SYSTEM AND METHOD FOR DETECTING ADVERSE ATMOSPHERIC CONDITIONS AHEAD OF AN AIRCRAFT

Abstract

System and method for detecting adverse atmospheric conditions ahead of an aircraft. The system has multiple, infrared cameras 8 adjusted to spatially detect 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 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 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

The present invention relates to a system and method for detecting adverse 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 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 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 SO₂ are less dangerous to aircraft, but detection of SO₂ can be used as an indicator of volcanic ash as these substances are often collocated and are transported together by atmospheric winds. Another important gas in volcanic clouds is water vapour (H₂O 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 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 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 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 as re-routing costs incurred by airline operators. The recent (14 Apr., 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 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 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 dioxide (SO₂) gas.

SO₂ 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 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 SO₂ clouds.

Ash and other particles can under the right conditions initiate ice particle 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.

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 of distinguishing volcanic substances from other substances in the atmosphere (e.g. meteorological clouds of water and ice).

WO2005031321A1, WO2005068977A1 and WO2005031323A1 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 adverse atmospheric condition.

U.S. Pat. No. 3,931,462 teaches the use of an UV video system for measuring SO₂ in plume from a smokestack.

U.S. Pat. No. 4,965,572 teaches methods and apparatus for detecting low level wind shear type turbulence remotely, such as by an infrared temperature detector.

U.S. Pat. No. 5,140,416 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.

U.S. Pat. No. 5,654,700 and U.S. Pat. No. 5,602,543 teach an adverse atmospheric condition 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 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 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 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 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 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, SO₂ gas and ice-coated ash particles. Preferably, the invention provides algorithms and processes for converting raw camera data to identify ash, SO₂ gas and ice coated ash.

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.

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.

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 which the imaging cameras are pre-calibrated for providing in-flight calibration values.

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.

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 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, SO₂ 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.

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, 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 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 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 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 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.

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 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 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 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 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 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 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:

FIG. 1 is a schematic of a single camera with filter, lens, shutter and protective window;

FIG. 2 is an example configuration for the multiple camera system;

FIG. 3 shows an ash cloud on the display;

FIG. 4 shows a diagram of radiative transfer calculation for a horizontal path in a clear atmosphere for three different flight altitudes; and

FIG. 5 shows a diagram of line strengths for the two bands of SO₂ at 8.6 μm and 7.3 μm. 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 p.m. Within this region, narrow (0.5-1.0 μm) bands are selected for detection of ash, water vapour, SO₂ 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 used to create an image. Unlike other types of infrared detecting equipment, microbolometers do not require cooling.

Typically this camera may contain 640×512 pixels×lines, have a noise equivalent temperature difference of 50 mK (or better) at 300 K in the 10-12 μm region, and provide sampling rates up to 60 Hz. Five collocated cameras are envisaged for the simultaneous detection of ash, SO₂ gas, H₂O gas, and ice-coated ash. Each camera has a detector that is sensitive to infrared radiation within the region 6-13 μm. Narrowband filters are placed over each camera to restrict the 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 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 FIG. 1. Infrared radiation from ahead of the aircraft enters the filter 1 of each camera and is focused through the 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 temperature controlled 6 and blackened on the side facing the optics.

An example configuration for the multiple camera system is shown in FIG. 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 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.

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 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 640×512×10×2 values, assuming a linear calibration. This process can be repeated for each narrowband 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 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.

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 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.

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 (BT_i,j,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:

$R_{i,j,k}=\frac{c_1v_k^3}{{}^{c_2v_k/{\mathrm{BT}}_{i,j,k}}-1}$

Where:

R_i,j,k is the radiance at pixel i, line j and filter k

v_k is the central wavenumber for camera filter k

BT_i,j,k is the brightness temperature

c₁ and c₂ are the Einstein radiation constants

The radiance R_i,j,k is determined from the pre- and post-calibration 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 calibration look-up table is different for every pixel. Then, the data for one pixel consists of the measurements: BT1, BT2, BT3, BT4 and BT5, where these represent brightness temperatures from each of the five cameras (e.g BT1 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 stream so that GPS coordinates, altitude (z), longitude (l), 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 and preferably faster.

In an embodiment the system uses filters at the following central wavenumbers (in cm⁻¹):

TABLE 1
Filter specifications for an embodiment of the present invention.
	Central
	wavenumber	Bandwidth
Filter	(cm−1)	(cm−1)	NEDT (mK)	Purpose
1	1410	100	200	H2O
2	1363	100	200	SO2
3	1155	100	200	SO2/ash
4	929	60	100	Ash/ice
5	830	60	100	Ash/ice

Ash Detection Algorithm

A pixel is declared to be ash if the following conditions are met at each instance:

DT1_Ash=(BT4−BT5)/Ta>T1_Ash(Ta,r,y,x)/Ta  (1)

DT2_Ash=(BT3−BT5)/Ta>T2_Ash(Ta,r,y,x)/Ta  (2)

Where T1_Ash and T2_Ash 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 DT1_Ash and DT2_Ash are non-dimensional quantities and are strictly indices.

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 likely to be influenced by volcanic ash; a higher value in unaffected areas.

H₂O Detection Algorithm

A pixel is declared to be water vapour affected if the following conditions are met at each instance:

DT_wv=BT1−Ta>T_wv(Ta,r,y,x)  (3)

Where T_wv 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.

No alert is sounded, but T_wv is used with the ice algorithm if that alert is sounded.

Ice-Coated Ash (ICA) Detection Algorithm

A pixel is declared to be ICA if the following conditions are met at each instance,

DT_ICA=(BT4−BT5)/Ta<T_ICA(Ta,r,y,x)/Ta  (4)

Where T_ICA 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.

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 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.

SO₂ Detection Algorithm

A pixel is declared to be SO₂ if the following conditions are met at each instance,

DT1_SO2=(BT1−BT2)/Ta<T1_SO2(Ta,r,y,x)/Ta  (5)

DT2_SO2=(BT3−BT5)/Ta<T2_SO2(Ta,r,y,x)/Ta  (6)

Where T1_SO2 and T2_SO2 are temperatures determined from pre-computed radiative 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 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 cloud is shown in FIG. 3. This is based on an ash cloud composed of silicate material and shows the DT1_Ash signal for 6 frames separated by a constant short time difference from two cameras imaging ahead of the aircraft. Highest concentrations of ash are indicated in red (or dark in FIG. 3 that is in black and white); the background sky is shown in light purple (or light grey in FIG. 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 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 μm) from an aircraft. FIG. 4 shows a horizontal path simulation of the radiance of the clear atmosphere from 700-1600 cm⁻¹ at three different flight altitudes. At 9.5 km the atmosphere appears very cold—the 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, SO₂, H₂O and ice-coated ash particles. These signatures can also be simulated by the radiative transfer model and the results 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.

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 μm according to the art. Generally, the instrument 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 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 thresholds are also incorporated into the detection algorithm by utilizing a filter near 8.6 μm 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 indices and scattering data for small particles, radii<30 μm). 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 robustness and to make the detection independent of the ambient air temperature.

SO₂ and H₂O look-up tables are also used. SO₂ has very strong absorptions near to 8.6 μm and 7.3 μm as FIG. 5 illustrates. The principle of detecting SO2 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 with SO2. Under normal conditions SO₂ has an extremely low abundance (<10⁻³ ppm), and so detection of SO₂ using these absorptions features is very effective in the case of volcanic clouds ahead of an aircraft.

Claims

1. A system for detecting adverse atmospheric conditions ahead of an aircraft, comprising

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 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 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 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 the display.

2. The system of claim 1 wherein the set of adverse atmospheric conditions includes volcanic ash, ice coated ash, water vapour and sulphur dioxide.

3. The system claim 2, wherein the system is arranged to seek to identify both ice coated ash and water vapour, and wherein the identification of water vapour is used to confirm an identification of ice coated ash.

4. The system of claim 1 wherein the threshold conditions are pre-computed using a radiative transfer model of the atmosphere.

5. The system of claim 1, wherein 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.

6. The system claim 1, wherein the measured parameters include pitch angle and ambient temperature.

7. The system claim 1, including one or more external blackened shutters against which said imaging cameras are pre-calibrated for providing in-flight calibration values.

8. A method for detecting adverse atmospheric conditions ahead of an aircraft 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 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 of the aircraft; and

iii) processing the identified likely occurrences of adverse atmospheric conditions to create a spatial image.

9. The method of claim 8, including the additional step of

iv) combining the image with images from other cameras and information on the aircraft flight path.

10. The method of claim 8, wherein the set of adverse atmospheric conditions includes volcanic ash, ice coated ash, water vapour and sulphur dioxide.

11. The method of claim 10, further comprising identifying likely occurrences of both ice coated ash and water vapour, wherein the identification of water vapour is used to confirm an identification of ice coated ash.

12. The method of claim 8, wherein the threshold conditions are pre-computed using a radiative transfer model of the atmosphere.

13. The method of claim 8, further comprising, for each camera, determining brightness temperatures from the detected infrared radiance, and wherein said identifying includes determining whether values related to the brightness temperatures meet the threshold conditions.

14. The method of claim 8, wherein the measured parameters include pitch angle and ambient temperature.

15. The method of claim 8, further comprising pre-calibrating the imaging cameras against one or more external blackened shutters, for providing in-flight calibration values.

16. (canceled)

17. 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 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 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 of the aircraft; and

iii) processing the identified likely occurrences of adverse atmospheric conditions to create spatial image.

18. The computer program product of claim 17, wherein the method further comprises combining the image with images from other cameras and information on the aircraft flight path.

19. The computer program product of claim 17, wherein the set of adverse atmospheric conditions includes volcanic ash, ice coated ash, water vapour and sulphur dioxide.

20. The computer program product of claim 17, wherein the method further comprises identifying likely occurrences of both ice coated ash and water vapour, wherein the identification of water vapour is used to confirm an identification of ice coated ash.

21. The computer program product of claim 17, wherein the threshold conditions are pre-computed using a radiative transfer model of the atmosphere.

22. The computer program product of claim 17, wherein the method further comprises, for each camera, determining brightness temperatures from the detected infrared radiance, and wherein said identifying includes determining whether values related to the brightness temperatures meet the threshold conditions.