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Field of view / field of regard
The following sections provide requirements that apply to these factors, regardless of
the system design and technologies.
Latency in general defines the time difference between a cause and an effect. In a
CVS system latency or display lag is the difference between a user input to a system
and the display of the system’s response to this input, e.g . the movement of the
platform or the movement of the pilot’s head, and the display of this effect on the
Latency in a CVS system can be partitioned into delays introduced by sensor inputs,
the processing chain and display outputs. Latency requirements are more stringent if a
correlation can be made between the non-lagged visualized outside world and the
displayed imagery or between combined imagery parts of the CVS system. For these
reasons, it is evident that lower latency in order to enable an acceptable conformality
is required. Studies have shown that higher system latency increases pilot workload
and reduce flight performance1. For pilotage of a rotary wing aircraft display delay for
head-down PFD symbology of up to 250 milliseconds was found to be permissible.
For display on an HMD allowing direct correlation with the outside world latency, i.e.
display delay, exceeding 132 ms significantly degraded performance2. Other studies
on a CVS system have shown that latency of 100 ms was still acceptable3.
Higher latency times (exceeding 200 ms to 300 ms) in HMDs have shown to induce
motion sickness4 and flight discomfort and uneasiness1. It was also shown that
excessive display latency times reduce the stability margin of a man machine system5,
i.e. bear the risk of pilot induced oscillations (PIO).
While for a CVS with 2D sensors only the so called “photon-in to photon-out” principle
gives an adequate description of system latency this concept is not sufficient for a
CVS with 3D sensors. For a CVS with 3D sensors another type of latency is
introduced by the refresh rate of the 3D sensor. Most 3D sensors are based on a
scanning principle with a refresh rate of the scenery of a few Hertz down to less than
one Hertz. While this lower refresh rate does not introduce any of the negative effects
of latency resulting in non-conformity of the displayed imagery as described above
(these can be fully compensated using the synchronously recorded navigation data)
they limit the ability of the system to depict a change in outside situation. One example
is an obstacle that suddenly comes into the field of view of the 3D sensor. A lower
1 Wildzunas, R. M ., Barron, T. and Wiley, R. W ., Visual Display Delay Effects on Pilot
Performance, Aviation, Space, and Environmental Medicine, Vol. 67, No. 3, Mar.
2 Barron , S., Lancraft, R., & Zacharias, G. (1980). Pilot/vehicle model analysis of
visual and motion cue requirements in flight simulation (No. NASA-CR-3312; REPT-
4300). Bolt, Beranek, and Newman, Inc., Cambridge, MA: Washington: NASA.
3 Norah K. Link, Ronald V. Kruk, David McKay, Sion Jennings, Greg Craig, “Hybrid
Enhanced and Synthetic Vision System Architecture for Rotorcraft Operations”,
Proceedings of SPIE Vol. 4713 (2002)
4 “Helmet-mounted displays: Sensation, perception and cognition”, issues/ edited by
Clarence E. Rash, Michael B. Russo, Tomasz R. Letowski and Elmar T. Schmeisser,
ISBN 978-0-615-28375-3, U.S. Army Aeromedical Research Laboratory,
5 Lusk, S. L., Martin, C. D., W hiteley, J. D ., & Johnson, W . V. (1990, Sept. 17-19,
1990). Time delay compensation using peripheral visual cues in an aircraft simulator.
Paper presented at the AIAA Flight Simulation Technologies Conference and Exhibit,
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