OZ9AEC

As promised yesterday, here is the video tutorial showing how to connect the demodulator output of a GNU Radio receiver to the input of the multimon software modem.

On July 10, 2010, I was listening to the linear transponder downlink of VO-52 when I suddenly noticed a very strong peak about 50 kHz below. I checked Gpredict and found out that it was the APRS downlink from the International Space Station RS0ISS on 145.825 MHz. Thanks to the flexible GNU Radio framework and the USRP, I could easily receive both satellites at the same time as shown on the video below.

You may have noticed the weird construction I used in the SSB/CW receiver V0.7 implemented in the GNU Radio companion. It was using three AGC2 blocks with different decay rates, running in parallel and selecting one of them using a selector. The reason why I didn't just use one block and set the decay rate of the block using an option menu or a slider was that this method didn't work! Today I decided to dig deeper into the code and see how to fix this.

On July 11, 2010, a total solar eclipse could be observed in the southern Pacific Ocean. During the eclipse, the AO-51 amateur radio satellite was entering daylight, coming in over South America.

When AO-51 enters daylight the solar arrays start charging the batteries. This can be observed in the telemetry as a sudden increase of the current from the solar arrays. This time; however, a sudden drop in the current was observed shortly after the current started to increase, see the graph below.

The time and place where this unexpected drop occurred suggested that the satellite might have experienced the solar eclipse that was about to happen over the southern Pacific. It was Masa san, JN1GKZ, who noticed this and reported it on the AMSAT BB where after Mark, N8MH sent out the telemetry graph shown above.

A quick simulation using the Celestia space simulator confirmed that this was indeed the case. The video below shows how it may have looked like from the satellite.

Watch on Youtube or download H.264 encoded .mov file (~100MB).

You can try it for yourself. Celestia is available as free download from http://shatters.net/celestia - it's a great space simulator for demonstration and educational purpose. You will have to add the data for the satellite. You can use my data to begin with, download from here: http://files.oz9aec.net/video/SolarEclipse/ao51.zip

Unpack the ao51.zip file to the "extras" folder of Celestia, start the program and find AO-51 (type ENTER, AO-51, ENTER, then press "g" to go to the satellite). Enjoy!

You may have already seen tooltips in the Gpredict user interface. I have made extensive use of them for providing short description of the functionality of the GUI controls all over the application. Lately, I have felt a desire to extend the usage of tooltips to also include graphical objects like satellites shown on the map or the passes shown in the "Sky at a glance" module.

Gpredict uses GooCanvas for graphics, which adds a the "missing" 2D canvas library to Gtk+. Since version 0.15 GooCanvas includes support for Gtk+-like tooltips for any objects on the canvas. This is really great since it simplifies the effort from messing with own popup windows to using single line API calls.