…a number of auto tuned DME receivers….more accurate than GPS …
Before they switched of SA, and pre-WAAS, that may have been true. I doubt that is the case today. It is just a hang-over from the pre-GPS-days, where that was the only way to do it. It is also sensible as alternative to GPS.
This plays havoc with some multi sensor navigation systems as most airliner FMC’s use a number of auto tuned DME receivers ( five in the B737NG) to update the FMC position as it is more accurate than GPS until it has to deal with a DME that has its zero datum adjusted.
How do they deal with it? I thought that perhaps airliner FMS systems don’t use terminal DMEs for their INS corrections – not least because the system is supposed to use a navaid within the DOC and a TDME has a very small chance of being in DOC when you are enroute at FL300+.
Before they switched of SA, and pre-WAAS, that may have been true. I doubt that is the case today
I don’t think a DME can resolve better than 0.1nm – can it? How accurate is the station in the packet return delay? 0.1nm = 500ns.
The underlying precision is going to be higher than 0.1NM, but if I remember correctly, the MLS [ping] had a special, higher resolution DME, indicating that the “classic” DME was not good enough.
as it is more accurate than GPS
This is dubious at best. DME uses widely spaced 3.5us pulses; GPS uses ~1us chips. So raw time accuracy should be at least 3 times better for the GPS C/A signal. The bigger ionospehric uncertainty is taken care of by WAAS/EGNOS. Furthermore, no contemporary GPS receiver (even by aviation standards) only tracks 5 signals at the same time.
This plays havoc with some multi sensor navigation systems as most airliner FMC’s use a number of auto tuned DME receivers
Then this is a fairly stupid implementation in the B737NG. The FMC needs a database anyway with the DME station coordinates. It would be easily to add another field which indicated how much less delay versus standard the station provides. Then the FMS could easily correct for that. Or it should simply not include that DME in its database or flag it as unavailable for INS drift correction.
I did not say that the DME issue played havoc with the B737NG system, perhaps I should have said some of the early systems had problems, the later systems have a fix for the DME adjusted zero problem.
As to the accuracy of DME vs GPS, I think that atmospheric factors and the fact that DME stations are fixed to the ground lends a lot to the accuracy, as I understand it effectively WAAS uses a station fixed to the ground to correct the GPS signals for the inherent inaccuracies of “raw” GPS over a small geographical area.
None of this escapes the fact that aviation has to have an alternative to satalite based systems and with the demise of Loran DME is the best of the rest, to my mind the abandonment of ground based hyperbolic navigation systems was a mistake as all the VOR, ADF & DME stations could have been replaced by very few Loran ( or omega VLF ) stations and GPS. The whole system would have had a great deal of redundency and be usable by both the aviation and marine industry’s.
No doubt someone will tell me I’m wrong !
I don’t see why the moving reference stations of GPS are a disadvantage versus the DME earth fixed reference stations. Yes, in the GPS case you need to continually update the ephemerids, while for DME you essentially only need to know its position once – but so what, the process is established and works.
GPS is moving towards multiple frequency bands, which improves the ionospheric model of the receiver greatly.
Personally, I’d be more worried about the deficiencies of the DME system, such as interference from other users of the same DME, the capacity limit, and that some receivers are quite prone to lock onto a reflected version of the signal.
Both models are proven and work. Originally, DME/DME area nav driven FMS had the edge, then after the US switched of the artificial errors in GPS they were at parity. Both systems pretty much allowed the same RNAV approaches to be flown. Recently with WAAS that has reversed it with LPV, while DME/DME can only do baro-aided VNAV on approaches.
So all in all, the performance difference is miniscule; the risk of a complete outage and jamming is theoretically higher for GPS, so a DME/DME backup would be fantastic for everyone to have, but in practice that has turned out to be a non-issue [until the big solar flare knocks out all GPS satellites, that is…]
Unfortunately, DME/DME rnav costs about 10-1000 times more than GPS rnav, with no practical implementation for light aircraft, so all of this remains idle theory – GPS rules.
so a DME/DME backup would be fantastic for everyone to have
I don’t think DME/DME is inherently expensive. A decent quality certified DME is best part of 10k (pick your favourite currency) which is about the same as a GPS. Both cost about 0.3k to build so the rest goes towards the player’s fixed costs, dealer discounts, etc. I think everybody aims at the same price band – where there are enough people paying but you can still make enough money to run the fixed costs of a certification-capable organisation, provide the necessary dealer support, etc. You can do DME/DME with a single DME – the transmitter is synthesised so frequency hopping is easy (probably using DDS; a dirty solution but OK with filtering, and a very modern way to do it). You don’t actually need two DMEs in the box, two antennae, etc.
The real problem is that GPS did the same to the GA nav business that the CD did to the vinyl record business. Only the Royal Institute of Navigation is still admirably carrying the correct flag, doing presentations at aviation shows on how GPS can fail, etc. So nobody is going to make such a product for GA. Not today.
The other problem with DME/DME is that it would be more or less useless at low level. Fly at say 1000-1500ft (most of UK’s VFR GA) and you will be very lucky to be receiving the required three DMEs – let alone be within their DOCs.
The other thing is that FOG (fibre optic gyro) costs are slowly falling and if you are happy to have an uncertified (but rock solid, military spec) inertial backup, driving some kind of portable moving map device via NMEA, you can get it for about 20k. The name I was thinking of escapes me but this is the sort of stuff. These boxes have a GPS antenna socket at one end, and output RS232 NMEA at the other end! If/when GPS goes, the NMEA output just carries on….
I don’t think low-level VFR needs a backup for GPS. If it all fails, people will get lost, but nobody will die.
What’s a CD?
I agree with Cobalt. GPS is fine why would you need a backup for GA? CDs were some mechanical version of MP3s….
