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APT & LRPT Weather Satellite Transmission Frequencies
| Satellite | Format | Frequency |
|---|---|---|
| NOAA 15 | APT | 137.62 MHz |
| NOAA 18 | APT | 137.9125 MHz |
| NOAA 19 | APT | 137.10 MHz |
| Meteor M2 | LRPT | Failed |
| Meteor M2-2 | LRPT | Failed |
| Meteor M2-3 | LRPT | 137.90 MHz |
| Meteor M2-4 | LRPT | 137.90 MHz |
⚡ What Causes the Center Spike?
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DC Offset in IQ Data: Most SDRs (especially RTL-SDRs) introduce a small DC bias in the I/Q signal. When you perform an FFT, this bias shows up as a strong spike at 0 Hz (the center of the spectrum).
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Imperfect IQ Balance: RTL-SDRs don’t have hardware IQ correction, so any imbalance between the I and Q channels leads to leakage at DC.
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No Built-in DC Blocking in FFT Display: SatDump doesn’t remove the DC spike from the waterfall or FFT display—it only blocks it behind the scenes before demodulation if you enable the right setting.
🛠 How to Deal With It in SatDump
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Enable DC Blocking: In the Configure tab for your satellite or in Offline Processing, look for a setting called DC Blocking. This won’t remove the spike from the FFT view, but it will prevent it from interfering with decoding.
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Offset the Tuning Frequency: Shift your center frequency slightly so the signal of interest isn’t sitting right on top of the DC spike.
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Use a Better SDR: Devices like Airspy or SDRplay have better IQ balance and often don’t suffer from this issue as severely.
🧪 Bonus Tip
If you're using a decimation factor, make sure your low-pass filter is properly configured—otherwise, aliasing can make the DC spike even more pronounced.