Sense and avoid
sensor selection
Unmanned aerial systems (UAS) integration
into National Aerospace System (NAS) dictate establishing regulatory standards
and equipment requirements to ensure UAS operational safety. Sense and avoid
technology is one of the important issues pertaining to safety of UAS. Pilots
of manned aircraft are responsible to see and avoid other traffic by relying on
visual detection, air traffic control radar separation, and other available
sensors installed on the aircraft. Unmanned aircraft pilots often have to rely
on the UAS camera sensor picture with limited field of view, which is not
sufficient for the sense-and avoid requirements for operation in the NAS.
That’s why proper selection, testing, and certification of sense-and -avoid
technology for UAS is important.
Electro optical/ infrared (EO/IR) sensor
is a sense-avoid technology selected for this paper. EO/IR is a non-cooperative
sensor, which means it does not rely on the technology carried by the intruder
aircraft. EO/IR is a passive sensor. Due to this sensor’s size, weight and
power (SWAP) considerations, it is suitable for use in the smaller size UAS
(less than 55 pounds). It is capable to operate during day and night in all
weather conditions. However, current EO/IR technology for SAA may have an
increased rate of false alarms generated due to clutter in images and weather
conditions such as fog or clouds.
EO and IR technologies are combined in a
single compact sensor create a complete SAA system. The EO sensor takes images in
the visible light spectrum with a charged coupled device camera. The infrared
sensor works within infrared spectrum, creating images based on temperature
differentiation.
EO/IR sensors have good performance in
terms of detection of azimuth, elevation and traffic coverage. However, the drawbacks
of this technology is that it is restricted range and has a limited field of
view (FOV). It is important to mention, that a tradeoff exists between the FOV
and detection range. For example, if the EO/IR sensor has a large FOV versus a
small FOV to passively scan, the distance at which it can detect an object will
decrease (Pearson, Moore, Ogdoc,
& Choi, n.d). The particular EO/IR sensor which is suitable for smaller
size UAS was developed by HoodTech Vision. Alticam AC-10 EO/IR sensor has small
size and weighs only 5,700 grams and measures 25.4 cm in diameter. It is designed
for both day and night and all weather operations. The sensor also features a laser
pointer, a laser range finder, and a pan-over tilt. Gimbal that tilts 45
degrees up and 90 degrees down with 360 digress endless pan capability. Since
power consumption is an important parameter to consider for small UAS, the AC-
10 was designed to use half the power of similar systems, freeing power for
additional sensors and saving fuel for increased mission range. Power supply
range is 24-32 VDC with 31 W continuous and 55 W peak consumption (HoodTech, n.d.).
It also has an increased FOV for better
traffic detection: the IR sensor has a horizontal FOV of 1.7°- 22° and EO
imager features 1.1°- 31.5° FOV. Since this particular sensor is enhanced with
laser pointer and range finder, its application for as a sense-and-avoid sensor
is greatly improved. The laser rangefinder operates in the 30 to 3000 meter
range and it is eye-safe. The slew rate of the gimbal
is 60° per second.
Figure
1. Alticam
AC-10 EO/IR sensor. Adapted from “Alticam
AC-10 specifications,” by HoodTech. (n.d.). Copyright by HoodTech.
1. The EO/IR sensor detects potential
intruders.
2. The laser subsystem confirms the
azimuth and elevation angles of potential traffic and estimates range of the
targets.
4. The gimbal with laser sensor slews to
the target bearing angle detected by the EO/IR system.
5. After this information has been
analyzed and, in case the intruder traffic presence is confirmed, the bearing
angles from the EO/IR and the range from the laser ranger are fused to estimate
the position and velocity of the intruder. In scenarios
involving multiple intruders, the gimbals/scanners may be employed to slew the
laser from one intruder to another (Ganguli, Avadhanam, Yadegar, Utt,
& McCalmont, 2011).
Additional specifications details of AC-10
sensor are presented in Table 1.
|
Sensor
|
Wavelength
|
Horizontal FOV
|
Pixels
|
Video output
|
Zoom
|
|
IR imager
|
3-5 μm
|
1.7°- 22°
|
640 x 480
|
NTSC
|
Optical 13X; digital 2X
|
|
EO imager
|
0.4-0.9 μm
|
1.1°- 31.5°
|
1280 x 720
|
NTSC
|
Optical 30X, Digital 0.5-2X
|
Table
1.
EO/IR sensor specifications. Adapted from Adapted from “Alticam AC-10 specifications,” by
HoodTech. (n.d.). Copyright by HoodTech.
References
