AIRCRAFTNO SE WITH A DEVICE FOR CALIBRATING AN IR DETECTOR AND
CALIBRATION METHOD
The field of the invention is that of the calibration of an infrared
matrix-array detector installed in a seeker located in the nose section of an
aircraft, notably when it is subjected to large thermal variations. By way of
examples of aircraft, mention may be made of rockets and missiles, etc.
The aim of the calibration is to compensate for variations in the
signals issued from each pixel of the detector, which variations are mainly
due to:
- the different size of each pixel, due to detector manufacturing
tolerances of about one micron;
- differences in pixel-to-pixel sensitivity at the wavelength in
question;
- the different viewing angle between pixels;
- the temperature of the external interface of the seeker (window or
dome);
- etc.
These signal variations create in the image a parasitic pattern
referred to as "fixed pattern noise" (FPN), which decreases detection
capability in the image. This FPN is:
- dependent on the temperature conditions of the optical
components of the seeker;
- specific to each detector;
- dependent on the integration time that will be used by the detector
during the mission; and
- different each time the detector is turned on.
To correct these signal variations, coefficients dedicated to each
pixel are established, for a given integration time, by equalizing the output
signals of each of the pixels when the detector and its optics are illuminated
with a uniform temperature source. The uniform temperature of the source,
used in this so-called "calibration" operation, is as representative as possible
of the operational scene (devoid of targets or satellites). When the aircraft is
flying at very high altitudes and high speeds, outside of the atmosphere for
example for an aircraft such as a rocket, the scene has an equivalent
temperature close to absolute zero. When the aircraft is flying at low altitudes
and low speeds, the scene has the temperature of the ground.
More precisely, the coefficients of each pixel of the image issued
from the detector are calculated in the following way. The detector captures
an image of the uniform temperature source set beforehand to a first
calibration temperature. The acquired image is not uniform because of FPN.
The discrepancy relative to the average value of the captured image is
determined for each pixel.
The same process is carried out for a second calibration
temperature. The 2 discrepancies measured for each pixel allow 2
coefficients called the gain and offset to be defined. During operational use,
the raw output value of each pixel will be modified as follows:
Corrected pixel = Ofl.et(,,+, Gain(,,, *raw pixel
where
li : is the inlegration lime
The 2 temperature values of the calibrating device with which the
coefficients are measured must be such that the flux received by the detector
at these 2 values is close to that which will be received by the latter in the
operational phase. For an optimal calibration, 2 values must be provided: the
first value being slightly below the estimated temperature of the scene, the
second slightly above.
The gain and offset are dependent on integration time. Therefore,
these operations must be repeated for each integration time envisioned for
the operational phase of the mission.
Conventionally, the uniform temperature scene is created using a
"black body" located in the seeker when the internal volume of the latter
allows it, as illustrated in figures l a and lb. These figures show an aircraft
nose section 100 equipped with a seeker 20 and a cone 10 that is ejected
when the seeker is operational. This ejectable cone thermally insulates the
seeker from the heat that will be generated by passage through lower layers
of the atmosphere. However, this thermal insulation is not perfect and
residual heat penetrates into the interior' of the cone. This seeker 20
comprises:
- a dome (or window) 21;
- a detector 22 of IR in the band 3 pm-12 pm;
5 - optics 24 able to form an image of a scene on the IR detector
placed in the focal plane 23 of the optics 24; the assembly (22, 24) made up
of the detector/optics for forming images of the scene is mounted on a
rotating device (simply represented by an arrow 26) able to pivot this
assembly by 180"; and
10 - a calibrating device 25 equipped with a black body 251 the
temperature of which is adjusted via the thermoelectric effect, and optics 253
able to direct the radiation emitted by the black body, said device being
located behind the detector 22 (relative to the scene).
In the calibration phase (shown in figure la) this detector/optics
15 assembly 22, 24 is directed by means of the rotating device so as to form the
image of the black body 251 on the detector 22: this detectorloptics assembly
in a certain way "turns its back" on the scene. The gain and offset are
calculated from this image of the black body. The temperature of the black
body is representative of the scene temperature to which the temperature
20 , estimated for the contribution of the dome is added.
Next, the calibration having been carried' out, this assembly is
pivoted by 180" so as to form the image of the scene on the detector and the
cone 10 is ejected, as shown in figure lb.
Using a blaclc body allows the calibration temperature value to be
25 adjusted and an excellent calibration temperature uniformity to be obtained.
However, it has as principal drawbacks:
- the loss of volume occasioned by the calibrating device, bearing in
mind that only a very small volume is available in the seeker and that
therefore the volume used by the calibration function counts as a
30 "loss" in operational use;
- a limited temperature range, notably as regards the low
temperatures that may be required on account of the scene
temperature;
- a temperature that changes fairly slowly; and
- the uncertainty in the estimations of the temperatures emitted by
the window or dome on the detector, which are required to determine
the temperature of the black body, bearing in mind that the
temperature of the window or dome makes, with that of the optics of
5 the detector, a significant contribution to FPN.
Another technique for obtaining a uniform temperature scene
consists in averaging a large number of images captured randomly in the
scene. In this case, it is assumed that all the pixels of the image have "seen"
10 an identical average signal and that any residual nonuniformity is the result of
the FPN to be corrected. This technique avoids the problem of volume loss
- but does not allow the equivalent of a very uniform source to be obtained,
unless a very large number of images are accumulated, thereby increasing
calibration time. This type of calibration is not suitable for aircraft the
15 calibration times of which are very short (typically shorter than 15 seconds)
andlor when the scene possesses bright spots (which are averageable only
with difficulty). In addition, only a single temperature value is available for the
calibration in this case.
The aim of the invention is to mitigate these drawbacks.
20 Specifically, there remains to this day a need for detectors to be calibrated
notably in the case of a very low scene temperature when the equipment is
subjected to very widely varying temperatures.
The proposed solution is based on a temperature reference, such
25 as a black body, located outside of the seeker, allowing the advantages of
the 2 prior-art solutions to be achieved without their drawbacks.
More precisely, one subject of the invention is an aircraft nose
section provided with an ejectable cone in which are located:
- a seeker equipped with an iR detector, a dome and optics able to
30 form an image of a scene on the IR detector placed in the focal
plane of the optics; and
- a calibrating device equipped with a temperature reference and
optics able to direct the radiation emitted by the temperature
i reference.
It is mainly characterized in that the calibrating device is placed
outside of the seeker, upstream thereof so that the calibrating optics are able
to form an image of the temperature reference on the detector through the
dome and the optics of the seeker, and in that this calibrating device is
5 ejectable with the cone.
This nose section makes it possible to satisfy without loss of useful
volume (internal to the seeker) a number of requirements such as:
- short calibration time;
10 - high-quality temperature reference uniformity (black body);
- calibration including the dome; and
- calibration froni two temperature values.
According to one feature of the invention, the calibrating device
15 furthermore comprises a cryostat in which the temperature reference is
located and comprising a cooler able to bring the temperature of the
reference to a preset temperature.
Preferably, the cooler is an open circuit cooler associated with a
reservoir of pressurized gas.
20 The preset temperature of the reference is for example comprised
between 100 K and 200 K.
Using a cryostat to encapsulate the black body allows:
- any condensation effects at low temperatures to be
avoided; and
25 - the temperature to be rapidly changed using a Joule-
Thomson type cooler associated with a reservoir of limited volume.
The assembly made up of the optics and detector of the seeker is
generally mounted on a rotating device having an amplitude of rotation lower
30 than 90".
The aircraft is typically a rocket or a missile.
Another subject of the invention is a method for calibrating the
detector of the seeker of an aircraft nose section such as described above
35 and the temperature reference of which is a black body, characterized in that
an image comprising pixels and the detector comprising at least one
integration time t,, it comprises the following steps:
calculating an offset and a gain for each pixel for an integration
time t, of the detector, from an image of the black body at a first
temperature TI", then from an image at a second temperature T2".
Since these images are formed on the detector through the dome
and the optics of the seeker, the temperatures TI" and T2" only
depend on the integration time t, and not on an estimated
temperature of the dome;
storing the calculated offset and gain in memory;
when there are a plurality of integration tines, reiterating these
steps of calculating and storing in memory an offset and a gain for
each other integration time t, of the detector with different
temperatures TI" and T2"; and
ejecting the calibrating device.
When there are plurality of integration times, the second temperature
used for the preceding integration time is advantageously that of the first
20 temperature of the following integration time.
I
This calibration is carried out once at the start of a mission under the
I
I operating conditions of the seeker i.e. for various integration times, the entire
optical system (including the dome) being accounted for so that there is no
need to modify the reference temperature to allow for the contribution of
25 optical parts, the seeker possibly being presented with two reference
temperatures bracketing the scene temperature.
Other features and advantages of the invention will become
apparent on reading the following detailed description which is given by way
30 of nonlimiting example and with reference to the appended drawings, in
which:
figures l a and lb, which were described above, schematically
show an exemplary nose section equipped with an IR detector and a device
for calibrating this detector, in the calibration phase (figure la) and in the
35 operational phase after the cone has been removed (figure I b), according to
the prior art; and
figures 2a and 2b schematically show an exemplary nose section
equipped with an IR detector and a device for calibrating this detector, in the
calibration phase (figure 2a) and in the operational phase after the cone has
been removed (figure 2b), according to the invention.
5 From one figure to another, the same elements are referenced by
the same references.
An exemplary nose section 100' according to the invention will
now be described with reference to figures 2a and 2b.
10 As in the example of the preamble, it is equipped with a seeker 20
and an ejectable cone 10 required to protect the seeker notably from the heat
generated by passage through lower layers of the atmosphere, and which is
ejected when the seeker is operational. The latter comprises:
- a dome 21;
15 - an IR detector 22; and
- optics 24 able to form an image of a scene on the IR detector
placed in the focal plane 23 of the optics 24.
According to the invention, the calibrating device 25' is provided
with a temperature reference that may be a black body 251 or simply a mirror
20 able to reflect to the detector the image thereof (narcissus effect). In the case
where a simple mirror is used, there is only one reference temperature, which
is equal to that of the detector of the seeker and which is therefore not
directly related to the temperature of the scene. This calibrating device is
indeed located in the ejectable cone 10 and fastened thereto, but it is outside
25 the seeker 20, upstream thereof (the upstream-downstream direction is that
of rays issued from the scene and reaching the detector) so that the image of
the temperature reference on the detector is formed through the dome and
the optics of the seeker.
Thus, this makes it possible:
30 - to preserve the internal bulk of the seeker for functional parts thereof;
- for the calibration to take account of the entire optical path, inclusive of
the dome of the seeker;
- to provide a plurality of calibration temperatures similar to those of the
scene and taking into account actual (and not estimated) dome
35 temperatures; and
- to obtain a correction corresponding to the exact state of the detector
at the instant of its use relative to a prior calibration carried out, for
example, in factory and the values of which will possibly have drifted.
The calibrating device 25' is equipped with focusing optics that
preferably comprise only mirrors 253; these optics allow a temperature
reference of small area to be used. Typically an area of about 10 mm
diameter may be used, which will form on the dome a beam of about 40 mm
diameter.
As the reference temperature is representative of the scene alone,
the temperature required for the temperature reference is colder than in the
prior art. So as to prevent any condensation on this temperature reference
251, the latter is advantageously located on a cold plate within the vacuum
chamber of a cryostat 254. In the simple case where a mirror is used as the
temperature reference, the calibrating device does not comprise a cryostat. A
temperature sensor located in proximity to the temperature reference allows
the system to determine the instant at which the images of the temperature
reference must be collected.
The temperature reference is brought to temperature by means of
an open circuit cooler (of the Joule-Thomson type) associated with a highpressure
gas reservoir (containing a gas such as argon or nitrogen) included
in the cryostat, so as to minimize the time taken to cool the temperature
reference and thus rapidly reach the low calibration temperatures, in contrast
to a black body based on the thermoelectric effect which not only takes
longer to cool but is also limited as regards the lower portion of the
temperature range. The volume of the reservoir may be small because the
duration of the calibration and therefore that of its operation is short.
The assembly made up of the detector 22 with its associated
optics 24 is preferably also mounted on a rotating device (represented by the
arrow 27) used when the seeker is tracking a target in the operational phase,
and optionally in the calibration phase to align the calibrating device with the
optics of the seeker and the detector. However, now an amplitude of rotation
of smaller than 90" is sufficient in the operational phase and an amplitude of
rotation of smaller than 10" is sufficient in the calibration phase. It is no
longer necessary to rotate this assembly by 180" between the two phases.
An electrical connection (not shown in the figures) allows
information to be transferred between the seeker and the calibrating device.
The information exchanged is at least the temperature of the device and the
temperature setpoint. This link is of course cut when the calibrating device is
5 ejected.
An exemplary method for calibrating the detector of a nose section
100' such as described and the temperature reference of which is a black
body will now be described. It mainly consists in calculating the corrections to
10 be applied to each pixel of the image of the scene. As indicated in the
preamble, correction coefficients (also called calibration coefficients) are
calculated for each pixel of an image of the black body. The gain and offset
depend on the integration time of the detector. Therefore, these operations
must be repeated for each integration time t, envisioned for the operational
15 phase of the mission.
The method comprises the following steps for each pixel.
- the black body 251 is brought to a first temperature TIo by means
of the cryostat 254 and a first image of this black body is formed on
the detector 22 through the dome 21 and the optics 24 of the
seeker;
- the black body is brought to a second temperature T2" by means
of the cryostat and a second image of this black body is formed on
the detector through the dome and the optics of the seeker, these
temperatures TIo and T2" depending on the integration time t,, but
not on an estimated temperature of the dome as was the case in
the prior art; and
- the offset and gain are calculated from the discrepancies
measured for each pixel between the two images, and stored in
memory.
30 These steps are reiterated for each other integration time t,, with
other first and second temperatures. These temperatures are preset
depending on expected scene temperatures.
To decrease the duration of the calibration, it is possible to take as
first temperature the second temperature used in the calculation carried out
35 for the preceding integration time, and thus avoid the need to change the
temperature of the black body between the second image of the preceding
integration time and the first image of the integration time in question. This is
illustrated in the table below for four integration times til, ti2, ti3 and ti4 and
five temperatures TIo, T2", T3", T4", T5". For the calculation of the
5 coefficients at the integration time til, T2" is the second temperature used for
the second image "image 1-2" and is also used for the first image "image 2-
2" of the calculation at the integration time ti2; etc.
10 Once the coefficients have been calculated for each pixel and for
each envisioned integration time, the calibrating device 25' is ejected with the
cone 10. It is not necessary to rotate the assembly made up of the detector
and its optics in order to pass into the operational phase.
15 In the field of rockets or high-altitude missiles (> 30 km), the flux
due to the scene is very low because it corresponds to a sky background.
The flux is mainly due to aerodynamic heating of the dome during flight. The
flux provided by the calibrating device must, in this case, be representative of
the additional heating of the dome after the cone has been removed added to
20 the heating (very little) due to the scene.
Since the flux that the calibrating device needs to provide is low,
the temperature setpoints for the black body thereof will also be low
(comprised between 100 K and 300 K). The contribution of the window or
dome is not estimated, in contrast to the case corresponding to figures l a
25 and l b in which the dome is not on the optical path travelled to form the
image of the black body on the detector.
The method was simulated with 4 integration times and with
TI0=256 K, TZ0=284 K, T3"=305 K, T4"=327 K and T5"=366 K. The
temperature increase between each plateau corresponds to an increase of
about 25 K in the temperature of the dome for each integration time. By virtue
of the rapid change in the temperature obtained using a Joule-Thomson type
cooler and the limited number of temperature changes, this calibration took
5 less than 30 seconds.
This calibrating method may notably be implemented by way of a
computer program product, this computer program comprising code
instructions allowing the steps of the calibrating method to be carried out. It is
10 stored on a medium that is readable by a computer, such as for example a
computer also used to synchronize the capture of the images with the
temperature sensed in proximity to the black body and optionally the various
integration times. The medium may be electronic, magnetic, optical,
electromagnetic or be a storage medium read using infrared light. Examples
15 of such media are semiconductor memories (random access memory RAM
or read-only memory ROM), tapes, floppy or magnetic disks or optical disks
(read only memory compact disks (CD-ROMs), readlwrite compact disks
(CD-RtW) and DVDs).
CLAIMS
5 1. An aircraft nose section (100') provided with an ejectable cone (10) in
which are located:
- a seeker (20) equipped with an IR detector (22), a dome (21) and
optics (24) able to form an image of a scene on the IR detector
placed in the focal plane (23) of the optics (24); and
- a calibrating device (25) equipped with a temperature reference
(251) and optics (253) able to direct the radiation emitted by the
temperature reference;
characterized in that the calibrating device (25') is placed outside of
the seeker (20), upstream thereof so that the calibrating optics (253)
15 are able to form an image of the temperature reference (251) on the
detector (22) through the dome (21) and the optics (24) of the seeker,
and in that this calibrating device (25') IS ejectable with the cone (10).
2. The aircraft nose section as claimed in the preceding claim,
20 characterized in that the calibrating device furthermore comprises a
cryostat (254) in which the temperature reference (251) is located and
comprising a cooler able to bring the temperature of the reference to a
preset temperature.
25 3. The aircraft nose section as claimed in the preceding claim,
characterized in that the cooler is an open circuit cooler associated
with a reservoir of pressurized gas.
4. The aircraft nose section as claimed in one of the preceding claims,
30 characterized in that the assembly (22, 24) made up of the optics of
the seeker and detector is mounted on a rotating device (27) having
an amplitude of rotation lower than 90".
5. The aircraft nose section as claimed in one of the preceding claims,
35 characterized in that the aircraft is a rocket or a missile.
13
6. A method for calibrating the detector of the seeker (20) of an aircraft
nose section (100') as claimed in one of the preceding claims 'and the
temperature reference of which is a black body (251), characterized in
that an image comprising pixels and the detector (22) comprising at
least one integration time ti, it comprises the following steps:
- calculating an offset and a gain for each pixel for an integration
time t, of the detector, from an image of the black body at a first
temperature TI ", then from an image at a second temperature T2",
10 these images being formed on the detector through the dome (21)
and the optics (24) of the seeker, and these temperatures TI" and
T2" depending on the integration time t, and not on an estimated
temperature of t??ed om;
- storing the calculated offset and gain in memory;
15 - when there are a plurality of integration times, reiterating these
steps of calculating and storing in memory an offset and a gain for
each other integration time t, of the detector with different
temperatures TI " and T2"; and
- ejecting the calibrating device (25').
7. The calibrating method as claimed in the preceding claim,
characterized in that when there are plurality of integration times, the
second temperature used for the preceding integration time is the first
temperature of the following integration time.
25
8. The calibrating metho-d as claimed in either of claims 6 and 7,
characterized in that at least one of the first or second temperatures is
comprised between 100 K and 300 K.
30 9. The calibrating method as claimed in one of claims 6 to 8,
characterized in that it is carried out with 4 integration times.
10.A computer program product, said computer program comprising code
instructions allowing the steps of the method as claimed in any one of
35 claims 6 to 9 to be carried out when said program is executed by a
computer.