Listening to the SO-50 FM satellite

  I'm not a big satellite guy, but I was checking the upcoming satellite passes on my Android smart-phone on New Year's Day and I found an excellent pass taking place that evening. It was predicted to be almost 14 minutes long (pretty much as long as it gets), and quite high at the peak (about 77 degrees). 

   My previous experiences with satellites has been a bit frustrating. It's more common to find passes about 7 to 9 minutes long, and that's an ideal figure. Really, the time the "bird" spends high enough over the horizon to actually receive without obstructions is only about half that amount. Likewise, the angle of elevation (how high it is above the horizon) is commonly in the single digits or maybe a dozen degrees. If you hold a fist straight out to the horizon, the height of the top of that fist is about five degrees. It's not very high. 90 degrees is, of course, straight up. 

   When I saw the pass predicted to peak at 77 degrees, I realised that it would offer several minutes for me to listen. So I dug out my three-element UHF Yagi and programmed the five channels that I would need into my UHF PuXing PX-777 HT. I don't have a real sat-comm antenna, like the Elk or Arrow, but my Yagi is small enough to work hand-held. Likewise, I didn't have a VHF beam, so I couldn't transmit. A real sat-comm antenna is both a VHF and UHF beam combined, and you hold it pointed at the moving satellite (and rotate it to adjust for best polarisation) during the pass. 

   The SO-50 is an FM satellite, while some others are SSB or digital (AX.25). Being FM, you can use most any HT for reception of the SO-50. You can look up the predicted passes for the SO-50 (or most any satellite) here. Just select "SO-50" from the drop-down and enter your location. There's a very good tutorial on how to "work" the SO-50 here, with lots of helpful advice. I'm not writing an exhaustive treatise on sat-comm here, but you should be able to listen to the SO-50 with the information contained in these links. 

  An FM satellite has been called "a repeater in the sky", and that's a very apt description. With only a handful of minutes and a single frequency to share, however, you shouldn't expect to have a long conversation. You're limited to just exchanging callsigns and locations. It's all pretty "rapid-fire", and listening will give you an idea of just how much territory the "bird" is covering. Hint: it's a lot of ground.

  As far as equipment required, here's what British ham G6LVB wrote at that linked page:

To operate SO-50, you need a radio that can receive FM on 70cm and transmit 5W FM on 2m with a 67Hz PL tone, i.e., a typical handheld radio these days. You can also use two separate radios, one on each band, equally well. If you use a single dual band radio, it’s extremely worthwhile using one which allows you to transmit on one band while simultaneously receiving on another. Ideally the radio should be able to tune to within 5kHz so that you can correct for Doppler. Pre-programming the radio’s memories will help here if you’re using an FM only radio.

  Hearing the satellite is the hard part. That's why I needed the Yagi. Now, satellites generally operate on both UHF and VHF: they transmit on UHF (the downlink, which is where you listen) and listen on VHF (the uplink, which is where you transmit). So you need a directional UHF antenna to hear the SO-50 (or most "birds", for that matter). The satellite transmitter is very weak (only about 100 mW) in order to preserve power. Your HT is a powerhouse compared to the satellite transmitter, so the beam provides the gain that you'll need to pick up the signal. Remember that FM isn't a "weak-signal" mode; it takes a substantial signal strength for the demodulator in your radio to separate a signal from the background noise. 

   Because the satellite is moving across the sky so quickly, the Doppler Effect comes in to play. The frequency that you will be able to hear the satellite on shifts downward as the pass progresses. If you just stay on the downlink frequency of 436.800 the entire time, you'll miss most of the action. The easiest way to deal with this is to program five channels into your HT ahead of time, and switch to the next lower-frequency channel as the signal fades over the course of the pass. Those frequencies are: 436.810, 436.805, 436.800, 436.795, and 436.790. Start with 436.810 at the start of the pass. It may be (as in my case), that the satellite is too low to hear anything at that frequency, so switch back and forth from 436.810 to 436.805 until you hear something. 

   The time that the SO-50 rises above the horizon is the AOS (Acquisition Of Signal). Remember, this is in an ideal situation; it will probably be later than that for it to rise above obstructions. You'll find the AOS time on the satellite pass prediction. The azimuth is the horizontal location of the satellite on the compass: 180 is straight south, for example. In general, satellites move from south to north. The azimuth figure gives you a place to start pointing your antenna. As the satellite rises, it's not too hard to follow it with your hand. Just point the beam at the place with the strongest signal, and don't forget to shift frequency downward. The LOS (Loss Of Signal) is where the satellite drops below the horizon. By looking at the azimuth of the LOS ahead of time, you can get an idea of the path the satellite will take. My Android app (HamSatDroid) shows a picture of the path, which can be helpful in tracking. 

  Of course, there are computer-controlled rotors that can track the signal based on the predicted pass, thus freeing up your hands to actually work your radio. But my experience has been that one guy can do it just fine -- especially if you are only receiving. 

  Aside from the ISS (International Space Station), the SO-50 is the only FM satellite available at the moment. Still, the Fox-1A is scheduled to launch in August. It will have FM, but the uplink and downlink are backwards from the SO-50: you transmit on UHF (435.180) and listen on VHF (145.960). There are two more Fox satellites (1B and 2) planned, but no word yet on their capabilities. 

  Otherwise, if you have an all-mode radio, you can work the FO-29 satellite on SSB or CW. With 100 kHz, there's more than one "channel" to share. And the FUNCube (AO-7) has SSB/CW with a more powerful 300 mW transmitter. 

  Personally, I'm more oriented to longer conversations and satellites aren't very useful to me. But it is exciting to pick one up and hone my tracking skills. It's highly unlikely that we'll ever see amateur satellites in higher orbits that allow longer passes, and these would also require specialised antennae and higher power to reach the extra distance. Once upon a time, there was talk of a possible FM repeater on the Moon. Of course, the cost of such a project is prohibitive, but it would allow nearly an entire hemisphere to talk for several hours if the phase is right. I guess that we can still dream, right?