Owner’s Manual | PCAS XRX
FREQUENTLY ASKED QUESTIONS 49
experience the following problems.
For an in-depth explanation of this, see page 18 (“Built-in Altimeter for Relative Altitude”).
Why does XRX need an internal compass?
The built-in compass allows for smoothing of the traffic direction shown. Since aircraft transponders do not
broadcast a constant flow of information, but rather send only when interrogated, a compass is used to track the
local movements and relate them to any traffic currently being displayed on the screen. The compass also
corrects for sampling averaging errors when turning. See page 17 (“Built-in Compass for Relative Bearing”) for
more information.
How does PCAS compare to TIS?
TIS, or Traffic Information Service, refers to the broadcast and receiving of traffic information from Ground-Based
Transmitters (GBTs) via a transmitted RF signal. Contained in this signal is the information ATC uses to display
traffic within the service area. TIS offers some advantages, but a few disadvantages when comparing to the way
PCAS delivers traffic information.
> TIS offers a fairly comprehensive view of traffic, usually displayed onto a moving map. However, TIS informa-
tion is limited to the service area, or area in which GBTs are equipped to send a TIS signal. As of this printing,
less than 50 TIS stations exist in the US. In addition, the Mode S TIS broadcast is being scaled down in prepa-
ration for a new datalink, ADS-B. Once outside the service area, traffic is unavailable. PCAS, on the other hand,
is not tied to a ground-based service. Traffic displayed on the PCAS screen is detected and computed
independently of a ground-based service. In fact, the only thing PCAS needs to function is a Mode C
transponder in the other aircraft (which all other collision avoidance systems need as well), which makes
PCAS truly a peer-to-peer system.
> TIS requires a Mode S transponder and a way to visually display the traffic information, usually on an MFD.
While the end result is clear and concise, the cost of such systems and installation can be expensive for
general aviation. PCAS is completely self-contained and does not require any additional hardware to
function. It’s portable as well, meaning that it can be taken from aircraft to aircraft, or removed for security.
> PCAS can derive its information from both ground-based interrogations as well as TCAS-equipped aircraft
flying overhead. Considering the ground clutter limitations of RADAR, PCAS often gets more information
from TCAS interrogations than ground RADAR, especially when flying low to the ground. TIS is a ground-
RADAR-based only, and aircraft flying below the RADAR horizon will not be detected.
> PCAS information is updated as soon as the target transponder sends information, which is usually several
times per RADAR sweep or TCAS interrogation. TIS information must be processed through several ground
systems, rebroadcast, received and displayed. Traffic information, particularly altitude information, can have
considerable delays.
> One other consideration is range calculation. PCAS displays slant range, or true distance, in estimated miles.
TIS displays range from an overhead-view perspective. For example, if the target aircraft is 2500 feet directly
above you, the closest PCAS will show the traffic is 0.5 NM. TIS would show the traffic on top of your position
in a 2-D fashion. However, TIS range calculations are time-based, which can be more accurate at distance. The
relative altitude would be indicated the same on TIS as with PCAS.
Is the audio beeping loud enough to be heard over a headset in the cockpit?
XRX incorporates an internal audible tone generator (3.2 KHz) which beeps to advise the pilot and passengers of
a traffic alert or advisory. This tone is high-pitched and at a volume that can be heard over the noise level in a
standard cockpit, similar to the stall warning horn or the outer marker tone. Because noise-cancelling headsets
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