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MAINTENANCE MANUAL
IRU POSITIONS
VOR/DME
POSITION
RADIO POSITION UPDATE MODE
GPS Position on Navigation Display
Figure 4
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MAINTENANCE MANUAL
LINE
SELECT
LINE
SELECT
KEYS
KEYS
BRIGHT FUNCTION AND
ADJUST MODE KEYS
ANNUNCIATORS
ANNUNCIATORS
CLEAR KEY
DELETE KEY
NUMERIC ALPHA
INIT/REF INDEXCONTROL AND DISPLAY UNIT
POSITION INITIALIZATION POSITION REFERENCE
POSITION REFERENCE
FMCS CDU - Displays
Figure 5
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MAINTENANCE MANUAL
(2)
The position reference (POS REF 2/3) page provides the ability to enable or to inhibit GPS data to the FMCS. To enable GPS, push the line select key adjacent to ENABLE>. The GPS data to the FMCS will be enabled, and the prompt will change to INHIBIT>. Push the line select key again, and GPS data will be inhibited and the prompt will change to ENABLE>.
(3)
The POS REF 3/3 page displays GPS latitude, longitude, and ground speed from both GPSSUs.
5. 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 MMR 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.
6. GPS Modes of Operation
______________________
A. The GPS Unit operates in the following modes:
(1)
Self-test
(2)
Initialization
(3)
Acquisition
(4)
Altitude aiding
(5)
Aided
(6)
Fault
B. During the self-test mode, the MMR tests its internal circuits to verify proper operation. The test occurs automatically on power-up. If the self-test fails, the MMR enters the fault mode.
C. The MMR 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.
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