pair
2) Channels 3 and 4 will be used for the second best DME/DME pair unless there is a manual procedure or route tuned station in which case channel 3 will contain this data.
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3) Channel 5 will primarily be used for storing ILS/DME information
(d)
The entire tuning procedure is explained in more detail in the tuning management section.
(e)
VOR/DME NAVMODE uses the DME slant range distance data along with the collocated VOR bearing data to compute airplane position. This data is used in geodetic equations in which latitude and longitude positions are calculated. The bearing of the airplane with respect to the VOR navaids north reference must be corrected to true north by adding the magnetic variance to the VORs north bearing.
(3) FMCS Optimal Position and Velocity Processing
(a)
The FMC combines the VHF NAV radio position and IRS inertial data in order to produce the most accurate airplane position and velocity.
(b)
Position optimization
1) In order to produce an optimum position, the FMC combines the position error filter and the complementary filter to ensure that the airplane position is valid.
2) The position filter combines IRS inertial position and the radio derived position. Two filters will be used, one for latitude and one for longitude.
3) The IRS is short-term stable while radio position is long-term stable. By realizing the long-term position and the inertial short-term position the drift error and Schuler errors can be determined. These errors are summed to produce a filtered airplane position error in both latitude and longitude. These filtered position errors will then be used by the complementary filter for airplane position.
4) The complementary filter combines the north and east velocity components from the velocity filter, the earths local radius, latitude and longitude errors from the position filter, radio position and mixed latitude and longitude positions from the IRS to continually update and enhance the IRS position. This filter is operational only when there is at least one valid IRS velocity and position and the radio position is valid.
(c) Velocity optimization
1) IRS velocity (the mixed velocity if all IRSs are valid) and radio position are combined to derive an estimate for the IRS velocity error. These derived velocity error components are then added to the IRS velocity components to produce filtered north and east velocity components.
2) Filtering the velocity components will only be done when in the DME/DME radio mode in order to cut down on biases. These errors will gradually be bled-off while not in the DME/DME NAVMODE. While the airplane is on the ground the error is bled-off at a quicker rate.
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MAINTENANCE MANUAL
D. Inertial and Wind Vector Computations
(1)
The inertial vectors are those vectors related to the airplanes north and east velocity components. They consist of:
(a)
Track angle
(b)
Ground speed (GS)
(c)
Flight path angle
(d)
Drift angle
(e)
Magnetic variation
(2)
The track angle filter combines the computed track angle and the track to derive the degree/frame track angle rate. The computed track angle is the true heading while the airplane is on the ground otherwise is computed by taking the arctangent of the velocity components supplied by the velocity filter. The degree/frame track angle rate is then calculated by combining the computed track angle with the previous value of track angle. The filter calculates the degree/frame track at a rate of 20 Hz.
(3)
Ground velocity magnitude is calculated by using the filtered velocity components. This value is calculated a 5 Hz rate.
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