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D. In the acquisition mode, the MMR searches for and locks onto the satellite signals. The MMR must find at least four satellites before it starts to calculate GPS data. The MMR 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 MMR can still acquire satellite signals. In this case, the MMR must look for all the satellites. This requires approximately 10 minutes.
E. The MMR enters the navigation mode after it acquires and locks onto at least four satellites. While in the navigation mode, the MMR updates and outputs positions, velocities, accelerations, and time.
F. If the MMR is unable to track at least four or more satellites, it enters the altitude-aided mode. With only three satellites in view, the MMR uses IRS inertial altitude plus the length of the earth's radius as the fourth range. The MMR re-enters the navigation mode when four satellites are acquired.
G. The MMR enters the aided mode during short periods (less than 30 seconds) of bad satellite coverage. In the aided mode, the MMR receives altitude, track, and ground speed from the IRS. The MMR uses these inputs to continue the calculation of navigation data. If the MMR cannot track a sufficient number of satellites for 30 seconds, it reverts to the acquisition mode.
H. The MMR enters the fault mode if a critical fault is detected. In this mode the navigation data is invalid, and the MMR is no longer useful for navigation on that flight. The other MMR continues to supply navigation data to the FMCS.
7. Operation
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A. Functional Description
(1)
The GPS can be divided into three segments: satellite, user, and control (Fig. 6).
(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.
(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 MMR selects the satellite constellation which provides the highest accuracy for position determination. The MMR automatically switches to new constellations 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.
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SATELLITESEGMENT
USER SEGMENT
CONTROL SEGMENT
GPS Segments
Figure 6
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(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.
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