Subject: [IGSLEO-6] Re: LEO Workshop summary
From: Jim Ray (USNO 202-762-1444) <[email-masked]>
To: [email-masked]
Cc: [email-masked] (Robert Weber (TU Wien))
Date: Thu, 12 Apr 2001 15:00:55 EDT

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IGS LEO Mail      12 Apr 12:00:59 PDT 2001      Message Number 6
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Author: Jim Ray

Regarding the nice summary of the LEO workshop, I previously raised a
question concerning the stated need for high-rate clocks:

>                               There was collective agreement that IGS 
> clocks at 30s or 10s would be of great value to these missions.
~~~~~ cut
> Summary requirements:
> - IGS 10-30sec clocks
> - Retrieval of occultation profiles require <0.1mm/sec velocity 
> determination and ~10cm LEO  POD
~~~~~ cut
>                           Currently they provide 30s clocks from 5min 
> estimates; proposed 1s from 5 min estimates & 1s ground data. Now 
> that SA is off, a new proposal is to investigate interpolation & 
> extrapolation for 1s GPS clocks from 5 min estimates & high rate 
> ground data.
~~~~~ cut
>                                   Velocity should be known to 0.1mm/s 
> (for occultation) and position to ~10cm.

I saw these requires again at the recent EGS meeting.

My questions concern the specific need for IGS clocks at a rate greater
than the present 5-min intervals.  Does this include only GPS satellites
or also tracking stations?  Now that SA is off, why can't this requirement
be met adequately by interpolation of the current 5-min clock values?

In the position paper by Jim Zumberge and Gerd Gendt for the IGS Network
Workshop (July 2000) -- Figure 2 -- I infer that the maximum interpolation
error between 5-min points (at 150 sec) would be about 1.7 cm = 57 ps.  I
seriously doubt that the precision of the IGS combined clocks is really
much better than 100 ps at each tabulated epoch, so the interpolation error
should not seem to be a great addition.

Maybe I'm missing something (e.g., the rate of change of the clocks?)
but it just does not seem obvious to me that samples more frequently than
5 minutes really buys you very much.  That would seem to be especially
true for the newer Block IIR oscillators which are very excellent frequency
standards.

Thanks and best regards,
--Jim