radiance and wherein said 20 identifying includes determining whether values related to the brightness temperatures meet the threshold conditions; and/or wherein the measured parameters include pitch angle and ambient temperature; and/or further comprising pre calibrating the imaging cameras against one or more external blackened shutters, for providing in-flight calibration values. 25

5. A computer program product stored on a computer readable medium, comprising a readable program which when executed by a computer causes the computer to carry out a method as claimed in claim 3 or 4.