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Re: ISS Objects Was: ISS over Mexico (fwd)



On Fri, 4 May 2001, Alex wrote:

> Wouldn't it be better if stations that push ISS objects through the ISS
> would update their keps automatically, especially unattended stations?
> During this pass we got to see two different objects representing ISS:
> 
> > W3ADO>AP28,NOCALL*   :;ISS_ 122 *041313z2747.92N\08733.77WS...
> > WA6LIE>APRK27,NOCALL*:;ISS_ 124 *041444z2332657N\11509.72WS...

That is why the EPOC day is included in the object name. Just because one
has newer keps means nothing.  What is important is whether the ISS has
maneuvered between the EPOC of the older one and the EPOC of the newer
one.  Since the Space shuttle departed several days ago, the ISS is no
longer boosting its orbit and so there should be no difference in the
predicted positions no matter which Keps you use (to the accuracy required
in this application).

> On a side note, if the object names were identical shouldn't the first
> station stop transmitting the ISS object when it receives an object with
> the same name from a different station?

Yes!  Exactly.   THere were several things considered in the current
design of the ISS-POSIT-UPLINK station software...

1)  One ISS-POSIT-UPLINK station is in Maryland and one is in California.
One serves HT/Mobile users on the East Coast and the other the West Coast.
Only people along the mississippi river will likely see both and then only
occassionally.

2)  If both stations are using the SAME keps, then the objects will be
identical, and they will replace each other as they should.  If one
stations keps are older, then there could be different solutions (if ISS
had maneuvered in the meantime) and so putting the EPOC in the object name
makes sure that users have the info they need to know which one to trust.
 
3)  If as you suggest, we let just any ISS object replace any other ISS
object regardless of the accuracy of the keps that generated it, then we
would have very low user confidence because sooner or later, somone's keps
are going to be wrong...

> The originating station should cease transmitting the Object/Item if it
sees somoene else transmitting the same object/item...

This would defeat the the point of having an East coast and a West coast
ISS-UPLINK-TRACKING station.  The Califirnia station would always be
transmitting first and the East coast station would always be canceled.

I do appologize for those stations in the geographical center of the USA,
that may occasisionally get duplicate posits, but that is why the EPOC is
in the object name, so that they can decide which is a better estimate.
Also, the CALIF and Maryland staions are 3000 miles apart and never hear
each other...

de WB4APR@amsat.org, Bob

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