Home' RTCA Documents for Review : C2 Link Systems MASPS_Draft Contents Appendix D
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Table D-2 in this OPA’s introduction section.
Note D-13: This Subsection is not applicable to the C2 Link System if the UAS uses an
alternate means of meeting the FAA ATC communications regulations.
Note D-14: This OPA has assumed that ATC Data Communications are not within the
scope of this MASPS. The most likely implementation of having ATC Data
Communications with the remote pilot would be via a ground-to-ground link
between the Data Communication service provider and the CS.
Note D-15: Currently, the FAA has not specified latency requirements on the VHF Voice
Radios used in the airspace. The assumption used in this document is that the
delays created by the airborne VHF radios and the propagation delays are
small when compared to the delays considered in the analysis of the FAA
ground systems. Since all aircraft will be using FAA approved VHF radios
that meet the ATC voice requirements, it is assumed that the ATC voice
requirements are the minimum for all users of the airspace even if only
operating in uncontrolled airspace.
Results from the OSA indicated that there was one use case that was the most demanding
and so needed to be considered when developing the C2 Link System’s RLTP to support
the remote pilots communicate activity in this OPA. That use case is:
C1-S1-US4.1. Remote pilot does not receive vector for separation instruction from
controller because of a C2 Link System interruption that was longer than the TET; this
hazard was assessed as being Major.
There are many use cases where it will be important for the remote pilot and controller to
communicate in a timely and reliable manner. However, this specific use case is the most
demanding because of the possible loss of separation that could occur. Consequently, this
use case was used as the basis for the following OPA Communication analysis.
The FAA has established an end-to-end latency requirement for all ATC voice
communication systems of 250 ms average, 300 ms 95% of the time, and a not to exceed
(NTE) value of 350 ms . The FAA conducted two studies in 2003 to develop these
requirements, see  and . Those studies included an assumed 40 ms delay between the
FAA systems and the crew in the aircraft; this 40 ms being in addition to the above end -to-
end requirement. These studies found that there was a significant increase in blocked
transmissions and therefore a need for subsequently repeated transmissions whenever the
total latency exceeded a 390 ms (350 ms + 40 ms) threshold. Consequently, the following
analysis is based on the current FAA position that, for successful integration of UAs into
the NAS, all UAS must meet the same communication requirements as those for manned
The diagram in Figure E-1 shows all the systems that must be considered when determining
the latency for voice communications that are relayed through the UA. This is a simplified
version of Figure C-7 in the OSED Appendix intended to show the systems that must be
included in the latency calculations to determine compliance with the FAA 390 ms
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