From: "Lemoine, Frank G. (GSFC-6980)" <frank.g.lemoine@nasa.gov>
To: "ids.analysis.forum@ids-doris.org" <ids.analysis.forum@ids-doris.org>
Date: Sat, 9 Oct 2010 03:56:45 -0500
Thread-Topic: TR : RE : DORIS residuals vs elevation. Comments.
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Subject: [ids.analysis.forum] TR : RE : DORIS residuals vs elevation.
 Comments.

Dear IDS analysis forum.

I forward below the following comments and discussion from Flavien Mercier (CNES).

Frank L.

**************************************************************
**************************************************************
________________________________________
De : Flavien Mercier [flavien.mercier@cnes.fr]
Date d'envoi : vendredi 8 octobre 2010 11:18
À : Lemoine, Frank G. (GSFC-6980)
Cc : Cerri Luca; Zelensky, Nikita P. (GSFC-698.0)[SGT, INC]; BERTHIAS JEAN-PAUL; frank.lemoine@gsfc.nasa.gov; Le Bail, Karine  (GSFC-698.0)[NVI INC]; Auriol Albert; Ferrage Pascale; Flavien Mercier
Objet : Re: RE : DORIS residuals

Hi Frank,

I agree with you regarding some aspects of the antennas
characterization. There are different subjects in this question of maps.

- for the Doris case, it we are strongly dependent from the pattern of
the passes in the antenna (ie the fluctuations are spatially very rapid
along a track path, and we have no observation between the different
possible tracks). Mixing the different satellites data will be
difficult, because these data reflect only the slope of the phase map
along a direction, which depends on the satellite track shape. So
efficient phase maps valid for any satellite seem difficult to handle
using Doppler residuals. On a lot of stations, the perturbations are at
high elevation (as shown in Nikita's plot), and are with small
wavelength, so they increase the rms, but will not degrade the
positioning for example.

- these effects observed in the residuals are very sensitive to the
local characteristics of the antenna implementation (see the metal roof
of Fairbanks for example). This means that they can only be
characterised in the complete environment for each station. This is also
the case for the antenna mounted on the satellite. It is difficult to
test this correctly on ground. Some stations have a correct behaviour
for the high elevation residuals, perhaps this shows better what
corresponds to an ideal implementation ?

- It is not reliable to use the observed residuals to improve the
measurements corrections, without external modelling. For example,
suppose that we characterise a correction pattern for a given location
of an antenna, then this pattern will correct the current errors of the
used solution, event if these errors are from an other origin than the
antenna (for example a bias in the station location). Same
considerations apply for tropospheric modelling studies for example :
the maps will be biased by the troposphere model used.

- the phase center determination for the antennas was performed using
elementary RF tests, and extracting from the measured phase pattern the
equivalent phase centers (for each frequency). Perhaps Albert Auriol can
give us informations on the residuals obtained in these measurements.

So I think that a common antenna correction map (for example using
dedicated tests on the starec antenna)  will not help a lot for the
improvement of Nikitas residuals. The main part at high elevation is due
to local implementation of the antenna

For the positioning, the problem is the consistency between the
satellites : the ground test uses a certain distribution of the
measurements to determine the phase centers, and this will not produce
the same effect for the different satellites, or the different
latitudes. I agree with you, there is something to study here.
However if we change the ground antenna map conventions, jump in the
coordinates series will occur. And we have no sufficient information on
the satellite side....

Flavien


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