GLIDEPATH (lateral)
For each half-scale needle deflection, the distance from the center of the glidepath to the aircraft is constant regardless of how far the aircraft is from runway. This is completely different from a ILS localizer beam t 17117y2410r hat has a width of about 700 feet at the runway threshold, but fans outward to about 1 NM when 10 NM from the transmitter.
When the COURSE needle moves fully to one side of the SALS' screen, the aircraft is 3125 feet from the center of glidepath no matter if the aircraft is 5 NM or 30 NM from the runway. MEDIUM needle deflection equals 1250 ft. and FINE, 250 ft.
GLIDESLOPE (vertical)
This imaginary line is a 3-degree inclined path to the runway. The glideslope needle can respond to a maximum of 1-degree above and below this path. As shown above, the height in feet of this plus/minus 1-degree scale decreases as the distance to the runway becomes shorter.
For example, when the aircraft is 30 NM from the runway, it can climb a maximum of about 3200 feet before the glideslope needle reaches its upper limit. It can descend the same amount before the needle reaches the lower limit. However, when the aircraft is 5 NM from the runway, it can climb a maximum of only 530 feet before the glideslope needle pegs the top end of the scale. The descent footage at that distance is also 530 feet.
FLIGHT PLANNING
It is important to note the glideslope radio distances above the ground (shown above) when planning a SALS approach especially if the terrain around the airport is mountainous. You would not want to be on the glideslope at 10 NM from touchdown if the terrain was 5000 feet at that point because the SALS' glideslope radio altitude at 10 NM is only 3200 feet.
Both glidepath and glideslope guidance is available up to 30 NM from runway.
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