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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

#31

Post by Scoundreller »

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…

> In general ADF is so inaccurate that its protected areas are massive

Great for listening to LW & MW(AM) radio stations in the cockpit though. Rumour has it some pilots would patch big sports games into the in-flight entertainment channels so you wouldn't miss them just because you're in the air.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#32

Post by Scoundreller »

aftbit wrote: post #9
VBprogrammer wrote: post #3

Earlier quoted context omitted.

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…

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…

I'm wondering if there's some doppler-shift assessment possible to detect (and possibly reject) spoofing/jamming? A fixed ground-based spoofer/jammer will be too consistent in its doppler-shift, unless it varies its frequency slightly, but that would only work well against specific targets, not broad areas.

> Radio waves experience Doppler shifts the same as sound waves do as objects move. The Doppler shifts for the real satellites will all be different as the object moves either towards or away from them depending on their position in the sky. However, the Doppler shift caused by the object’s motion due to a spoofer is the same for all the satellites signals because they are all arriving from the same direction. This uniformity of Doppler is another indication of spoofing. Again, only the most sophisticated spoofer can account for an object’s motion to adjust its Doppler shift for each individual satellite, and to do so requires tracking the object’s course.

https://safran-navigation-timing.com/resiliency-in-pnt-gps-g...

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#33

Post by cjrp »

Scoundreller wrote: post #32
aftbit wrote: post #9

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. 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…

I'm wondering if there's some doppler-shift assessment possible to detect (and possibly reject) spoofing/jamming? A fixed ground-based spoofer/jammer will be too consistent in its doppler-shift, unless it varies its frequency slightly, but that would only work well against specific targets, not broad areas. > Radio waves experience Doppler shifts the same as sound waves do as objects move. The Doppler shifts for the…

I’m not sure if they still use it, but there was an Inertial Reference System on board - you set your location at the start of the flight, and then it can (roughly) give you a position using dead reckoning. A large disagreement between this position and the GPS position would result in an alert.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#34

Post by actionfromafar »

mschuster91 wrote: post #22
VBprogrammer wrote: post #3

Earlier quoted context omitted.

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

Depends on what you mean adequately. We probably shouldn't carpet bomb them. But there's a lot more that could be done - turning the weapons donations to Ukraine from a drip-drip of the spigot to a firehose should wrap the war up.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#35

Post by scoopr »

aftbit wrote: post #9
VBprogrammer wrote: post #3

Earlier quoted context omitted.

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…

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…

Not just near battlezones, just being near a jammy neighbour is enough: https://yle.fi/a/74-20086338

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#36

Post by rlpb »

chipsa wrote: post #29
rlpb wrote: post #27

Earlier quoted context omitted.

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.

That's a good point. But if you're 50nm from a DME, I think you're unlikely to be able to get a lock in practice.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#37

Post by Scoundreller »

cjrp wrote: post #33
Scoundreller wrote: post #32

Earlier quoted context omitted.

I'm wondering if there's some doppler-shift assessment possible to detect (and possibly reject) spoofing/jamming? A fixed ground-based spoofer/jammer will be too consistent in its doppler-shift, unless it varies its frequency slightly, but that would only work well against specific targets, not broad areas. > Radio waves experience Doppler shifts the same as sound waves do as objects move. The Doppler shifts for the…

I’m not sure if they still use it, but there was an Inertial Reference System on board - you set your location at the start of the flight, and then it can (roughly) give you a position using dead reckoning. A large disagreement between this position and the GPS position would result in an alert.

Definitely still a thing and I think that’s the goto when GPS fails. Of course there’s drift that accumulates. I think the primary threat to commercial aviation without GPS is guided landing systems where you need the location precision.

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#38

Post by tjohns »

rlpb wrote: post #36
chipsa wrote: post #29

Earlier quoted context omitted.

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.

That's a good point. But if you're 50nm from a DME, I think you're unlikely to be able to get a lock in practice.

For altitudes above 12,900 ft AGL, the official service volume for a DME is 100-130nm.

Below that it's considered "line of sight"... and some quick math shows that you'd be able to get line of sight >50 nm for all altitudes above 1700 ft AGL (which is very low).

Source: https://www.faa.gov/air_traffic/publications/atpubs/aim_html...

Re: Decoding DME aircraft radio navigation system with the LimeSDR

#39

Post by rlpb »

tjohns wrote: post #38
rlpb wrote: post #36

Earlier quoted context omitted.

That's a good point. But if you're 50nm from a DME, I think you're unlikely to be able to get a lock in practice.

For altitudes above 12,900 ft AGL, the official service volume for a DME is 100-130nm. Below that it's considered "line of sight"... and some quick math shows that you'd be able to get line of sight >50 nm for all altitudes above 1700 ft AGL (which is very low). Source: https://www.faa.gov/air_traffic/publications/atpubs/aim_html...

Yes but only to about 100 aircraft at a time, favouring stronger (and therefore closer) signals: https://en.wikipedia.org/wiki/Distance_measuring_equipment#S...

If you're 100nm away, chances are there are more than 100 aircraft nearer to the DME than you from at least one of the two required DMEs. Especially if GPS has failed and many aircraft are trying to use backup DME-DME. Unless you're in a very sparse area.

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