Yes DME gives you precise MAPt definition but yes not required if one has a watch for timed descents, there are examples of NDB only approaches: Oussant, Bergerac, Lanzarotte but I doubt anyone fly them, the dispatch rates on these are less than SVFR !
Conceptually, no matter how you fiddle with NDB only it will be difficult to have it working well for both radial guidance and as position fix, either it’s going all over the place when you are over it or you have no clue how far you are, maybe works better like in Russia with two of them and single ADF? the whole exercise of having it as GPS backup is mix of CYA with cost saving by HIAL airports 
Ibra wrote:
maybe works better like in Russia with two of them and single ADF?
They’d normally have two ADF! :)
Or, to be precise, a single indicator with two needles.
Airborne_Again wrote:
With the NDB in the OM position, I don’t see much benefit with the addition of a DME in most cases
Don’t you get lower system minima? I read NCO.OP.111 to allow 350 ft MDH with an NDB alone, but down to 300 ft MDH with NDB/DME.
huv wrote:
Don’t you get lower system minima?
Yes, but in my experience few NDB approaches have minima that low anyway.
I learned that Avignon Caumont LFMV did loose the ILS17 approach. Now there only GPS and NDB appoach…
I wonder how hard it would be to build a VOR using SDR (software defined radio)? I think ~100 MHz is well within the capabilities of SDR these days, especially since no modulation is required, so you’d just need a bunch of RF power amps and a Raspberry Pi.
Edited to add: a quick google shows several SDR VOR receivers, so it should be possible to build a transmitter as well. How many antennas do you need? I know the real thing has quite a lot (I sometimes drive by the SJC VOR which is really a monument to something or other) but how many do you actually need? 6?
Airborne_Again wrote:
huv wrote:
Don’t you get lower system minima?
Yes, but in my experience few NDB approaches have minima that low anyway.
Well I agree. Also I am told that DME does not seem to make a lot of difference for the procedure designer except for stepdown options.
However, it definitely makes a difference for the pilot flying the approach, whether there is a DME to give added situational awareness, or not. To make an approach in IMC with an ADF as the only nav instrument is in principle just as viable today as it was 70 years ago when it was the norm, but I think most pilots today would find it somewhat uncomfortable or downright scary.
huv wrote:
eLORAN would be an ideal low cost backup for GPS as it has very good coverage, is practically unjammable and I guess it would be straightforward to include the receiver in a GPS navigator – but there is no real drive for that yet.
Here is a new legitimate argument in favour of it.
UK had VDF approaches, maybe worth getting back to those instead or as addition to HIAL NDB at least someone is watching for you? They still ask in PPL exams about accuracy of VDF in Class A is +/-2deg or +/-5deg, it’s not like there is any Class A CTR now nor VDF controllers in NATS
D&D unit still use “RT fixes” on 121.5 (not even VDF) and they even give QDM to land in any airport (just like PAR), helps some PPL who are caught inside or above clouds or in low visibility flying with map & stopwatch and no transponders (those who never use VOR, GPS or tablet), I heard the story of someone over the channel back from Calais who landed safely in Lydd with QDMs from D&D with simple “RT fixes”, I imagine having a transponder or proper VDF in airport would have been even more easier, maybe better than using GoogleMap
Ibra wrote:
They still ask in PPL exams about accuracy of VDF in Class A is +/-2deg or +/-5deg, it’s not like there is any Class A CTR now nor VDF controllers in NATS
Are you sure that “Class A” refers to the airspace class and not the fix accuracy class? There are class A, B etc. fixes with various accuracy.
NATS only handles enroute airspace in the UK, right? In that case it’s unlikely they have VDF. But airport towers may. A few decades ago all towers in Sweden had VDF but there are fewer and fewer now. My impression is that they are kept as long as they work, but if they break they are decommissioned.
(I’ve actually used VDF in anger once when my aircraft’s single NAV receiver stopped working over the Baltic sea. I could certainly have made landfall on dead reckoning, but as I was approaching controlled airspace, VDF seemed like a better bet. And the military airport close to me (ESCM) used to have a VDF instrument approach!)
