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The navigation message to the satellites.
The upload stations are in Ascension island, Diego Garcia island, and Kawajalein island.
GPS Accuracy
Civilian users have access to standard positioning service (SPS). SPS has an accuracy of 15 -25 meters in 95% of the position fixes. For security reasons, the American Department of Defense intentionally degrades the accuracy for civilian users to 100 meters in 95% of the fixes.
Military users have access to precision positioning service (PPS). PPS has an accuracy of 18 meters or less in 95% of the fixes.
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Ranging
The multi-mode receivers (MMRs) use the principle of ranging to measure the distance between the MMR on the airplane and the satellites. The MMR has in memory the location of the satellites in their orbits at any time. It is possible for the MMR to know the position of each satellite because they follow a known orbit.
The MMR measures the time it takes for a radio signal to go from a satellite to the airplane. Since the MMR 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.
However, since this is a one-way range measurement, the MMR must know exactly at what time the satellite sent the radio signal. The MMR compares the satellite signal to a signal that the MMR makes at the same time as the satellite. The difference between the two signals (time bias) is the time the satellite signal took to get to the MMR.
Each satellite has atomic clocks to keep accurate time. All the satellites have precisely the same time. The MMR in the airplane has an internal clock but it is not atomic, so it is not as accurate. Thus, it is not possible for the MMR to have precisely the same time as the satellite.
The MMR assumes that its internal clock is off by some clock bias. This clock bias is an unknown that the MMR must calculate. The clock bias is the difference between the MMR time and GPS time.
To calculate the airplane position (latitude, longitude, and altitude) and the clock bias, the MMR must know the position of at least four satellites. The MMR then measures the distances to all the satellites at the same time, and solves for these four unknowns with four range equations:
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Latitude
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Longitude
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Altitude
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Clock bias.
GPS Time
All the satellites synchronize to universal time (coordinated) (UTC). The satellites transmit this time to the MMR. The accuracy of the satellite UTC is approximately 100 nanoseconds. The MMR transmits UTC on an ARINC 429 format. The MMR also transmits a very accurate time mark once per second.
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GPS Modes of Operation
The multi-mode receiver (MMR) operates in these modes:
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Acquisition mode
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Navigation mode
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Altitude aided mode
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Aided mode.
Acquisition Mode
The MMR looks for and locks on to the satellite signals. The MMR must find at least 4 satellites before it starts to calculate GPS data. The MMR accepts this data from the flight management computing function (FMCF) in the AIMS when the MMR is in the acquisition mode:
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Position
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Velocity
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Time
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Date.
The MMR uses the FMCF data to calculate which satellites are available at the present airplane position. This helps the MMR get the signals from those satellites available.
If the AIMS data is not available, the MMR can still acquire satellites signals. However, the MMR takes longer to acquire the satellite signals because it has to look for all the satellites. When the MMR finds the satellites, it calculates which it can use.
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The MMR takes approximately 75 seconds to acquire the satellite signals when the AIMS data is available. The MMR takes approximately 4 minutes (maximum of 10 minutes) to acquire the satellites when the AIMS data is not available.
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