DME client visualisation

ADS-B, ACARS, VOR, ILS, Airband AM
keukenrol
Posts: 7
Joined: Sat Apr 08, 2023 7:50 pm

DME client visualisation

Post by keukenrol »

Hi

I was wondering if it would be possible to visualise DME pulse trains from air to ground stations? Based on the following website this would mean a single interrogation pulse (bit) takes only 3.5 microseconds. https://www.studyaircrafts.com/dme

With a sample rate of 2.4MSPs this would give us approximately 8.4 samples for a single pulse.
I am currently doing some research on this topic and would like to know if someone with basic programming experience in c and python would be able to develop their own plugin for sdrangel? This is a reception only topic!

Long story short, DME frequencies can be found online and the ground and air stations are 63MHz apart. To allow multiple aircraft to send to the same station, every aircraft has its own pulse train on which the ground station replies by repeating it. I would then map the amount of "connected clients" on a given beacon, possible also trying to determine their pulse train.

Thanks!
srcejon
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Re: DME client visualisation

Post by srcejon »

To visualise the pulses, you can use the scope within the channel analyzer plugin (set scope to show magnitude).
keukenrol
Posts: 7
Joined: Sat Apr 08, 2023 7:50 pm

Re: DME client visualisation

Post by keukenrol »

The pulses are of such short durations that it seems no pulses are there. I am relatively close to a ground station but I am unable to see pulses from both aircraft and ground station on their frequencies respectively.
srcejon
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Location: London
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Re: DME client visualisation

Post by srcejon »

Post a screenshot or IQ file, maybe?
keukenrol
Posts: 7
Joined: Sat Apr 08, 2023 7:50 pm

Re: DME client visualisation

Post by keukenrol »

I have been trying a few different settings, in the end I went for a wideband scanner antenna and the max gain of the SDR (49.6dB) in order to be able to see them. The the pulses seem to be shifted in frequency too, which might be due to reflections.
The next step would be to identify unique sets of pulse trains and map them.
Each green blob is in fact a pulse train, but it is happening so fast that it seems like a single pulse.
Attachments
dme.PNG
dme.PNG (265.18 KiB) Viewed 1962 times
srcejon
Posts: 327
Joined: Thu Jan 19, 2023 2:30 pm
Location: London
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Re: DME client visualisation

Post by srcejon »

Use the Channel Analyzer to view the signal power vs time in the scope.
keukenrol
Posts: 7
Joined: Sat Apr 08, 2023 7:50 pm

Re: DME client visualisation

Post by keukenrol »

Allright, starting to get there. I will look for the documentation to get one shot triggers working and zoom in on a specific pulse.
Attachments
dmechan.png
dmechan.png (247.57 KiB) Viewed 1938 times
keukenrol
Posts: 7
Joined: Sat Apr 08, 2023 7:50 pm

Re: DME client visualisation

Post by keukenrol »

Hi there

I am struggling to get the one shot trigger working. I have setup the rising edge trigger to about 300m on the scale. In freerun mode, a lot of pulses often pass this level.
However, it never seems to trigger. When it does trigger, the pulse is not visible on the scale and I have to restart SDRangel since both oneshot and freerun buttons are now greyed out. I also get this behaviour when the channel scope is paused and i use the time offset and time range sliders to zoom in on a pulse.

Since the DME signal is pulse modulated, I set the sample rate to a max stable of 2.4MSps for the RTL SDR dongle.This should give a sample period of 0.416 microseconds.
With a pulse width of 3.5 microseconds I suppose this should be enough to visualise the pulse. It will be difficult to visualise the pulse amplitude modulation however.

Any tips to visualise it like the following image? Apart from increasing the sample rate.
https://www.researchgate.net/publicatio ... e-pair.png
Nicely explained here: https://www.skyradar.com/blog/navaids-a ... me-and-vor

My setup:
sdrangel_dme.png
sdrangel_dme.png (1001.05 KiB) Viewed 1853 times
I have managed to manually find a pulse once:
DME_PULSE.png
DME_PULSE.png (46.63 KiB) Viewed 1853 times
Thanks!
srcejon
Posts: 327
Joined: Thu Jan 19, 2023 2:30 pm
Location: London
Contact:

Re: DME client visualisation

Post by srcejon »

I suspect the buttons are getting greyed out, because you are viewing a saved trace, rather than live trace. E.g in the first pic, M:00 to the right indicates live data, whereas M:01 in the second pic indicates a saved trace. (It can automatically switch to saved trace when you adjust the offset - This is a little confusing and has caught me out before - you have to switch back to M:00).

See: 13 Memory trace here: https://github.com/f4exb/sdrangel/blob/ ... /readme.md
srcejon
Posts: 327
Joined: Thu Jan 19, 2023 2:30 pm
Location: London
Contact:

Re: DME client visualisation

Post by srcejon »

Also note, that it's the entire pulse that is 3.5us. The pulses within the pulse are shorter (around 1/5th by the look of it), so I think you need a higher sample rate. Some RTL SDRs may do 3.2MSa/s, but that's still probably not enough, although above the Nyquist frequency, so you could interpolate.
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