[amsat-bb] Re: ARISSat telemetry and Doppler tuning

Phil Karn karn at philkarn.net
Thu Aug 18 21:01:33 PDT 2011

On 8/18/11 6:59 AM, Alan P. Biddle wrote:
> Ron,
> That behavior is common around TCA, Time of Closest Approach.  At that time
> everything is changing rapidly, and slight errors in QTH position, system
> clock, and Keps can be magnified. 

As everybody knows, even tiny errors in the Keplerian mean motion (MM)
will accumulate over time and put the satellite ahead of or behind where
it really is. That will show up as a large frequency error when Doppler
is changing rapidly at closest approach, but a small (or no) error near
AOS and LOS when Doppler isn't changing rapidly anyway.

So instead of manually tuning the radio to compensate for unpredicted
Doppler, why not manually tune the element set? I recommend manually
adjusting the "mean anomaly (MA) at epoch" until the satellite is on
frequency. This parameter says where the satellite is within its orbit
relative to perigee at the epoch time, so changing it slides the
satellite forward and backward along its orbit. If this is indeed the
main error in the element set, then correcting it should cause the
satellite to track normally even through closest approach.

This has the added advantage of correcting your antenna pointing as well
as your radio tuning.

This is probably better than tuning the mean motion (MM) as it would be
difficult to distinguish between the two during a single pass. The MM is
the time derivative of MA, so the effect on orbital position from a
given change in MM depends on how long it's been since the epoch; the
older the eleset, the greater the resulting change in position from a
given change in MM. A given change to MA always moves the satellite by
the same amount regardless of how long it's been since the epoch.

Properly adjusting the MM separately from the MA would require careful
Doppler measurements of a number of orbits.


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