The Doppler navigation used a x-shaped antenna in the belly - it rotated so that when each side was reading an equal groundspeed the angle relative to centreline = drift.
There’s more info on a couple of other pprune pages and a good photo at
Photos: Hawker Siddeley HS-121 Trident 3B Aircraft Pictures | Airliners.net
The Special Reactive Instant Navigation Geographic Current Location Indicator Pointer” (SPRINGCLIP) fixed just above the map in the pictures is holding a bare metal rod I think, initially there was a pen with a little ink reservoir on it. The idea was to have a permanent “flight recorder readout” of actual track flown for record purposes e.g. to improve airspace usage etc., most F/Os were glad that got dropped pretty early as there was often more ink on fingers than map! Also the maps had to be route specific so started to be significant cost as single use items.
Drift could be fed to the autoflight system, in fact IIRC the steering was actually looking to keep commanded track not heading per se. Just below the compass you can see a horizontal rectangle with 4 white marks on it (can’t remember what they are). This covered a drift selector, with it open you set drift yourself (e.g. to fly radar heading set it to zero) and with it closed as in the picture drift was auto. (I think!!)
The current “actual drift and G/S” are just above the map in the picture. As PeteMcleland has just pointed out this could be adjusted because Doppler wasn’t always reliable especially above e.g. smooth water or snow. The adjustment was by an X shaped switch at the rear of the centre pedestal. Move the switch > change the drift and groundspeed readouts. As Peter says the actual display motors could also be adjusted to drive the pointer around the map.
Voice activated autopilot. This was a very advanced feature only occasionally demonstrated to a certain category of SLF. It was only available after the introduction of the Cockpit Voice Recorder and required good outside visibility. The mike for the CVR was located in the overhead towards the back of the centre panel. Remember the Gripper had 3 pilots. Remember F/Os flew alternate legs as RHS pilot and “Flight Engineer” (all qualified in both seats). Also we operated with noise muffler headsets and used intercom as standard.
Suitable SLF asks usual questions – gosh isn’t it small in here, do you understand all those knobs and dials, what do you do all the time ‘cos it’s all automatic now isn’t it?” 2 front pilots deal with SLF questions, P3(F/E) apparently oblivious of visitor.
Response to question 3 “yes it’s so automatic we don’t even have to turn any knobs any more, it’s voice activated”
“NO!?!”
“want a demo? OK lean forward a bit and speak into that mike.” (You see where the SLF needed appropriate qualifications now) “ Say clearly “TURN LEFT”.
SLF: “TURN LEFT!”
P3's left hand resting nonchalantly on rear of centre console increases right drift ……. Autopilot senses drift error, banks aircraft left …………
You get the drift I’m sure….
Steve