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Display Unit (CDU) is installed in the CPG right-hand
console. The CDU (fig 3-11) identifies those
helicopters equipped with the AN/ASN-128 DNS.
The CDU (fig 3-12) are those helicopters equipped
with the AN/ASN-137 DNS. The CDU used with the
AN/ASN-137 DNS is not part of the DNS, but is used
to communicate with the DNS.
The DNS provides doppler velocity, and navigational
position and steering information for the helicopter. The
DNS is the navigator in the non integrated system.
The HARS provides attitude reference (heading, pitch,
and roll) to the DNS. Navigational accuracy may
degrade at altitudes above 10,000 feet AGL, and at pitch
and roll angles greater than 30 degrees. The DNS is
capable of providing position readouts in either the
Military Grid Reference System (MGRS) form of
Universal Transverse Mercator (UTM) or in latitude and
longitude (LAT/LONG) coordinates. Navigation
calculations are accomplished by the DNS in LAT/LONG
and then converted to UTM (if necessary) for display
purposes. Coordinate data may be entered in either
coordinate format. The AN/ASN-128 DNS has 10
internal storage locations for coordinate data. The
AN/ASN-137 DNS has 20 storage locations. Primary
power to operate the DNS is provided through the DPLR
circuit breaker on the pilot overhead circuit breaker
panel. In addition, the AN/ASN-137 DNS also uses
power provided through the MUX FAB R circuit breaker
on the CPG No. 1 circuit breaker panel.
3.14.4 DNS Antenna. The AN/ASN-128 and AN/ ASN-
137 DNS use a common antenna (RT-1193A). The DNS
antenna is located in a combined antenna/ra-dome and
(RT) housing protected by the fairing on the bottom
center fuselage area as shown in figure 3-1.
3.14.5 AN/ASN-128 DNS Controls and Displays. The
controls and displays are on the front of the CDU
CP1252 (fig 3-11) located on the CPG right console.
The function of each control is described in table 3-12 .
3.14.6 Modes of Operation. The three basic modes of
operation are as follows:
a. Test Mode. The TEST mode contains two functions:
LAMP TEST mode, in which all display segments
are lit, and TEST mode, in which system operation is
verified. In the LAMP TEST mode. system operation is
identical to that of navigate mode except that all lamp
segments and the MEM and MAL indicator lamps are
lighted to verify their operation. In TEST mode, the
system antenna no longer transmits or receives
electromagnetic energy; instead, self-generated test
signals are inserted into the electronics to verify
operation. System operation automatically reverts into
the backup mode during test mode. Self-test of the DNS
is done using Built-In-Test Equipment (BITE) and all
units connected and energized for normal operation.
Self-test isolates failures to one of the three units of the
DNS. The CDU (except for the keyboard and display) is
on a continuous basis, and any failure is displayed by
turn-on of the MAL indicator lamp on the CDU. The
signal data converter and (RTA) are tested by turning the
MODE selector switch to TEST. Failure of these
components is displayed on the CDU by turn-on of the
MAL indicator lamp. Identification of the failed unit is
indicated by a code on the display panel of the CDU.
Continuous monitoring of the signal data converter and
RTA is provided by the MEM indicator lamp. The MEM
indicator lamp will light in normal operation when flying
over smooth water. However, if the lamp remains lighted
beyond 10 minutes over land or rough water, there is a
malfunction in the doppler set. Then, to determine the
nature of the malfunction, the operator should turn the
MODE selector switch to TEST. As the keyboard is
used, operation is verified by observing the alphanumeric
readout.
Change 3 3-37
TM 1-1520-238-10
Figure 3-11. CDU CP1252, Used With AN/ASN-128 DNS
Table 3-12. AN/ASN-128 Control and Indicator Functions
Control/Indicator Function
MODE Selector
OFF Turns off electrical power to the set.
LAMP TEST Checks the operation of all lamps.
TEST Initiates a (BIT) exercise for the DNS.
UTM Selects (UTM) as the navigational mode of operation.
LATT/LONG Selects LAT/LONG as the navigational mode of operation.
BACKUP Places navigation set in true airspeed plus remembered wind mode of operation. If true
airspeed is not available, places navigation set in remembered velocity mode of
operation.
DISPLAY Selects navigation data for display.
selector
WIND SP/DIR Windspeed in kilometers per hour (km/h).
(Left Display)
3-38 Change 3
TM 1-1520-238-10
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