Why does Signal strength vary over a range of 10 dB on a 24 hour cycle?
I have an Outernet patch antenna and preamp mounted outdoors in a fixed position accurately pointed at Inmarsat-4 98W from Columbus, Ohio. The receivers (RTL-SDR and an Ettus USRP) are indoors. I’m seeing signal strength vary from -115 dB to -125 dB (as indicated by RSSI) on a 24 hour cycle. At night the signal is strongest and low in mid morning. The SNR is following the variation in signal strength over a range of 13 dB to 3 dB. Atmospheric water vapor attenuation should be negligible at 1.5 GHz. And, with a 30 plus degree beam width antenna satellite orbital movement should not matter. The timing of the variation makes me think it is caused by sky noise. Can there be that much noise at this frequency?
Does anyone have a good explanation for this signal variation?