HRDF
 

I'm a bit long in the tooth but I've noticed one or two airports with an annotation of 'HRDF' or 'High Resolution Direction Finder' being depicted.
Anyone any Idea what these might be?

No idea - which airports are these? The DRDF we used to use was really useful for radar (lost aircraft, aircraft below radar cover, providing a rough QDM, or even simply as a freecall was being received to direct your eyes to the right part of the radar display). Also for tower, assuming the aerial was somewhere on the central part of the airport, meaning you could often find a stuck mic (or anonymous tranmission!) aeroplane on the airport. Unfortunately it was deemed surplus to requirements, with all the other fancy kit we had, and seemed to disappear from many uk airports.

Interesting if someone's seeing the benefit again!

I agree DRDF was useful as one of your 'tools' for discovering aircraft, especially where radar couldn't see and even in some circumstances for FISOs which might (for instance) be unable to transmit speech but could receive carrier wave only transmissions.
In this case it is called ''High Resolution DF' and the examples I have are at Little Rissington and Manston.

I believe HRDF refers specifically to those DF sites that form part of the D&D Auto-Triangulation network. The two examples you mentioned are both part of this network. The other term is DRDF
but I believe this is used in the military airfield context rather than part of the auto-T network in the sense that it has both UHF and VHF whereas VDF would be VHF only as is commonly the case at a civil unit.

From the RSP website it was announced that as part of the requirements for re-opening Manston Airport, the HRDF would have to be re-sited. I think that may be going on at present but no official information has been made to confirm that.

That confirms what I suspected about the D & D auto triangulation system at Swanwick Mil; at Northolt there's an annotation in AD 2.19 :-
'UDF/VDF....243.0......Not in use by Northolt frequencies, D & D only'

It is coming back . in Maastricht UAC we have automatic DF function that an be activated on request to help identifying the aircrfat transmitting, very helpful when you have 50+ labels ( aircraft) some overlapping on your scope, very accurate too, So this 1880 technology is still quite useful today,

It is many decades since I last had anything to do with DF so the memory is a bit hazy but HRDF was the term used by the MOD/RAF for its new DF systems bought in the early 80s. The predecessor was termed CADF. The HR is High Resolution. It was higher resolution than its predecessor but still not particularly accurate. NATS bought the same system for its airports around the same time. The same equipment is often termed DRDF outside of the MOD with the D being Digital or Doppler depending upon your preference.

The system supported VHF and UHF, you just slotted in receiver modules for the channels you wanted to cover. The MOD DF network formed the back bone of the Distress and Diversion triangulation system but when located on an airfield also included the local frequencies with displays in the Tower. Many of those airfield locations are no longer RAF airfields, some are no longer airfields at all, but the equipment remains part of the D&D network. Project Marshall was supposed to update and rationalise that equipment but I have no idea how whether it did or still will, indications from Manston are that it hadn't in the original timeframe at least.

Northolt is a slightly special case. Northolt Approach is at Swanwick and the workstation they use has no provision for either a DF display or integration of DF equipment with the radar display because it's a standard NATS Workstation and NATS sees no need for it at approach positions in the London TMA. It remains part of the D&D network though.

DF still in use at EGHH and very useful for the reasons stated in this thread

The Little Rissington installation and the one at Manston are also 'stand alone' in that they don't feed to the airfields where they are located but are part of the the Swanwick Mil system.
At Farnborough we got rid of the old green 'CADF' CRTs and had Furneaux DRDFs installed way back in about '76; the RAF looked at these and eventually installed DRDF too but as usual it took them quite a while as we were funded by MOD(PE) rather than MOD(Air) and we got 16 inch Cossor displays along with SSR prior to the AR15 MOD (Air) system.
We kept the Furneaux displays through Watchman until they were replaced by TAG in 2002 under a NATS contract after much discussion with TAG who didn't want to fund DRDF. I think they supplied Frequentis DFs. A few years after I retired I heard they had withdrawn the DF displays completely; pity because by then I had become a FISO and a single channel DF would have been useful to assist in detecting infringers of the Heathrow CTR from where I was based at Fairoaks.