[amsat-bb] Re: SDX transponder timing delay measurements?

Tom Clark, K3IO K3IO at verizon.net
Tue Sep 12 12:24:48 PDT 2006

Ken wrote:

>> As a design goal, it would be helpful to the 
>> OD process if the delay could be made consistent 
>> within about +/-5 micro-sec -- i.e., 1500 meters 
>> (smaller is better).  It is also important to
>> quantify the standard deviation to that bias is 
>> since that is also factored into the OD process.

Let me chime in since this is one of my specialty areas.

It has been my hope/plan that the delays will be very accurately
calibrated, and that we will have "certified" ground stations that are
also well calibrated. I've been pushing to have ALL local oscillators on
EAGLE and all timing clocks (remember that the 100-200 kb/s sampling
clock is yet one more LO in the system) derived from a single, high
stability clock. In addition to ranging, it is my hope that we can have
an amateur, global time synchronization capability at the usec level.

When (occasionally) it is time to do the ranging task, it might be
necessary to swap in a different, optimized software into the SDX --
this would be my backup just in case the delay is not deterministic when
normal SDX is running.

Even if the delays are not as accurately determined as I hope, I see no
reason that they will not be stable over times of minutes. With this
being true, then we can do very accurate DIFFERENTIAL delays using N
"certified" ground stations. If you do this at ~20,000 km range with
stations ~1 earth radii, the difference observable only suffers from a
factor ~3 degradation. Every ground station pair you add (the number of
pairs M increases with the number of stations N as M=(N)*(N-1)/2) gives
a statistical improvement of of sqrt(M-1); hence the geometrical loss
from using a differenced observable is  easily made up on the ground.

73, Tom

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