[ARC5] [Milsurplus] AN/ARN-9 Air-Track ILS

[Mike Morrow]

> I'm not familiar with GS in this range.  What is this system and when was it 
> used?  I am using modern GS equipment in the 320 MHz range.

My previous posting discussed some of the differences between the USN's AIR-TRACK ILS and the USAAF's SCS-51 ILS.  So, to re-cap:

--- AIR TRACK ILS (USN) ---
The AIR TRACK ILS was developed around 1935 and proposed for USN carrier and land base use.  Only the USN showed significant interest.  It was never accepted for carrier use, but it did appear at some patrol aircraft bases.
 
The system consisted of:
MF LOCALIZER 300-800 kHz, modulated at 90 and 150 Hz.
VHF GLIDE SLOPE 90-95 MHz, modulated at 60 Hz.
MF MARKER BEACON 300-800 kHz, modulated with unspecified distinctive tone.

The same MF frequency was used for both LOCALIZER and MARKER BEACON, distinction arising from different audible modulation tone.

The GLIDE SLOPE approach path was NOT constant, but more parabolic.

Ground equipment:
YB-* - COMPLETE ILS, YC-* plus YD-*.
YC-* - MF LOCALIZER and VHF GLIDE SLOPE transmitters.
YD-* - MF MARKER BEACON transmitter.

Aircraft equipment:
(ZAX - 1940)
RU-* - MF LOCALIZER and MF MARKER BEACON receiver.
ZA-* - VHF GLIDE SLOPE receiver, LOCALIZER converter.                                         
(AN/ARN-9 - 1943)
R-23 or -24/ARC-5 with MX-19/ARC-5 audio adapter - MF LOCALIZER and
   and MF MARKER BEACON receiver.
R-43/ARN-9 - VHF GLIDE SLOPE receiver with LOCALIZER converter.

The USN abandoned the difficult-to-use AIR TRACK ILS in 1944, adopting instead the USAAF's SCS-51 ILS.

--- SCS-51 ILS (USAAF) ---
The SCS-51 ILS was developed around 1942.  The USAAF played the major role in its development after the USA entered WWII.  

The system consisted of:
VHF LOCALIZER 108-110 MHz (6 channels), modulated at 90 and 150 Hz.
UHF GLIDE SLOPE 332-335 MHz (3 channels), modulated at 90 and 150 Hz.
VHF MARKER BEACON 75 Hz, modulated at 1300 Hz.

The GLIDE SLOPE approach path, unlike the AIR TRACK, is constant slope.

Ground equipment (typical):
AN/MRN-1 - VHF LOCALIZER transmitter.
AN/CRN-2 - UHF GLIDE SLOPE transmitter.
AN/MRN-3 - VHF MARKER BEACON transmitter.

Aircraft equipment (typical):
(1943 and later)
RC-103-A - VHF LOCALIZER receiver (BC-733-*).
AN/ARN-5 - UHF GLIDE SLOPE receiver (R-89/ARN-5).                                         
RC-193-A - MARKER BEACON receiver (BC-1333).

The SCS-51 ILS was adopted by the USN in 1944.  By 1948 it had been adopted as the standard military and civil aviation ILS, where it remains today with the addition of more LOCALIZER and GLIDE SLOPE frequencies, but otherwise identical.

Mike / KK5F