S 394-025
(2) Apply the sealant on all the mounting screw hole counter-bored areas (do not fill the recessed areas of the screw heads; cotton may be used to fill recesses while sealing) to make a smooth antenna surface (AMM 51-31-01/201).
NOTE: PR1826 and PR1828 sealants are recommended when a short cure
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time is important.
S 144-026
(3) Remove the unwanted sealant from around the antenna base
(AMM 51-31-01/201).
H. ADF Antenna Test
S 864-028
(1) Supply electrical power (AMM 24-22-00/201).
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747-400 MAINTENANCE MANUAL
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(2) Close the circuit breaker for the applicable ADF antenna:
(a) P7 Overhead Circuit Breaker Panel
1) 7B9 NAV RADIO ADF L
2) 7B20 NAV RADIO ADF R
S 714-030
(3) Do an operational test of the ADF antenna.
(a)
Connect the headphone to the captain's jack panel (B54).
(b)
Set the ADF/VOR selector switch on the captain's audio control panel to the ADF position.
(c)
Turn the ADF system volume control and the cockpit speaker volume control to a correct setting.
(d)
Set the VOICE/RANGE switch on the captain's audio control panel to the B (Both) position.
(e)
Use the FMC to tune the ADF receiver to a local broadcast station's frequency.
(f)
Make sure the audio sound can be heard loud and clear from the headphone.
I. Put the Airplane Back to Its Usual Condition
S 864-032
(1) Remove electrical power if it is not necessary (AMM 24-22-00/201).
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2.
3.
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747-400 MAINTENANCE MANUAL
GLOBAL POSITIONING SYSTEM - DESCRIPTION AND OPERATION
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General
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A. The global positioning system (GPS) is a satellite-based radio navigation system that calculates airplane position and instantaneous time with a high degree of accuracy. The GPS carrier frequencies are 1575.42 megahertz and 1227.6 megahertz. Commercial receivers use the 1575.42 megahertz frequency only.
B. The GPS consists of two multi-mode receivers (MMR) and two active antennas (Fig. 1). Power is applied to the system through the MMR L and MMR R circuit breakers on the P7 overhead circuit breaker panel.
C. The GPS receives position data from the inertial reference system (IRS). The GPS supplies information to the flight management computer system (FMCS), the modular avionics and warning electronics assembly (MAWEA), and the data management unit (DMU) (Fig. 2).
D. The GPS antennas receive position and time data from several satellites. The MMRs calculate the airplane position by determining the distance from the known location of the satellites in view. The GPS and other navigation systems send data to the FMCS for use in the FMCS calculation of airplane position.
GPS Antenna
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A. The GPS antenna receives L-band radio frequency signals from the navigation satellites. The antenna is small and flat (approximately 0.75 inch thick, 3 inches wide, and 5 inches long). There is a single coaxial connector at the bottom of the antenna.
B. The GPS antenna impedance is 50 ohms. A masking angle of 5 degrees is used to reject the signals from satellites less than 5 degrees above the horizon.
C. Satellite signals reflected from the surface of the ocean can change the polarization of the signals. The antenna is designed to receive signals with the correct polarization and to reject the reflected signals.
GPS Position Determination
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