5 Group had developed its own method of target marking under W/Cdr Leonard Cheshire (claimed to be “three times better” than 8 Group’s Pathfinder squadrons) which was necessary if pinpoint precision targets such as the viaduct over the Dortmund-Ems canal were to be destroyed, though it could be a very dangerous business for the crews allocated to the role.

 

A critical factor in a successful attack is an accurate assessment of the wind strength and direction.  This was achieved in two ways, used either concurrently or separately.  Selected Lancasters equipped with API attachments and radar aids were routed so as to find a wind in the proximity of the target.  Resultant wind velocities are passed back by W/T to Group HQ who in turn re-issued a bombing wind (Breeze) to the main force at H minus 5 minutes.

 

Initially Primary Blind Marker (PBM) Lancasters with specially trained crews using H2S Mk III drop green TIs in the general target area, these markers act as a datum point for Mosquito markers and for visual flare dropping.  As a last resort they can be used as a marking point for bombing if visual marking is not possible for any reason.

 

Target marker Lancasters dropped flares from an altitude of several thousand feet which illuminated the general area of the target.  This provided enough light for the Master Bomber to visually identify the target accurately, he then flew in very low in his Mosquito in a shallow dive to 1000’ or so to drop his target indicators, exposing himself to the risk of intense fire from light flak.  He then became the Target Controller directing other target markers in their Mosquitoes to drop further indicators should it become necessary to keep the aiming point clearly marked or to make changes to the aiming point if needed as the Main Force Lancasters dropped their bombs.

 

In order that smoke from exploding bombs should not obscure the target, delayed action fuses were fitted timed at half an hour or more before exploding, however as a counter measure the German defences included ground flares which attempted to cover the area with smoke.

 

The timing of the marking is critical.  Assuming that H hour is the Time Over Target (TOT), the following applies:

 

H minus 12 minutes: the Primary Blind Markers drop green Tis on instruments aiming at the centre of the target area.

 

H minus 9 minutes to H minus 5 minutes: flare forces illuminate the target area with sticks of flares.

 

H minus 8 minutes (or as soon after as possible) the Mosquito visual markers identify the chosen marking point and drop a red target indicator from approximately 800 feet.  This is assessed and if accurate is backed up by the remaining Mosquito marker aircraft.  If the first TI is inaccurate, the Master Bomber will order the marker force to cancel this marker b