I’ve been pondering potential upgrades to my satellite capabilities. Right now, I’m using a very popular combination: an arrow antenna and a Kenwood TH-D7A. Often, at the beginning of passes, where satellites are still relatively distant, I get very weak signals, and can’t often hear the satellite well until they are maybe 15 or 20 degrees over the horizon. In talking (both on the bird, and via email) with Mark Spencer, WA8SME, I’m beginning to see why his recommendation of adding a preamplifier to the receive side of my setup might result in better overall performance.

Mark wrote a very nice article for the Amsat Journal entitled Why can’t I hear AO-51? which is simply great and clearly explains how you can figure out the path loss between the satellite and you. It turns out that the path loss from you to the AO-51 is probably somewhere around -126db on 440mhz, but the uplink loss from you to AO-51 is only about -112db, or nearly 14 db stronger.

What’s the difference? Well, first of all AO-51 is running only 1w of total power and my Kenwood cranks out above 5. That’s almost 7db stronger right there. What’s the rest of the difference? Mostly the difference in free space loss between 2m and 70cm. The approximation for path loss that is most often used for freespace
path loss is:

path loss = 32.44 + 20 log(d) + 20 log(f)

given in db for a distance of d kilometers and frequency f in megahertz.

Assuming the distance is constant, for a given frequency, the difference is the is about 9.5 db stronger on 2m than on 70cm. (20 * log(145.92)-20*log(435.3) is just about 9.5). It turns out that AO-51 is a little deafer than your HT, but has about 4db more gain in its antenna compared to your rubber duck, so overall, you have a much easier time getting signals to it than it does getting to you.

What this suggests to me is that a receive preamp may very well be a good idea. I’m looking into getting one soon.

Okay, I was bored waiting for potential Ande passes, so I tuned into Genesat. I got a few telemetry packets, although the frequency seemed to be 5khz low from the published frequency of 437.075Mhz.

KE7EGC>UNDEF,TELEM:GeneSat1.orgB4511B0A00000000000000260069009F7113722166CC790B02F0
KE7EGC>UNDEF,TELEM:GeneSat1.orgFA511B06000000000001002600690001EA02BA27081CBF026621
KE7EGC>UNDEF,TELEM:GeneSat1.org04521B8A010E018901DD010100A4015F2DCB1C226D951D1C023A
KE7EGC>UNDEF,TELEM:GeneSat1.org36521B73010F017101DE010000A4019F992216023D996D22E207
KE7EGC>UNDEF,TELEM:GeneSat1.org3B521B72010F016E01DE010000A4015F2DCB1C226D951D1C023A
KE7EGC>UNDEF,TELEM:GeneSat1.org40521B0100000001000100250069005F2299221C02202274231C
KE7EGC>UNDEF,TELEM:GeneSat1.org45521B0200000002000000260069009F07BC13A0230013281C02

Not sure what any of it means, but there it goes.

More interesting reading, from Tom Clark on the AMSAT mailing list.

Well, I’ve been experimenting a bit more with some weather satellite reception, and on the off chance that anyone cares, I thought I’d write down some of what I learned. Last night (after dark) I decided to record an NOAA-17 pass. This time, I used my Yaesu VX-3R, which has a wideband FM setting, and my totally inappropriate Arrow Yagi that I have been using for satellite reception. I recorded the pass using a little Sony voice recorder. This was my result:

Bigger version, recorded with the Sony Voice Recorder…

It was during the dark, so I didn’t really expect to see much detail, but I noticed lots and lots of horizontal streaking. A moment’s thought made me realize that it was probably because the voice recorder compresses the audio, resulting in these short term artifacts. So, this morning, I lugged my laptop out into the yard and recorded some pristine, uncompressed audio. Here’s the result:

Full Size Version, recorded on my laptop…

What have I learned? That with a gain antenna that tracks, and the wideband fm setting, you can probably record some pretty reasonable images. Yes, it’s suboptimal: you get more noise in the image than the approrpriate bandwidth would allow. But still, they aren’t too bad.

I’m gonna work on improving my decoder some (45 lines of code really isn’t enough), and then maybe work on this some more when I’m on my christmas break.

Addendum: Sadly, when I tried to record a pass of NOAA18 over the ocean, I found another difficulty: intermodulation interference. I could hear a strong local FM station, and what sounded like aircraft audio superimposed with the satellite downlink. The results were far from stellar.

Ugly NOAA18 Pass, lots of interference…

Addendum²: A pass of NOAA-15 was happening just as sunset, and was going to be east of my position. Unfortunately, just as it was getting interesting, it shifted to a different mode or something, and I lost the super-cool looking grazing earth. But I did find that there are some places where perhaps I’m not as directly blasted by all the many sources of interference that I heard in the previous pass, so the overall pass is somewhat less noisy.

The full image from NOAA-15.