Mime-Version: 1.0
Message-Id: <p06240802c3684bc717a7@[134.157.27.36]>
In-Reply-To: <p06020407c367b658d4ca@[129.116.190.133]>
References: <C3676A04.BC70%flemoine@puuoo.gsfc.nasa.gov>
 <p06020407c367b658d4ca@[129.116.190.133]>
Date: Tue, 20 Nov 2007 09:33:45 +0100
To: IDS_Analysis_Forum <ids.analysis.forum@cls.fr>
From: Pascal Willis <willis@ipgp.jussieu.fr>
Subject: Re: [ids.analysis.forum] DORIS on-site meteorological data 
 (response)
Content-Type: text/plain; charset="us-ascii" ; format="flowed"

Hi Frank,

following John's email, to be more precise, we fix the dry part, 
using the a priori altitude information to do the correction and we 
estimate the remaining part in our process (called wet).

There is time constrains between successive passes (whatever the 
satellite could be) and the constraint depends on the time interval 
between the 2 passes.

the tropospheric mapping function is also an issue.

Best regards
Pascal

>Hi Frank,
>
>Do you apply any apriori to the zenith delay estimates? Do you 
>estimate the full zenith delay or only the wet part? I believe these 
>will influence the effect of missing met info.
>
>We have two results that are related to your question.
>
>With PRARE, GFZ did not always have real met info, and so some 
>climatological values were substituted in the data we were given. We 
>found it important to replace these with 'real' met info (from 
>ECMWF, I think). However, we were estimating only the wet part, so 
>it would not be surprising that we needed good a priori pressure 
>values so that the dry part could be computed with some confidence. 
>We also noted that even when we had in situ measurements, they were 
>not necessarily unbiased. We compared the met data with ECMWF and 
>removed biases where we thought it necessary.
>
>With DORIS, we found it beneficial to constrain the zenith parameter 
>estimates at the few percent level, at least in terms of recovering 
>good station coordinate estimates. In this case, we were estimating 
>the full zenith delay (wet and dry). The models are certainly good 
>to a few percent, and it is not unreasonable to use this to 
>constrain the solution. However, this again assumes reasonably good 
>values for the met info. I think this is described in one of our 
>early DORIS papers...perhaps "Absolute Positioning Using DORIS 
>Tracking of the SPOT-2 Satellite, Geophysical Research Letters, 
>19(20), 2039-2042, October 23, 1992 (M. M. Watkins, J. C. Ries, and 
>G. W. Davis)".
>
>I would expect that estimating the full (wet+dry) zenith delay, 
>without any apriori constraint, should be relatively insensitive to 
>not having the true met info. But if one is applying some apriori, 
>or assuming that the dry part is known so that one needs estimate 
>only the wet part (both are good strategies to improve the results), 
>then I would think that one needs at least a reliable value for the 
>pressure.
>
>Best regards,
>John


-- 
------------------------------------------------------------------------------ 
Pascal Willis
Institut de Physique du Globe de Paris	Ph : +33-(0)1-44-27-24-84
Etudes Spatiales et Planetologie	FAX: +33-(0)1-44-27-73-40
4, place Jussieu, Case 89		Em : 
willis@ipgp.jussieu.fr              
Paris 75252 Cedex 5, France		http://www.ipgp.jussieu.fr/~willis/
------------------------------------------------------------------------------