TM  1-1520-240-10            3-3-19 3-3-38.  Modes of Operation. Control  of  the  AN/ASN-128B,  including  selection  of modes and displays, and entry and readout of data is performed  via  the  Computer  Display  Unit  (CDU)  front panel. The AN/ASN-128B has four basic modes of op- eration: off, navigate, test and GPS landing. In navigate mode three submodes may be selected manually or au- tomatically. These are combined mode (default or prima- ry mode of operation), GPS only mode, or Doppler only mode. 3-3-39.  Off Mode. In the off mode the system is inoperable. However, the Edge  lighting  is  lighted  by  an  external  aircraft  power source and is independent of the AN/ASN-128B mode switch setting. Edge lighting may not be available if the helicopter is modified with the night vision MWO. 3-3-40.  Navigate Mode. In the navigation mode (MGRS or LAT/LONG) position of the CDU MODE switch) power is applied to all system components, and all required outputs and functions are provided.  The  Doppler  radar  velocity  sensor  (DRVS) measures aircraft velocity, and converts analog heading, 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 avai- lable. 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 com- bined 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 automati- cally 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 naviga- tion. If Doppler mode is selected and the Doppler be- comes 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 opera- tion 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 electron- ics 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 malfunc- tion 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 navi- gation 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 en-