Meaning of abbreviation 'S.I.'

[Nick Beale]

I'm looking at the ORB of No. 221 Squadron for 1942 (Wellngton anti-shipping ops from Egypt) and I've come across 'S.I. badly jammed'.

This seems to be referring to ASV radar but what does it stand for: search indicator, search instrument, special item or … ?

[Chris Goss]

Special Instrument?

[Nick Beale]

Originally Posted by Chris Goss

Special Instrument?

Could be. Another unit's ORB refers to a "special Wellington" of 221 Sqn.

[Chris Goss]

Could also be Special Installation

[BigWeeker]

Hello, Nick
I, too, am researching Wellingtons in the ME, and have, courtesy of TNA, many of the relevant ORB’s. I put in a request for the same query a while back and the general consensus was that it stood for Special Installation.
Regards
Andy B

[Nick Beale]

Thanks Andy. One day, I guess one of us will find a source that spells it out in full — but probably only if we are looking for something else.

[Nick Beale]

I found this in a Middle East operational Research report. It refers both to S.I. and Special Interference but as I read it, they are two different things (i.e. the radar itself and the effects of jamming).

Also, a title that I've just found in the TNA catalogue: AVIA 19/198 "Guard for special installation aerials".