Home' RTCA Documents for Review : C2 Link Systems MASPS_Draft Contents Appendix D
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The FAA Column gives the latency (in ms) specified or me asured by the FAA for each
type of voice switch. The value includes delays from all systems between the controller
and the FAA’s VHF Radio (but does not include any delay in the FAA radio itself because
the FAA considers it to be negligible). The propagation column is the calculated
propagation delay (in ms) for the range required in each type of environment (en route or
terminal). The UAS column gives the difference between the FAA NAS level specification
for maximum latency (e.g., 390 ms NTE) and the data in the FAA column; this represents
the allowable latency for a UAS operating in that environment. The final column explains
the specific rationale for the calculations that resulted in the number in the UAS column.
With this information, the resulting latency standard for a UAS operating in controlled
airspace and managed by a Center would need to be 152 ms on average (the shortest time
within the UAS column in Table E-8) and 178.5 ms for 99% of the time (from the second
row of Table E-8). For airspace managed by a Terminal Radar Approach Control
(TRACON), the standard would be 154.7 ms NTE (from the last row of Table E-8). The
measured data (in the FAA column of Table E-8) has much lower latency than the
specification requires for the current Center’s Voice Switching and Communication
Systems (VSCS). Unfortunately, there is no measured data available for terminal systems.
The Interim Voice Switch Replacement System (IVSR) is the most recently procured VHF
system that has been put into service. But there are four other legacy systems in use as of
the writing of this document; unfortunately, there is no latency data available for those
older systems. The NVS latency requirement is much stricter than the current VSCS
system. So, in the future, there will be more time for the UA voice relay when in en route
airspace (i.e., Class E and A). Consequently, the most demanding latency, based on the
above analysis, is the (390 - 235 - 0.3 =) 154.7 ms (rounded to 155 ms); this was derived
using the Terminal Voice Switch (IVSR) specification (from the second row of Table E-
8). This assessment is based on the consideration that an NTE standard will be more
difficult to meet than a mean based standard (i.e., the mean requirement of 152 ms of the
VSCS) and the fact that the terminal environment has a lower tolerance for disruption
caused by repeated voice communications.
Note D-16: If measured data for terminal voice systems is made available by the FAA, this
requirement should be revisited in future versions of this MASPS.
Short C2 Link System Interruptions
From the perspective of efficiently supporting the remote pilot’s voice communication
activities, C2 Link System interruptions should be kept short enough so that the
information being communicated to or from the remote pilot is not significantly impacted
and thus not resulting in the communicator’s intent being unclear. The phraseology used
by all pilots and controllers is limited and has some context ba sis to help with
understanding. However, even though the phrases used are typically only a few words in
length, each word carries important information that is critical to the clear understanding
of the communicator’s intent. So, in the analysis used here, an interruption of longer than
one word is assumed to cause a request to repeat the transmission. That request would
somewhat degrade the efficient operation of the communicators.
In this analysis, the following typical information exchange is used as the basis for
communication exchanges for the use cases and scenarios being considered. Although in
some exchanges the phrases may be longer and in other exchanges they are shorter, for this
analysis, it is viewed that the following phrases are average in length and therefore
appropriate for the demanding situations being considered.
Controller: “Callsign, turn right to heading X degrees”
Pilot: “Callsign, turning right to heading X degrees”
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