TM  1-1520-240-10 3-3-16            3-3-35.  Horizontal   Situation   Indicator   Mode   Se- lect.  (fig. 3-3-6) The HSI MODE SELECT panel is on each pilots instru- ment panel below HSI. The panel allows the pilot to se- lect  the  navigation  mode  to  be  displayed  on  the  HSI, command AFCS heading select feature, and visually in- dicates marker beacon passage. The panel is divided into three sections labeled CRS, BRG, and MKR BCN. a.  The CRS (course) section consists of four push- button selector switches labeled VOR SEL, GPS/DOP SEL, FM SEL, and CMD SEL. Pressing VOR SEL, GPS/ DOP SEL, or FM SEL causes the output of the selected navigation set to be electrically connected to the course deviation indicator on the corresponding HSI and lights SEL legend in the switch. These switches are electrically interlocked so only one set may be selected at a time. Selecting another navigational modes, automatically dis- engages the mode in use, turns out the SEL legend of the mode in use, engages the selected mode, and lights the SEL legend of the selected mode. Each pilot may inde- pendently select different navigational modes for display on his HSI. b.  Pressing CMD SEL engages the AFCS heading select feature, and will illuminate the SEL legend on the switch. Due to electrical interlocks between the two HSI control panels, both CMD SEL switches cannot be en- gaged at the same time. The engaged switch is indicated by the lit SEL legend. If the opposite CMD SEL switch is pressed when operating with heading select, the HDG on the AFCS panel will release and heading select will be disengaged. Control of the course deviation indicator in the VOR mode relative to bearing and course deviation to or from a VOR is assigned to the pilot who has CMD SEL engaged. If the pilot who does not have CMD SEL engaged makes an adjustment on his CRS knob, it will have no effect on the CDI relative to the course. c.  Pressing VOR/ADF pushbutton on the BRG side selects the navigational aid, the bearing of which will be indicated by the HSI No. 2 pointer. The switch is divided into two segments labeled VOR and ADF. If VOR seg- ment is lit, the No. 2 pointer will indicate the bearing to a VOR station to which the VHF navigation set is tuned. Pressing the switch will cause the VOR segment to go out and the No. 2 pointer will indicate the bearing to the station to which the ADF set is tuned out and light the ADF  segment.  The  opposite  action  occurs  if  the  ADF segment is lit and the switch is pressed. d.  A MKR BCN light will illuminate during marker bea- con passage. The light labeled O comes on when pass- ing through an outer marker beam. The light labeled M comes on when passing through a middle marker and finally the light labeled I comes on when passing through an inner marker. As each light illuminates, a distinctive tone identifying the beacon will be heard over the inter- phone (table 3-2-1). The lights have a press-to-test fea- ture; pressing any one of the three lamps will cause all MKR BCN lamps to illuminate on both pilots HSI MODE SELECT panels. 3-3-36.  Doppler/GPS  Navigation  Set  AN/ASN-128B (DGNS). The AN/ASN-128B DGNS is an AN/ASN-128 LDNS with embedded GPS receiver. The AN/ANS-128B in conjunc- tion with the aircraft’s heading, vertical references, and position and velocity updates from its internal GPS, pro- vides accurate aircraft velocity, position and steering in- formation from ground level to 10,000 feet. The system provides  world-wide  navigation,  with  position  readout available   in   both   Military   Grid   Reference   System (MGRS) and Latitude and Longitude (LAT/LONG) coor- dinates. Navigation and steering is performed using LAT/ LONG  coordinates  and  a  bilateral  MGRS-LAT/LONG conversion routine is provided for MGRS operation. Up to 100 destinations may be entered in either format and not necessarily the same format. 3-3-37.  Controls, Displays and Function. The control and displays for the AN/ANS-128B are on the front panel (fig. 3-3-7). The function of each control is as follows: NOTE The MODE switch is locked in the OFF posi- tion and must be pulled out and turned to get into or out of OFF position. CONTROLS/ INDICATOR FUNCTION MODE switch Selects navigation set mode of operation. OFF In this position the naviga- tion set is inoperable; non- volatile RAM retains stored data. LAMP TEST Checks operation of all lamps. TEST Initiates built-in-self-test ex- ercise for the Doppler and GPS functions of the naviga- tion set.