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transponder, but with everyone's transponder broadcasting position, altitude and velocity data. Its
advantages are its lower relative cost (no onboard interrogator required to activate transponders) and its
ability to provide S&A information in both VMC and IMC. Its disadvantage is its dependence on all
traffic carrying and continuously operating transponders. In this scenario, UA should have the capability
to change transponder settings while in flight.
The passive non-cooperative scenario is the most demanding one. It is also the most analogous to the
human eye. A S&A system in this scenario relies on a sensor to detect and provide azimuth and elevation
to the oncoming traffic. Its advantages are its moderate relative cost and ability to detect non-transponder
equipped traffic. Its disadvantages are its lack of direct range or closure rate information, potentially high
bandwidth requirement (if not autonomous), and its probable inability to penetrate weather. The
gimbaled EO/IR sensors currently carried by reconnaissance UA are examples of such systems, but if
they are looking at the ground for reconnaissance then they are not available to perform S&A. An
emerging approach that would negate the high bandwidth requirement of any active system is optical flow
technology, which reports only when it detects an object showing a lack of movement against the sky,
instead of sending a continuous video stream to the ground controller. Imagery from one or more
inexpensive optical sensors on the UA is continuously compared to the last image by an onboard
processor to detect minute changes in pixels, indicating traffic of potential interest. Only when such
objects are detected is their bearing relayed to the ground.
Once the "see" portion of S&A is satisfied, the UA must use this information to execute an avoidance
maneuver. The latency between seeing and avoiding for the pilot of a manned aircraft ranges from 10 to
12.5 seconds according to FAA and DoD studies5. If relying on a ground operator to S&A, the UA incurs
the same human latency, but adds the latency of the data link bringing the image to the ground for a
5 Tyndall Air Force Base Mid-Air Collision Avoidance Study; FAA P-8740-51; see also Krause, Avoiding Mid-Air
Collisions, p. 13
UAS ROADMAP 2005
APPENDIX F – AIRSPACE
Page F-9
decision and the avoidance command back to the UA. This added latency can range from less than a
second for line-of-sight links to more for satellite links.
An alternative is to empower the UA to autonomously decide whether and which way to react to avoid a
collision once it detects oncoming traffic, thereby removing the latency imposed by data links. This
approach has been considered for implementation on TCAS II-equipped manned aircraft, since TCAS II
already recommends a vertical direction to the pilot; but simulations have found the automated maneuver
worsens the situation in a fraction of the scenarios. For this reason, the FAA has not certified automated
collision avoidance algorithms based on TCAS resolution advisories; doing so would set a significant
precedent for UA S&A capabilities.
The long-term FAA plan is “to move away from infrastructure-based systems towards a more
autonomous, aircraft-based system” for collision avoidance6. Installation of TCAS is increasing across
the aviation community, and TCAS functionality supports increased operator autonomy. Research and
testing of Automatic Dependent Surveillance-Broadcast (ADS-B) may afford an even greater capability
and affirms the intent of the aviation community to support and continue down this path. Such equipment
complements basic S&A, adds to the situational awareness, and helps provide separation from close
traffic in all meteorological conditions.
COMMAND, CONTROL, COMMUNICATIONS
Data Link Security. In general, there are two main areas of concern when considering link security:
inadvertent or hostile interference of the uplink and downlink. The forward (“up”) link controls the
activities of the platform itself and the payload hardware. This command and control link requires a
sufficient degree of security to insure that only authorized agents have access to the control mechanisms
of the platform. The return (“down”) link transmits critical data from the platform payload to the
warfighter or analyst on the ground or in the air. System health and status information must also be
delivered to the GCS or UA operator without compromise.
Redundant/Independent Navigation. The air navigation environment is changing, in part, because of the
demands of increased traffic flow. Allowances for deviation from intended flight paths are being
reduced. This provides another means for increasing air traffic capacity as airways and standard
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