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"NO ALT" REPLIES
Many aircraft have encoding altimeters that require warm-
up time. The replies received from warming-up encoders do
not contain altitude data. TCAD will indicate NO ALT
from these aircraft, until their encoder is provides altitude
data. These replies often come from aircraft interrogated
while on the ground. Thus, TCAD may display NO ALT
even in locations where altitude reporting is required.
Occasionally a NO ALT indication can be generated from
sources other than non-Mode C traffic. These replies come
from operations unrelated to civilian air traffic control, and
are more likely to occur along the coasts. There is no
difference between these replies and non-Mode C replies
and the data is treated as traffic. TCAD priority
computations weight nearby Mode C traffic above the non-
Mode C, so altitude-equipped traffic will be prioritized and
displayed appropriately. Good management of shield size
will minimize extraneous indications.
RANGE CONSIDERATIONS
Range is displayed in indicated Nautical Miles (iNM).
Actual range is equal to iNM when the threat aircraft’s
transponder output power is normal, and the signal is not
shadowed by the airframe. Transponders used aboard
general aviation aircraft typically provide close agreement
between iNM and actual range. Higher speed aircraft, such
as airliners, often transmit stronger transponder signals,
usually two times the power of general aviation
transponders. In this case the actual range will be about
40% greater than the displayed iNM, thus offering greater
margins for these high-speed aircraft. For example, an
airliner showing 3.0 iNM may actually be four miles or
more away. An airliner showing 0.7 iNM is likely to be a
mile away.
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