May 10th, 2013

Today Jan, PA3FXB and I used ISS passes to improve doppler compensation.

The first pass of three was disappointing because no reflections could be detected. In the second one I found Jan shortly after his window opened. But it sounded more like a strange kind of music than CW, Jan stated after listening to the audio file, I recorded of his signal. The reason were too long steps in timing for the frequency correction. I reduced it for the last try from 100 ms to 10 ms. After some recalibration of frequency and rotor control I started the system again when the ISS rose.

I could hear Jan without any manual corrections immediately after he started transmissions. The frequency tracking worked quite smoothly and reading his CW was possible without any problems.

PA3FXB as received by DJ5AR with QRG tracking every 10 ms

The audio file contains Jan´s signal in three periods. From 0:00 to 1:00 the signal raises while being on the upper flat part of the doppler curve shown above. During the second period from 1:00 to 2:00 the signal passes the steep part and even an adjustment every 10 ms leads to a kind of jumping signal. When receiving the third period, the signal was quite stable again and faded out, while the ISS moved to the eastern horizon.

In between his TX periods Jan tried to catch my signal without doppler compensation, just playing with the RIT. He successfully got a short part of my transmission:

DJ5AR as received by PA3FXB without QRG tracking

Doppler compensation on my TX part did not work. It seems that the SDR does not accept CAT commands for frequency change while transmitting. So we will work on another strategy, where TX frequencies remain fixed and both stations will do full doppler compensation only while receiving.