Changing default transponder settings in Gpredict

Almost all satellite tracking software I tried so far never got the transponder settings for
doppler control right out of the box. There is the theory and then there’s reality. In reality
I still had to look for my downlink signal in order to find the right frequencies. Now one
thing that I like to live by is that when I have to repeat certain things over and over, I like
to automate them. Same goes for setting the correct up- and downlink frequencies when
operating linear satellites.
In the current version (1.3) of Gpredict, there is no editor to change settings for transponders,
so your favorite editor (vi I’m assuming 🙂 ) will be your friend here to add a new transponder
to the config file of the satellite.

Let’s do this for VO-52. First we need to get the catalogue number of the satellite as Gpredict
organizes it’s information based on that. If we click on the desired satellite and look at the
highlighted field, we get that piece of info.

Now that we know the number (in this case 28650 for VO-52) we can go look for the matching file
that Gpredict uses to store the information in. It’s located in ~/.config/Gpredict/trsp

Let’s look at the file itself and see it’s format:

mschulz@shack:~/.config/Gpredict/trsp> cat 28650.trsp

[Indian Beacon]
DOWN_LOW=145936000

[Indian U/V Lin]
UP_LOW=435220000
UP_HIGH=435280000
DOWN_LOW=145870000
DOWN_HIGH=145930000
INVERT=true

[Dutch Beacon CW]
DOWN_LOW=145860000

[Dutch U/V Lin]
UP_LOW=435225000
UP_HIGH=435275000
DOWN_LOW=145875000
DOWN_HIGH=145925000
INVERT=true

As you can see, it’s pretty straight forward. First we have a label for the transponder followed by the
values for up- and downlink frequencies and if it’s an inverting transponder. Based on that, all we have
to do is add a new section for our matching frequency pair to the end:

[New U/V Lin]
UP_LOW=435220000
UP_HIGH=435280000
DOWN_LOW=145873000
DOWN_HIGH=145930000
INVERT=true

Note that I only changed one side, in this case the downlink frequency to keep things simple. The way
I determined how to change it was also pretty easy. I just set my uplink frequency in the middle of the
passband where Gpredict would put it also. Then I tuned in my downlink signal and noted the difference
in frequency from the passband middle, which gave me the value by which I had to change the settings
in the config file. As you can see, the new setting is 2kHz lower than the original one which is roughly
where I want it to be to have to only fine tune my downlink signal.

Have fun tuning your Gpredict settings and see you on the birds.

73 Michael

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