RTL-SDR for NOAA15/18/19 Satellite Weather-Summer 2025 – Telecommunications, Navigation & Electronics

Introduction

Several years ago, I looked at receiving NOAA15/18/19 satellite weather using various configurations with a Windows 10 laptop and a Raspberry Pi4B (Ref.1/5). I also simulated the NOAA-APT transmission using Scicos (Ref.6). I thought it would interesting to revisit NOAA and try some different ideas I picked up working with Cubesats.

Satellite Tracking

Fig.1 N2YO Satellite Path NOAA19 July 5th_2025

Figure 1 shows a plot of NOAA19 for July5th_2025. The website n2yo.com gives 10day predictions for NOAA15/18/19 and you can pick the best pass for your particular case. You can also use Gpredict to give you immediate visibility of the satellite footprint.

Equipment Setup

Figure 2 shows the receiving equipment setup. Normally I use a Vee dipole mounted on a tripod at my home location looking East. This time I was at Toronto Harbourfront so the Vee was mounted on a small table tripod. I noticed that I was able to improve the signal gain by holding the Vee in my hand and adjusting the azimuth and elevation. This gave significant improvement. Previously I saved the decoded output as an audio wav file which I then adjusted in Audacity. This time I followed the procedure I used for decoding Cubesats and sampled at 250Ksps and saved the reception as a baseband IQ file. This has the advantage that you can play the signal back, adjust the Doppler shift, and experiment with NBFM/WBFM decoding and then save the decoded output as an audio wav file. The baseband IQ file can also be used in GNURadio for further study.

Fig.2 SDR# NOAA19 Rx Setup for IQ File Saving

Figure 2 shows the receive configuration. The RTL-SDR gain is nominally set at 19.7dB but can be adjusted for best SNR depending on local conditions. Sampling is set at 0.250Msps. A baseband signal recorder is used to save the signal as a complex IQ wav file.

Fig.3 SDR# NOAA19 Rx Setup for IQ File Playback

Figure 3 shows the playback configuration. A Baseband File Player is used to playback the previously recorded IQ file. I used NBFM with audio gain set in the middle to produce a signal at about -0.5/0.5 in Audacity. The SDR# audio setting was to VBcable input and Audacity to VBcable output. I adjusted for Doppler shift using the centre carrier. Approaching it was 137.103MHz decreasing to about 137.099MHz.

Fig.4 Audio WAV File in Audacity Adjusted Gain

Fig.5 Spectrum of Audio WAV File Showing 2.4KHz Tone

Figure 4 shows the demodulated audio wav file in Audacity. I used the Amplify effect to raise the gain slightly. You can check the quality of the demodulation simply by measuring the SNR of the 2.4KHz tone, which in this case is: -16.6dB – (-33.1dB) = 16.5dB as in Figure 5.

Signal Decoding

Fig.6 Audio WAV File Decoded in NOAA-APT

Fig.7 Resampled Audio WAV File in WxtoImg Decoder.

Once the audio wav file is ready, then I next decoded it in NOAA-APT (Ref.7). This is an excellent decoder with a complete guide with examples on the website. The decoder also has a re-sampling feature to convert the 48KHz audio wav file to a 11025Hz rate to be used in the WxtoImg decoder (Ref.8) as in Figure 7.