Glideslope - dots

[claudemp]

Hi,

Can any body help me?
What dots represent (height or angle) in a glideslope?

For exemple if i'am 2 dots below glideslople, how many ft/meters am I below glideslope or degrees(angle).

Hope my question is clear.

Best Regards,

[dixi188]

IIRC one dot deviation is half of a degree.

A quick google search will come up with some stuff.

Hope this helps.

[Clyde Parthangel]

Without getting too technical, it depends on the glidepath angle. For the Glidepath, full scale deflection of the CDI - five dots or 2.5 dots depending on the type of instrument - is defined by the Displacement Sensitivity of the Glidepath. This is known by ILS tecchies as the 'Width'. It is dictated by ICAO Annex 10 as ±0.48 Theta, where Theta is the Glidepath angle (between 2° and 4°). For a 3° Glidepath angle, this works out for full scale deflection as 4.44° (full scale fly down) and 1.56° (full scale fly-up). However, linearity across this span is not guaranteed. It is reasonably linear to half scale but outside that all bets are off! For this reason, the engineers and flight inspectors measure at ±0.24 Theta. However, please bear in mind that despite what the CPL textbook diagrams show, a glidepath is not sharply defined. It is produced by the mixing of signals reflected off an imperfect surface (the ground in front of the glidepath) so if you imagine the quality of the reflection that you see in a creased and crumpled piece of aluminium foil that has been flattened out, you might get some idea of what the glideslope signal 'looks like' in reality.

[TinyTim2]

The receivers are calibrated to show each dot as a particular difference in modulation , which is an indication of deviation from the beam centre , irrespective of range from touchdown . So a display of one dot at 5 nm should be the same relatively as a dot at 500 yds . However the display on modern aircraft such as A320 is computed and may be different .