Home' RTCA Documents for Review : DO-230H FRAC Contents 168
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Relative aperture is an issue with most zoom lenses at night; as the lens is zoomed in on a target, the
numerical aperture quickly increases and imaging performance degrades accordingly. Camera performance
should be validated under actual operating conditions, especially at night.
Thermal Imaging Sensors
Thermal imaging sensors, also known as Forward-looking Infrared (FLIR) sensors, sense radiated heat and
do not depend on visible illumination. This means that day and night performance can be nearly the same,
and that targets can be imaged in modest fog and in the presence of most smoke, heavy rain and leaves.
The trade-offs are less image detail and monochromatic displays. Compared to visible CCTV imagers,
FLIRs have less resolution because of their longer wavelengths; the resolution of a midwave 3 to 5 micron
FLIR will be approximately one-quarter (1/4) that of a visible camera and a long wave 8-10 micron FLIR
will have approximately one-tenth (1/10) the resolution of a visible camera. FLIR detectors, even if
uncooled, are significantly more expensive than CCD and CMOS video detectors. FLIR optics, which use
germanium or other types of crystals, are similarly more expensive than video camera lenses and the
selection of focal lengths is more limited. It may not be affordable to realize comparable range performance
with FLIRs and video cameras, and this must be evaluated when considering the use of FLIR sensors.
FLIR detectors are available with or without cryogenic cooling. Cooling a thermal detector improves its
performance, but it is costly and the operational reliability of cryogenic coolers is limited. For airports, in
situations where detection ranges are modest, uncooled detectors are the preferred choice. At the present
time, most commercial uncooled FLIRs use detectors made from vanadium-oxide (VOX), but other
materials are available including amorphous silicon (aSi) and barium strontium titanate (BST) which offer
similar performance. BST detectors, however, tend to be noisy and blurry and this can detract from using
BST-based thermal imagers with video analytics.
Because they can detect longer wavelength energy, FLIRs can provide target assessments at night and in
bad weather, both of which are essential for airport perimeter coverage. FLIRs can do this without regard
for variations in visible light including strong point light sources which may be employed on the airside
and along terminal roadways at night.
FLIRs can be used in most cases with video analytics because white-on-black background target images
often provide sufficient contrast for the analytical functions to perform acceptably. As in the case of video
sensors, validating FLIR performance under actual operating conditions should be done before FLIRs are
specified as ISSA sensors.
Perimeter intrusion suggests breaches at outlying fences. In an airport regulatory context, however, the
security perimeter typically includes fences, vehicle and pedestrian gates, tenant facilities (such as
commercial airlines), military facilities, water boundaries, and an “interior perimeter” consisting of a large
number of potential breach points inside the terminals, offices, and commercial buildings.
The airport’s Perimeter Intrusion Detection System (PIDS) must provide essential information on the
intrusions for airport security to discriminate between true security threats, and such anomalous, but
relatively common false alarms as malfunctioning access controls, employee misuse of credentials,
procedural failures such as piggy-backing or escort guidelines, or misinterpretation of alert/alarm data at
the Security Operations Center (SOC).
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