INSTALLATION MANUAL
89000007
Page 4-5
15 Nov 2013
© Honeywell International Inc. Do not copy without express permission of Honeywell.
NOTE:
The TIS-A output and GPS input speeds are not separately controlled on the
KT 74. Not all combinations of GPS input and TIS-A output will be usable if
the external devices operate on fixed bit rates and are different to each
other.
4.3.11 GPS System Certification Level
An important metric for ADS-B ground system behavior is the SDA level. It is
intended to reflect the probability that the GPS position source is providing
erroneous information, and is based on the certification standard that was
used by the GPS vendor. This will be indicated in the form of a letter code (A
to D) on the data plate or installation documentation for the GPS in
accordance with the standards DO-178B and DO-254, for example “DO-
178B level C”. If both standards are reported but at different levels, use the
lower standard (higher letter).
4.3.12 GPS NACv
Another metric that the ADS-B ground system uses to help it track the
aircraft is NACv. NACv is a design feature of the GPS receiver. It represents
the error bound for velocity that the GPS may report in
acceleration/deceleration or turning maneuvers. You can find this information
from your GPS installation manual.
4.3.13 Measurement Unit
Entry of the following configuration items may be in meters or feet. Select the
required measurement unit from the choice on the display.
4.3.14 Aircraft Length and Width
On the ground, ADS-B transmits encoded aircraft size information which is
used by ATC to identify taxiing routes and potential conflicts. When
configured for ADS-B, the KT 74 will ask for the aircraft length and width
(wingspan), in meters, and will calculate the appropriate size code for
transmission.
4.3.15 GPS Antenna Offset
The GPS antenna offset is used together with the aircraft length and width to
manage taxiway conflicts. A typical GPS installation does not report the
geographic position of the center of the aircraft, or even the tip of the nose
of the aircraft; instead it usually reports the location of the actual GPS
antenna (not the GPS receiver). In normal flight operations this distinction is
of no practical importance at all, but if ADS-B is used to manage taxiway
conflicts, a significant offset in antenna position could mean that the aircraft
is not in the same place as the ADS-B reported position. Although primarily
intended for position correction on large transport aircraft, General Aviation
aircraft can also have a significant offset. For example, if the aircraft has a
long tail boom and the GPS antenna is on the top of the tail, the GPS
position could be 15 feet (4.6 m) or more from the nose of the aircraft.
