[HN Gopher] Decoding DME aircraft radio navigation system with t...
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Decoding DME aircraft radio navigation system with the LimeSDR
Author : wolframio
Score : 84 points
Date : 2024-07-16 06:31 UTC (16 hours ago)
(HTM) web link (destevez.net)
(TXT) w3m dump (destevez.net)
| deutschepost wrote:
| VORs are pretty interesting. At first i thought every plane had a
| small antenna array on board to check the direction to the VOR.
|
| But the direction is just calculated by the phase shift between
| an omnidirectional and a directional signal. So it can be
| implemented very cheaply on every plane.
| VBprogrammer wrote:
| ADF uses radio direction finding techniques. It's not very
| accurate and is all but deprecated except for some corner cases
| where the legislation hasn't yet allowed for overlay approaches
| (replacing a real radio beacon with its true position via GPS /
| IRS).
|
| There are some convincing reasons for being cautious on general
| on that front. Accidents have happened because some beacons
| have been substituted for the wrong location or even things
| like slant range being different from true range. In general
| ADF is so inaccurate that its protected areas are massive.
| aftbit wrote:
| There's also a very real chance that commercial aviation may
| find itself operating in a GPS-denied environment, at least
| in various edge cases. For example, combat zones often employ
| GPS spoofing and jamming, and because radio waves don't
| exactly respect borders, this can sometimes affect civilian
| equipment outside of the combat zone. This has happened at
| least near Israel, Iraq, and Ukraine in the last few years.
| In other cases, truckers or rideshare drivers that want to
| spoof their company trackers end up parking too close to an
| airport and impacting planes on an RNAV approach.
| ryandrake wrote:
| > There's also a very real chance that commercial aviation
| may find itself operating in a GPS-denied environment, at
| least in various edge cases.
|
| Yea, it's kind of terrifying that we are slowly putting all
| of our eggs in the GPS basket. I love GPS but when lives
| are at stake, you need a redundant backup navigation system
| that is robustly deployed and reliably works.
| ammar2 wrote:
| The VOR Minimum Operation Network[1] in the US is
| basically supposed to be that. They're decommissioning a
| lot of the VORs but at least guaranteeing that you'll be
| 100NM away from a working VOR and an airport with an
| approach that can be accomplished with VORs for the
| initial fixes.
|
| Still definitely feels like putting a lot of reliance on
| GPS but at least there's a backup for the worst case.
|
| [1] https://www.faa.gov/about/office_org/headquarters_off
| ices/at...
| tjohns wrote:
| There's also a DME Minimum Operational Network, for
| airliners that can use DME-DME RNAV. (That's too
| expensive for smaller aircraft to install, though.)
| cameldrv wrote:
| It's too bad that DME/DME RNAV isn't more widely
| available. The only real reason it's so expensive is that
| there isn't much demand for it since GPS (usually) works
| fine. Electronics-wise, it's not much more complicated
| than a transponder. Unlike a GPS, it does have to
| transmit, so it will always be somewhat more expensive
| than GPS.
|
| The other problem is that there's a limit to how many
| aircraft a DME station can serve at a time (about 100),
| but I believe that could be greatly expanded if aircraft
| weren't pinging the DME so often. A position fix every
| second is generally fine, and it could be even more
| infrequent if you have a cheap inertial system to fuse
| with it that can fill in the track for a few seconds
| between pings.
| rlpb wrote:
| One thing I have yet to understand is why DME-DME is
| preferred over VOR-VOR. Because the latter can support
| unlimited aircraft, unlike DME.
| toomuchtodo wrote:
| > you need a redundant backup navigation system that is
| robustly deployed and reliably works.
|
| Could existing cellular network base stations be this
| ground based backup navigation system?
|
| https://people.engineering.osu.edu/sites/default/files/20
| 22-...
| someguydave wrote:
| Sure it could be done but good luck convincing cell phone
| vendors to adopt liability for aircraft safety
| tjohns wrote:
| No. After about 5000 ft AGL (give or take) you can't pick
| up cell tower signals at all, since the antennas are
| directional and pointed towards the ground.
|
| This is a deliberate design decision, because even a low-
| altitude aircraft would have hundreds of cell towers in
| sight and would overwhelm the network when handsets tried
| to register on all of them.
|
| But also: Pilots like being able to have guarantees about
| system accuracy. We get notices anytime even a single GPS
| satellite is out of service (even though there are 31 of
| them), and have software tools in the aircraft to predict
| if there will be any signal degradation along our route
| (RAIM). I can't imagine having anything near that level
| of guarenteed safety with an ad-hoc system like
| described.
| nradov wrote:
| I agree. Starlink could potentially be used as an
| alternative to GPS (and other similar constellations).
| But it will probably be a long time, if ever, before it's
| certified for civil aviation navigation. And it's also
| vulnerable to jamming.
|
| https://www.technologyreview.com/2022/10/21/1062001/space
| x-s...
| mikewarot wrote:
| Starlink and GPS always face denial of service from
| exterior forces, or simply not being able to launch
| replacements up into orbit in a timely manner.
|
| We should ALWAYS have full coverage from ground based
| navigation systems, and the pilots should be required to
| use them on a regular basis.
| _djo_ wrote:
| It's currently happening in the Eastern DRC too, affecting
| not only scheduled commercial traffic but humanitarian
| flights as well.
| tjohns wrote:
| NDBs/ADF are much more common outside the US, where there
| isn't a huge installed VOR network or where there is a need
| for long-range airways over remote terrain. (Canada, I'm
| looking at you.)
| mschuster91 wrote:
| > replacing a real radio beacon with its true position via
| GPS / IRS
|
| That's the last thing you want to do. Here in Europe, we're
| dealing with serious issues because the Russians are jamming
| GPS from somewhere in Kaliningrad, but unfortunately we can't
| respond adequately without legitimately risking WW3.
|
| [1] https://www.bbc.com/news/articles/cne900k4wvjo
| jojobas wrote:
| Easiest to understand with a flashing + rotating light analogy.
|
| When a rotating, say one turn in 10 seconds, green light is
| pointing north, a red light flashes.
|
| If you see a green flash 1 second before a red flash, you're on
| the 36 degree radial.
| platz wrote:
| Wouldn't you be on the 324 degree radial
| lisper wrote:
| Depends on the direction of rotation which the GP didn't
| specify.
| 0xfaded wrote:
| Here is the authoritative video of an English radio engineer
| explaining how VOR works.
|
| It truly is 60s level wizardry.
|
| https://youtu.be/tZ2gG1v9Xg8
| venti wrote:
| If you want some in-depth technical details on how VORs work, I
| found this video very helpful. It goes into many of the details
| of the analog radio engineering:
|
| https://www.youtube.com/watch?v=tZ2gG1v9Xg8
| psunavy03 wrote:
| You can also compare and contrast how VOR/DME and TACAN works,
| to see two different solutions to one problem (polar coordinate
| measurement in 3D space.)
|
| The mechanical differences are quite obvious when you look at a
| VOR, a TACAN, and a VORTAC, but the engineering behind them is
| interesting.
| toomuchtodo wrote:
| Animats wrote a great comment on the topic here [1] as well.
|
| [1] https://news.ycombinator.com/item?id=40400619
| Animats wrote:
| Cute.
|
| He set up his receiver near the VOR, though. So he doesn't get
| any useful distance info from it. He can hear the aircraft's
| query and the fixed station's reply, but near the DME station,
| the difference will be constant, just the fixed delay.
|
| The next step is to put the receiver far from the DME station.
| Then, the time delay measured will indicate the aircraft to DME
| station distance minus the aircraft to receiver distance. I think
| this lets you locate the aircraft somewhere on a hyperbola,
| similar to the way GPS and LORAN work off time differences. If
| you have two receivers at different locations, you should be able
| to get two hyperbolas and locate the aircraft.
|
| This is really a 3D problem, because altitude. So you get quadric
| surfaces and need 3 receivers. Preferably four, because there are
| multiple solutions. Two is enough to get a rough aircraft
| location for test purposes.
|
| This has potential as a ground backup for ADS-B. ADS-B tells you
| where the aircraft nav system thinks it is. This is telling you
| where it really is, if it's using a VOR/DME at the moment.
|
| But not who it is. That's not in the DME poll.
| tjohns wrote:
| We already do something vaguely similar with MLAT, measuring
| the time delay from transponder signals at different receiver
| sites.
|
| MLAT data can be used for either unofficial situational
| awareness in non-radar facilities (to display non-ADS-B
| aircraft), and in some limited cases can be fed directly into
| official radar displays when running in sensor fusion mode.
| rlpb wrote:
| > if it's using a VOR/DME at the moment
|
| This is the crucial thing. The article says
|
| > The pilot will usually tune the radios to the stations that
| are part of the procedure that the aircraft is flying (although
| the pilot is free to tune to other stations as a cross check),
| so the kind of aircraft that we expect to see in the recording
| are those operating on the Madrid Barajas airport, not those
| flying high en route.
|
| The article author has it right. Nowadays most aircraft are
| using GPS to navigate, and only use DME if on a specific
| approach procedure that requires it. In practice, this has far
| narrower scope than ADS-B.
|
| Another commenter has it right - if instead of an experiment
| you actually want to locate aircraft without (or not) using
| ADS-B, you're far better off doing MLAT on Mode S, though you
| do need multiple spatially separated receivers for that.
| Aircraft are far more likely to have a Mode S transponder and
| have it switched on than they are to be using DME on the
| frequency you choose to monitor.
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