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Decoding DME aircraft radio navigation system with the LimeSDR

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Re: Decoding DME aircraft radio navigation system with the LimeSDR

#21

Post by tjohns »

toomuchtodo wrote: post #15
ryandrake wrote: post #10

Earlier quoted context omitted.

> 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.

> 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/2022-...

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.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#22

Post by mschuster91 »

VBprogrammer wrote: post #3
deutschepost wrote: post #2

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.

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 loc…

> 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

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#23

Post by lisper »

platz wrote: post #8
jojobas wrote: post #5

Earlier quoted context omitted.

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.

Wouldn't you be on the 324 degree radial

Depends on the direction of rotation which the GP didn’t specify.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#24

Post by Animats »

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.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#25

Post by tjohns »

Animats wrote: post #24

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 receiv…

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.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#26

Post by rlpb »

Animats wrote: post #24

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 receiv…

> 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.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#27

Post by rlpb »

tjohns wrote: post #14
ammar2 wrote: post #11

Earlier quoted context omitted.

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/a…

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.)

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.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#29

Post by chipsa »

rlpb wrote: post #27
tjohns wrote: post #14

Earlier quoted context omitted.

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.)

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.

Probably the required accuracy. VOR is on the order of a degree for accuracy. DME is around 0.1nm. So if you’re 50nm from the VOR, then you may have a position fix error of 0.87nm across the radial, if I did my math right.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#30

Post by ominous_prime »

rlpb wrote: post #26
Animats wrote: post #24

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 receiv…

> 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. T…

Even if an approach uses DME, depending on the aircraft or company operating procedures they still may not be using DME, because GPS is a valid substitute for DME in an approach and more convenient if you’re already otherwise using GPS RNAV.

In fact it’s quite common to shoot approaches that have DME specified fixes in an aircraft that doesn’t even have a DME transceiver.

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