Subject: RE : ITRF2000 orbits and SAA model
From: "John C. Ries" <[email-masked]>
To: [email-masked]
Date: Thu, 8 Feb 2007 15:49:09 -0600

Hi Luca, Everyone,

At 7:52 PM +0100 2/8/07, Cerri Luca wrote:
>2) Long term frequency variations
>We apply the SAA model AFTER having removed the long term frequency 
>drift (difference between
>on-board time and TAI is modeled as a second order polynomial). So 
>we would expect to find
>a zero mean in the short term frequency correction coming from the SAA model.
>Just as an example, If I calculate the mean of the short-term 
>frequency correction over cycle
>184, I obtain a non zero value (~ 0.013 Hz). this leads to a clock 
>offset of about 5.5 microsec
>after 10 days (on the 2GHz nominal frequency). In figure 3 I show 
>the difference between two
>doris only orbits, with and without SAA correction; the mean along 
>track difference is about
>1 cm, a bit less then what I was expecting. Nikita, do you find any 
>difference in the along-track
>>doris/laser bias when using the SAA model?

Upon further thought, I guess I would expect the SAA model to restore 
the frequency to the correct mean.

Note that my bias-drift parameter, combined with editing the affected 
data, has the same net result as the SAA model, which is to restore 
the right frequency drift. The downside of my approach is that I have 
to sacrifice a lot of the data. Borrowing a figure from the previous 
email to illustrate what I do, I cut out the data in the vicinity of 
the large frequency change, leaving a set of data that can be 
approximated with the same slope. This likely works less well when 
the SAA effect starts to become very serious.

>
>3) SAA and network degradation
>Starting from around cycle 60, we see a degradation of the doris 
>network (figures 4 and 5).
>There are about 10 beacons disappearing, 8 of which are not in the 
>SAA region. This should
>have an impact in the comparison between SAA and non SAA orbits. Any 
>difference brought by
>the SAA correction would be stronger when the number of measurements 
>outside the SAA is reduced.
>I wonder if this can explain much of the different trend that is 
>observed in the Z bias before
>cycle 91 when using the SAA model. What do you think ?

I found my earlier analysis, which I showed at the 2004 DORIS 
workshop. What I found was essentially zero sensitivity to the SAA 
effect for Z, whether I just left the 'bad' data in, edited it out, 
or applied my global bias-drift parameter. This is consistent with my 
other test where I scaled all the DORIS heights (by about 7 mm). The 
RMS of the residuals improves as you remove the error, but the orbit 
seems insensitive.

However, an important caveat...I still use the 'dynamical' approach 
for my processing. Only slow variations in the orbit can be 
accommodated. 'Reduced dynamics' approaches, however, if the dynamics 
are sufficiently relaxed, could accommodate the reference frame 
variations that would occur within a day, and it is easy to show that 
the 1/rev empiricals are correlated with the Z-centering.

My question for Nikita would be how 'relaxed' the empirical 
parameterization was for those SLR/DORIS orbits.

Best regards,
John

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