AN/ARN-101 Tactical LORAN navigation system question

[yahya]

I am looking for more information on this old navigation system made by Lear-Siegler in the late 1970s. It apparently saw use in the F-4E/G, RF-4C, AC-130, and OV-10. Was it a pure LORAN-C or did it contain an additional inertial navigation system as a backup?

The links below provided some vague hints on the real life operation of the AN/ARN-101.

AN/ARN-101 in the Desert Storm

 

[Kiltonge]

Here are the Lear-Siegler and Litton submissions for the ARN-101 competition. Both included inertial.

 

[yahya]

Thank you very much for the submission. Did the Lear Siegler set use the Loran antenna in the shape of a "dog house" or a "towel rack"?

 

[Kiltonge]

Towel-rail for ARN-92, doghouse for 101.

 

[yahya]

Thank you for the reply and the information.

By the way, the April 26, 1976, issue of the AWST has a piece on the ARN-101 with a picture as below (hot-linked), which suggests that the ARN-101 could use either LORAN or Omega as sources of navigation data.

If you happen to know how the set operated, does this mean that the INS was always on, and that the position data was supplemented either by LORAN or by Omega to correct the INS drift?

 

[Kiltonge]

Well that is curious, none of the USAF docs I've read said anything about Omega. I wonder if it was a switch position 'reserved for future use'.

Incidentally the INS selected for the ARN-101 was the Kearfott SKI-2300, beating Litton's submission.

The default mode was for LORAN-C/D to periodically update the INS. In the event of a scramble, the INS could be aligned in flight entirely from LORAN in about three minutes.

A couple of significant weapons-delivery upgrades that the 101 introduced, on top of cueing Pave Tack and Maverick, were CCIP for guns / bombs / rockets and an "in-range" indicator for Shrike which maximised stand-off range for pilots, instead of them having to best-guess the launch parameters.

The big unfulfilled potential of 101 was the Helmet-Mounted Sight, which in testing cued Pave Tack to wherever the pilot was looking or, conversely, provided in-sightline cues so he could turn his head to look at what the WSO had found with Pave Tack. For the RF-4C the HMS would also have provided a 'fly here' mode in which the autopilot was driven by the 101 to overfly whatever the pilot looked at.

 

[Kiltonge]

For novelty value, here's a Lear-Siegler project logo.

 

[yahya]

Thank you very much indeed for sharing interesting details.

Initially, I thought that the ARN-101 was the last nav system with CDU before the classic Phantom II jets were phased out in the US. However, at http://www.f4phantomparts.com/NAV.html there is a picture of an alleged F-4's CDU by Smiths (image hot-linked below), which clearly shows the operational mode knob to include the Doppler, INS, GPS or INS/GPS mixed mode, albeit without Loran. Do you recognize this FMS? Was it indeed used in the F-4s?