The final set of internal environments to be considered is temperature and altitude. The most 
obvious considerations are whether or not the sensor will be installed in a pressurized compartment and 
whether or not the temperature of the compartment will be controlled. Assuming the answer to both is 
no, as is typically the case, what then are the effects upon the sensor design? To maintain sensor 
resolution, optical alignment must remain precise over a large range of temperatures. The aircraft 
could be required to fly a mission profile which involved takeoff from an airbase in summer, where the 
ground temperature is 90°F, resulting in internal compartment temperatures of 140°F. High altitude | 
transit flight to the target area then subjects the sensor to a cold environment of perhaps -20°F. This 
is followed by the penetration imaging run where some intermediate temperature is experienced by the 
sensor. Again, the sensor opto-mechanical design must be considered. To provide a sensor structure 
with inherently low susceptibility-to-vibration, an analysis was performed which identified materials 
and structural shapes that yielded a mechanically stable structure. It is of equal importance, in | 
light of the temperature altitude environment, that the structure be thermally stable. Therefore, the | 
sensor design should be modeled and analyzed to quantify the effects of the different thermal expansion 
rates of the materials, given their specific shapes, lengths and thicknesses. The goal is to achieve a 
design in which thermal expansion motions tend to cancel at steady state temperatures. Of course, 
material selection and component design must be accomplished within the requirements dictated by the 
vibration environment. 
The guidelines to achieving this are to restrict the design to structures which are symmetric, and 
minimize the types of materials used so as to match thermal expansion coefficients as much as possible. 
For exemple, consider again the detector/mirror arrangement discussed previously. For good stiffness 
the mir—or mount could be fabricated from stainless steel. The mirror itself could be glass with a 
front surface reflective coating, polished steel, or perhaps some other material. From a thermal 
viewpoint, however, it is important that the mirror and mount be of the same material. If this were 
not the case, the mirror and mount could act as a bimetallic strip over temperatures. This would 
reszit in loss of the mirror's optical figure and perhaps a misalignment of the entire optical system. 
Perhaps a more difficult aspect of the temperature altitude environment is one which arises from the 
aircraft aerodynamics, relatively independent of the gross environment. During high speed flight the 
friction of the air passing the aircraft skin produces heating of the skin and any stagnant air trapped 
in surface irregularities. For high subsonic and supersonic flight at low altitude, skin temperatures | 
of 85°C are common. If the sensor aperture is exposed, as is the case for an infrared linescanner | 
imazing through a slot in the aircraft skin, there will be heating of the external components of the 
optical system. In addition to the thermal effects already considered, aerodynamics heating of the 
aperture can mask the scene being imaged. This can be dealt with in two ways. The first is to recess 
the sensor so that its aperture lies well below the skin. This results in a larger slot in the skin 
for the same clear viewing aperture. Unfortunately, the slot can have adverse effects on local airflow 
which can affect aircraft performance. Usually, systems imaging through a slot employ fairings around 
it to minimize the aerodynamic effects and further isolate the sensor aperture. It is interesting to 
note that disturbances in the boundary layer air at the aperture do not affect image quality due to long 
wavelength of the infrared energy. 
A second technique is the use of infrared transmitting windows to isolate the sensor and provide a 
clean aerodynamic.skin. The choice of materials available is very limited; gerranium, zinc sulfide, itil 
and zinc selenide being the most common. 
The use of the IR windows has several major drawbacks however. They are relatively expensive and 
difficult to fabricate. They are subject to erosion and wear, making them difficult to maintain. And, | MN 
their transmission decreases rapidly with temperature.  Documented experience over the past five years | 
on RF-4, F-14, and FlllC aircraft has shown in the case of the AN/AAD-5 Infrared Linescanner system that 
system performance can be maintained in this environment using the recessed aperture, faired slot | | 
design. To date, no high performance infrared reconnaissance system has successfully demonstrated 
comparable image quality obtained through an infrared window. 
SUMMARY 
In order to successfully develop and deploy an infrared reconnaissance sensor the systems engineer 
must have a thorough understanding of the proposed mission and hence the mission environment. The. 
external environment will dictate the resolution, field-of-view and thermal sensitivity of the sensor 
system. The mission will determine the choice of aircraft and flight profile, and hence the internal 
environment in which the sensor must meet its specified performance. Since no sensor system can 
perform under all combinations of internal and external environments to their extremes, a nominal 
environmental set must be determined and this set must be bounded by reasonable expectation of its 
extent about the nominal case. To be successful, the entire reconnaissance system must be considered, 
not just the infrared sensor. 
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