Sven Grahn’s Space Tracking Notes talks about his efforts in using radio to track satellites. There is all sorts of really good stuff in here, a lot of it having to do with tracking the secret launches of Russian satellites during the Cold War. Very, very neat stuff.
I’ve been reading up a bit on antenna design, particularly the design of Yagi style antennas, and decided to give it a whirl. In particular, I decided to try to design a Yagi that actually is supposed to work on 137.62 megahertz, to see what is possible. I actually am not displeased with the paper specs for the resulting four element 137Mhz Yagi. I designed it using the suite of programs written by David Kirkby, G8WRB. Here’s the resulting pattern at the design frequency.
Here is the specification for the resulting design…
NOTES Optimised with the genetic algorithm
FREQUENCY 137.620000
MIN_FREQUENCY 137.000000
MAX_FREQUENCY 138.000000
STEP_FREQUENCY 0.020000
ELEMENTS 4
DRIVEN 1
PARASITIC 3
ANGULAR_STEP 2.000000
#DATA_DRIVEN x y length diameter voltage(r) voltage(i)
DATA_DRIVEN 0.38625 0.00000 1.02524 0.00475 1.00000 0.00000
#DATA_PARASITIC x y length diameter
DATA_PARASITIC
0.00000 0.00000 1.08740 0.00475 reflector
0.95210 0.00000 0.94818 0.00475 D1
1.78574 0.00000 0.92937 0.00475 D2
And here is the analysis over the entire 137-138 Mhz band.
# Driven=1 parasitic=3 total-elements=4 design=137.620MHz # Checked from 137.000MHz to 138.000MHz. f(MHz) E(deg) H(deg) R jX VSWR Gain(dBi) FB(dB) SideLobes(dB) 137.000 58.1 79.6 28.25 -4.35 1.790 9.202 26.440 0.000 137.020 58.1 79.6 28.24 -4.22 1.789 9.205 26.547 0.000 137.040 58.1 79.6 28.23 -4.08 1.788 9.209 26.650 0.000 137.060 58.1 79.5 28.23 -3.95 1.788 9.212 26.750 0.000 137.080 58.1 79.5 28.22 -3.81 1.787 9.216 26.845 0.000 137.100 58.1 79.5 28.21 -3.68 1.786 9.219 26.936 0.000 137.120 58.0 79.4 28.20 -3.55 1.786 9.222 27.023 0.000 137.140 58.0 79.4 28.20 -3.41 1.785 9.226 27.104 0.000 137.160 58.0 79.4 28.19 -3.28 1.785 9.229 27.179 0.000 137.180 58.0 79.3 28.18 -3.14 1.784 9.233 27.250 0.000 137.200 58.0 79.3 28.17 -3.01 1.784 9.236 27.314 0.000 137.220 58.0 79.3 28.16 -2.87 1.784 9.240 27.371 0.000 137.240 58.0 79.2 28.16 -2.73 1.784 9.243 27.423 0.000 137.260 57.9 79.2 28.15 -2.60 1.783 9.247 27.467 0.000 137.280 57.9 79.2 28.14 -2.46 1.783 9.250 27.505 0.000 137.300 57.9 79.1 28.13 -2.33 1.783 9.254 27.535 0.000 137.320 57.9 79.1 28.12 -2.19 1.783 9.257 27.558 0.000 137.340 57.9 79.0 28.11 -2.05 1.783 9.261 27.574 0.000 137.360 57.9 79.0 28.10 -1.92 1.783 9.264 27.582 0.000 137.380 57.9 79.0 28.09 -1.78 1.783 9.268 27.582 0.000 137.400 57.8 78.9 28.08 -1.64 1.784 9.271 27.575 0.000 137.420 57.8 78.9 28.07 -1.51 1.784 9.275 27.561 0.000 137.440 57.8 78.9 28.06 -1.37 1.784 9.278 27.539 0.000 137.460 57.8 78.8 28.04 -1.23 1.784 9.282 27.509 0.000 137.480 57.8 78.8 28.03 -1.10 1.785 9.286 27.473 0.000 137.500 57.8 78.8 28.02 -0.96 1.785 9.289 27.429 0.000 137.520 57.8 78.7 28.01 -0.82 1.786 9.293 27.379 0.000 137.540 57.7 78.7 28.00 -0.68 1.786 9.296 27.322 0.000 137.560 57.7 78.6 27.98 -0.55 1.787 9.300 27.258 0.000 137.580 57.7 78.6 27.97 -0.41 1.788 9.304 27.189 0.000 137.600 57.7 78.6 27.96 -0.27 1.788 9.307 27.114 0.000 137.620 57.7 78.5 27.95 -0.13 1.789 9.311 27.034 0.000 137.640 57.7 78.5 27.93 0.01 1.790 9.315 26.948 0.000 137.660 57.6 78.4 27.92 0.15 1.791 9.318 26.858 0.000 137.680 57.6 78.4 27.91 0.29 1.792 9.322 26.763 0.000 137.700 57.6 78.4 27.89 0.43 1.793 9.326 26.664 0.000 137.720 57.6 78.3 27.88 0.56 1.794 9.330 26.561 0.000 137.740 57.6 78.3 27.86 0.70 1.795 9.333 26.455 0.000 137.760 57.6 78.3 27.85 0.84 1.796 9.337 26.345 0.000 137.780 57.6 78.2 27.84 0.98 1.797 9.341 26.233 0.000 137.800 57.5 78.2 27.82 1.12 1.799 9.345 26.118 0.000 137.820 57.5 78.1 27.81 1.26 1.800 9.348 26.000 0.000 137.840 57.5 78.1 27.79 1.40 1.801 9.352 25.881 0.000 137.860 57.5 78.1 27.78 1.55 1.803 9.356 25.760 0.000 137.880 57.5 78.0 27.76 1.69 1.804 9.360 25.637 0.000 137.900 57.5 78.0 27.74 1.83 1.806 9.364 25.512 0.000 137.920 57.4 77.9 27.73 1.97 1.807 9.367 25.387 0.000 137.940 57.4 77.9 27.71 2.11 1.809 9.371 25.260 0.000 137.960 57.4 77.9 27.70 2.25 1.811 9.375 25.133 0.000 137.980 57.4 77.8 27.68 2.39 1.812 9.379 25.005 0.000 138.000 57.4 77.8 27.66 2.54 1.814 9.383 24.876 0.000
This was mostly just an exercise, to understand how the software works. It’s kind of silly to use a linearly polarized antenna for the circularly polarized polar orbiting satellites (3db mismatch), but I’d still be interested in hearing any comments about this design from knowledgeable antenna design people.
It’s just that the weather has been too crappy for me to stand outside with my laptop and do the recordings. But today, the weather dawned nice, and so I did manage to record a nice western pass of NOAA17.
I keep wondering if the whole KISS principle (a personal favorite of mine) might be sensible to apply more thoroughly. Diane, VA3DB pointed me at a satellite design I hadn’t seen before: a picosat that would carry aerogel ultracaps as well as traditional nicads. It was dubbed Volksat. I think there are lots of sensible and good ideas contained here.
Well, I’ve made some headway on a project that I thought would be cool to write: porting G3RUH’s Plan 13 Satellite Prediction algorithm to a more palateable language than BASIC. I chose python, and it appears to be mostly working. It reads in the TLE orbital elements (same ones I use in “predict” or “gpredict”) and then allows you to create a bunch of satellite objects, and query their positions over time.
Here’s a screendump of a simple test program that I was running this morning:
Satellites which are above the horizon are marked in bold. They are sorted by elevation. The datafields displayed are elevation, azimuth, latitude and longitude of the subsatellite point, velocity, the Doppler velocity, and the frequency of a signal Doppler shifted from 435.845Mhz (just a value I did to check, since I was using PolySat CP3 at the time, which has APRS telemetry downlinked on that frequency). The code requires some additional cleanup, and once I have it all ready to go and documented, I’ll make it available. I think it will have a lot of uses.
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.
Well, during a road trip with my wife to San Diego and back, I managed to begin to type up my notes for an upcoming tutorial article (cross fingers) on reception and decoding of weather satellite imagery. I basically reimplemented what I had, stripping it down to its barest essence, and trying to make it easy to understand, yet still capable of creating good imagery. Oddly enough though, in the process, I inadvertently seemed to introduce some aliasing artifacts which I must admit, are puzzling me mightily. Oh well.
This morning I decided to try to record a low 25 degree maximum pass of NOAA17 that occurred to the east. It starts out fairly noisy because I have a pretty high horizon to the northeast. The first is my “advanced” decoder, which is about six times longer than the simpler decoder I wrote.
The second is the same data file, processed with my “simple”, easy-to-follow decoder. The only serious feature it is lacking is the sync detection and rectification, which as soon as I can figure the simplest way to add, I’ll try to get in. The simple version is two pages of code, not including the data for the filter tables.
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.
Yawn. Recorded another satellite pass. Decoded it with my software. Played with it in GIMP. Part of the pass spoiled by an oddly synchronous signal, which seemed to also be Doppler shifted. Problem on the satellite? I don’t know.
Well, I was awake for a decent daytime pass of NOAA17, so I wandered out into my front yard, and recorded the pass. It was a westward pass, covering from Canada all the way down to Baja California in the south, and was reasonably noise free over a great amount of it. I hauled it into gimp and did a bit of judicious image editing, and this is what I came up with:
I consider this to be pretty darned good for as ad-hoc as my approach to satellite reception actually is.
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.
Okay, this isn’t that impressive, but let me explain.
I recorded about 4.5 minutes of audio from one of the weather satellites, using my small pocket recorder and a Kenwood TH-D7A. In most respects, I shouldn’t expect anything good to happen. I’m using a cheap little voice recorder. I’m using a receiver that has insufficient receive bandwidth (this is probably the worst problem) and I’m using an antenna that’s tuned for a completely different band (still, the signal strength seems excellent). Still, all that aside, you can see the clear outline of Baja.
Well, I thought it was kind of neat. Makes me wish I had the right receiver though.
Addendum: I tried to record a few minutes of the satellite using the yagi and my old Radio Shack PRO-60 scanner set to wideband mode. Unfortunately, I didn’t have the right splitter setup, so I can’t track the antenna by hand as well, and it looks like I had some significant interference in the middle. I only caught the tail of the pass, so I didn’t really get much, but it might indeed be better, even though the bandwidth is way too large.
Apparently the peakcock mantis shrimp packs a mighty wallop, which can even shatter the glass of aquariums. They also are responsible for the introduction of a new word to my vocabulary: “shrimpoluminescence”. Catch the linked video below.
USATODAY.com - Shrimp spring into shattering action
The speed of the strike (up to 50 mph, or 23 m/s) creates cavitation bubbles between the shrimp’s hammer-like heel and the struck snail. The bubbles collapse, and generate heat, light, and sound. The shell shatters with a flash too-fast-to-see, and a bang. Watch the flash (called shrimpoluminescence for another species) in the video, slowed by a factor of 900. (Courtesy of Sheila Patek, Wyatt Korff and Roy Caldwell/UC Berkeley) Though the mantis shrimp’s tough heel is impregnated with hard minerals, still she must shed the pitted, damaged surface every few months, and grow new heel armor.
I have heard of the word triboluminescence before, which might provide a few minutes of goofy fun crushing Wintergreen lifesavers.
[tags]Science,Tribolumiscence,Shrimpoluminscence,Shrimp,Livesavers[/tags]
Addendum: Link to a bonus cartoon that only makes sense if you are as geeky as me, or read the above links.
I was a little bit disappointed when I found out the total was a bit short of what the header proclaimed, but you should still check out 1001 things to do with liquid nitrogen
LN2 also works great for sweeping and cleaning hard floors such as concrete or wood.
Get a couple liters in a container, and dump it on the floor in the direction
you want the debris to travel. It picks up everything in it’s wave and if it hits a wall,
the wave will boil off and deposit the junk there. Now all you have to do is go
around the perimeter and sweep up the clutter.
