737 Flight Crew Operations Manual
The FMC uses crew entered flight plan information, airplane systems data, and data from the FMC navigation database and performance database to calculate airplane present position, and pitch, roll, and thrust commands required to fly an optimum flight profile. The FMC sends these commands to the autothrottle, autopilot, and flight director. Map and route information are sent to the respective pilot’s navigation displays. The EFIS control panels are used to select the desired information for navigation display. The mode control panel is used to select the autothrottle, autopilot, and flight director operating modes.
Global Positioning System (GPS)
Two GPS receivers receive GPS satellite positioning signals. The left and right GPS receivers are independent and each provides an accurate airplane geographical position to the FMC and other aircraft systems. GPS operation is automatic.
GPS Displays
POS REF page 2/3 shows the left and right GPS latitude and longitude position. POS SHIFT page 3/3 shows the left and right GPS position relative to the FMC position. NAV STATUS page 1/2 shows the GPS currently in use by the FMC for position calculation.
When the POS (position) switch on the EFIS control panel is selected, the navigation display shows the left and right GPS symbols. The GPS symbols are identical and show as a single symbol when the GPS receivers calculate the same position.
An amber GPS light illuminates to indicate a failure of both GPS sensor units. Failure of a single GPS sensor causes the light to illuminate when either system annunciator panel is pushed.
GPS Data
FMC logic selects the position from one of the GPS sensor units as the primary update to the FMC position. When GPS position data is available, radio updating can also occur. If all GPS data becomes unavailable, the FMC position will be determined by radio or inertial (IRS) updating.
GPS navigational information can be manually deselected on the NAV OPTIONS page 2/2. No other controls are provided because the operation of the GPS is completely automatic.
Copyright . The Boeing Company. See title page for details.
11.20.2  March 29, 2004
Flight Management, Navigation -Navigation Systems Description
737 Flight Crew Operations Manual

GPS System Schematic
GPS GPSTRANSMITTER TRANSMITTER
GPS
TRANSMITTER GPS TRANSMITTER
GPS SENSOR UNITS
ANTENNA ANTENNA
GPS SENSOR UNIT GPS SENSOR UNIT LEFT
RIGHT
FLIGHT MANAGEMENT COMPUTER
SPO  FRE  2/ 3 
FMC  OP S  GS 
FMC Position  N47o 32.4  122o 18.6 W  KT437 
I R S  L 
N47o 32.4  6W122o 18.  KT435 
I R S  R 
N47o 32.4 W122o 18.7  KT438 
Nav Sensor  G P S  L 
Positions  N47o 32.4  W122o 18.6 
G P S  R 
N47 32.4 W122o 18.6o 
R A D I O 
N47 32.4o  W122o 18.8 
GPS  UP DA TE 
ON OFF >  GPS UPDATE 
ON/OFF 
NAV OPTIONS 
2/2 

Copyright . The Boeing Company. See title page for details.
April 28, 2000  11.20.3
Flight Management, Navigation -Navigation Systems Description 737 Flight Crew Operations Manual

Inertial System
The inertial system computes airplane position, ground speed, and attitude data for the DUs, flight management system, autoflight system, and other systems. The major components of the inertial system are the air data inertial reference units (ADIRU), an inertial system display unit (ISDU), IRS mode select unit (MSU), and an IRS transfer switch. For information about the air data part of the system, see chapter 10. The ADIRUs provide inertial position and track data to the FMC, and attitude, altitude, and airspeed data to the CDS. Each ADIRU has an IRS section and an air data section.
Inertial Reference System
Two independent IRSs are installed. Each IRS has three sets of laser gyros and accelerometers. The IRSs are the airplane’s sole source of attitude and heading information, except for the standby attitude indicator and standby magnetic compass.
In their normal navigation mode, the IRSs provide attitude, true and magnetic heading, acceleration, vertical speed, ground speed, track, present position, and wind data to appropriate airplane systems. IRS outputs are independent of external navigation aids.
IRS Alignment
An IRS must be aligned and initialized with airplane present position before it can enter the navigation mode. The present position is normally entered through the FMC CDU. If the present position cannot be entered through the FMC CDU, it may be entered through the ISDU keyboard. The airplane must remain stationary during alignment.
Normal alignment between 78 degrees 15 minutes North or South is initiated by rotating the MSU switch from OFF to NAV. The IRS performs a short power test, during which the ON DC light illuminates. When the ON DC light extinguishes and the ALIGN light illuminates, the alignment process begins. Airplane present position should be entered at this time. Alignment time varies from five minutes to seventeen minutes depending on airplane latitude.
　
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