DME is the future

How long?

Difficult to say... The heading information comes from a slaved gyro, it should be (and stay!) accurate to within a couple of degrees. Magnetic variation is stored in the database and corrected automatically. The TAS measurement is also very accurate, the only real unknown is the wind. I guess that you could fly for half an hour in DR mode and still be within 10NM of the computed position. But I can't really tell because I've seen the "DR mode" message only for a few seconds during turns at low level, when the GPS antenna points away from the good satellites and not enough DMEs are in range.

Before GPS caused the bottom to drop out of the market, there was a US player who produced a moving map box which used VOR/DME or something like that. From vague memory, it may have been Narco or Collins. I don't recall ever seeing any DME/DME product for anything resembling sub-jet GA.

it should be (and stay!) accurate to within a couple of degrees

Of course but for how long?

I guess that you could fly for half an hour in DR mode and still be within 10NM of the computed position

10nm error after 30 mins at say 300kt?

That (10nm in 150nm) is an error of 3.8 degrees, assuming I got the arctan(10/150) right, which is about 8 degrees per hour. That is not very good, but way better than any solid state gyro AFAIK.

OTOH, a "slaved gyro" should never be dead reckoning the heading. Normally they get the heading from a fluxgate magnetometer like a KMT112. Then obviously your heading will be continually tweaked, and any error will come from "sideways" drift on the constant heading.

Of course but for how long?

As long as the fuel lasts :-)

10nm error after 30 mins at say 300kt? That (10nm in 150nm) is an error of 3.8 degrees, assuming I got the arctan(10/150) right, which is about 8 degrees per hour. That is not very good, but way better than any solid state gyro AFAIK.

That was more or less the basis of my guess. We have two independent directional gyros for pilot and co-pilot with independent fluxgates and we usually see a difference of 2 to 3 degrees between them (the comparison monitor will display an alert if the difference exceeds 5 degrees). So I think that 3 degrees off is almost the worst case.

OK but that is not an inertial nav system.

That is a function which uses a slaved compass system to dead-reckon.

In zero wind that could be very good, but if you fly into a crosswind of say 50kt, for half an hour, you will be 25nm off track after that half an hour, but your heading will still be spot on

And at the sort of speeds you are talking about that is quite possible because at 300kt you could cross very different weather systems very quickly.

I have to say that I have never heard of such a product or function, but then I know nothing about jet FMSs...

Re my earlier comment about NMEA... NMEA is actually used in avionics. The King GPSs use a bought-in GPS module which outputs serial NMEA. This actually comes out of a pin on the KLN94. There is the more common "aviation" data stream also which includes the programmed route etc. But you could easily convert NMEA into something you could drive a moving map with. For example Oziexplorer will take serial NMEA so if somebody nuked all the GPS sats, you could just carry on flying, for €20k for that FOG box The fact that all private aviation would be grounded on Day 1 of any significant hostilities (as it was in WW2) is another matter...

I don't really believe anybody will mandate DME/DME RNAV capability for IFR GA, because that would end all light IFR GA, and nobody who is actually in charge of aviation policy in the (relatively) free world actually wants that.

I don't really believe anybody will mandate DME/DME RNAV capability for IFR GA, because that would end all light IFR GA, and nobody who is actually in charge of aviation policy in the (relatively) free world actually wants that.

I can't imagine that either. Not in the near future at least. And in the medium future, I rather believe in a system (already proposed a long time ago, can't remember it's name) that uses "pseudo GPS satellites" in fixed positions on the ground that can be received and evaluated by ordinary GPS receivers. This would enable high-precision navigation with already existing on board equipment even if the satellites are shut down or destroyed.

I believe the more relevant threat is jamming of the satellite frequencies.

But you could jam all other frequencies (NAV and COM) too! And jamming only works locally.

Is it a lot easier to jam GPS than to jam the VHF navaids, because of the power.

What is much harder is to spoof GPS i.e. do it so that your GPS receiver reports a good fix.

However GPS jamming for aircraft receivers is a lot harder than GPS jamming for ground receivers, because the aircraft antennae are largely protected from stuff coming up from the ground.

GPS jamming would become really big if GPS was used for road pricing - one of the big propositions for Galileo which would help to create those mythical 100000 jobs. But it would still be ground based.

I suspect Loran might come back as an RNAV backup, but I think that by the time the powers to be think this through, the will to do something about GPS vulnerability disappears - because most aviation would cease immediately in the event of significant military action, anyway.

GPS jamming would become really big if GPS was used for road pricing

It is, in Germany! Only for trucks on motorways so far, but maybe soon for cars as well. No attempt of jamming has been made so far.

Maybe in trucks there is no point, because they have had tamper-proof tachographs for many years.

But "the big plan" for Europe was to have billing per km, and for that you would need to track the vehicle as it drives along.

Obviously there are also ways to make that tamper-proof, relatively, e.g. by recording any speed on the wheels while the GPS is not showing any motion, but I think there are bigger fish to fry for the tax collectors of Europe than try to basically suppress driving...