Gpredict, GQRX and RTL-SDR


I have been using GQRX and Gpredict for a couple of months. They are both very
nice programs. Today Sept 1, I think that I finally have it figured out. I have
been leaving them up continuously 24-7 for about a month without receiving anything
with Gpredict and GQRX while I would always catch some telmetry frames using my Windows 10 machine, SDRSharp and Orbitron.

I think that I know why now. Today, I was able to be at the computer for several
passes of AO-85, AO-91 and AO-92 and received over 100 frames using Gpredict and GQRX. But I had to do several thinks to accomplish that. I also noticed that the Hardware Frequency reading on the Receiver Option Tab didn’t match the satellite that was overhead at the time and the kHz offset was very large. The the frequency range of the satellite above the waterfall didn’t match the frequency range of the satellite overhead.

I tried a couple of things by manually setting the correct frequency to the Hardware frequency by inputting the correct value in the Receiver Frequency box and then zeroing the Channel Filter offset value to zero then again setting the receiver frequency to the correct value. I’m using a RTL-SDR dongle for my receiver. Another observation is that it doesn’t appear that Gpredict passes the Downlink Frequency to the Hardware Frequency when the satellite becomes active by either clicking on the satellite shown on the world map on Gpredict or when is within the radio window. It would seem that Gpredict should pass the value in the update transponder database to the GQRX Hardware value then offset the frequency for Doppler from the Hardware Frequency value and then center that value in the frequency reading just above the waterfall.

I’m curious whether anyone else has seen anything similar.

Mike, NA5SS


Hi Mike,

The hardware frequency in gqrx is only relevant for debugging. It just tells you what the oscillator in the receiver hardware is tuned to and it is the frequency you have at the center of the spectrum display.

However, since SDR samples a large bandwidth, let’s say 1 MHz, you can receive anywhere from hardware_freq - 500 kHz to hardware_freq + 500 kHz using “software tuning” without changing the hardware frequency. So, the actual receive frequency is what you see on the big digits above the spectrum display.

When gqrx receives a new frequency through the remote control interface, it will always try software tuning first and only change the hardware frequency if the new frequency is outside of the above-mentioned range.

I hope this helps.