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* Latitude
* Longitude
* Altitude
*
tBIAS.
GPS Time
All the satellites synchronize to universal time (coordinated)
(UTC). The satellites transmit this time to the receiver. The
accuracy of the satellite UTC is approximately 100
nanoseconds. The receiver transmits UTC on an ARINC 429
format. The receiver also transmits a very accurate time mark
once per second.
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GPS Modes of Operation
The GPS operates in these modes:
* Acquisition mode
* Navigation mode
* Altitude aided mode
* Aided mode.
Acquisition Mode
The GPS looks for and locks on to the satellite signals. The GPS
must find at least 4 satellites before it starts to calculate GPS
data. The GPS gets data from the air data inertial reference
system (ADIRS). The GPS uses this data from air data inertial
reference unit (ADIRU) when it is in the acquisition mode:
* Present position
* Altitude.
The GPS uses this data and its internal database to calculate
which satellites are available at the present airplane position.
The GPS takes less time to get to the navigation mode with this
data.
If the ADIRU data is not available, the GPS can still acquire
satellites signals. It takes longer to acquire the satellite signals
because it has to look for all the satellites. When the GPS finds
the satellites, it calculates which ones it can use.
The GPS takes approximately 75 seconds to acquire the
satellite signals when the ADIRU data is available. The GPS
takes approximately 4 minutes (maximum of 10 minutes) to
acquire the satellites when the ADIRU data is not available.
Navigation Mode
The GPS enters the navigation mode after it acquires and locks
on to at least 4 satellites. When the GPS is in the navigation
mode, it calculates GPS data.
Altitude Aided Mode
With four satellites available, the GPS stores the difference
between the ADIRU inertial altitude and the GPS altitude.
The GPS stores the difference between inertial and GPS altitude
so that it can estimate the GPS altitude when only three
satellites are available.
In the altitude aided mode, the GPS sums the airplane altitude
from the ADIRU and the length of the earth radius as the fourth
range.
The GPS enters the altitude aided mode only after these three
conditions are true:
* The GPS was in the navigation mode
* There are only 3 satellites available with good geometry for
position fixes
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* The GPS stored the difference between inertial and GPS
altitude in memory.
The GPS starts normal operation again when a fourth satellite
comes into view.
Aided Mode
The GPS enters the aided mode during short periods (less than
30 seconds) of bad satellite coverage. An example of bad
satellite coverage is where at least four satellites are available
but the airplane banks and loses satellite reception.
In the aided mode, the GPS receives inertial altitude, track
angle, and groundspeed from the ADIRU. The GPS uses the
ADIRU data to go back quickly to the navigation mode when
there is good satellite coverage again. The GPS output is NCD
in this mode.
If the GPS cannot track any satellites for 30 seconds or more,
the GPS goes to the acquisition mode.
GPS Frequencies
The satellites transmit to the GPS 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
　
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