UAS
Beyond Line of Sight Operations
Shawn
M. Wyne
ASCI
638 – Human Factors in Unmanned Systems
Embry-Riddle Aeronautical
University
Abstract
The operation of Unmanned
Aircraft Systems (UAS) normally requires a connection to the aircraft through a
datalink. This paper explores the method and technique of datalink through a
beyond line of sight (BLOS) connection. The technical and procedural
implications are described, as well as an analysis of benefits and drawbacks of
such a system. Finally, several applications are suggested that may see a
benefit of BLOS operations.Keywords: UAS, BLOS, beyond line of sight, datalink
UAS
Beyond Line of Sight Operations
Unmanned Aircraft Systems
(UAS) utilize a number of techniques to receive control inputs and return data
to the controller. The MQ-9 Reaper aircraft is manufactured by General Atomics
Aeronautical Systems (GA-ASI) with the express capability to operate beyond
line of sight (BLOS) from the control station (GA-ASI, 2015). Radio waves have
a limitation in that the curvature of the Earth prevents their transmission
very far. In order to extend this range, the radio waves need to have a method
of reaching an aircraft that is not blocked by the planet. The technique used
by the MQ-9 is to bounce the signal off of a satellite in geosynchronous orbit.
For many technical and practical reasons, the signal currently chosen for use
is in the Ku-band of the radio spectrum. The command signal departs the ground
station, arrives at a Ku Satellite Communications Antenna (SATCOM), and is
directed at the satellite. The satellite that receives the signal then
re-transmits it down to the aircraft, which has an 18 inch satellite dish in
the nose (GA-ASI, 2015). The return link signal follows the same path back to
the Ground Control Station (GCS). This extends the range of the aircraft to
anywhere within the footprint of the satellite, which is hundreds of miles
wide. Figure 1 shows the arrangement of systems to achieve this range. Of
course this process is neither simple nor cheap. The aircraft is only built in
one configuration, but the GCS requires extra equipment to utilize the BLOS
link. A specific link manager assembly and ground modem are needed to convert
signals into a digital form that is required. The next complicated piece is the
SATCOM antenna. This is not a small piece of equipment and requires independent
power and controls for operation. It must be precisely aimed and calibrated by
experts before use. Of course the most expensive piece of the puzzle is the
satellites themselves. Ku-band satellites are a limited resource, and used by
many commercial enterprises. MQ-9, and any UAS, are unlikely to have dedicated
satellites, so bandwidth is typically purchased from existing satellites. Bandwidth
is limited, and is directly related to cost. Ongoing operations require a dedicated planning team to schedule the appropriate time and frequencies needed. Operating in this manner also requires communication specialists to coordinate with the satellites for frequencies and numerous other details needed to get a good satellite connection. Government operations also encrypt their signals, so security personnel are needed to keep cryptologic devices current. The last pieces are the ones not unique to UAS operations: the maintenance personnel and the flight crew. For the flight crew, the procedure is actually fairly simple. The communications specialists inform the crew what satellite settings to configure, and beyond that it is simply turning the equipment on. Because the satellite is so exacting, it either works or it does not. If not, the crew cannot do anything besides changing settings.
The most significant advantage to this mode of operation is the greater range achievable by the aircraft. Because the signal is digital, it allows the inclusion of voice radio, high definition video, and all in an encrypted and secure transmission. There are also limitations involved. Notably, there is a greater time delay between the sending and receiving of signals. Ku radio waves, like all electromagnetic waves, travels at the speed of light. Over a short distance, the time is extremely short. But the distance up to and back down from the satellite is cumulative. Control and response of the aircraft is impacted, since the signal must make two trips between the pilot commanding a turn, and seeing the resulting bank. Additionally, since the signal is digital, there is an unavoidable delay in the processing of the signal. The cumulative effects of these delays can exceed 1.5 seconds (GA-ASI, 2015). While it may seem short, it is long enough to make landing an aircraft under this mode difficult enough that it is prohibited as a rule.
Depending on the goals and performance of the UAS, this delay does not have to impact its use negatively. But human factors issues do arise from the delays inherent in BLOS operations. Time delays between control inputs and observed responses induce a margin of instability that is prone to error (Wickens, Gordon, and Liu, 1998). Pilot induced oscillations are a continual risk, and the emergency procedures in the technical manual explicitly contain a boldface procedure to recover from this dangerous error (GA-ASI, 2015). The delay is not insurmountable through training, but it is always present. Due to the long duration of flight capability by larger UAS, BLOS operations are a useful tool even for commercial applications. A single aircraft could monitor hundreds of miles of pipelines in a single flight. Where terrain is uneven and might block LOS signals, an aircraft could search for wildfires across the entire state of Colorado. Even if the land is flat, a UAS could survey coastal damage after a hurricane along the entire coast of Florida. In aviation terms, the distance allowed by an LOS signal is extremely small. Many of the uses of UAS large enough to accept BLOS equipment would benefit from the expansion of its range.
References
General
Atomics Aeronautical Systems, Inc. (2015, August 4). Flight Manual, USAF Series MQ-9 Aircraft, Serial Numbers 004, 006, 008,
and Above. California: General
Atomics – ASI.Wickens, C, Gordon, S, and Liu, Y. (1998). An Introduction to Human Factors Engineering. New York: Addison Wesley Longman, Inc.
Appendix
Figure 1. UAS Command and
Control Diagram reprinted from “Flight Manual, USAF Series MQ-9 Aircraft,
Serial Numbers 004, 006, 008, and Above,” by GA-ASI, 2015.
