Subject: FW: RE : ITRF2000 orbits and SAA model (Comments by Luca Cerri)
From: "Frank G. Lemoine" <[email-masked]>
To: <[email-masked]>
Date: Thu, 08 Feb 2007 19:43:14 +0000

------ Forwarded Message
From: "Cerri Luca" <[email-masked]>
Date: Thu, 8 Feb 2007 19:52:08 +0100
To: "John C. Ries" <[email-masked]>, "Frank G. Lemoine"
<[email-masked]>, "Nikita Zelensky" <[email-masked]>
Cc: <[email-masked]>, "Mercier Flavien"
<[email-masked]>, "Houry Sabine" <[email-masked]>, "Perrachon
Pascal" <[email-masked]>,
<[email-masked]>, "Bruce J Haines" <[email-masked]>,
"Lemoine Jean-Michel" <[email-masked]>, <[email-masked]>,
<[email-masked]>, <[email-masked]>,
<[email-masked]>
Subject: RE : ITRF2000 orbits and SAA model

Dear all, 

There seem to be many subjects to be discussed concerning the Doris
instrument on Jason,
and I hope to bring a contribution to this discussion.

1) Ionospheric correction and SAA
Frank is right. As it has been explained in the past, the current way to
calculate 
the ionospheric correction is not optimal, as the result depends on the on
board 
and ground frequency biases (and on orbit error as well). A more correct way
to 
compute the ionospheric correction has been proposed (the approach is
similar to GPS), 
which depends only on the measured quantities (phase measurements on the two
signals). 
In the case of Jason, due to the behaviour of the on board oscillator,
the difference between the current and the improved correction becomes
significant 
in those passes that are very perturbed bye the SAA.
In figure 1, obtained for cycle 184, I show on the first line the difference
between 
the new and the current ionospheric correction when the model is NOT
applied, 
on the second line the on board frequency correction obtained from the SAA
model, 
on the third line the difference between the two ionospheric corrections
when 
the SAA model is applied. I use different colors to identify clearly
different passes. 
The difference between the two corrections is clearly reduced when using the
SAA model. 
In the very near future we will adopt the new ionospheric correction, which
is completely 
independent from any frequency offset.
In figure 2 I show the difference between two doris-only orbits obtained
with the two 
different ionospheric corrections.
Note the difference is completely negligible (same result with or without
SAA model).

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?

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 ?

Best regards

Luca

-----Message d'origine-----
De : John C. Ries [mailto:[email-masked]]
Envoyé : mercredi 7 février 2007 22:20
À : Frank G. Lemoine; Nikita Zelensky; Cerri Luca
Cc : [email-masked]; Mercier Flavien; Houry Sabine; Perrachon
Pascal; [email-masked]; Bruce J Haines; Lemoine Jean-Michel;
[email-masked]; [email-masked]; [email-masked];
[email-masked]
Objet : Re: ITRF2000 orbits and SAA model

Hi Everyone,

This seems an opportune time to bring up an issue that has been at
the back of my mind regarding the SAA model. It's not surprising that
the second half of the first segment in the plots Frank sent out is
affected by the SAA model, since this is where the effect starts to
get pretty large. A question that has bothered me since we first
started considering this SAA model is this:

The SAA model is designed to compensate for the frequency
acceleration that the USO experiences. However, does it also 'undo'
the 'damage' in the preprocessing where a cycle-mean value for the
frequency drift is applied by CNES (where the SAA model is not used)?
This was based on the idea Pascal Willis originally introduced....an
excerpt from which I include as an attachment. If the SAA model
accommodates the short period variations in the frequency drift, does
it also 'undo' the effect on the 10-day mean?

You may recall that I introduced in my processing a 'global frequency
drift' parameter to compensate exactly for this, and it clearly
increased as the SAA effect got worse. I showed that the fits were a
little better, but more important, the apparent biases in the station
heights went back to near zero. My DORIS fits with this model are
0.351 mm/s, but this probably cannot be directly compared to the GSFC
fits of 0.3695 mm/s because of my relatively stringent editing
(probably more severe than anyone else uses).

If this global drift bias is left in, it would cause an apparent
'global station height' change. I would not have expected a big
effect, but we are only talking about about 4 mm in the plots Frank
sent out. With an imperfect distribution of stations N-S, a global
height change might cause the orbits to drift along the Z-axis. When
this global frequency drift is 'reset' when the new USO is used, it
could explain the features in Frank's plots. If the GSFC orbits
included this 'global frequency drift' parameter, would the drifts
disappear?

I can repeat the first 90 cycles with and without my 'global
frequency drift' parameter, both using ITRF2005 for added clarity. If
this explanation is correct, I should see a similar drift in my
orbits. As Frank pointed out, the station coordinate solutions should
also show a scale drift, that would 'reset' at the time of the USO
change.

As an additional comment about ITRF2005...it is quite clear that
ITRF2005 must be scaled downward by about 1 ppb for the best results.
It is unambiguous in the LAGEOS-1, LAGEOS-2 and Jason-1 SLR results.
Not applying the scale leaves in about a 6 mm height bias, and with
the uneven distribution of SLR stations, can measurably skew the
orbit. It is possible this is less clear in the altimeter crossovers,
since correlated errors in the body-fixed frame can cancel out, but
it's clear in the SLR residuals. My understanding is that it is the
recommendation of the ILRS that the SLR coordinates from ITRF2005 be
scaled down by 1 ppb. Whether this is also done for DORIS is left to
the user...I see no strong benefit other than a very slight decrease
in the DORIS residuals.

Best regards,
John

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