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pitch and roll into digital form. This data and Embedded
GPS Receiver (EGR) velocity and position data are then
sent to the CDU for processing. Baro altitude is used for
aiding the GPS when only three space vehicles are available.
Present position is computed by using one of three
navigation submodes which can be selected manually or
automatically. These submodes are as follows:
3-3-41. Combined Mode (Default or Primary Mode of
Operation).
Doppler and GPS position and velocity data are combined
to provide navigation. This mode is used when a
minimum of three (with baro) or four space vehicles are
available. GPS Estimated Position Error (EPE) is less
than approximately 150 meters, and the Doppler is not in
memory. If GPS becomes invalid (e.g., due to increased
EPE), the AN/ASN-128B will automatically switch to
Doppler mode until a valid GPS status is received. If the
Doppler becomes invalid (e.g., flight over glassy smooth
water causing memory), the AN/ASN-128B will automatically
switch to GPS mode if GPS is valid or an alternate
Doppler mode if the GPS is not valid.
3-3-42. GPS Mode.
GPS positions and velocities are used for navigation by
the Doppler navigation processor in the CDU. If GPS
mode is
selected and the GPS becomes invalid (para. 3-3-41
above), the AN/ASN-128B will not navigate.
3-3-43. Doppler Mode.
Doppler position and velocity data are used for navigation.
If Doppler mode is selected and the Doppler becomes
invalid (para. 3-3-41 above), the AN/ASN-128B
will automatically switch remembered velocity since a
TAS sensor is not available. If Doppler mode is manually
selected at the start of the flight an initial present position
must be obtained and entered prior to flight. Navigation
is performed in a latitude/longitude for computational
convenience only. At the same time, distance, bearing
and time-to-go to any one of 100 preset destinations are
computed (as selected by FLY-TO-DEST.
3-3-44. Test Mode.
The test mode contains two functions: LAMP TEST, in
which all display segments are lit; and TEST , in which
system operation is verified. In lamp test, system operation
is identical to that of the navigate mode except that
all lamp segments and the MAL indicator lamp are lighted
to verify their operation. In TEST, the RTA no longer
transmits or receives electromagnetic energy; instead,
self generated test signals are inserted into the electronics
to verify operation of the DRVS. At this time a self test
is performed by the GPS and navigation computations
continue using remembered velocity. In the TEST mode,
Doppler test results are displayed on the CDU front panel
for the first 15 seconds (approximate). At the end of this
period either GO ALL is displayed if there is no malfunction
in the navigation set, or a failure code is displayed if
a malfunction has occurred. A rotating bar on the display
indicates that the GPS has not completed self test. If the
navigation set is maintained in the TEST mode, no navigation
data can be displayed on the CDU front panel. If
a Doppler malfunction is detected, the MAL indicator
lamp lights and DF is displayed. At the completion of
GPS self test (up to two minutes), The rotating bar is
replaced with a complete test result code. the failed unit
and the failed circuit card are also indicated by a code on
the CDU display.
The CDU is continuously monitored for failures, using its
own computer as built-in-test-equipment (BITE). Any
BITE malfunction the MAL indicator lamp on the CDU to
light. If the MODE switch on the CDU is set to TEST,
identification of the failed LRU is indicated by a code on
the display panel. Aircraft heading, pitch and roll are also
displayed in this mode by depressing the ENT key after
Doppler test is completed. GPS test status is displayed
if the ENT key is depressed a second time. Malfunction
codes are automatically latched and can only be cleared
by recycling the CDU power via the CDU mode switch
(OFF/ON).
3-3-45. GPS Landing Mode.
In the GPS landing mode, the Doppler navigation system
provides information to the HSI indicator for real-time
landing guidance to a touch down point previously enTM
1-1520-240-10
3-3-20
tered in any of the 100 fly-to destinations. The landing
approach is determined by present position and the entered
touch down altitude, glideslope and inbound approach
course.
3-3-46. CDU Operation.
Various required operating data, such an initial present
position (if GPS is not valid or Doppler mode is selected).
destination coordinates with or without GPS landing
　
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