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A 737-300/400/500MAINTENANCE MANUAL
GPSSU-2P6 CIRCUIT BREAKER PANEL
GPS Interface Schematic
Figure 2
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GPS DATA OUT
GPS DATA OUT
TIME MARK 1 OUT
TIME MARK 2 OUT
GPSSU Schematic
Figure 3
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A 737-300/400/500MAINTENANCE MANUAL
G. An internal clock is used as a time reference for measuring the distance to each satellite being tracked. The clock is assumed to be wrong by an amount called the clock bias. This parameter is calculated at the same time the satellite distances are calculated.
H. The processor, satellite data, analog to digital conversion, and RF signal integrity are checked by a continuous BITE monitor. When a critical fault is detected, all ARINC 429 outputs are invalid. The fault is recorded in non-volatile memory to aid shop maintenance.
I. ARINC 429 transmitters send satellite measurements and navigation data to the flight management computer system (FMCS). If a failure is detected, a failure discrete is sent to the master caution unit.
3. 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. The length of the coaxial cable to the GPSSU is limited to minimize signal losses. 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.
4. GPS Position Determination
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A. Each GPSSU uses the principle of ranging to measure the distance between the GPSSU and the satellites. The GPSSU memory has an almanac of the location of the satellites in their orbits at any time.
B. The GPSSU measures the time it takes for a radio signal to go from a satellite to the airplane. Since the GPSSU knows the location of the satellite, and that the radio signal travels at the speed of light, it can calculate the distance to the satellite.
C. However, since this is one-way ranging, the GPSSU must know precisely at what time the satellite sent the radio signal. The GPSSU compares the satellite signal to a signal the GPSSU generates at the same time as the satellite. The difference between the two signals is the time the signal took to get to the GPSSU.
D. The GPSSU assumes that its internal clock is off by some clock bias. This is an unknown that the GPSSU must calculate. This clock bias represents the difference between the GPSSU time and the GPS satellite time.
E. To calculate the airplane position (latitude, longitude, and altitude) and the clock bias, the GPSSU must use data from at least four satellites. The GPSSU measures the distances to all the satellites at the same time. It then solves for the four unknowns, latitude, longitude, altitude, and clock bias by the use of four equations.
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A 737-300/400/500MAINTENANCE MANUAL
5. GPS Displays
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