# What are outer marker horizontal dimensions at 3 degrees ILS path?

What is the outer marker "distance" along the flight path of a 3 degree ILS?

I provide my two approaches below, but as they contradict each other, I am asking this question to learn the proper answer.

Approach #1: Wikipedia says OM horizontal dimensions is an ellpsoid of 2400 x 4200 ft (I assume the former is the dimension parallel to aircraft's path ) at 1000 ft. I assume vertically OM signal is of conical shape, therefore its horizontal size expands and should be (proportionally) larger at higher altitudes. At 3 degree glideslope, altitude range for OM crossing is between 1200 and 2100 ft (4 NM and 7 NM respectively). This gives an answer to my question equal to a range between 2400 * 1.2 = 2880 and 2400 * 2.1 = 5040 ft.

Approach #2: Flying at 100 kts, I can hear OM for ca. 12 seconds at 4.4 DME. This translates to 2000 ft of OM length at height of 1500 ft. Empiria does not give a range, but still contradicts wikipedia and is much more reasonable number, as I cannot remember hearing OM for almost half a minute while flying over...

• While the ideal model of the radio signal is concical, I believe the actual radio signal shape is closer to a paraboloid (egg shape). – Ron Beyer Oct 11 '16 at 14:40
• That would be a good explanation, I like it. However, it would only fit the Wikipedia and empiric data if it the egg were less than 2000 ft high - decreasing size above 1000 ft :). So I still wonder if anybody knows the exact numerical solution to this question. – user2530062 Oct 11 '16 at 14:45