## The radio-navigation subsystem ILS (Nominal ILS signals)

The masked subsystem block ILS, which is contained in the library NAVLIB, is used to compute nominal values of the glideslope and localizer signals, for a given position of the aircraft. It also checks the validity of the signals, using the separate subsystem block ILStest. See the references below for more details about the variables and parameters from the block ILS.

### Inputvector

      uils = [xe ye H]'

      xe: X-coordinate of aircraft in Earth-axes, [m]
      ye: Y-coordinate of aircraft in Earth-axes, [m]
      H : altitude of aircraft above sea level, [m]

Note: these inputvariables are usually extracted from a non-linear aircraft model. The block ILS computes the nominal values of the ILS signals, using the actual aircraft position. Therefore it does not give correct results whenever a small-deviations model is used for the aircraft dynamics! The Beaver model is a good example of this type of nonlinear aircraft model.

### Outputvectors

      yils1 = [igs iloc]'
      yils2 = [epsilon_gs Gamma_loc]'
      yils3 = [xf yf Hf dgs Rgs Rloc]'
      yils4 = [LOC_flag GS_flag]'

      igs       : nominal localizer current, [micro-Ampere]
      iloc      : nominal glideslope current, [micro-Ampere]

      epsilon_gs: angle between line through aircraft's c.g. and
                  glideslope antenna and nominal glide path, [rad]
      Gamma_loc : angle between ground-projection of line through
                  aircraft's c.g. and localizer antenna, and runway
                  centerline, [rad]

      xf        : X-coordinate of aircraft in runway-fixed reference
                  frame XF-YF-ZF, [m]
      yf        : Y-coordinate of aircraft in runway-fixed reference
                  frame XF-YF-ZF, [m]
      Hf        : altitude of aircraft in runway-fixed reference
                  frame XF-YF-ZF, [m]; Hf = -zf
      dgs       : distance from aircraft's c.g. to nominal glide path,
                  measured perpendicularly to nominal glide path, [m]
      Rgs       : 2D-distance from c.g. of aircraft to glideslope an-
                  tenna (as seen from above), [m]
      Rloc      : 2D-distance from c.g. of aircraft to localizer an-
                  tenna (as seen from above), [m]

      LOC_flag  : flag which is set to one if localizer signal cannot
                  be received with appropriate accuracy, else,
                  LOC_flag = 0
      GS_flag   : flag which is set to one if glideslope signal can-
                  not be received with appropriate accuracy, else,
                  GS_flag = 0

Note: igs is proportional to epsilon_gs, iloc is proportional to Gamma_loc. Both igs and iloc are limited to +/- 150 [micro-Ampere]. For more information about the definitions of the variables, consult the references below.

### Parameters that need to be specified in dialog-boxes of the masked subsystem ILS

- position of the origin of the runway-fixed reference frame XFYFZF (given by the coordinates xRW, yRW, and the altitude above sea level HRW, measured relatively to Earth-fixed reference frame)
- heading of the runway, psi_RW
- nominal glideslope angle, gamma_gs
- X-distance from runway threshold to localizer antenna, xloc
- X-distance from runway threshold to glideslope antenna, xgs
- Y-distance from runway centerline to glideslope antenna, ygs

Double-click the block ILS if you want to specify these variables (the units of measurements are given in the dialog-boxes).

### More information

The block ILS can be used in combination with other blocks that implement steady-state errors or noise signals in the glideslope and localizer currents. The blocks GSerr and LOCerr determine steady-state errors in the glideslope and localizer currents, respectively, while GSnoise and LOCnoise determine glideslope and localizer noise. The combination of all these blocks has been illustrated in the system ILS example. All ILS-related blocks have been gathered in the ILS sublibrary of the block-library NAVLIB.

Note: the autopilot simulation models APILOT2 or APILOT3 provide a good demonstration of the practical use of the radio-navigation models from NAVLIB.

### References

Apart from the FDC user-manual, the following reference contains more information about the ILS signals:

1. M.O. Rauw: A Simulink environment for Flight Dynamics and Control analysis - Application to the DHC-2 'Beaver', part I: Implementation of a model library in Simulink. Delft University of Technology, September 1993