Random avionics internals

Is DME/DME really more accurate than GPS? I find that a little hard to believe, but I could be wrong.

We always joked that NARCO stood for “Not A Radio Company”. Our previous radio was a Narco and we had no end of trouble with it (replaced with a GTR225), particularly the high voltage display which was very unreliable – the connector design being a serious weak spot with it, so you’d lose digit segments at random.

Snoopy wrote:

More accurate than gps and ins.

Certainly more accurate in INS, but in no way more accurate than GPS, even without SBAS!

E.g. I recall having seen somewhere (so sorry, no sources) that DME/DME can be used for RNAV 1 only with A/P or F/D because the positionsing tolerances are large enough that you can’t get RNAV 1 precision with the flight technical tolerances of hand flying on raw data. OTOH with GPS the positioning is tight enough that hand flying on raw data is fine for RNAV 1.

IIRC a DME can only handle a maximum of 100 aircraft at any one time.
Secondly DME being a measure of the hypoteneuse, distance changes with altitude.
In the days before GPS, yes DME/DME with INS was the most accurate but not now.

DME/DME was never certified for use under 1nm (at best it’s RNAV1 with 3DME stations at right altitudes and maybe RNP1 with +5 DME stations overhead Schiphol), the DME arcs are +/-1nm and departures & arrivals are RNAV1

GA aircraft GPS is accurate enough for 0.3nm specs, in practice, it’s rarely 100m off where you should unless you fly all days along it’s just the signal is not smooth to track & fly at those levels (same as DME/DME at 1nm)

KNS80 made RNAV with DME/VOR but there was no database involved and it was 5nm Airway navigation, I would not put my life on it for approach & departure besides it rarely worked bellow 500ft agl when departing from Stapleford toward some “fake VOR” 5nm away from LAM & BKP

Peter wrote:

I think Narco did an RNAV product

The NS800 is a really good piece of equipment, like a KNS80, but might not be what you were thinking of.

On Narco being unreliable, the screen on my 890 has bad contacts and is now useless. Am I better off getting an overhauled 890 from the USA (e.g. Bennett), or replace it with an overhauled KN64 (e.g. Mendelsohn)? Unless there’s someone who specifically repairs DMEs?

I stand corrected, it’s not as accurate. The reason DME/DME updating still makes sense is in conjunction with INS and GNSS. The FMC can compare and determine „valid“ position data using 2 of 3 sources compared to one on one.

For RNP apps the radio nav has to be disabled.

In answer to Peters question Narco did build a multi sensor RNAV unit called the STARNAV . It used VOR/DME and GPS with the option of LORAN input.I don’t think it was ever DME/DME but with a fast auto tuned DME unit this would have been the next logical step.

It is a pity NARCO went bust they had some good ideas and kept King honest, but the variable reliability of their products was their undoing the 12d NAV/com & NS800 exhibited superb reliability but their ADF was rubbish, I still have a Narco ADF and it has exhibited superb reliability in its new role as a doorstop.

Narco totally lost the plot with their last new product. Who else would invest R&D into a new Mode A/C transponder when everyone else had moved on to Mode S.
http://www.aeroelectric.com/Installation_Data/Narco/narco-at-165-install.pdf