(a)  The primary source of air data for the FMCS is by definition DADC No. 1. The validity of air data is checked using the information coded in the Sign/Status Matrix (SSM) of the ARINC 429 outputs of both DADS. If the FMC program detects an invalid condition in the SSM of any digital word from DADC No. 1, the FMC will then use DADC No. 2 as its source of air data.
 G.  Optimum Navigation Mode Determination
 (1)  The precedence of navigation modes for the FMCS navigation function is as follows, with the inputs to the FMCS as shown:
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 Navigation Mode_______________  Inputs
 DME/DME/IRS  Two DME ranges. IRS latitude and longitude. IRS north and east velocity. IRS altitude and vertical speed. IRS true and magnetic heading. Pressure altitude and true airspeed.
 DME/VOR/IRS  One DME range and one VOR bearing. IRS latitude and longitude. IRS north and east velocity. IRS altitude and vertical speed. IRS true and magnetic heading. Pressure altitude and true airspeed
 Dead-Reckoning  IRS latitude and longitude. IRS
 (D/R) with IRS  north and east velocity. IRS altitude and vertical speed. IRS true and magnetic heading. Pressure altitude and true airspeed.
 Airplane on Ground  IRS latitude and longitude. IRS north and east velocity. IRS altitude and vertical speed. IRS true and magnetic heading. Pressure altitude.
 (2)  The FMCS is programmed to select the navigation mode that provides the best determination of airplane present position and velocity. This selection depends upon sensor status, and availability of navigation aid (DME, VOR/DME, VORTAC) ground stations. The selection process is automatic and controlled solely by the FMC. The FMC contains a nine-state Kalman filter. This filter is used in the DME/DME/IRS and DME/VOR/IRS modes, for navigation aid correction to IRS position and velocity solutions. Also the filter reduces transients in position and velocity. The transients may be caused by differences in correction from one navigation aid to another.
 H. Position Determination
 (1)  Navigation function processing consists of computations for both the horizontal and vertical position of the airplane. For vertical navigation, the process computes values of altitude and Flight Path Angle (FPA), using data from the IRS and DADC. When the airplane is on the ground the FPA is set to zero. The more involved process of horizontal navigation primarily consists of a dead-reckoning position computation, and a Kalman filtering process for estimating dead-reckoning errors. In the order that they are accomplished, the FMCS navigation processing tasks are:
 (a)  
Navigation mode selection

 (b)  
On-ground navigation

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 (2)
(3)

(4)

(5)

(6)

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737-300/400/500MAINTENANCE MANUAL
 (c)  
Dead-reckoning

 (d)  
Track angle calculation

 (e)  
Kalman filtering

 (f)
Position updatingEvery five seconds the FMC checks the status of all the navigation sensors and makes a selection of the optimum navigation mode available. The transition from a radio-updated navigation mode (DME/DME/IRS, or DME/VOR/IRS) to IRS dead-reckoning occurs 15 seconds after radio measurements are lost. However, this 15-second period does not include time while in the cone of silence of a tuned navigation aid. When the airplane is on the ground, the navigation mode selected is the On-Ground Navigation Mode.On-Ground Navigation

 (a)  
When the airplane is on the ground, the On-Ground Navigation mode is used, because signals from navigation aid stations are not reliable for position determining. On the ground, the system position and velocity states are initialized, and the navigation outputs are simply the current IRS values. Once the airplane is in the air, and the IRS and DADC are valid, other modes of navigation can be selected.
　
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