[time-nuts] Arduinos in time and near space

[Jim Lux]

On 1/19/14 6:41 PM, Lizeth Norman wrote:
> Hi all,
> Funny how this topic of the arduino time library comes around. Have been
> following your conversations regarding the precise nature of arduino time
> (gps time aware)
> Is it possible to write (assuming the poor little creature would do it) a
> piece of code, that given your lat/long, the time and a two line element
> set for an orbiting object, such as the ISS, that would give you the
> acquisition of signal time/loss of signal time and so forth?

Of course, and it could easily generate real time look angle (az and el).

You might want to go with something like a teensy3 rather than an 
Arduino: faster, lower power, a LOT more memory space, and cheaper.


I don't know that anyone has ported a decent orbit propagator (like SGP4 
from celestrak.com, which is sort of the standard) to the Arduino 
environment.  I don't think it would be all that hard.

Having done this kind of thing quite a few times now, I would recommend 
you work in XYZ  (either Earth Centered Fixed or Earth Centered 
Inertial) kind of coordinates and do things with matrix computations, 
rather than trying to do the trig solid geometry approach. (e.g. don't 
use what's in the ARRL books).

ISS is at one XYZ coordinate, you're at another XYZ, so the vector from 
you to ISS is (ISS-You).  The Doppler can be computed from the 
difference in the two velocity vectors.  Look angles are computed by 
looking at your local vertical vector.

If you need better than 1 degree pointing, you DO need to take into 
account that the earth is not spherical.



> Am interested in this as part of a balloon project. My part of the payload
> would be a small 5w radio with a modem, driven by an arduino. Was thinking
> APRS on the high frequency bands, as well as the digipeater on the ISS.
> BTW: Had a look at the scheduler in the Arduino Time (official site)
> download. Looks like it'd be pretty easy to set up.
> Norm n3ykf
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