[amsat-bb] mobile satellite operation...
kf6kyi at gmail.com
Wed Jul 23 14:18:00 PDT 2008
During one of the many recent discussions, somebody mentioned that
Doppler tracking wasn't really that difficult, all one needed to do was
download InstantTrack or whatever on some old, ancient PC, and that you
would need to have a PC around to track the satellite anyway.
But here's the thing: I _don't_ need a pc to track the sats. I operate
handheld with my little TH-D7A in one hand, and my trusty Arrow in the
other. Before each pass, I run a little Python satellite prediction
program that I wrote to dump information about the pass, which looks like:
AO-51 will be visible from grid CM87ux starting in 04:44:31 at 01:44:03
01:44:03 +0.0° 132.0° ↘ 18.8°N 102.4°W AOS
01:45:00 +3.3° 127.9° ↘ 22.3°N 103.2°W
01:46:00 +7.2° 122.4° ↘ 25.9°N 104.0°W
01:47:00 +11.4° 114.8° ↘ 29.5°N 104.9°W
01:48:00 +15.8° 104.4° → 33.1°N 105.9°W
01:49:00 +19.8° 90.3° → 36.7°N 106.9°W
01:50:00 +22.1° 72.7° → 40.3°N 108.0°W
01:50:18 +22.3° 67.1° ↗ 41.4°N 108.3°W MAX
01:51:00 +21.5° 54.1° ↗ 43.9°N 109.2°W
01:52:00 +18.3° 37.9° ↗ 47.5°N 110.5°W
01:53:00 +14.1° 25.5° ↗ 51.1°N 112.0°W
01:54:00 +9.7° 16.5° ↑ 54.6°N 113.6°W
01:55:00 +5.7° 10.0° ↑ 58.2°N 115.6°W
01:56:00 +2.0° 5.2° ↑ 61.7°N 117.9°W
01:56:35 +0.1° 2.9° ↑ 63.7°N 119.4°W LOS
At home, I know basically where the compass points line up, and for away
from home contacts, I carry a little compass. With a tiny bit of
practice and a reasonably accurate digital watch, you get pretty good at
just tracking the antenna naturally over the course of a pass. With the
FM birds, I just listen for the signal getting raspy, and tune down in
frquency as needed.
For me, I don't want to carry a PC. Or a laptop. I have my little FT-817
that's actually pretty good to use on a strap around my neck, but if you
have to sling a laptop and operate it all simultaneously, you're
So, here's the idea: the Python library I wrote is actually pretty
simple. It's a direct port of G3RUH's Plan 13 algorithm, and runs fast
enough to be entirely useful. It also has the capability of doing
Doppler translations at a reasonable rate, even on a fairly modest
microcontroller. (In fact, the way I started on this project was noting
that G6LVB's tracker implements the same algorithm to provide automatic
antenna tracking). So, why not build a little battery powered
microcontroller unit that provides Doppler tracking for the FT-817ND?
You could load the orbital elements onto (say) and SD card on your PC in
the house, and then jam the little thing into a smallish battery powered
microcontroller, and it would provide automatic Doppler tuning. Then,
truly mobile operation would be possible on the linear birds, without
having to bring laptops or juggle with the reasonably fast Doppler of VO-52.
Yes, the setup isn't quite ideal: you don't get full duplex, but in most
other respects, it would be totally adequate.
You could even use an off the shelf controller like:
which has a USB->host connection, two serial ports, and costs $129. This
establishes a sort of greatest upper bound on how much such a project
need to cost.
What do people think? Will the lack of full duplex kill me in this
endeavor? What am I overlooking?
Mark "trying to turn ham radio into just button pushing" VandeWettering
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