Copyright . The Boeing Company. See title page for details.
December 7, 2001  11.20.5
Flight Management, Navigation -Navigation Systems Description 737 Operations Manual

Radio Navigation Systems Automatic Direction Finding (ADF)
An automatic direction finding (ADF) system enables automatic determination of

magnetic and relative bearings to selected facilities.
One or two ADF receivers are installed. The ADF bearing signals are sent to the
pointers on the EHSIs and RDMIs/RMIs. The audio is heard by using the ADF
receiver control on the audio selector panel.

If heading or track information is lost or invalid, ADF bearing pointers on the
EHSI will not be displayed, and the ADF bearing pointers on the RDMI/RMI will
not display the correct magnetic bearing. Relative bearings indicated by pointers
may be correct if the receiver is operating.

Distance Measuring Equipment (DME)
Two DME systems are installed. Each may be tuned automatically by the FMC or manually on the VHF navigation control panel. The position of the AUTO/MANUAL switch determines the tuning mode.
With the AUTO mode selected, the FMC autotunes DME receivers as necessary for position updating. During normal operations, two different DME signals or a collocated DME and VOR signal provide an accurate radio geographical position to the FMC.
On some airplanes a dual scanning DME system is installed. This consists of two DME interrogators, each of which rapidly alternates between the manually tuned frequency (VOR/ILS) and an automatically tuned frequency (for FMC use). No AUTO/MANUAL switch is present.
DME distance is displayed on the EHSIs and RMDIs (on airplanes with RDMIs) when the receivers are tuned to a VOR/DME station or a collocated DME and localizer facility.
Instrument Landing System (ILS)
Two ILS receivers are installed. They are controlled by manual tuning on the VHF navigation control panels.
ILS information is displayed on the EADIs and EHSIs. On some airplanes, the ILS
localizer and glideslope can also be displayed on the standby horizon. LOC updating of the FMC occurs only after the ILS is manually tuned.The tuned ILS frequency is displayed in the VOR/ILS display mode.
Copyright . The Boeing Company. See title page for details.
11.20.6  December 6, 2002

Flight Management, Navigation -Navigation Systems Description 737 Operations Manual
Very High Frequency Omni Range (VOR)
Two VOR receivers and two control panels are installed. The VHF navigation control panel is used to select VOR and ILS frequencies. Each may be tuned automatically by the FMC or manually on the VHF navigation control panel. The position of the AUTO/MANUAL switch determines the tuning mode.
On airplanes with dual scanning DME, no AUTO/MANUAL switch is present. VORs are tuned manually.
VOR information is displayed on the EHSIs and RDMIs/RMIs. VOR–DME radio updating is available in the automatic or manual modes if a valid, in–range, VOR–DME station is selected.
Left and right VOR bearings are displayed on the RDMI/RMI when a valid in–range VOR station is tuned and the respective VOR/ADF bearing pointer switch is in the VOR position. The EHSI displays course deviation when operating in the VOR mode.
VHF NAV Transfer Switch
Should either VOR receiver fail, the VHF NAV transfer switch enables selection of the opposite VHF NAV receiver for display.
ATC Transponder
Two ATC transponders are installed and controlled by a single control panel. The ATC transponder system transmits a coded radio signal when interrogated by ATC ground radar. Altitude reporting capability is provided.
On airplanes with TCAS, TCAS is controlled from the transponder panel. The TCAS system is described in Chapter 15.
Weather Radar
The X band color weather radar system detects and locates various types of precipitation bearing clouds along the flight path of the airplane and gives the pilot a visual indication in color of the clouds’ intensity.
In WX mode, the radar displays a cloud’s rainfall intensity by displaying colors contrasted against a black background. Areas of heaviest rainfall appear in red, the next level of rainfall in yellow, and the least rainfall in green.
In WX/T mode, the radar displays normal precipitation and precipitation associated with turbulence. When the radar detects a horizontal flow of precipitation with velocities of 5 or more meters per second toward or away from the radar antenna, that target display becomes magenta. This magenta area is associated with heavy turbulence. The detection of turbulence is automatically limited to a 50 nautical mile range, regardless of the selected range.
　
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