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 E.  The GPSSU enters the navigation mode after it acquires and locks onto at least four satellites. While in the navigation mode, the GPSSU updates and outputs positions, velocities, accelerations, and time.
 F.  If the GPSSU is unable to track at least four or more satellites, it enters the altitude-aided mode. With only three satellites in view, the GPSSU uses IRS inertial altitude plus the length of the earth's radius as the fourth range. The GPSSU re-enters the navigation mode when four satellites are acquired.
 G.  The GPSSU enters the aided mode during short periods (less than 30 seconds) of bad satellite coverage. In the aided mode, the GPSSU receives altitude, track, and ground speed from the IRS. The GPSSU uses these inputs to continue the calculation of navigation data. If the GPSSU cannot track a sufficient number of satellites for 30 seconds, it reverts to the acquisition mode.
 H.  The GPSSU enters the fault mode if a critical fault is detected. In this mode the navigation data is invalid, and the GPSSU is no longer useful for navigation on that flight. The other GPSSU continues to supply navigation data to the FMCS.
 8.  Operation_________
 A.  Functional Description
 (1)  
The GPS can be divided into three segments: satellite, user, and control (Fig. 5).

 (2)  
The satellite segment is a group of satellites that orbit approximately 10,900 nautical miles above the earth. Each satellite completes an orbit once every 12 hours. The full group of operational satellites consists of 21 primaries and 3 spares.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
SATELLITE SEGMENT
USER SEGMENT
CONTROL SEGMENT

GPS Segments
Figure 5

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
F09040
A
737-300/400/500MAINTENANCE MANUAL
 (3)  
The user segment includes all GPS users. Each user receives the signals from four or more satellites and calculates the recevier's position. Based on information from the internal almanac, the GPSSU selects the satellite constellation which provides the highest accuracy for position determination. The GPSSU automatically switches to new constallations as the satellites progress in their orbits and the airplane proceeds on its flight path.

 (4)  
The purpose of the control segment is to monitor and correct satellite orbits and clocks, to calculate and format the satellite navigation message, and to periodically update the message via the upload stations. The message consists of data concerning the future positions of the satellites, as well as corrections for satellite time.

 (5)  
The control segment consists of one master control station, five monitor stations, and three upload stations. The stations are located in a world-wide arrangement to optimize satellite control. All monitor stations are equipped with atomic clocks and multi-channel receivers that receive signals from the satellites in view. By receiving the same signals and navigation messages as the users, the monitor stations can determine whether the satellites are providing optimal performance data.

 (6)  
The master control station directs all operations in the control segment. It continuously receives the data collected by the monitor stations. This data is used to formulate the navigation message which contains individual orbital parameters and clock corrections for all satellites. The navigation message is relayed to the satellites three times a day by the upload stations.

 B. Control
 (1)  Do these steps to put the system in operation.
 (a)  
Close the GPS 1 and GPS 2 circuit breakers on the P6 and P18 panels.

 (b)  
Check GPS operation. 1) Push the INIT REF key on one of the FMCS CDUs. 2) Push the NEXT PAGE key on the CDU. 3) Make sure that the POS REF 2/3 page shows the position of

 GPS L and GPS R.

 NOTE: It can take up to 10 minutes for GPS data to be
　
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