(2)
The position shift (POS SHIFT 3/3) page shows the position of each sensor unit (and FMC) in relation to the FMC system position. The RNP/ACTUAL display shows the computed accuracy in relation to the required accuracy for FMC navigation.
(3)
The navigation status (NAV STATUS 1/2) page shows the update status of various navigation systems (including GPS) used by the FMC. Data from these systems are used in the navigation solution.
(4)
The navigations options (NAV OPTIONS 2/2) page provides the ability to select the GPS update mode. The line select key adjacent to ON/OFF> toggles the GPS data to the FMC on and off. The selected mode is highlighted on the CDU display.
6. GPS Signals
___________
A. All GPS satellites simultaneously transmit at the same frequency. A spread-spectrum modulation technique enables pseudo random codes and navigation data to be transmitted simultaneously at the same frequency without interference. Each satellite has a unique pseudo random code.
NOTE: A pseudo random code is a signal with a distinct pattern which is
____
used to identify a particular satellite.
B. GPS signals are transmitted at very low power, often below that of thermal noise. Spread-spectrum modulation is used to transform the signal from high power to a large band width. This technique spreads the carrier power over a wide frequency sprectrum.
C. The GPSSU knows the transmitted carrier frequency, the type of modulation, the pseudo random code, and the code bit rate. The receiver generates a copy of the signal expected from each satellite. The respective codes from each satellite are aligned, synchronized and then tracked.
D. The receiver decodes the spread spectrum signal by knowing what to look for and where to look, below the noise level for repeating patterns. The signal is multiplied by the selected code and is converted to a high-power, narrow-band signal. The signal is demodulated to obtain the navigation data.
7. GPS Modes of Operation
______________________
A. The GPSSU operates in the following modes:
(1)
Self-test
(2)
Initialization
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A 737-300/400/500MAINTENANCE MANUAL
NAVIGATION OPTIONS
FMCS CDU - Displays
Figure 4
F10217
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A
737-300/400/500MAINTENANCE MANUAL
(3)
Acquisition
(4)
Altitude aiding
(5)
Aided
(6)
Fault
B. During the self-test mode, the GPSSU tests its internal circuits to verify proper operation. The test occurs automatically on power-up. If the self-test fails, the GPSSU enters the fault mode.
C. The GPSSU enters the initialization mode if the self-test passes. This mode lasts about 30 seconds while the signal processing sections are initialized with values of latitude, longitude, and altitude. During this mode there are no navigation outputs.
D. In the acquisition mode, the GPSSU searches for and locks onto the satellite signals. The GPSSU must find at least four satellites before it starts to calculate GPS data. The GPSSU uses position data from the IRS that was obtained in the initialization mode in order to calculate which satellites are in view. If the IRS position data is not available, the GPSSU can still acquire satellite signals. In this case, the GPSSU must look for all the satellites. This requires approximately 10 minutes.
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