Rwy20 wrote:
I am sure someone will make an app for this.
There are plenty of Apps for celestial naviagtion, e.g.:
http://appcrawlr.com/ios-apps/best-apps-celestial-navigation
Rwy20 wrote:
Sounds like it should be feasible with a smartphone with camera?
Maybe one could measure the angle between sun/moon and horizon using the smartphone camera, but the precision is not good enough. One arc minute or 1/60 of a degree off shifts the position by 1NM, this is why sextants need to be of a certain physical size. And for seeing stars, the smartphone camera lacks sensitivity and dynamic range.
There are some drones today that use pattern recognition to navigate, comparing images from an online camera with stored images. Kind of like VFR 
No good flying over water though. I remember the first Google maps on my phone 10 years or more ago. My phone had no GPS, but Google managed to pinpoint the position rather well using cell towers. Today with 100x more cell towers, I would guess one can do without GPS and get almost GPS accuracy? at least up to a certain alt.
DMEs doesn’t seem to be going away anytime soon, anyone know why exactly?
Le Sving
DME can provide GPS like performance for FMC equipped aircraft, the B737NG uses 6 DME fixes to check the GPS position.
The whole idea being not to be totally reliant on satellite data for navigation. It is unfortunate that GA has been totally fixated on GPS and failed to see the wisdom of multi sensor navigation systems,
Before Narco went bust they produced the STARNAV box that was capable of RNAV using VOR/DME, GPS & LORAN.
By the time the STARNAV had hit the market GA had become totally obsessed with GPS as it is today.
I was told recently that the marine industry is now looking at a computer driven IR sextant system that can provide navigation data to almost GPS accuracy, I shal have to ask my contact at the marine collage at fleetwood how this is progressing.
Today with 100x more cell towers, I would guess one can do without GPS and get almost GPS accuracy? at least up to a certain alt.
The tower based positioning works only by triangulating and assuming the signal attenuation is constant per distance, which won’t work if there is terrain or other sources of attenuation e.g. you walking the other side of a house, into a forest, etc.
I was told recently that the marine industry is now looking at a computer driven IR sextant system that can provide navigation data to almost GPS accuracy, I shal have to ask my contact at the marine collage at fleetwood how this is progressing.
You can do this today for about 20k (a box with a GPS antenna input and an NMEA output, with FOG gyros inside). It won’t be airliner grade nav (that would be c. 100k) but it would be plenty good enough for a few hours of sailing or flying.
A_and_C wrote:
It is unfortunate that GA has been totally fixated on GPS
No it’s very fortunate for the airlines, because if all GA planes started to hammer those few DME with their pulses as well (and possibly 6 frequencies in parallel), they would all be way above their capacity. Some already are.
The problem is not reliance on Satnav, the problem is not exploiting the full redundancy Satnav offers; why does no aviation grade receiver cope with GPS L2 and L5 signals, and Galileo, Glonass and Beidou?
Beidou-1 (the test system) had the user terminals transmit to the satellites; this severely limited capacity (and made user terminals expensive). They must have realised this problem, as the general positioning service (Beidou-2) does not require the user terminals to transmit, they’re receive only, just as GPS, Galileo and Glonass. It’s just aviation that hasn’t yet realised the madness of having the user stations transmitting to the positioning infrastructure.
BTW, a few DME channels (frequencies) have been decommissioned to make space for the GPS L5 signal – that should tell you something about which direction DMEs are headed…
tomjnx wrote:
No it’s very fortunate for the airlines, because if all GA planes started to hammer those few DME with their pulses as well (and possibly 6 frequencies in parallel), they would all be way above their capacity.
That’s true! Anyway, DME/DME is nice to have, but does not work at low level, especially during approaches, because usually not enough ground stations are in view.
A real alternative would be “ground based GPS satellites” (as those proposed by this company here: http://www.popsci.com/technology/article/2011-08/ground-based-analog-gps-could-make-positioning-technology-accurate-inches). Quite inexpensive and if they use the same frequencies and data formats as GPS, existing receivers could be used without modification.
Another (comparatively) cheap backup to GPS would be a radio altimeter coupled to a terrain model, e.g. the one already used in terrain warning units. Cruise missiles have been navigating by this method for decades with an accuracy of some tens of meters.
The problem is not reliance on Satnav, the problem is not exploiting the full redundancy Satnav offers; why does no aviation grade receiver cope with GPS L2 and L5 signals, and Galileo, Glonass and Beidou?
How does more satellites help?
A little jammer suspended under a baloon will jam the whole lot really well, over quite a large area.
All my (few) reception losses were obviously jamming, like the one around Athens September 2015.
Cruise missiles have been navigating by this method for decades with an accuracy of some tens of meters.
Wasn’t there a problem with snow obscuring the terrain, in the USSR?
They had to do a fresh pass with a satellite-borne radar to get the latest ground profile, before launching the cruise missiles.
Peter wrote:
How does more satellites help?
Because they offer more frequencies quite far apart. L5 is in the same protected aviation band as DME.
Sure, a jammer could disturb L5 as well, but then DME wouldn’t help, as it’s in the same frequency band.
And it’s easy to disturb VOR and ILS, to the point of causing wrong indication in the cockpit without any warning sign that something might be amiss. With GPS you at least get a flag…
what_next wrote:
Another (comparatively) cheap backup to GPS would be a radio altimeter coupled to a terrain model, e.g. the one already used in terrain warning units. Cruise missiles have been navigating by this method for decades with an accuracy of some tens of meters.
The early cruise missiles don’t use a radar altimeter to navigate. They have a terrain map and have a terrain following radar. This is where the “USSR thick snow” issue came in. I don’t think anyone would suggest GA uses terrain following radar to get around… it would not work over N France anyway
INS is the only way forward if you want jam resistant navigation, but you want the sort of accuracy which we in GA are used to (closely avoiding prohibited areas, etc) then you need to do occassional corrections. Airliners do them using DME (or GPS in the newer ones, I am told). GA could use GPS, because GPS works ~99.999% of the time and if it actually got shut down then any plausible reason would have GA banned anyway.
Historically, airliners didn’t need to navigate all that accurately (10nm was OK) because over the sea it didn’t matter that much and once you were in range of land you got VOR fixes and radar vectors. Then once you intercepted the LOC you were sorted. The flight was nearly all in CAS anyway.
As I wrote earlier you can do it with a 20k box, and that price is highly unlikely to come down especially if you want some paperwork with it. So you could drive a tablet from it, etc. If I really thought GPS was to become unreliable this is what I would get – instead of a panel full of avionics eye candy which doesn’t actually do more than my KLN94. I would probably mod the KLN94 to use the data out of that box 
Narco (and IIRC Collins did something similar in the 1990s) were too late, as the bottom was dropping out of the navigation market due to GPS. That situation is still there, only stronger. I really don’t believe anybody is going to bring in Loran or anything else. It would be used only by GA and nobody in Europe is going to spend a single penny to support GA.
