Commandline for direct decode with rtl_fm #31
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It works with using Nooelec SDR stick. |
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I guess you could do something like (ppm 0, frequency 402.0 MHz) I recommend using rs41mod Here the sample rate is 48kHz, rtl_fm has 16bit samples. Here are some examples for baseband data (see description) |
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With rtl_rm/rtl_sdr you need to know the ppm and the exact frequency of the signal. rs41mod decodes a lot better than the old rs41ptu. |
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Thanks for the detailed answer. It works now much better Whats that number When next RS41 passes I'll try if I look for frequeny via SDRSharp and Nooelec RTL Dongle has a TCXO so luckly it is exact enough so frequencies don't be corrected. |
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The numbers in parantheses are the number of errors the error correction code was able to correct. It can correct up to 2x12=24 errors. If there is no number, then there were no errors in the frame, i.e. you know you have a good signal. If there are too many errors, it is possible that sub-packets still have have valid CRC, this is indicated by "0" (in the square brackets "[11110]") and the packet (e.g. GPS3 in your example) is displayed. "1" means CRC error for that packet. If you are sure the signal is strong, maybe there is still some dc/frequency offset. Try if the additional option "--dc" for the decoder makes a difference. Although with TCXO the frequency offset shouldn't be such a problem. |
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If you are unsure about the signal quality, you can save the rtl_fm IQ data into a file and add a wav header or convert to float32, such that sdrsharp or gqrx resp. can replay the file. If rtl_fm uses only a very simple lowpass filter before decimation/downsampling (try option Then prepare the IQ data for viewing in gqrx or sdr#, gqrx (raw s16 -> raw f32): sdr# (raw s16 -> wav s16): |
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Hi rs1729, thanks for all your support. Just tried with and without dc Option. With -E dc option So I'd say makes not so much difference. Viewing spectrum in SDR Sharp via rtl_tcp shows about 16 dB SNR. Tried a few minutes later with about 18 dB SNR, which changes quite fast So now I even get temperature, pressure and humidity. Receive quality is better now, but why is there now temperature , pressure and humidity in addition? Just by coincidence that RS41 is transmitting this data now? Or just because of the better signal quality? |
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On https://radiosondy.info/sonde.php?sondenumber=S2251325 there is current RH reported about 5 % whereas here is always 2% shown. Am I missing some parameters? EDIT: Just quitted and restarted rs41mod and now Temperature and humidity data is gone. However it can't be cached data displayed because Temperature and pressure changed often between frames. |
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PTU data is only displayed when the required coefficients are received. They are distributed over 51 frames, you can see the config/calib data with option Above about 12-14km when the radiosonde reaches the stratosphere, the rel. humidity usually drops down to 0-1%. At the poles this region is lower at approx. 8km altitude, near the equator rather 17km. Though the rel. humidity is difficult to measure, sometimes the sensors show higher values, errors up to 5% RH are not unusual. And it is not that easy to calculate the rel. humidity from the data, the sensors are temperature dependent and are not simple to model. Example You can compare the data here |
It looks like in sdr# the calculated dB is not only depending on the chosen filter bandwidth but also settings like sample rate or if you zoom in. So maybe it is the displayed sample rate or FFT length. |
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Hello,
trying to decode RS41 data as described in https://github.com/rs1729/RS/tree/master/rs41
However I'm missing an option to directly pipe into from rtl_fm. How is this possible to follow live data ?
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