Don't know about glideslopes dropping away but at about 50' a fixedwing aircraft enters "flare mode" & ignores the glideslope. From here on in it is all about Radaly closure rate. In fact on the A320 if the glideslope fails below 100' it will just carry on & land anyway. All you will know about it is that there is no glideslope deviation indication & the glideslope deviation scale flashes.