This week @ULSF we have a cool NOAA APT catch by Alex @Csete.
This image from the North Pole was captured from Denmark.
This video shows the tracking sequence as illustrated by Alex’s GPredict software.
“I used GNU Radio and the USRP to receive and Gpredict … to predict the AOS/LOS times and to know where to point the [hand held] antenna during the pass. What I did was to record the received spectrum (250kHz as seen on the video from last week) in raw format. This data is pretty much a digital representation of what was on the air at that time at 137.x MHz +/- 125 kHz (At 137 MHz the Doppler shift will be less than +/- 5 kHz). The recorded data is about 1.7 GBytes for a 15 minute pass.
“After the pass, I replayed the recorded spectrum and ran it through the channel filter, FM demodulator, audio recorder and image decoder. It still feels like listening live the only difference being the signals don’t come from the antenna but from a recorded file. It has the advantage that I can always restart if I make a mistake during decoding and it is also very useful for experimenting with the receiver chain.
“The idea with the DX was to try to receive signals when the sat is still far away (low elevation). Due to the FAX-like transmission, the image is transmitted line by line and the satellite always transmits what is right below it (sub satellite point/line). It’s like a camera always pointing down to Earth and only capturing one line at a time. So, the lower the elevation angle, the farther away will the transmitted image come from. Obviously, there is a limit to how far away this can be because the satellites are in a low Earth orbit (between 800-900 km).
“Receiving while the satellite is still at low elevations can be very easy if there is clear line of sight or very difficult if there are trees in the way. I got mixed results this weekend:
The best accomplishment is probably the one from the North pole, though it’s rather noisy and difficult to recognize.”