[time-nuts] Time and frequency practical exercise 2018 late quarter; precision measure of 432mhz band Sat in Lunar Orbit

Bob kb8tq kb8tq at n1k.org
Sat Nov 18 11:14:10 EST 2017


> On Nov 18, 2017, at 5:38 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> Hi,
> On 11/18/2017 02:16 AM, Hal Murray wrote:
>> kb8tq at n1k.org said:
>>> Ok, 1 Hz at 437.5 MHZ is roughly 2 ppb. That is pretty much “slam dunk�
>>> accuracy with a GPSDO. Much easier to obtain and set up in a school
>>> environment. The key will be orbit estimation for the +/- doppler part of
>>> it.  Orbit estimation is not quite a slam dunk sort of thing. The GPSDO
>>> would also give accurate location. Even with good orbit data, the solution
>>> still requires a good location estimate.
>> What is the orbital period?  It would be fun to plot the Doppler over time
>> and see if you can get something that looks like a big chunk of an orbit.
>> Ugh.   What is the Doppler due to the Earth's rotation?
> You need to compensate for your position, because it would lower due to longitude naturally.
> Yeah, and then the moon isn't in perfect circular orbit either.

Effectively you have multiple things spinning around in various ways. The “orbit” isn’t a single thing
in this case. Earth gravity “lumps and bumps” impact GPS orbits. There are a lot of things to have
lumps and bumps in this caae. Depending on the orbit relative to this and that, you may have a fairly 
limited observation time on each pass. This and that accumulate on the “dark” part of the orbit, but 
you can’t observe it as it happens. You simply see a jump (versus prediction) at the start of the next


> The sat is also in a not so perfect orbit, so it would also needed to be measured and characterized.
> Fun problem.
> I realized that my on the fly least square algorithms would be nice to adapt to this problem.
> Cheers,
> Magnus
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