I had a look what software is available ... / 2014-01-13 - Koos van den Hout

2014-01-13 1 year ago
I had a look what software is available for predicting satellite passes in the ubuntu ham radio software repositories.
predict-g1yyh in an xterm screenshot
predict-g1yyh in an xterm
I found PREDICT satellite tracking and orbital prediction program which does all the calculations given recent Keplerian elements which are available from sources like Keplerian elements at Amsat and Current NORAD Two-Line Element Sets at celestrak. Predict comes in two versions: predict in a version from May 2006 and predict-g1yyh. They look a bit different but the calculations are the same in the end.

Important part (to me) is the software also does the calculations for doppler shift. Doppler shift does occur measurably in radio signals when the two stations have a high enough speed difference between them. Since amateur radio satellites are low earth orbit satellites, the speed is measured in kilometers per second and the shift is there and should be taken into account. Shifts in the 70cm band are high enough that you need to retune the radio. As visible in the screenshot predict does calculate the frequencies when the satellite is visible.

gsat screenshot
gsat graphical client for predict screenshot
The interesting part is that predict and predict-g1yyh can also run as a server program where clients can access the data. One of the interesting clients is gsat from package predict-gsat. With gsat a world map is plotted with the home location and the satellite footprint visible and the data for antenna direction and doppler shift is shown.

gpredict screenshot main window
gpredict screenshot main window
I also found package gpredict which is totally not related to predict. In my opinion gpredict has a more modern interface (it's gtk+ based) but it lacks support for doppler shift. This makes it somewhat less interesting for radio use.

gpredict screenshot radio control with doppler shift
gpredict screenshot radio control with doppler shift.
Update: gpredict can support doppler shift when it thinks it can control a radio. Configuring a radio with RX only support will make the 'Radio Control' window available where you can select the satellite and its services when it is known to gpredict under 'Target' or enter the uplink and downlink frequencies of the satellite by hand, select 'track' and see the result of the doppler shift calculations.

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