U.S. patent application number 13/695171 was filed with the patent office on 2013-05-30 for system and method for detecting adverse atmospheric conditions ahead of an aircraft.
This patent application is currently assigned to NORSK INSTITUTT FOR LUFTFORSKNING. The applicant listed for this patent is Cirilo Bernardo, Alfredo Jose Prata. Invention is credited to Cirilo Bernardo, Alfredo Jose Prata.
Application Number | 20130135470 13/695171 |
Document ID | / |
Family ID | 44356186 |
Filed Date | 2013-05-30 |
United States Patent
Application |
20130135470 |
Kind Code |
A1 |
Prata; Alfredo Jose ; et
al. |
May 30, 2013 |
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.
Inventors: |
Prata; Alfredo Jose;
(Lillestrom, NO) ; Bernardo; Cirilo; (Kaleen,
AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Prata; Alfredo Jose
Bernardo; Cirilo |
Lillestrom
Kaleen |
|
NO
AU |
|
|
Assignee: |
NORSK INSTITUTT FOR
LUFTFORSKNING
Kjeller
NO
|
Family ID: |
44356186 |
Appl. No.: |
13/695171 |
Filed: |
April 28, 2011 |
PCT Filed: |
April 28, 2011 |
PCT NO: |
PCT/EP11/56805 |
371 Date: |
February 14, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61329353 |
Apr 29, 2010 |
|
|
|
Current U.S.
Class: |
348/144 |
Current CPC
Class: |
H04N 5/247 20130101;
H04N 5/33 20130101; G01S 3/781 20130101; G01S 19/53 20130101 |
Class at
Publication: |
348/144 |
International
Class: |
H04N 5/33 20060101
H04N005/33 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 29, 2010 |
NO |
20100625 |
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.
Description
[0001] 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.
[0002] 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.
[0003] 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 SO.sub.2 are less dangerous to aircraft, but
detection of SO.sub.2 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 (H.sub.2O 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.
[0004] 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.
[0005] 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.
[0006] Other adverse atmospheric conditions include the toxic gases
emitted by volcanoes and industrial plants. Of particular
importance and abundance is sulphur dioxide (SO.sub.2) gas.
[0007] SO.sub.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
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.sub.2 clouds.
[0008] 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.
[0009] Jet aircraft at cruise altitudes (above 15,000 feet), travel
rapidly (>500 km hr.sup.-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).
[0010] 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.
[0011] U.S. Pat. No. 3,931,462 teaches the use of an UV video
system for measuring SO.sub.2 in plume from a smokestack.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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, SO.sub.2 gas and ice-coated ash
particles. Preferably, the invention provides algorithms and
processes for converting raw camera data to identify ash, SO.sub.2
gas and ice coated ash.
[0017] The system preferably monitors the field of view of the
aircraft.
[0018] The cameras of the invention may be uncooled microbolometer
collocated cameras.
[0019] In one embodiment of the system the pitch angle and ambient
temperature are accounted for in the look-up table.
[0020] 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.
[0021] Preferably, the threshold conditions are pre-computed using
a radiative transfer model of the atmosphere.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] Preferably the system is arranged to detect at least the
three volcanic substances (ash, SO.sub.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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] Preferred embodiments of the present invention will now be
described by way of example only and with reference to the
accompanying drawings, in which:
[0035] FIG. 1 is a schematic of a single camera with filter, lens,
shutter and protective window;
[0036] FIG. 2 is an example configuration for the multiple camera
system;
[0037] FIG. 3 shows an ash cloud on the display;
[0038] FIG. 4 shows a diagram of radiative transfer calculation for
a horizontal path in a clear atmosphere for three different flight
altitudes; and
[0039] FIG. 5 shows a diagram of line strengths for the two bands
of SO.sub.2 at 8.6 .mu.m and 7.3 .mu.m. The response functions for
the filters of the system are also shown.
[0040] 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 .mu.m) bands are selected for detection of ash, water
vapour, SO.sub.2 gas and ice coated ash. The preferred detection
method is to use wide-field-of-view, rapid sampling, imaging,
uncooled microbolometer cameras.
[0041] 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.
[0042] Typically this camera may contain 640.times.512
pixels.times.lines, have a noise equivalent temperature difference
of 50 mK (or better) at 300 K in the 10-12 .mu.m region, and
provide sampling rates up to 60 Hz. Five collocated cameras are
envisaged for the simultaneous detection of ash, SO.sub.2 gas,
H.sub.2O gas, and ice-coated ash. Each camera has a detector that
is sensitive to infrared radiation within the region 6-13 .mu.m.
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.
[0043] 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.
[0044] 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.
[0045] 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.times.512.times.10.times.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.
[0046] 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.
[0047] 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.sub.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 = c 1 v k 3 c 2 v k / BT i , j , k - 1 ##EQU00001##
Where:
[0048] R.sub.i,j,k is the radiance at pixel i, line j and filter
k
[0049] v.sub.k is the central wavenumber for camera filter k
[0050] BT.sub.i,j,k is the brightness temperature
[0051] c.sub.1 and c.sub.2 are the Einstein radiation constants
[0052] The radiance R.sub.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.
[0053] 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).
[0054] 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.
[0055] In an embodiment the system uses filters at the following
central wavenumbers (in cm.sup.-1):
TABLE-US-00001 TABLE 1 Filter specifications for an embodiment of
the present invention. Central wavenumber Bandwidth Filter
(cm.sup.-1) (cm.sup.-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
[0056] A pixel is declared to be ash if the following conditions
are met at each instance:
DT1.sub.Ash=(BT4-BT5)/Ta>T1.sub.Ash(Ta,r,y,x)/Ta (1)
DT2.sub.Ash=(BT3-BT5)/Ta>T2.sub.Ash(Ta,r,y,x)/Ta (2)
Where T1.sub.Ash and T2.sub.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.sub.Ash and
DT2.sub.Ash are non-dimensional quantities and are strictly
indices.
[0057] 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.
H.sub.2O Detection Algorithm
[0058] A pixel is declared to be water vapour affected if the
following conditions are met at each instance:
DT.sub.wv=BT1-Ta>T.sub.wv(Ta,r,y,x) (3)
Where T.sub.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.
[0059] No alert is sounded, but T.sub.wv is used with the ice
algorithm if that alert is sounded.
Ice-Coated Ash (ICA) Detection Algorithm
[0060] A pixel is declared to be ICA if the following conditions
are met at each instance,
DT.sub.ICA=(BT4-BT5)/Ta<T.sub.ICA(Ta,r,y,x)/Ta (4)
Where T.sub.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.
[0061] 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.
SO.sub.2 Detection Algorithm
[0062] A pixel is declared to be SO.sub.2 if the following
conditions are met at each instance,
DT1.sub.SO2=(BT1-BT2)/Ta<T1.sub.SO2(Ta,r,y,x)/Ta (5)
DT2.sub.SO2=(BT3-BT5)/Ta<T2.sub.SO2(Ta,r,y,x)/Ta (6)
Where T1.sub.SO2 and T2.sub.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.
[0063] 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.
[0064] 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.sub.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.
[0065] 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 .mu.m) from an aircraft. FIG. 4 shows a
horizontal path simulation of the radiance of the clear atmosphere
from 700-1600 cm.sup.-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,
SO.sub.2, H.sub.2O 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.
[0066] 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 .mu.m according to the art.
Generally, the instrument would look in the horizontal or slightly
upwards (aircraft usually have a 3.degree. 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 .mu.m that has sensitivity
to volcanic ash.
[0067] 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 .mu.m). 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.
[0068] Normalisation of the temperature differences is done to
provide some robustness and to make the detection independent of
the ambient air temperature.
[0069] SO.sub.2 and H.sub.2O look-up tables are also used. SO.sub.2
has very strong absorptions near to 8.6 .mu.m and 7.3 .mu.m 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 SO.sub.2 has an
extremely low abundance (<10.sup.-3 ppm), and so detection of
SO.sub.2 using these absorptions features is very effective in the
case of volcanic clouds ahead of an aircraft.
* * * * *