Weekly update (Week 45+46)

Week 45 did not bring too many news for MarconISSta. The system is still operational and measurements were taken in some of the bands outside of the VHF bands (AIS bands, Space Operation/ MetSat bands). In absence of other news, find some plots below.

Look at these waterfall plots. The constant signals at 137.5 MHz, ~137.2 MHz etc are self-induced artefacts. From 11:30 there is a lot of activity and for some signals the frequency changes. Normally, the frequency of signals is higher during approach of the source, while it is lower while moving away. For some of the signals, this “Doppler shift” is in the opposite way than expected. Any idea what this is? We do. One hint: It keeps recurring over various parts of the world, mainly over North America (http://www.isstracker.com/historical).

2018-11-02T11-00-00_LNA_30_PGA_12_TIA_0_VHF_WIDE.csv.png

 

Any idea what the signal at 144 MHz to 144.2 MHz at around 00:30 on November 4th might be?

2018-11-04T00-00-00_LNA_30_PGA_12_TIA_0_VHF_WIDE.csv.png

We won’t receive new data this week, as the ground control team is busy with other real-time Ops. New data is expected on November 22nd 18.

2 thoughts on “Weekly update (Week 45+46)

  1. “Doppler shift” in the opposite way – LimeSDR has lots of intermodulation artefacts (albeit weak) when there’s a strong signal. It mixes with something in it and creates these “ghosts” Some are moving upwards when the frequency of strong signal increases, others are moving downwards frequency wise. I have listened to FM radio on some UHF frequencies (I believe so) and when I fine tuned the SDR window’s center (essentialy retuning it) the station disappeared. When just moving the sliding “selector”, i.e. the decoder filter , it stayed there. So, I suspect intermodulation in the LimeSDR.

    It could be measured with a clear sine wave generator, sweeping from 0 to the max while watching the spectrum. Some artifacts would then move up, some down and with varying speed (higher order intermodulation moves away with integer times freq delta).

    Sorry for bad description. Maybe not true. But worth mentioning. I love LimeSDR. 🙂

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  2. Thanks for your comment!

    You are right about the “ghosts” and the LimeSDR’s affinity to include artefacts. However, the shift clearly is “Doppler shift” related. We are in contact with some operators at 137-138 MHz to validate whether any of them has seen these signals as well. The other theory is – like you mentioned – intermodulation artefact. The Doppler-shifted signal in that case would be transmitted in a higher band and a mirrored artefact is seen in 137-138 MHz. We keep working on this analysis.

    Also look at the week 47 post. There we have some real artefact of other systems!

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