Page 21 D633W101-BEJ Sep 05/2002
GPS -THEORY OF OPERATION -3
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There are only 3 satellites available with good geometry for position fixes
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The MMR stored the difference between inertial and GPS altitude in memory.
The MMR starts normal operation again when a fourth satellite comes into view.
Aided Mode
The MMR enters the aided mode during short periods (less than 30 seconds) of bad satellite coverage. An example of bad satellite coverage is poor satellite geometry, where at least four satellites are available but they are not spread out far enough so the MMR can make an accurate position fix.
In the aided mode, the MMR receives altitude, heading, and speed from the flight management computing function (FMCF) of the AIMS. The MMR uses the FMCF data to go back quickly to the navigation mode when there is good satellite coverage again. The MMR output is NCD in this mode.
GPS Frequencies
The satellites transmit to the MMR in the airplane on the L1 (1575.42 MHz) and L2 (1227.6 MHz) frequencies. The satellites downlink satellite status data to the monitor stations on 1783.74 MHz.
The upload stations send information to the satellites on 2227.5 MHz.
Autonomous Integrity Limit
The MMR has a receiver autonomous integrity monitor (RAIM) function. The RAIM monitors the status of the satellites that the MMR uses for calculations. The output of the RAIM function is an estimate of the GPS position error. The autonomous integrity monitor value goes to the flight management computing function (FMCF) of the AIMS. The FMCF uses the autonomous integrity monitor to determine if it can use GPS data for navigation.
Values Calculated by the MMR
These are some of the values that the MMR calculates:
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Latitude
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Longitude
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Altitude
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Universal time (coordinated) (UTC)
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Date
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North/south velocity
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East/west velocity
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Vertical velocity
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Track angle
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Autonomous integrity limit
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Satellite position
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MMR status.
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D633W101-BEJ Sep 05/2002
777 AIRCRAFT MAINTENANCE MANUAL
GPS -THEORY OF OPERATION -3
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D633W101-BEJ Jan 05/2002
GPS -FUNCTIONAL DESCRIPTION
Power
The power supply makes various dc voltages from the 28v dc input to the multi-mode receiver (MMR). The MMR sends 12v dc on the center conductor of the antenna coax to the amplifier in the GPS antenna.
Satellite Signal Processing
The low noise amplifier (LNA) receives and amplifies the satellite signals from the GPS antenna. The receiver detects the satellite signal and sends it to a digital-to-analog converter (D/ A). The D/A sends the digitized signal to the microprocessor. The microprocessor calculates the airplane position and other GPS data. The GPS data goes to the flight management computing function (FMCF) in the left and right AIMS cabinets.
The MMR makes time mark pulses once per second. The time mark pulses goes to the AIMS cabinets.
Inputs
The AIMS cabinets use inertial reference data from the FMCF in the left and right AIMS cabinets during initialization. The MMR also uses this data in the aided and altitude aided modes.
The left MMR receives test and fault report requests on the central maintenance computing system (CMCS) bus from the left AIMS cabinet. Built-in-test equipment (BITE) circuits test and report faults to the AIMS cabinets.
Outputs
The left and right MMRs send position and time mark data to the left and right AIMS cabinets.
The GPSSUs also send position data to the ground proximity warning computer (GPWC). The GPWC uses this data in its terrain awareness and terrain clearance floor functions.
EFFECTIVITY BEJ ALL 34-58-00
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