Subject: [ids.analysis.forum] Re: surface forces
From: "John C. Ries" <[email-masked]>
To: Remko Scharroo <[email-masked]>, Steve Klosko <[email-masked]>
Cc: "[email-masked]" <[email-masked]>, Cerri Luca <[email-masked]>, Eelco Doornbos <[email-masked]>, Marek Ziebart <[email-masked]>, Frank Lemoine <[email-masked]>, Scott Luthcke <[email-masked]>, Michiel Otten <[email-masked]>, Eric Leuliette <[email-masked]>, "Mercier Flavien (contact)" <[email-masked]>, "[email-masked]" <[email-masked]>, Nikita Zelensky <[email-masked]>
Date: Fri, 5 Mar 2010 21:04:17 -0600

Hi Remko,
>
>
>You are absolutely right that a radial offset of the CoM is hardly 
>"tamed" by the tracking data. I haven't done the exercise lately, 
>but I do remember checking this out in the ERS-1/2 era when I was 
>doing both short-arc (one rev or less) and long-arc (5.5 days) 
>solutions.
>
>In the long-arc solutions I could move the CoM up or down several 
>centimeters without affecting the orbit even as little as a 
>millimeter. Yes, the tracking residuals worsened, but the orbit 
>stayed basically in place.
>
>So in the framework of TOPEX and a "wobbling" solar panel, the 
>dynamic orbit determination will still position the CoM in the right 
>place, only the tracking data residuals would be slightly impacted. 
>To get the right sea level, we would need to make a geometric 
>correction to the altimeter range because the vertical distance 
>between the antenna feed and the CoM changes over time.

I agree that a constant bias in the CoM cannot be accommodated with 
the parameters we usually adjust, and so it is left in the residuals. 
For this reason, we can estimate the vertical antenna phase offset, 
for example. But if we were to estimate a CONSTANT radial 
acceleration, it would easily go into that parameter and bias the 
orbit. This is exactly what happened with the early JPL orbits for 
T/P, until they switched the reduced-dynamics to use 1/revs.

In the same way, a variable CoM, oscillating at the orbital period 
(as the solar arrays do) can go into the orbit since the 1/revs will 
accommodate that signal. Thus there is at least the potential for T/P 
to have been affected by the solar array warp. But given that we 
don't know the initial deployment angle error, we don't have precise 
models for the warping effect from heating (or the temperatures) 
without ground tests, and there are multiple hinges in the array that 
add to the uncertainty, we really have no hope of forward modelling 
this. Hence it hasn't been considered very much.

We did do a test in 1998 where Dochul Yang looked for a correlation 
in the crossovers for the first 50 cycles of T/P with the solar array 
warp (based on Kubitschek's dissertation), and we saw nothing 
convincing. (and he did check that the effect wasn't just cancelling).
>
>
>Let me remind everybody, that TOPEX seems NOT the issue in this 
>whole story. TOPEX does not seem to exhibit the 58-day cycle in sea 
>level, whereas Jason-1 and Jason-2 DO. One of the images in Eric 
>Leuliette's presentation, comparing Jason-2 to Envisat clearly shows 
>the S2 aliased variation, whereas we know Envisat cannot have an S2 
>alias (or rather, it is aliased to a constant).

This is the interesting part. I would ask for two clarifications

1) Is the 58-day cycle not seen because T/P is noisier or is the 
series from T/P good enough that we are sure that signal isn't there? 
i.e., is the determination of a lack of a 58-day cycle a clear and 
confident one? It's very important to this discussion to be sure 
about this. Is there evidence of a periodicity at twice that in any 
of the series?

2) Are the peaks associated with high beta prime or low? In the Jason 
POD paper that's about to be published, it was shown that the RMS 
differences were significantly larger during high beta prime, and the 
Z shifts were maximized where the cross-track differences were 
maximum, which was also at high beta prime. I saw the same thing in 
my tests.

The second item puzzles me more than a little. The geometry is right 
for SRP maximizing the cross-track effect at high beta-prime, but I 
have to wonder why the cross-track 1/revs don't 'whiten' the 
cross-track differences. It seems to me that the cross-track 1/revs 
ought to be trying to accommodate the cross-track orbit error, and 
they should succeed in an average sense, with some level of scatter 
about that. Yet the comparison in the POD paper, and indeed in my own 
test, the Z-shift is almost a pure scaling of the beta-prime angle. 
But as this is a purely 120-day signal, it is not necessarily 
relevant to the problem at hand.

So I come back to two scenarios.

1) T/P, with some sort of 58-day orbital error, has biased the S2 
tide to remove it from the T/P orbit. Now S2 shows up in Jason-1 and 
2. We can't test S2 with the sun-synchronous satellites. And S2 is 
hard to test with GRACE as it is also long-period there. This all 
depends on how much T/P has influenced the tide models being used. If 
the tide models used a lot of Jason-1 data, then this scenario 
becomes implausible.

2) Something besides SRP is affecting the Jason-type spacecraft, 
where a 1/rev variation along-track (or radial, or both) is occurring 
that is modulated by two-beta prime (for example, something that 
would one-beta-prime but where the yaw flip has created a symmetry 
that makes it two-beta-prime). If this is really occurring just in 
high beta-prime, then it is quite puzzling since this would seem to 
be the most steady temperature regime, and where the solar panels are 
articulating the least.

Best regards,
JR