Instrument landing SYSTEMS

Indications from the AN/ARN.31 are  displayed

on the course indicator and give both localizer and 

glide-slope guidance to a pilot making an instrument landing.                                     

Both localizer and glide.slope operate on 20 fixed frequency channels. The Localizer frequencies range from 108.1 to 111.9 MHz  and the glide.slope from 329.3 to 335.0 MHz. The glide.slope frequency is paired with the localizer frequency and selected on the control panel.


ARN-31 system

The ARN-31 receiver set dates from  1956, using subminiature tubes and some early germanium  diodes. The system has two receiver boxes and a common power supply. The receivers differ only in the frequency band.


   R-625/ARN31      Localizer    Receiver    108.1 -111.9 MHz  in 100kHz steps (!)

   R-626/ARN31      GlideSlope Receiver    329.3 -335.0 MHz  in 300kHz steps

   PP- 2792/ARN  Power supply

   C– 1464 / ARN     Nav set control

   ID250/ARN          LOC /GS course deviation indicator



The R-626 / ARN-31  receiver is a single conversion superhet, with four IF stages at 18.9MHz. The bandwidth is approx. 140kHz (-6dB) or 200kHz ( -30dB), adequate for the 300kHz channel spacing as used in 1957. Nowadays, the spacing is 150kHz to accommodate 40 channels and these receivers are obsolete.


Circuit diagram with some background is here


The C-1464/ARN31  ILS control panel controls both the  Localizer and Glide slope receivers with a single, 20-position knob.

Glide Slope Receiver

R-626 / ARN-31