Comments on “Effect of Wet Snow on the Null-Reference ILS System”

Walton’s paper [l] describes a situation for the Instrument Landing System (ILS), glide path antenna that occurs during a period of time when a deep blanket of snow transitions from freshly fallen to the melted condition. During this time period the air, snow, and Earth reflection interaction may cause the ground reflection component of the guidance signal (image) to nearly vanish. This reduction in image strength has the potential to cause the glide path angle to change and to cause a decrease in the displacement sensitivity (indicated guidance error versus aircraft glide-path offset) of the vertical guidance signal. Current FAA Category I, 11, and I11 operating procedures [4] allow snow cover that does not exceed 18 in for null-reference and capture-effect antennas without downgrading of service or category. These procedures also allow the glide path guidance signal to be radiated without integrity monitoring of the image component of the radiation.